Victor L. Perez and Jessica Chow

Introduction

Proper functioning of limbal stem cells is crucial to the maintenance of a stable and healthy ocular surface. Limbal stem cells provide a renewal source of corneal epithelial cells and act as a barrier to the extension of conjunctival epithelial cells onto the corneal surface. Limbal stem cell deficiency (LSCD) results in breakdown of the corneal epithelium and poor wound healing, eventually leading to conjunctivalization and opacification of the cornea.¹ Symptoms of LSCD include redness, irritation, photophobia, and decreased vision. Early findings on slit lamp examination include corneal neovascularization, pannus, and loss of the palisades of Vogt. As the condition progresses, punctate epithelial keratopathy and frank epithelial defects may develop. Due to decreased wound healing, these epithelial defects may become persistent, which can lead to stromal scarring, ulceration, and perforation. Conjunctivalization of the cornea may occur, where the corneal epithelium is replaced with a conjunctival epithelial phenotype.² Staining of the conjunctivalized epithelium with fluorescein may occur. If conjunctival stem cells, which are thought to reside in the fornices, are also compromised, the entire ocular surface may become keratinized.^3,4

Causes of limbal stem cell deficiency include both inherited and acquired etiologies. Aniridia is a condition in which anterior segment dysgenesis results in a decreased number of limbal stem cells. Aniridic individuals are born with a normal ocular surface. However, as the patient ages, limbal stem cell dysfunction manifests as corneal epitheliopathy which begins in the peripheral cornea and progresses to involve the central cornea, leading to corneal ulceration, scarring and decreased vision. Other congenital causes of LSCD include Peters anomaly and ectodermal dyplasia.¹ Autoimmune disorders can also result in limbal stem cell deficiency (Fig. 33.1). Typically, these conditions cause chronic inflammation of the conjunctiva with secondary involvement of the limbus. Stevens–Johnson syndrome, toxic epidermal necrolysis, and ocular cicatricial pemphigoid are all diseases in which chronic conjunctival and limbal inflammation diminishes the stem cell population. Limbal lesions, such as conjunctival or corneal intraepithelial neoplasia (CIN) are also associated with limbal stem cell deficiency.⁵ Stem cell deficiency is thought to result from replacement of normal stem cells with neoplastic cells. Pterygia have also been associated with LSCD, presumably, due to chronic limbal inflammation.⁶ Direct trauma to the limbus by alkali, acid, or thermal injury is another common cause of LSCD.³ Other acquired causes of LSCD include infectious causes, such as herpesviruses and trachoma. Recent reports of mustard gas-induced LSCD have also been described.⁷

The categories above represent the most common causes of LSCD as reported in the literature. However, another group of patients develop LSCD secondary to iatrogenic etiologies, defined as directly resulting from intervention or treatment by a physician or surgeon. Many of these cases are multifactorial, and while they are treated with the same methods as those used for classically described causes of stem cell deficiency, awareness of induced etiologies may be useful for both the early recognition and prevention of LSCD in these patients. This chapter will review examples of iatrogenic causes of LSCD (Box 33.1).

Multiple Ocular Surgery-Induced Iatrogenic Stem Cell Deficiency

Limbal stem cell deficiency can occur as a result of multiple ocular surgeries (Fig. 33.2). Puangsricharern and Tseng classified limbal stem cell deficiency etiologies into two groups, resulting from either hypofunction or aplasia of stem cells.⁶ In the latter category, patients had a clear history of limbal stem cell destruction by chemical/thermal burns, Stevens–Johnson syndrome, severe microbial keratitis, contact lens-induced keratopathy, and a previously undescribed group of patients had undergone multiple surgeries or cryotherapies to the limbal region. Patients had clinical signs of LSCD, including corneal vascularization and irregular corneal epithelium, as well as impression cytology demonstrating goblet cell-containing conjunctival epithelial cells on the corneal surface.⁶

In 1998, Schwartz and Holland described a group of patients with limbal stem cell disease not secondary to a known diagnosis, which they termed ‘iatrogenic limbal stem cell deficiency.’8,9 The patients were similar to the above group in Puangsricharern’s study who had undergone ‘multiple surgeries or cryotherapies of the limbal region.’ Schwartz’s study comprised 14 eyes of 12 patients. All eyes had a history of prior ocular surgery involving the corneoscleral limbus, as well as concurrent external disease, including pterygium, keratoconjunctivitis sicca, rosacea, herpes simplex keratitis, or corneal edema. Eleven eyes had received long-term topical medications. In all eyes, the superior quadrants that corresponded to the areas of prior limbal surgery were affected by corneal scarring and neovascularization. Many of the eyes had undergone multiple ocular surgeries. Nine eyes had undergone prior ICCE, three had ECCE, and nine had penetrating keratoplasties.

