Although penetrating keratoplasty (PK) is a regulated surgery, all steps are likely to present incidents. In this chapter we will review the possible complications that may appear during the surgical act, as well as the maneuvers that may be useful to avoid or correct them.

COMPLICATIONS WITH ANESTHESIA

A PK can be performed under general or local anesthesia with equal safety. The importance of both lies in achieving, among other things, the complete akinesia of the eyelids and the extra ocular musculature, in order to eliminate intraocular pressure (IOP) elevations in relation to muscle contraction. General anesthesia is indicated in patients at high risk of choroidal hemorrhage, in pierced eyes, at pediatric ages and in those with poor cooperation during surgery.

Unlike the general one, which does not produce ocular complications, the forms of local anesthesia, both the peribulbar with blocking of the eyelids and the retrobulbar¹, can produce very serious ocular complications. Edge² found 7 perforations in 50,000 surgeries (0.014%). These 7 eyes were long, with a posterior staphyloma that would be the most important predisposing factor. Orbital hemorrhages, optic nerve trauma and muscle injuries can also occur. Other less frequent complications are intradural injection, which could result in respiratory arrest and intravenous injection that would lead to arrhythmias. To avoid these complications, a refined technique is required. In case of any of the mentioned problems we must postpone the surgery until its resolution.

EXPOSURE OF THE GLOBE

The choice of eyelid speculum is important to avoid involuntary eyeball compressions. Such compression increases the IOP and produces a distortion of the globe that facilitates poor trepanation and greater astigmatism³. Usually we use Lieberman's speculum with aspiration, capable of effectively compensating the pressure of a tight orbit.

FIXING THE GLOBE

The fixation methods are mainly two: the sutures passed under the rectus muscles (upper and lower) and the fixation rings. The first serve to immobilize and stabilize the eye. In case of injuring one of the arteries that accompany the muscles, a hematoma with postoperative restriction in the vertical gaze can occur. This complication, although infrequent, is reduced with the use of completely atraumatic needles – vascular type. An aggressive needle could even puncture the eyeball and cause a retinal hole and a possible retinal detachment. If it occurs, McNeill⁴ suggests the application of cryotherapy on the perforated area.

Fixation rings like Flieringa’s serve to stabilize the globe and prevent scleral collapse⁵. They are used in aphakic patients, in pediatric age, in high myopic patients and in all those in whom we suspect a low scleral stiffness. They are fixed to the sclera by 6-8 sutures of vicryl or 7-0 silk, which are passed from the limbus to the periphery. During the placement of these rings can occur from a small hyposphagma, by damaging a vessel of the episclera, to eye perforations. The prophylaxis of these problems is based on a correct technique and the use of spiked needles. There are conflicting opinions about the usefulness of the rings. Some authors especially value their use as a point of attachment during trepanation, while Vajpayee⁶ and Rudd⁷ argue that, if not properly sutured, they could distort the balloon and ovalize the cornea and induce more astigmatism.

PREPARATION OF THE GRAFT

Obtaining a graft under the appropriate conditions is a critical step for PK and should be performed before trepanation of the receptor⁸. It can be done from a whole donor eyeball or from a previously separated and preserved corneoscleral cap. As it is known, trepanation on the epithelial side gives rise to a graft 0.25-0.50 mm greater than on the endothelial side⁹. It is essential to use a very sharp material, especially when carving by pressing (die cutting). Materials that are not sharp, poor centering of the trephine and poor grip of the globe or donor tissue can make the graft defective⁸. Carving the graft in a hypotonic, hypertonic globe or with marked oscillations of tension will provide a non-cylindrical graft. A tearing of the Descemet¹⁰ may occur. The resulting button will be irregular, oval, beveled or staggered, which will hinder its suture in the recipient bed and will produce greater astigmatism or other postoperative complications.

The loss of endothelial cells is another important intraoperative complication, since the viability of the graft depends on them. Brightbill¹¹ argues that die-cutting is preferable to rotary cutting for the donor cornea. Once the button is obtained, it is convenient to protect it (in a Petri dish) with the endothelium covered with viscoelastic. For this function, Miyata¹² concludes that the combination of Viscoat and Healon protects better than the second agent alone. Although it is rare, it is not impossible that the graft may fall on the transfer from the receiving area to the receiving eye, with the result that it is contaminated or lost. Apart from the precaution, it is advisable to work in both phases in neighboring areas of the surgical field itself. In any case, it is advisable to have more than one donor cornea for the intervention, before any anomaly when carving a new graft and, if there is no other cornea, to defer the surgery⁸.

TREPANATION OF THE RECEPTOR

Before trepanning the patient's cornea, it is essential to make a well-centered mark with the trephine. The cornea should be dried, which will allow to see the groove created when pressing with the trephine³. An eccentric trepanation will result in an increased risk of immunological rejection as well as an increase in postoperative astigmatism³. To avoid this complication, we must mark the corneal geometric center. In eccentric pupils a certain correction can be made, but it should be remembered that a pupil in pharmacological meiosis may not coincide with the physiological one. The marking mode consists of measuring the vertical and horizontal meridians and dividing their length by two. In the place where they both cross, one will find the corneal center. If it has not been marked correctly, it should be re-dried and marked again until a perfect centering is achieved.

The trepanation should be performed perpendicular to the cornea, with a wide rotation movement and ideally in a single time⁸. Applying the trephine repeatedly may cause irregularities. The tone of the eye will influence the correct carving of the receiver. If any of these guidelines is not followed, the receiving bed may have bevels, steps or flaps, which will hinder the correct apposition of the graft. Any residual aileron must be resected (with Vannas’ scissors), in order to regularize as much as possible the edge of the receiving window. In cases with edema, inadvertent remnants of Descemet's membrane may remain in situ. If they are extensive, they can lead to graft failure due to endothelial damage⁵. Therefore, it must always be checked that AC has been completely penetrated and any remaining tissue in the window must be trimmed.

In vascularized corneas there may be some bleeding at the time of trepanation. Although it usually stops spontaneously and especially when applying the graft, such bleeding can be a problem in an aphakic eye if it falls into the vitreous chamber. If the vascularization is abundant, it will be indicated to treat it previously. Multiple techniques have been described, such as photocoagulation¹³, using photodynamic therapy¹⁴, or diathermy, either a few weeks earlier – an occasion to associate the injection of an anti-vasopressor agent – or at the beginning of surgery. This can be done with a bipolar coaxial diathermy of fine tip (23 G) or, as Dua proposes, with unipolar diathermy applied through a needle driven into the tissue.

HANDLING AND INJURIES OF IRIS

The iris is the first structure that the trephine can injure once it has crossed the cornea and the anterior chamber (AC) is emptied. A sudden penetration or compression of the eyeball predisposes to this accident. The alternating movement of the trephine makes it harder to damage the iris. Care should also be taken when trimming the button with scissors⁸. This complication occurs more frequently in very thin corneas or in perforated eyes with atalamia and the iris attached to the endothelium. To avoid this complication Gruber et al¹⁵ recommend filling the AC with a viscoelastic prior to trepanation, as well as the use of cyanoacrylate adhesive in corneal perforations. If damage to the iris occurs, it must be repaired with 10-0 polypropylene sutures with atraumatic needle.

