Associated with aniridia keratopathy
JOHN M. FREEMAN
MECA Eye & Laser Center, 6485 Poplar Ave, Memphis, TN 38119, USA
John M. Freeman — MD, ophthalmologist, director of the Center, tel. (901) 767-3937, е-mail: emilyh@mecavision.com
Introduction
Aniridic keratopathy (AK) is the slow opacification of the cornea seen in congenital aniridia which usually does not become visually significant until the second decade of life or later. AK results from a failure of the surface epithelium and in some cases does not appear to be clinically present at birth. Clinically the condition is observed as a creeping pannus of conjunctival like surface cells that move in from the periphery to slowly cover the entire cornea. The surface changes lead to neovascularization and scarring of the corneal surface and anterior stroma, and this eventually leads to opacity formation in the cornea. The early changes can create general ocular surface discomfort, photophobia, inflammation, and recurrent erosions. In the late stages, the corneal opacity causes loss of vision and there are no known clinically proven medical treatments that definitively slow or halt the progression of the condition. Once corneal opacity develops, surgical restoration of a clear visual axis can be challenging and the two main options are either keratoprosthesis or limbal stem cell transplants followed by corneal transplant. Over the last two decades significant insights have been gained into both normal corneal epithelial maintenance and the abnormal corneal maintenance seen with congenital aniridia. These insights are bringing us closer than ever to developing new therapies for the treatment of aniridic keratopathy based on a molecular and cellular understanding of the disease.
Maintenance of Normal Corneal Epithelium
In the normal cornea, the clear corneal epithelium is maintained by the ocular surface complex that includes a variety of mechanisms that function on the anatomic, cellular, and molecular level. Every aspect of the external eye and surrounding anatomy plays some role in this ocular surface complex. Normal lid function, the fornices, the tear film, the lacrimal gland, and the neuro-secretory reflex are all well known anatomic components of this complex. On the molecular level, a variety of growth factors and cytokines have been identified in corneal epithelial homeostasis including members of the epidermal and fibroblast growth family, hepatocyte growth factor, transforming growth factor and insulin-like growth factor [1]. On the cellular level corneal limbal stem cells play the central role in maintaining the corneal epithelium. Corneal limbal stem cells are maintained in the limbal stem cell niche at the periphery of the cornea in the palisades of Vogt. The limbal stem cells provide a reservoir of cells with constant proliferative potential that maintain the corneal epithelium. They provide a steady state homeostasis of tissue specific terminally differentiated cells while maintaining a reservoir of stem cells whose mitotic divisions can be amplified in response to varying demands. Limbal stem cells divide to create transient amplifying cells (TAC). Transient amplifying cells migrate and divide to become wing cells which then become terminally differentiated corneal epithelial cells that are eventually desquamated on the corneal surface. The proliferative potential of the daughter cells appears to diminish throughout the differentiation process. In humans, the average lifespan for a corneal epithelial cell is estimated to be approximately 10 days as the surface cells are lost and replaced by the deeper cell layers [2]. In response to sudden loss of corneal epithelium, surrounding cells start covering the epithelial defect by migration and cell spreading, while the deeper transient amplifying cells and stem cells provide the reconstitutive mitosis to replace the lost cells. The limbal stem cells also provide a barrier function to prevent conjunctival type cells from growing onto the cornea. Traditionally, aniridic keratopathy has been understood as a limbal stem cell deficiency while it may be more of deficiency in the overall process of corneal epithelial homeostasis.
Findings in Aniridic Keratopathy
The prevalence of aniridic keratopathy while not universal increases with age and has been reported in 46-90% of aniridics [3-6]. The timing of AK onset and progression appears to be variable but usually onset develops by the teen years [7, 8] and there can be a difference in progression between the eyes in a given individual. In one study involving self-reporting, the average age of diagnosis was 20 years+/-12.2 years [9]. There is agreement that the condition develops in stages moving slowly from the periphery to the center of the cornea. There are at least two different scales for staging AK. The one from Sweden and Norway has been employed on the greatest scale and involves the following grading: grade 0: no corneal involvement; grade 1: peripheral invasion of blood vessels; grade 2: total circumference of limbus has conjunctivalizaion; grade 3: conjunctivalization and vessels have reached the center and the stroma is involved; grade 4: opacified cornea [10].
A useful technique for observing the early stages of aniridic keratopathy at the slitlamp is the phenomenon of «late-staining». «Late-staining» involves the instillation of topical fluorescein dye and the cobalt blue filter just like an epithelial defect. However, the abnormal conjunctival type tissue that signifies the limbal stem cell failure requires 3-6 minutes of time to uptake the dye and will show a weaker glow than a normal epithelial defect. This technique is particularly useful in exams under anesthesia on small children where the subtle findings of early stage AK can easily be missed.