All eyes exhibited a chronic, progressive epitheliopathy that began in the peripheral cornea and extended centrally, in a pattern consistent with limbal stem cell deficiency. The epitheliopathy was initially sectoral and located in an area of previous limbal surgery, in contrast to classically described causes of LSCD which involve the entire limbus. The authors noted that the epitheliopathy did not resolve with therapy for dry eye management or with cessation of topical medications, implying a permanent alteration in the stability of the ocular surface. They hypothesized that direct surgical trauma to the limbus resulted in localized stem cell deficiency, resulting in increased susceptibility to further damage from both external disease and toxicity from long-term topical medications. All patients had presumed multifactorial LSCD secondary to prior corneoscleral limbal surgery combined with topical medications.⁸

Sridhar et al. reported three cases of impression cytology-proven LSCD following limbal surgeries.¹⁰ One patient had undergone multiple pterygium surgeries, while the other two had undergone therapeutic penetrating keratoplasties. In all cases, limbal stem cell deficiency was confirmed by detection of goblet cells on the surface of the cornea by impression cytology. Surgical trauma to the limbus was the predisposing factor for the development of LSCD in all three cases, while use of topical medications in cases of therapeutic penetrating keratoplasty was felt to be a contributing factor.

Glaucoma Surgery and Iatrogenic Limbal Stem Cell Deficiency

The development of limbal stem cell deficiency in patients who have undergone glaucoma procedures is multifactorial and, due to a combination of mechanical insult from surgery and toxic injury from antimetabolite and chronic topical medication use.8,11 Antimetabolites, such as 5-fluorouracil and mitomycin C that are used in conjunction with glaucoma surgery are well known to cause corneal toxicity.^11–13 5-fluorouracil (5-FU), a cell-cycle-specific antimetabolite, inhibits fibroblast proliferation and is used to prevent scarring and closure of the filtration sites in glaucoma procedures. In the acute phase, the cornea exhibits epithelial toxicity and sloughing following the administration of 5-FU, is likely due to its antimetabolite effect on corneal epithelial progenitor cells. With prolonged and recurrent use of 5-FU, damage to the slower-cycling limbal stem cells can result in corneal conjunctivalization and persistent epithelial breakdown.¹³ In 2000, Pires et al. reported two cases of iatrogenic limbal stem cell deficiency in post-trabeculectomy patients who had each received over 105 mg 5-FU injections in the postoperative period. Limbal stem cell deficiency was confirmed by impression cytology. One patient with partial limbal stem cell deficiency recovered well after amniotic membrane transplantation while the other, who exhibited total limbal stem cell deficiency, required limbal stem cell transplantation to stabilize the ocular surface.¹³

Mitomycin C (MMC), a potent DNA cross-linker, is an antimetabolite that targets dividing cells. MMC is usually applied during glaucoma surgery by soaking the drug in sponges and placing them in contact with the conjunctiva near the filtration site. However, this technique can lead to wound leak, due to delayed conjunctival healing.12,14 Sauder and Jonas reported a case series of seven patients who underwent trabeculectomy with a modified technique, in which mitomycin C was administered subconjunctivally prior to opening of the conjunctiva. In this series, three patients developed signs and symptoms of limbal stem cell deficiency, including avascularity of over 50% of the limbal circumference, dry eye, corneal epithelial irregularities, and corneal opacities. The other four patients did not show similar sequelae, but, of note, were each followed for less than 14 months, compared to the three patients with MMC-related complications that were followed for at least 2 years. The authors concluded that subconjunctival MMC in conjunction with trabeculectomy could result in delayed postoperative complications, including late limbal stem cell deficiency, and should therefore be avoided.¹⁴