There is controversy about the need to perform a peripheral iridotomy, but in general it is accepted that its presence reduces the risk of Urrets-Zavalía syndrome, especially if materials (viscoelastic, air, fibrin, blood) remain in the AC. We always practice it in phakic or aphakic eyes without vitrectomies. In pseudophakic patients it would be more excusable if there are no subsequent synechiae. Although it is important that the iridotomies are performed at the base of the iris, a too basal cut or excessive traction of the iris root can cause hemorrhage or iridodialysis. Bleeding of the iris can be controlled by viscoelastic, an air bubble, coaxial bipolar diathermy or even with the application of epinephrine¹⁰.

CRYSTALLINE LENS INJURY

The lesion of the lens capsule is rare, but it can occur if we perform a sharp trepanation or in case of tipping the scissors too much during an iridectomy. The perforation of the anterior capsule forces the lens to be removed and an intraocular lens placed. For this reason, it is advisable to perform preoperative biometry in all phakic patients who undergo PK. The instillation of pilocarpine to contract the pupil before surgery offers some extra protection to the lens⁵.

COLLAPSE OF THE EYE GLOBE AND «POSITIVE VITREOUS PRESSURE»

By removing the diseased cornea, we create a large opening in the globe and therefore this will tend to collapse depending on the (lack of) stiffness of its walls. This is often perceived, by the surgeon, as "positive vitreous pressure". Although the vitreous can change volume if it is hydrated or if a liquid moves behind it (e.g., a misdirected irrigation), in reality there is no vitreous pressure differentiated from that of the eyeball as a whole. When the iris and the lens protrude after trepanation, it is usually due to the fact that the sclera is not very rigid (children, high myopic), that the choroid is thickened by venous congestion, or that there is external compression by a poorly placed or ineffective speculum, or contraction of the orbital or palpebral muscles. Preoperative massages or compressions (Honan's balloon) or hyperosmolar agents lower the eye pressure by exiting or extracting intraocular fluid – possibly from the vitreous, although not exclusively. The anti-Trendelenburg position reduces orbital and choroidal congestion. In any case, avoiding these phenomena reduces the risk of certain complications, and regardless of the mechanism of the protrusion, this will decrease if we remove volume from the vitreous chamber as practicing a small dry vitrectomy.

The protrusion of the iris and the lens may be more complicated if there is zonular weakness. It could lead to crystalline dislocation associated with vitreous loss. In all cases in which there is vitreous in the AC, it must be removed either by means of surgical sponge and scissors or a mechanized anterior vitrectomy.

EXPULSIVE HAEMORRHAGE

Suprachoroidal or expulsive hemorrhage is the most feared intraoperative complication in PK. According to the series, its frequency has been estimated between 0.47%¹⁷ and 3.3%¹⁸. For Purcell¹⁹, the most important risk factors to consider are age, arterial hypertension, perioperative tachycardia, glaucoma, previous trauma, hypotonia, inflammation and Valsalva maneuvers. If it is not detected and controlled in time, the intraocular content can be expelled in its entirety. The reduction of the IOP and more especially of the vitreous pressure would decrease the choroidal blood flow which would drastically reduce the possibility of expulsive hemorrhage. Although it is not possible to trepan without applying a certain pressure, one should try to minimize this⁸.

The treatment of an intraoperative expulsive hemorrhage requires certain cold blood and to know the steps to follow. The first thing will be to cover the exit of the intraocular content. The most easily available means is the tip of a finger, although there are specially designed instruments that can be kept on hand in high-risk cases. Second, one or more posterior sclerotomies are performed. It is recommended to place them in the intermuscular quadrants and between 5 and 15 mm from the limbus. If a main choroidal bag is identified, it will be the preferred place to practice these. They must be large enough to allow blood to drain, although it is rare that clots can be removed⁵. It is debated whether it is useful to introduce a blunt spatula through the sclerotomy to favor drainage or if it can be counterproductive.

As soon as the pressure decreases, one should try to suture the graft to close the eyeball. Once the graft is in place, we can focus on reforming the AC and assessing the state of the lens, the iris, or the possible realization of an anterior vitrectomy¹⁹. In an aphakic patient, the reformation of the globe with BSS or viscoelastic can help to reduce the bags, but it is advisable to consult with the vitreous-retina surgeon before making any maneuver in the posterior segment. In the end, it is not interesting to leave the eyeball hypotonic, since it may favor re-bleeding in the postoperative period, but it should also be monitored that the IOP does not rise acutely. If the bleeding is not controlled, it can amount to a spontaneous evisceration of the eye. If the bleeding has been very important and the retina accesses the anterior segment and is incarcerated in the corneal scar, the prognosis becomes darker, but it is preferable to avoid as much as possible a primary evisceration, before evaluating the options in the first days of the postoperative period.

COMPLICATIONS DURING SUTURING

Intraoperative complications related to sutures, apart from errors in the technique of placement, include: poor burial of the knots, suture that is too loose or too tight, dehiscence and transfixing suture³. Both the independent stitches and the continuous suture can be broken, although the repair of the second is more laborious. If a suture is badly placed, we should not hesitate to change it to avoid future complications. Apart from a refined technique, to place the sutures in the most precise way, it is recommended that the cornea surgeon have a slit lamp and a keratoscope in his microscope (see chapter 3.2.1).

The postoperative period of penetrating keratoplasty (PK) is a period that can be extended for life and therefore its correct management can contribute to its long-term success as well as the quality of the donor tissue, the surgical technique or the patient's preparation.

THE IMMEDIATE POSTOPERATIVE PERIOD

Once the surgery is finished, a drop of antibiotic and corticoid is instilled and then a monocular bandage with plastic protector is done.

Care during the first 24 hours after a PK should focus on the maintenance of the wound tightness, which is assessed by the Seidel¹ test, the degree of graft edema and the state of the anterior segment. With the application of topical fluorescein, we can confirm the restoration of the graft epithelium, the degree of tension of the sutures and the control of possible elevations of intraocular pressure (IOP). For this we have the non-contact tonometer, the dynamic contour tonometer, Perkins’, Goldmann’s and the rebound tonometer. The contact surface of the latter is smaller than that of Goldmann, so it is not affected by the scar. There are possible complications associated with direct contact with the ocular surface, such as abrasion, transmission of infectious agents, or chemical injuries. The non-contact tonometer is an easy and quick way to measure IOP after PK. In certain cases, it can cause wound dehiscence in the immediate postoperative period, although this is very infrequent^2,3.