Clinical studies of aniridic keratopathy with confocal microscopy have revealed that at least some aniridics early on have normal appearing palisades of Vogt and corneal epithelium [11] and thus have some functioning limbal stem cells and a somewhat normal appearing niche. These studies have found that the progression of aniridic keratopathy coincides with loss of organization of the palisades of Vogt, loss of central corneal sensation, loss of central subasal nerve density, and invasion of the corneal surface with blood vessels and inflammatory cells. All of these eventually culminate with corneal opacity and loss of visual acuity. In contrast to other limbal stem cell deficiencies such as those from chemical burns or inflammatory conditions such as Stevens-Johnson syndrome or ocular cicatricial pemphigoid, AK displays 360 degrees of peripheral pannus while maintaining clear central cornea epithelium for years. In other limbal stem cell deficiencies, total peripheral pannus would lead to loss of clear central epithelium in short order, usually weeks to months. This suggests a difference in the exact nature of stem cell failure.
Molecular genetics and basic research on aniridic keratopathy
Most cases of congenital aniridia result from a heterozygous mutation in one of the pax 6 genes on chromosome 11. Over 200 different mutations have been identified in the pax 6 gene involving congenital aniridia [12]. Classically, pax 6, a transcription factor that has been conserved in many species, has been studied as a developmental gene. Pax 6 has over a 1000 potential downstream targets and is known to be active during organogenesis of the eye, brain, and pancreas [13]. It is considered a «multi-level regulator of ocular development» and much research has elucidated the complex role of pax 6 during development of the eye [14]. However, in contrast to foveal hypoplasia and aniridia which represent obvious defects of development, aniridic keratopathy appears to be a transition from a fairly normal epithelial system to a defective system that occurs slowly over decades. Thus aniridic keratopathy points to an ongoing adult role for pax 6 gene function and that is exactly what animal models of pax 6 mutations have revealed over the last decade. Pax 6 appears to play an active role in corneal epithelial homeostasis [15] and pax 6 gene deficiency leads to increased corneal epithelial turnover [16]. This increased corneal turnover may be partly from increased cell fragility from defective cell adhesion molecules and defective extra-cellular matrix [17]. This adult role for pax 6 provides an opportunity to develop therapies that can halt or reverse the progression of AK.
Medical Treatment
Currently there are no known clinically proven treatments that definitively slow, halt, or reverse the progression of AK. However, several treatments have proven helpful to treat the symptoms of early and late stages of AK and most are based on extrapolation of treatments and techniques used in similar clinical conditions. It is likely that there is some mild benefit to these approaches but it is hard to measure. A general principle is to protect the corneal epithelium from stress when avoidable. For example during cataract or glaucoma surgery, the limbal area and epithelium should be avoided and protected during surgery whenever possible. Decreasing exposure to wind and sun is also likely beneficial as UV exposure increases corneal epithelial cell loss and would likely accelerate the progression of the condition. Use of preservative-free topical glaucoma agents and lubricant drops is also likely beneficial. Topical cyclosporine likewise helps to break any cycle of inflammation and dryness that could progress the condition. Autologous serum derived from the patient’s own serum has proven somewhat helpful in improving symptoms of AK and may slightly improve clinical findings of the condition [18]. Autologous serum has growth factors that are known to be involved in epithelial maintenance and has demonstrated ability to help heal neurotrophic corneal ulcers in other conditions. Recurrent erosions and dry eyes in general can be treated with conventional methods with the exception that epithelial debridement should probably be avoided.
Surgical Treatment
In AK, once corneal opacity develops across the visual axis and visual acuity drops below baseline, surgery is the only option to restore a clear visual axis. Corneal transplants alone do not work for the long term because AK involves failure of the ocular surface which is maintained by the host in a corneal transplant. The two main techniques utilized to restore the visual axis in AK are limbal stem cell transplants (keratolimbal allografts KLAL) and keratoprosthesis. Limbal stem cell transplants restore the maintenance of a corneal epithelium while keratoprostheses do not require a normal ocular surface. Neither technique is without risk and the decision to proceed with either one is difficult for the doctor and patient.
Limbal stem cell transplants for aniridic keratopathy
Limbal stem cell transplants are usually harvested from cadaveric sources in the United States and directly transplanted to the patient, but other countries have employed ex-vivo expansion to expand the tissue before transplant [19]. Regardless of technique, systemic immunosuppression is required for survival of the allografts. The systemic immunosuppression required is similar to that used in renal transplants; however, there is no standard protocol for length or type of immunosuppression. In one study using directly transplanted KLAL tissue, 75% of the aniridic eyes had a stable ocular surface with an average follow-up of 4.7 years after limbal stem cell transplant. In this study success was very dependent on systemic immunsuppression as only 29% of those who could not take systemic immunosuppression had a long-term stable ocular surface [20]. This study utilized systemic oral prednisone briefly and longer term tacrolimus and mycophenolate. A study with 3 year follow-up using ex-vivo expanded allografts showed only 22% of aniridic eyes had improved ocular surface. This study only used systemic immunosuppression with cyclosporine and prednisone for no more than 6 months [21] which may account for the lower success. If significant stromal scarring is present after the limbal stem cell transplant, subsequent corneal procedures such as deep anterior lamellar keratoplasty or penetrating keratoplasty may be required. Doing limbal stem cell transplants earlier, before formation of deep stromal scarring, avoids the need for these subsequent procedures.