It is essential to inform the patient of the possible problems that may arise, in such a way that they are aware of their situation and respond quickly to any incident. This is one of the most important factors in the success or failure of a keratoplasty. In addition, the patient must be given all the necessary instructions in writing, together with the recommendation for a strict follow-up, with frequent visits during the first year.

POSTOPERATIVE TREATMENT

The treatment administered will depend on the nature of the problem that has led to the PK, of the possible intraoperative complications and the subsequent evolution. Its main objective is to prevent infection, reduce inflammation and keep the graft transparent. For this we will use different active principles that we discuss below.

Topical antibiotics

We will use a topical antibiotic 3 times a day until the epithelium is repaired. Some surgeons prefer to lengthen the treatment until the first month and then suspend it to avoid generating resistance. The currently most used option is topical third and fourth generation quinolones, since they are bactericidal antibiotics of broad spectrum, good tolerance and have little epithelial toxicity.

The first week after PK is the time of greatest risk and therefore of important recognition of early endophthalmitis⁴. Most of the pathogens will be Gram +, which usually inhabit the natural flora of the conjunctiva. Immediate prophylaxis should cover Gram + pathogens. In contrast, late endophthalmitis, associated with the use of therapeutic contact lenses and erosions at the level of the sutures, are most often produced by Gram - pathogens.

Corticosteroids

The treatment will start the same day of the intervention with a systemic corticoid. The dose will depend mainly on the weight of the patient. 1 mg/kg/day is recommended in a single dose, with a gradual reduction every week at a dose greater than half of the previous dose until its suspension. This reduction will be slower the longer the treatment time has been. In cases of high-risk keratoplasty, each case will be individualized, and it will be valued to add another type of systemic immunosuppressant in a prolonged manner.

Topical corticosteroids are an excellent way to treat inflammations limited to the anterior segment of the eye and are the "gold standard" for the prevention of immune rejection in PK. There are different protocols regarding its use⁵. Therapeutic levels can be reached in the vitreous and retina if the patient is aphakic or pseudophakic and/or has had a vitrectomy, but it should never be the route of administration for cases in which only the posterior pole is affected. The formulations of prednisolone are those that reach higher therapeutic levels, followed by dexamethasone.

Although fluorometholone, loteprednol and rimexolone are very ineffective in the treatment of intraocular inflammations, they may be sufficiently effective as a preventive treatment to avoid rejection in PK. The dose will vary according to the degree of inflammation. As a general rule, the more intense and prolonged the treatment, the longer the period of descent of the dose should be. The adverse effects to be monitored are cataracts in very prolonged treatments and the increase in IOP, which in corticoid responders is usually after the first 15 days. It is preferable to use eye drops without preservatives whenever possible, since they are effective in preventing the development of epithelial disorders of the cornea after PK⁶.

There is still no consensus on how much time corticosteroids should be used. We recommend the guideline presented in table 1. Shimmura-tomita et al⁷ conducted a long-term comparative study with the objective of evaluating the efficacy and safety of the use of topical corticosteroids in low-risk PK. They observed that the use of 0.1% topical fluorometholone at low doses one year after PK was beneficial for the prevention of rejection. If sutures persist, it is advisable to maintain corticosteroid treatment at least once a day and in high-risk keratoplasty it may be preferable to use a more potent maintenance corticosteroid once a day. It is also convenient to avoid the use of phosphates in those cases with chronic epitheliopathy to avoid the risk of calcification. In high-risk PKs, each case will be individualized, and the addition of a systemic immunosuppressant will be considered.

Artificial tears

It is advisable to administer ocular lubricants without preservatives in a dose of 4 to 12 times a day for the prevention of epithelial drug toxicity and in order to improve corneal hyposensitivity during the first years.

Mydriatics and cycloplegics

In addition to the topical corticoids for the control of inflammation, we can associate a mydriatic eye drop for the control of the pain produced by the ciliary muscle spasm, to prevent the formation of synechiae or to avoid the risk of pupillary block. The most widely used cycloplegic-mydriatics, from higher to lower potency and duration of action, are atropine, homatropine, cyclopentolate and tropicamide. The most used are those of medium type, such as cyclopentolate, in a drop pattern every 8-12 hours that decreases as the process is controlled, until a single administration at night. This allows accommodation during the day and interferes less with daytime vision until it is completely removed.

Hypotensors

If we find an elevated IOP, the use of a hypotensive agent, preferably a beta-blocker and/or an alpha-2-adrenergic agent, is indicated before a prostaglandin due to the pro-inflammatory effects and the association with macular edema, especially in aphakic eyes⁸, of the latter. We also prefer to avoid it in herpetic keratopathies. The miotics should be avoided, due to the greater formation of anterior synechiae. The management of post-PK glaucoma is crucial, elevated IOP is detrimental to optic nerve fibers and corneal graft endothelial cells (see chapter 3.3.2).

Approximately between 10% and 20% of patients treated with long-term topical corticosteroids will develop ocular hypertension, so it is very important to maintain a close control¹⁰.

Immunosuppressants

Before surgery and during the first postoperative days, patients should be differentiated according to the risk of rejection. High risk cases will be those that present, at least, one of these situations:

- Previous grafts.

- History of herpetic keratitis.

- Chemical injury.

- Keratopathy by silicone oil.

- Infections or inflammations «hot».

- Vascularized cornea.

- Preoperative glaucoma.

- Anterior synechiae.

- Extreme ages.

In these cases, the treatment will start the same day of the intervention with an intravenous corticoid and the next day with an oral regime. The dose will depend mainly on the weight of the patient. It is recommended 1-2 mg/kg/day with a gradual reduction. Systemic corticosteroids should be avoided as much as possible or monitored in diabetic, hypertensive, immunosuppressed patients, a history of peptic ulcer or severe gastroesophageal reflux and psychiatric disorders due to the possible exacerbation of their disease.

In addition to the topical and systemic application of corticoids, systemic immunosuppression must be used for at least 12 months after a high-risk PK. Immunomodulatory therapy must be managed by an ophthalmologist expert in the management of this type of drugs, although always in collaboration with a group of specialists who can assume the complications derived from the treatment.

The immunosuppressive drugs of choice at present are mycophenolate mofetil 1 g/12 h, which alters the DNA synthesis of T and B lymphocytes, and cyclosporine A (CsA) 5-10 mg/kg/day, whose function is to block the production of IL-2, inhibiting the proliferation of cytotoxic LT^11,12. Another immunosuppressant drug is tacrolimus, which has the same mechanism of action as CsA.

CsA is usually the first drug that we associate with corticoids, although, depending on the diagnosis, the drug of choice may be another. Thus, in herpetic keratopathies we prefer mycophenolate for its synergistic action with antivirals. The initial dose of CsA is usually 5 mg/kg/day divided into two doses (it can be increased to a maximum of 10 mg/kg/day). The response is usually observed after a few weeks. The decrease in CsA is done gradually, 10% each month until the effective minimum. This dose is maintained for a variable period of time depending on the diagnosis and evolution of the patient, and in principle, not less than one year.

Regarding topical treatment with CsA at different concentrations, there is not enough evidence to demonstrate its usefulness for the prevention of rejection, especially in high-risk keratoplasties^13-16.

Systemic immunosuppression may promote side effects, so patients should be monitored by an internist to rule out infections and possible neoplasms among other diseases before starting immunosuppressive treatment. In addition, during the treatment it is very important to perform liver and kidney function tests. If any contraindication to the use of an immunosuppressant is found, the indication of a high-risk PK should be reconsidered.

Herpetic keratopathy

In cases of previous herpetic keratitis, it is recommended to start antiviral treatment orally (Acyclovir 400 mg/12h) from the same day of the intervention¹⁷ and for 12 months, although it seems that prolonging the treatment for 18 months could decrease the rate of recurrence. There are also other agents such as Valacyclovir (500 mg once a day) that is more bioavailable than Acyclovir and can be taken with food without losing effect. It will also be important to maintain this oral treatment while we are using topical corticosteroids. It has been seen that the rate of herpetic recurrence drops to 0-5% in the first 2 years compared to 36-50% in those patients not treated¹⁸.

Ocular surface problems

The existence of previous ocular surface disease contributes to hinder epithelization, delay healing and decrease the transparency of the corneal graft, so we must perform a very thorough care and treatment of these structures before and after surgery by palpebral hygiene, topical and/or systemic anti-inflammatories and oral tetracyclines. The treatment of persistent epithelial defect after PK is controversial. Traditionally compressive bandage was used until the epithelial closure, but it has been shown that once the epithelium crosses the graft-receptor junction, the epithelial closure time is practically the same if good lubrication with drops and gels is maintained. We also have the option of using a soft contact lens as a bandage or in extreme cases performing a lateral tarsorrhaphy or occlusion of the lacrimal point^19,20.

Management of the suture

The ideal suture material should maintain the apposition of the tissues with a correct and constant tension and induce the minimal tissue reaction throughout the healing period. Regarding the configuration of the suture, two main models are distinguished: continuous suture and independent radial stitches. If we have performed the first one, we can leave it permanently if the result is satisfactory or withdraw it one year after the surgery. The suture with independent stitches can be manipulated by the selective removal of them. We prefer not to remove any points before the first 3 to 6 months (depending on age, etc.), except for certain exceptions. If a suture is loose, broken or vascularized, its removal is indicated^21,22. It is convenient to use an antibiotic and topical corticosteroid 3 times a day for at least a week after removal of sutures. With the use of polyester (Mersilene) of 11-0 it is possible to leave some stitches for longer periods and even permanently. The fact that this type of material hardly loses tension allows obtaining excellent refractive results, as well as increasing the stability and safety of the graft-receptor scar²³.

Postoperative follow-ups

The next controls should be done in the order specified in Table 2. At each control visit, a review of symptoms and signs will be made in the slit lamp that can help us identify any complications. In some of the visits, complementary tests will be performed, evaluating the visual acuity without correction, with pinhole and with correction. Corneal topographies, endothelial counts, campimetries, optical coherence tomographies of both anterior and posterior segments, ultrasound, etc. should also be performed.

In terms of the patients resuming their usual work and daily life, it is necessary to warn against performing violent physical exercises or great efforts for a few months to avoid trauma and other complications. The continued use of protective glasses on all carriers of a PK is recommended. Due to the variability of astigmatism, we will not give, in principle, a definitive correction until its reduction and stability have been achieved, except in cases of low visual acuity in the contralateral eye. Once all sutures are removed, different refractive surgical procedures may be necessary to correct residual ametropia²⁴. With regard to children, it is important to rehabilitate them using glasses or hydrophilic contact lenses for prolonged use to avoid or recover amblyopia²⁵.

Rejection of the graft

Rejection is the main cause of graft failure and one of the main aspects that postoperative follow-up must address. This topic is the subject of another chapter in this section. We will only remember the crucial importance of patient education so that it is able to identify symptoms and take immediate action, such as starting topical corticosteroids with great frequency and going to the specialist as soon as possible.

The results of penetrating keratoplasty (PK) have improved thanks to advances in the conservation of donor corneas, surgical techniques, materials and adjuvant pharmacological treatment. However, there are a number of complications that can occur postoperatively and lead to graft failure if not treated properly and in time. It is therefore important to know them, to know how to prevent them and, in case they arise, to act correctly and with alacrity to resolve them.

COMPLICATIONS RELATED TO THE SCAR

Lack of airtightness

The lack of tightness with escape of aqueous humor in the postoperative period is usually due to sutures that are too superficial, loose, perforating or that distort the tissue. A disparity between the graft and the receiving window can also influence. It may present with flattening of the anterior chamber (AC) if it is very important, but in many cases it is only detected by the Seidel test, so this must be part of routine exploration in the early postoperative period. The filtering scar may be responsible for complications as serious as hypotonia and choroidal detachment, the epithelization of AC, graft failure and even endophthalmitis¹.

This complication is avoided by selecting correctly the size of the graft and the window and with a correct suture, checking the tightness at the end of the intervention. Do not hesitate to replace a badly placed suture that causes poor coaptation of the graft or add all the necessary to obtain the tightness. In the postoperative period, a discrete Seidel + can be observed for a few days with occlusion with a compressive bandage or therapeutic contact lens (LCT), but if it is copious, persistent or the AC becomes narrower it will be necessary to complete or revise the suture.

Ectasia and reopening of the scar

The rupture of radial stitches or of a continuous suture in the early postoperative period (Figure 1) or the premature removal of these are the main causes of these complications². The education of the patient is essential to avoid accidents or rubbing that can cause a rupture of the suture and the permanent use of protective glasses should be insisted on. Although it is not easy to assess the consolidation of the graft-host junction, there are signs such as its whitish opacity and the presence of deep vessels that indicate a sufficient degree of cohesion to initiate removal of the sutures³. The treatment will vary according to whether or not the intraocular content is exiting. If it is only an ectasia of the scar, it may be sufficient to place sutures in said area again. Herniation of the iris or exit of other intraocular structures such as the lens or vitreous will require a previous assessment and a specific surgical treatment (Figure 2).

Abscess of the suture

Sutures are one of the most important risk factors for the appearance of an infectious keratitis, and secondary inflammation could cause the activation of an immune reaction (Figure 3). Leahey et al⁴ presented a series of 18 cases of suture abscesses in 773 patients operated on with PK (2.3%). In this series, the germs that were isolated most frequently were Gram + (83%). To resolve this complication, the infected site should be removed and treated aggressively with antibiotics reinforced empirically and according to the antibiogram as soon as we have the results of the cultures.

Induced astigmatism

The result of a successful PK due to transparency may be distorted by high or irregular astigmatism. The average astigmatism after PK is usually between 4 and 6 diopters (D)⁵. This is due to multiple factors that include those that may depend on the patient, the donor, the surgical technique or the postoperative period. Specific mention has been made to eccentric trepanation, the disparity between the graft and the receiving window, the misalignment of the tissue or the incorrect placement of the sutures. A comparison between continuous, independent or mixed sutures did not find a difference in astigmatism⁶.