Besides variable success, the main drawback for limbal stem cell transplants is the systemic immunosuppression which can damage the overall health of the individual. Aniridia has an association with diabetes and obesity that may make the immunosuppression more risky. In addition, the systemic immunosuppression requires close coordination with an immunosuppression specialist as most ophthalmologists are not well-versed in managing these potentially dangerous drugs.
Keratoprosthetic for Aniridic Keratopathy
Currently, the Boston Keratoprosthetic (K-pro) is the main keratoprosthetic used in the United States and probably world-wide. The Boston K-pro utilizes a PMMA front piece with 3mm optic that is fastened to a surrounding carrier cadaveric cornea by a backplate. Usually a fresh cadaveric cornea is used but frozen and stored irradiated corneas have been used with success. It does not require a normal ocular surface and a conjunctivalized surface may actually be advantageous. Two main drawbacks of keratoprosthetics are one, inability to accurately measure intraocular pressure and two, the ongoing risk of endophthalmitis. Intraocular pressure must be estimated clinically by touch. Because the device in essence connects the non-sterile outside world with intraocular space, the potential for infection is always present. Patients are kept on topical antibacterial antibiotics to suppress bacterial growth and some clinicians advocate a povidine iodine rinse at monthly clinic visits and chronic anti-fungal drops [22]. In addition to the topical drops, a bandage contact lens must be worn continuously. The contact lens keeps the surface from becoming dehydrated and developing epithelial defects. An advantage for aniridics, for whom intraocular lenses can be problematic, is that the power of the keratoprosthetic can be customized to the axial length and phakic status of the eye. Also, the power of the contact lens can be adjusted to fine tune the refractive status of the patient.
The long term success of the Boston K-pro in aniridics from initial reports seemed promising [23] while a larger recent series pointed out that there are still significant challenges for these devices in aniridics [24]. Initial series reported 100% retention of the device with median follow-up of 17 months but a more recent large study with mean follow-up of 28.7 months had a retention of only 77% [24]. Retroprosthetic membrane is the most common reported post-operative complication with rates between 25% and 77% in different series. This may be particular important in aniridics with the potential for aniridic fibrosis syndrome [25]. Fortunately, devastating complications such as corneal melts, device extrusion, and endophthalmitis have been relatively rare, but can occur with the Boston Keratoprosthetic. Also, as measurement of intraocular pressure becomes less precise with the K-pro, most clinicians recommend placement of a tube shunt for better glaucoma control.
To summarize the surgical treatment of aniridic keratopathy. neither limbal stem cell transplants nor the Boston Keratoprosthetic have long-term success rates higher than 90% and both require meticulous, attentive care by the doctor and patient. Limbal stem cell transplants entail more risk to the body and require help from systemic immunosuppression specialist. However, risk to the eye is lower and unlikely to be devastating. The Boston K-pro entails more risk to the eye in that more rapid developing catastrophic events are possible.
Future Treatments
We are possibly on the cusp of a new era in treatment of congenial aniridia. Researchers in Canada in 2014, reported on new molecular based treatment for congenital aniridia named START therapy. This treatment involves suppressing premature termination codons in pax 6 gene nonsense mutations with topical or systemic medications. This treatment given in the postnatal period yielded dramatic results in pax 6 heterzygous mice by recovering more normal phenotypes in retinal, lens, and corneal tissue. These same researchers are initiating a clinical trial in humans using the same drug topically in aniridics with in-frame nonsense mutations. It is estimated that approximately 50% of pax 6 mutant aniridics have such mutations. Thus now knowing the precise mutation in a given aniridic is critical to determining if a patient could potentially benefit from the trial.
Summary
Aniridic keratopathy represents a challenging condition to treat because restoration of the visual axis entails significant risk without high enough success rates. From studies in animal models, a more refined model of the condition has emerged that goes beyond a basic stem cell deficiency. This model highlights the ongoing dynamic mechanisms involved in corneal epithelial homeostasis and creates potential for treatments that can slow, halt, or reverse the condition before the more invasive risky current procedures are needed. Future therapies will likely rely on restoring more normal pax 6 gene expression in the adult such as START therapy, or rely on replacement of components downstream of pax 6 that are critical for ongoing corneal homeostasis.
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