There are multiple techniques to treat astigmatism after PK. Up to about 3.5 D many patients tolerate glasses well. Above this value, contact lenses that are permeable to gas may be useful. It is possible to manipulate the continuous sutures in the first postoperative months and the independent stitches can be removed selectively for a longer period. For this to be effective, it is necessary that the material cannot be resorbed; that's why we prefer polyester (Mersilene 11-0) to the classic nylon. When the sutures have already been removed, the different astigmatism surgery techniques can be applied, such as arcuate incisions⁷, wedge resection⁸, laser ablation (LASIK or PRK) or phakic⁹ or pseudophakic intraocular lenses.

EPITHELIAL COMPLICATIONS

Persistent epithelial defect

The appearance and persistence of an epithelial defect in a PK (Figure 4) may be due to limbal insufficiency, defective quality of the graft or poor lubrication of the ocular surface. Kim et al¹⁰ studied donor factors that could be associated with an early epithelial defect, which they found in 27 of 39 patients (62.5%). The only statistically significant factor was the time between graft preservation and surgery. The closure of any epithelial defect should be a priority in the postoperative period of a PK. Among the options for its treatment, we find topical treatments, compressive bandage¹¹, temporary tarsorrhaphy, use of corneal or scleral LCT¹², and the placement of an amniotic membrane over the defect. Panda et al¹³ compared the tarsorrhaphy and the monocular dressing and concluded that the defects closed 5 days before the average with the first. It is advisable to reduce topical treatment with corticosteroids and to use eye drops without preservatives.

Filamentous keratitis

Rotkis et al¹⁴ studied a series in which 31 of 114 patients operated on PK (27.2%) had corneal filaments. Its appearance is due to the excessive formation of mucus due to conjunctival irritation and the presence of a fragile epithelium to which it can adhere. The filaments can be found at any location on the cornea although they are more often seen at the graft-host junction. Its treatment consists of removing them with a hemostat or tweezers, a copious use of lubricants and mucolytic agents such as N-acetylcysteine.

Epithelial and subepithelial rejection

The epithelial rejection appears as an elevated line that crosses the surface of the graft and advances through it. Although it is not frequent, when it appears it does it between 7 and 14 days after surgery. It represents a replacement of the donor epithelium by that of the recipient. It is accompanied by ciliary hyperemia and desquamation of the epithelium. Its prophylaxis and treatment are based on corticosteroids, with increased doses should it occur. Sub-epithelial rejection (sometimes called "chronic stromal") is presented as nummular infiltrates reminiscent of those typical of adenovirus infection. Although by themselves they do not pose a threat to the graft, the inflammation that accompanies them could favor an endothelial rejection.

STROMAL COMPLICATIONS

Infectious keratitis

It can occur early or late. Harris et al¹⁵ found that the most important risk factors were epithelial defects (55%), the size of LCT and the presence of exposed sutures (31%). Al-Hazza and Tabbara¹⁶, observed bacterial keratitis in 113 of 947 PK (11.9%), concur with the previous ones and add trichiasis as an additional factor (39% in their series). The most commonly associated microorganisms are Gram + (S. epidermidis and S. pneumoniae). In late forms, fungi play an important role, particularly C. albicans¹⁵. The presentation of an infectious keratitis in a PK requires taking cultures from the area and starting the reinforced empirical antibiotic treatment that will be adjusted with the result of the antibiogram (Figure 5).

Infectious crystalline keratopathy

Described by Gorovoy et al¹⁷ in 1983, it is called the appearance of whitish corneal deposits with a characteristic appearance of snowflakes or fragments of broken glass (Figure 6). It is due to the formation of calcium phosphate crystals due to the presence of Gram + cocci in the stroma, especially S. viridans – also involved in the formation of dental tartar. Although these are generally non-aggressive opportunistic germs – usually associated with epithelial defects – they should be treated like any other infectious keratitis.

Recurrence of the underlying disease

In pathologies of genetic origin such as dystrophies, the tendency to relapse in general will be present and will depend on whether the cells involved replenish the graft (Figure 7). Therefore, they tend to occur more frequently in those in which participation in their pathogenesis is primarily epithelial, since the receptor's keratocytes have more difficulty in accessing the graft. Corneal degenerations secondary to systemic processes also tend to relapse if these have not been resolved. Marcon et al¹⁸ retrospectively studied the recurrences of dystrophies in patients operated on by PK and concluded that those of Bowman's membrane did so in 88% of the cases, averaging two years after surgery. Therefore, the current trend is to treat these superficial dystrophies with laser ablation (PTK) or superficial lamellar keratoplasty.

In PK for keratoconus, relapse has sometimes been described as actually either a progression of the ectasia in the recipient cornea or an alteration of the graft-host junction – in which the pathogenic mechanisms of keratoconus can participate. Thinning occurs in this area with the absence of a Bowman layer in the histology¹⁹ (Figure 8). These situations usually respond well to wedge resection and in cases of extreme ectasia may require reconstructive keratoplasty in horseshoe or crown (see chapter 5.8.7).

Stromal rejection

Although rare, isolated rejection of the stroma can be seen in the form of stromal infiltrates, neovascularization, or keratolysis. The treatment consists of both topical and systemic corticosteroids.

ENDOTHELIAL COMPLICATIONS

Primary failure of the graft

It is defined as a graft that from the first day presents edema and fails to recover transparency. It is due to a lack of viability of the endothelium of the graft, either due to poor tissue quality, excessive surgical trauma or a combination of both causes. Wilhemus et al²⁰ found cold preservation for more than 7 days of corneas older than 70 years as a risk factor. Our experience with fresh whole eyeballs indicates that age does not influence survival (see chapter 3.6). When a primary failure occurs, it is recommended to wait about 3 weeks with hyperosmolar treatment before replacing the graft or performing an endothelial transplant.

Endothelial rejection

It is the most frequent and serious type of rejection that often leads to graft failure. The possibility of avoiding irreversible failure depends on the earliest possible diagnosis and aggressive treatment, which is why the education of patients is crucial. Its frequency has been estimated between 8% and 37% according to different series, although it depends a lot on the underlying pathology. Possible predisposing factors include the youth of the recipient (<40 years), presence of vascularization, anterior synechiae, large diameter of the graft (> 8.5 mm), limbus transplant, “hot” PK, pediatric PK, among others²².

Typical endothelial rejection occurs with "keratic" endothelial precipitates, whether dispersed or forming a line (Khodadoust’s) that usually migrates from the periphery (Figure 9), as well as edema of the graft. It may be accompanied by ciliary hyperemia and reaction in AC (Tyndall +). The most characteristic symptom is foggy vision without other discomforts, which may occur occasionally. The prevention and treatment of rejection is the subject of the following chapter.

Late endothelial failure

The late failure of the graft is understood as a decompensation after having maintained transparency for a longer or shorter time, without having appreciated signs or history of rejection or other cause of failure. Ing et al²¹ found a 21% rate of graft failure at 10 years. The possible causes are a scarce donor endothelium, or its loss increased by episodes of rejection perhaps subclinical, or mechanical factors such as the presence of synechiae, etc. Currently it tends to be treated with an endothelial transplant, except for irreversible stromal opacity or high astigmatism, where a new PK is still indicated.

INTRAOCULAR COMPLICATIONS

Anterior synechiae

The presence of anterior synechiae may seem innocuous but is associated with serious complications such as angle-closure glaucoma and rejection or late graft failure. Tragakis and Brown²³ found that 87% of patients with synechiae developed graft edema versus 15% in their absence. Large diameter grafts, a filtering scar, a pupillary block or the difficulty of reforming the AC at the end of surgery can favor its formation². When they are extensive it has been recommended to release them to avoid the aforementioned secondary complications.

Hernia of iris

It is usually the consequence of an opening of the scar, in particular with hypertension. If the herniated iris is not necrotic, it is preferable to try to preserve it by correcting the hernia, taking care that it is not epithelialized as it could lead to the formation of an epithelial cyst in the AC².

Hyphema

It is an infrequent complication in the absence of trauma or de-coagulation. It can occur when a synechiolysis, iridectomy or iridoplasty was needed. It usually reabsorbs spontaneously but if it does not or cause ocular hypertension, it may be necessary to evacuate it (Figure 11).

Paralytic mydriasis (Urrets-Zavalía syndrome)

It was described in 1963 as the appearance of a fixed and dilated pupil, with atrophy of the iris and posterior synechiae²⁴. It is most likely due to a pupillary block in the early postoperative period with sphincter ischemia (Figure 12). It can occur between 2.2% and 5.8%²⁵ of the PKs. It is avoided by performing peripheral iridotomies, not leaving foreign materials (viscoelastic, air, etc.) in the AC and applying a cycloplegic eye drop at the end of the intervention.

Glaucoma

In a series of 502 PKs, Foulks²⁶ found 91 (18%) who presented chronic postoperative glaucoma, 44 (9%) hypertension in the immediate postoperative period and 13 (2%) glaucoma by angular closure. This is favored after a PK in cases where there was already a previous glaucoma and in aphakic eyes. Apart from damage to the optic nerve, the increase in ocular pressure can cause the loss of endothelial cells²⁷. Therefore, its pre and postoperative controls are crucial for graft survival. The treatment of hypertension and glaucoma in a PK is the subject of another chapter (3.3.2).

Cataracts

The PK can accelerate the development of cataracts mainly by the use of corticosteroids after the intervention.

Suprachoroidal hemorrhage

Although suprachoroidal hemorrhage is known more as an intraoperative complication, it can occur postoperatively. Absolute rest should be recommended and observation until it is reabsorbed². If there is contact between the bags, one should consult with the vitreous-retina surgeon about the suitability of its surgical drainage.

AC epithelization

Although it was a more frequent complication in the past²⁸, its possibility should be considered in a PK operation. It appears as an epithelial membrane that covers the corneal endothelium and other structures of the AC causing anterior synechiae and graft failure (Figure 13). Its cause is the entrance of epithelial cells inside the eye due to fistulisation of tracts of penetrating sutures or dehiscence of the wound. Its treatment requires aggressive surgery as before an oncological problem, with a replacement of the PK and “en block” resection of the affected structures. Otherwise the eye can move towards absolute glaucoma and subsequent phthisis bulbi. In the case of a small area of epithelialization, the use of intra-cameral 5-fluorouracil has recently been described²⁹.

Endophthalmitis

It can be due to the use of contaminated corneal material, the introduction of germs in the eye during or after surgery or secondary to infectious keratitis². Leveille, in a series of 1876 PKs presented only 4 cases (0.2%) of postoperative endophthalmitis³⁰. The careful preparation of the donor material, the correct preparation of the recipient eye, a purified surgical technique and a preventive treatment with antibiotics, are recommended to avoid this complication. It is an emergency in which the main objective will be to save the eye. It is advisable to follow the guidelines of the Endophthalmitis Vitrectomy Study Group³¹ to treat these patients.

The cornea transplant is the most successful tissue transplant in humans. This is partly due to an improvement in the surgical technique, in the processing of the donor tissue, as well as in the medical management of the rejection, which has given rates of graft survival of up to 95% in some series¹. Of the 36,000 corneal transplants performed annually in the United States in the last decade², immune rejection constitutes 27.9% of cases of failure, only exceeded by primary failure due to loss of endothelial cells, which causes 30.1% of them³.

Rejection is a complex immune reaction that can cause decompensation of the graft. In general, it presents at least one of the following characteristics⁴:

• Development of epithelial and/or endothelial line and stromal rejection band.

• Inflammatory reaction in the anterior chamber with keratic precipitates (KPs).

• Corneal edema in previously transparent graft with or without signs of inflammation.

• Start at 3 weeks or more after transplant.

• Inflammation limited to the graft.

• Start of the process in the vicinity of blood vessels.

• Centripetal movement of inflammation.

While the development of new immunomodulatory therapies yields promising results, the ophthalmologist's challenge continues to be to recognize those patients at high risk of rejection, be able to transmit to patients the need for an early diagnosis and the rapid establishment of aggressive treatment.

PATHOGENESIS

The cornea has a lower incidence of rejection than any other type of transplant carried out in the body, despite not being routinely performed with compatible HLA patients.

The cornea is normally an immunologically privileged tissue⁵, and this immune privilege is actively maintained by multiple mechanisms, among which the absence of blood and lymphatic vessels, the relative scarcity of mature antigen-presenting cells (APCs) in the central area of the cornea and the presence of modulating factors of the immune response in the aqueous humor, as well as the expression of Fas ligand or CD 95L^5,6, basic elements of the so-called Immune Deviation Associated with the Anterior Chamber (IDACA)⁷.

The recognition by the immune system of foreign histocompatibility antigens (HLA) in the graft cells initiates a series of mechanisms that result in sensitization (afferent arm of the response). There are two forms of presentation of alloantigens to T cells, direct and indirect. During direct allosensitization, the donor APCs migrate from the graft to the lymphoid tissue of the host and present their antigens. In contrast, in the indirect pathway, the receptor APCs collect, process and present the antigens to the T cells in the presence of their own HLA molecules. Both pathways are involved in the pathogenesis of rejection, however, their exact contribution is still unknown⁸. After the stimulation of immune cells in the regional lymph nodes, the immune system organizes a response to these antigens that manifests when the effector cells reach the graft (efferent arm of the response). This response is orchestrated by CD4 T lymphocytes, while these, together with CD8 T lymphocytes, are the main effector cells⁹.

Risk factors

The survival of the transplant depends on the conditions in which it is performed. It has been seen that the probability of survival of a low-risk transplant is close to 90%, while in the case of a high-risk transplant this percentage does not reach 50%¹⁰.

The characteristics of both the donor and the recipient that confer a high risk of immune rejection of the graft are:

• Vascularization: A high risk of rejection associated with neovascularization of the recipient cornea has been demonstrated, either preoperatively or postoperatively, this being the statistically most potent factor in multivariate prediction of rejection risk. It has been seen that the risk increases when the area of ​​stromal vascularization increases, and any transplant performed with 2 or more quadrants of deep stromal vascularization is considered high risk¹¹.

• Diameter of the graft: In the past, the finding that large-diameter transplants (> 8 mm) were more susceptible to rejection¹² was explained by the proximity of the limbal vessels, but it has now been observed that this fact is related to the presence of Langerhans cells in the peripheral cornea¹³.

• Successive transplants: a cornea with a previous failed transplant due to any cause is considered high risk, and the risk is greater if the failure was secondary to rejection¹⁴. The failure rate due to rejection increases in these cases from 8% to 40% when there have been 2 or more previous transplants, and this risk is high even if the recipient's cornea is avascular¹⁵.

• Previous inflammation of the ocular surface: transplants performed in the presence of active inflammation are more prone to be rejected¹⁶, especially in diseases of the ocular surface such as severe dry eye, ocular pemphigoid, Steven-Johnson syndrome or chemical injuries. Therefore, it is necessary to achieve the best possible control of the inflammation before transplanting.

• Atopy: An increased risk of rejection has been observed in patients with atopic dermatitis, even in the absence of clinically evident conjunctival allergy¹⁷. In addition, it has been seen that atopy increases the risk of rejection in patients with keratoconus, one of the main indications of a transplant, due to the systemic immune alterations that this entails¹⁸.

• Age of the recipient: a rigorous statistical evaluation of the prospective data of the Collaborative Corneal Transplantation Studies (CCTS) confirmed that a young age of the recipient (<40 years) is a strong risk factor for the rejection of allografts¹⁵.

• Other risk factors: The contact of the graft with the vascular system of the donor through previous synechiae increases the risk of rejection¹⁵. Loose sutures, either by the induction of vascularization or by the stimulation of local inflammation phenomena have also been shown to increase the risk of graft rejection. In addition, the previous use of medications for glaucoma and filtering surgery are a significant risk factor for rejection¹⁹.

When any of the aforementioned risk factors is identified, the ophthalmologist must consider that he/she is faced with a high-risk transplant and increase the level of suspicion in case of any episode of inflammation that occurs.

CLINICAL APPEARANCE

All the layers of the cornea can be rejected, and in this way the rejection is classified into 5 types:

A) Epithelial rejection: It appears in 10-14% of the cases, between 1-13 months of the transplant, more typically around the third month. It begins with a linear pattern that extends from the periphery of the graft, usually in the vicinity of a vessel at the graft-host junction and progresses towards the center (Figure 1). The epithelium behind the rejection line has an irregular, whitish appearance and is replaced by epithelium of the receptor. Superficial infiltrates may appear that in turn progress towards the center, called Kaye points²⁰. Its course is usually self-limited, but immediate attention must be given due to its close association (by 74%) with other types of rejection.

B) Chronic stromal rejection: Its frequency oscillates between 2,4-15%. It appears between 6 weeks and 20 months postoperatively, in isolation or associated with other forms of rejection. It is characterized by subepithelial infiltrates located only in the donor cornea, reminiscent of those that follow adenoviral conjunctivitis, but in the absence of hyperemia or other signs of conjunctivitis (Figures 2 and 3). They show a very good response to treatment with topical corticosteroids, and rarely lead to graft failure.

C) Hyper-acute stromal rejection: Characterized by the sudden emergence of peripheral veiling (haze) in a previously transparent cornea, which progresses rapidly in 24-48 h to the entire cornea. It is associated with loss of epithelium, followed by the development of intense neovascularization of the stroma. The clinical picture may be difficult to distinguish from a strongly vascularized corneal abscess, but in the case of rejection there is usually no hypopyon. When severe, it may progress to graft necrosis, descemetocele and perforation⁴.

D) Chronic focal rejection or endothelial rejection: It is the most frequent, most symptomatic and devastating form. Its incidence varies between 2-44%, occurring as early as 3 weeks and as late as 35 years after transplantation, although in most cases it appears around 8 months²¹. On examination, the eye appears hyperemic with signs of inflammation in the anterior chamber, which is usually mild or moderate. The KPs can be seen diffusely or linearly, forming a Khodadoust line or K line (Figure 4). It begins at the periphery of the graft around an area of ​​neovascularization, and advances towards the center of the cornea. Stromal edema and folds in Descemet may be diffuse or segmental and lie between the edge of the graft and the rejection line. This type of rejection can be classified as⁴:

• Definitive: inflammation appears in the anterior chamber, KPs limited to the donor cornea, and K line.

• Probable: with all the previous findings but without K line.

• Possible: gradual edema of the graft appears but no signs of inflammation or K line.

E) Rejection in previous graft: The rejection in a cornea with successive transplants is characterized by²²:

• It can occur in the first two weeks, since the recipient is previously sensitized, although it is more frequent to occur after 4 weeks.

• There is involvement of the edge of the receiving cornea.

• The K line may be absent despite severe inflammation in the anterior chamber.

• It carries a higher risk of graft failure despite proper treatment initiation and will require prolonged use of corticosteroids to control it.

Any episode of inflammation in a transplant patient should be considered as a possible rejection and should be evaluated and treated immediately to preserve graft survival.

Differential Diagnosis

Among the situations that challenge the clinician's ability to diagnose rejection, the most complex is the recurrent inflammation that appears in cases of herpetic keratouveitis. The occurrence of rejection is more frequent in patients transplanted due to herpetic disease, and repeated episodes of inflammation have a darker prognosis for the graft²³. In these cases, the presence of a typical epithelial dendrite-form lesion or the absence of K line help to differentiate the cause of the inflammatory episode. In addition, in herpetic uveitis KPs are not limited to the graft area, but also settle in the recipient's cornea.

In the overgrowth or epithelial invasion, a whitish membrane can be seen on the posterior surface of the graft, on the iris, or accumulations of epithelial cells in the anterior chamber, which can be confused with signs of rejection (Figure 5). In these cases, the corticoid treatment will be ineffective. Finally, slow and indolent infections can simulate rejection, especially in cases of crystalline keratitis caused by fungi or alpha-hemolytic streptococci (Figure 6).

PREVENTION

The prevention of graft rejection can be carried out with preoperative, intraoperative and postoperative measures.

Preoperative measures:

1. Reduction of the antigenic load of the donor tissue:

• Exposure of tissue to ultraviolet light: it is thought that it reduces the rejection rate by depleting the Langerhans cell number and preventing the activation of T lymphocytes²⁴.

• Depletion of local macrophages of the corneal button by subconjunctival injection of clodramate liposomes²⁵.

• Anti-lymphangiogenic treatment: lymph-angiogenesis provides exit routes for APCs from the cornea to regional lymph nodes²⁶. The elimination of ipsilateral cervical lymphatic drainage lymph nodes leads to the derogation of the allo-response and to immunological ignorance, but in humans cervical lymphadenectomy before corneal transplantation is not feasible given the risk-benefit ratio²⁷.

• Conservation in organic culture media: there is evidence that in corneas preserved in this way there is a reduction of Langerhans cells and dendritic cells, which could be effective to reduce the possibility of rejection²⁸.

• The pre-treatment of the graft with hyperbaric oxygen and the use of heterologous antibodies has been shown to reduce the antigenicity of the graft²⁹.

2. Minimize antigenic differences between host and donor:

The disparity of HLA class I or class II leads to a greater frequency of rejection of the corneal graft. In recent years, extensive research has addressed the role of HLA and ABO compatibility for transplant survival^30-32, and several studies suggest a beneficial effect of HLA I compatibility for graft survival. However, the Collaborative Corneal Transplantation Studies (CCTS) could not confirm this, and found that HLA-A, -B and DR compatibility does not substantially reduce the probability of corneal graft failure; that a cross-match between donor and positive recipient does not dramatically increase the risk of failure; and that the adaptation of the ABO blood group, which can be achieved with a relatively small expense, can be effective in reducing the risk of graft failure³³.

Intraoperative measures: There are intraoperative factors that contribute to the prevention of graft rejection; including avoiding graft shifting from its central position, by a meticulous suture and a good apposition between the donor and recipient cornea. The de-epithelialization of the graft has been suggested by multiple authors³⁴, arguing that it acts as a source of class I and II antigens. However, it is not currently considered that the de-epithelialization of the graft helps in the prevention of rejection.

Postoperative measures: These are aimed at controlling or reducing the recipient's immune response against the donor tissue. In addition to the role of immunosuppression, the importance of good suture management, such as premature removal of vascularized sites and steroid prophylaxis when removing sutures, must be considered.

The findings in the literature about the suppression of the host immune response for the prevention of rejection can be summarized as follows:

- No significant differences have been detected in the incidence of rejection between the short-term or long-term topical postoperative use of corticosteroids³⁵.

- Treatment with systemic cyclosporine (CsA) has not been shown to reduce the incidence of long-term rejection in high-risk keratoplasty^36,37.

- Grafts treated with topical CsA contain a significantly lower number of infiltrating T lymphocytes, indicating that it actively inhibits the entry of these cells into the tissue; however, it has not been shown that the incidence of rejection in the first transplant decreases³⁸, or after previous episodes of rejection³⁹.

- Mycophenolate mofetil (MFM) significantly reduced rejection episodes in high-risk keratoplasty⁴⁰. In addition, it has been shown that MMF and CsA have a similar effect when it comes to maintaining survival and preventing graft rejection⁴¹.

- Sirolimus and MFM in combination are effective in extending transplant survival in the majority of high-risk patients⁴².

- Topical tacrolimus 0.03% has been shown to be effective in preventing rejection in high-risk patients without increasing intraocular pressure⁴³.

- Tacrolimus in the form of a biodegradable polymer implantable in the anterior chamber as a sustained delivery system of the drug has been shown to be effective in prolonging graft survival in high-risk transplants⁴⁴.

- Several anti-VEGF drugs have been shown to be effective in reducing vascularization in high-risk transplantation, thereby reducing the incidence of rejection in both animal models^46,47 and humans^48,49.

- The use of anti CD4 and blocking co-stimulatory molecules, such as Ig-CTLA4, CD28 and CD154 (CD40 ligand) are new approaches towards the management of graft rejection that have shown good results in animal models^50-52.

The protocol used by the Cornea Unit of the University Clinical Hospital of Valladolid includes systemic immunosuppression with inhibitors of T lymphocytes or antimetabolites depending on the characteristics of the patient in cases of previous corneal vascularization of 2 or more quadrants, grafts > 8 mm or re-grafts for previous rejection.

TREATMENT

Immunological rejection in a transplanted cornea can lead to corneal graft failure. Adequate management requires early detection and aggressive therapy of the rejection episode. The options that we count on to deal with these episodes are:

Corticosteroids

Topical, periocular or systemic corticosteroid therapy is the treatment of choice for the acute rejection reaction^53,54. The treatment regimen in acute rejection consists of hourly instillation of topical corticosteroids (usually 1% prednisolone acetate) until the rejection process is stopped or reversed. This therapy can be complemented with periocular corticosteroids (triamcinolone) when the topical route is unfeasible, or systemically, both orally and intravenously, in severe cases⁴, recommending that oral corticosteroids be administered at a higher dose than routine (1.5 mg/kg/day) and continued for 6 to 8 weeks.

Cyclosporine A

The main action of the systemic CsA is preventive. It acts on the afferent arm of the immune response, although it also has some inhibitory influence on the efferent arm by inactivating the cytotoxic T lymphocytes. The combined treatment of intravenous methylprednisolone in pulses and oral CsA has been shown to be safe and effective in reversing the rejection process. It can also protect the graft from future episodes of rejection⁵⁵. Although some studies have shown encouraging data about topical CsA, to date controlled clinical trials have not been able to demonstrate important results with respect to treatment with topical corticosteroids^56,57.

REJECTION IN LAMELLAR GRAFTS

The new lamellar transplant techniques have postulated a lower risk of rejection due to limited exposure of the donor cells to the response of the immune system.

Deep anterior lamellar keratoplasty (DALK): Its most obvious advantage over penetrating keratoplasty (PK) is that in the first case the corneal endothelium is not subject to immune rejection. A lower incidence of rejection episodes was determined in the DALK group, with similar frequencies of epithelial rejection (5.2% and 4.2%, respectively for PK and DALK) and stromal (4.8% and 6.3% respectively)⁵⁸. The clinical characteristics of epithelial and stromal rejection after DALK were similar to those observed in rejection after PK. It has been observed that epithelial rejection precedes the stromal in most cases, and this appears as an area of ​​acute edema and veiling (haze), followed by vascularization – particularly at the interface – in some cases⁵⁹ (Figure 7). Close monitoring and appropriate prophylaxis are also necessary after DALK, especially in high-risk patients.

Endothelial lamellar keratoplasty (ELK/DSAEK): In a large series of 598 eyes undergoing ELK, episodes of rejection were observed in 12% in 2 years. Almost 35% were asymptomatic and were diagnosed during a routine examination. The initial signs included KPs and edema, and in no case was a rejection line observed⁶⁰. Most cases are controlled with topical or oral medication, but some may progress to graft failure⁶¹.

Descemet-endothelial keratoplasty (DEK/DMEK): DEK is associated with a significant reduction in the risk of graft rejection compared to ELK and PK^62,63, with a rejection rate of 0.1% -0.7%.