4.3 Pterygium

4.3.1 Introduction

A pterygium has been deÞned as a triangular-shaped, elastotic degeneration of the conjunctiva, consisting of bulbar conjunctival epithelium and hypertrophied subconjunctival connective tissue, occurring medially and laterally in the palpebral Þssure, and encroaching onto the cornea [1]. Epidemiological studies suggest an association with chronic exposure to sunlight, with an increased geographical prevalence in a Òperi-equatorial pterygium beltÓ of latitudes 37¡ north and south of the equator [2]. The examination of more than 64,000 Aborigines and 40,000 non-Aborigines throughout rural Australia in the early 1980s demonstrated an increased prevalence of pterygium in Aborigines [3]. Furthermore, the pattern of pterygium prevalence was tested against UV zones with the lowest prevalence of pterygia seen in the lowest UV intensity zone and vice versa [3]. Aborigines live most of their lives outdoors with most of their communities having makeshift housing in surroundings of bare ground [3]. These Þndings help to demonstrate a strong correlation between UV irradiation and the prevalence of pterygium. Indeed, the UV type B light in solar radiation has been found to be the most signiÞcant environmental factor in pterygium pathogenesis [2].

Studies have recently demonstrated that a pterygium could possibly not be just a degenerative lesion, but the result of an uncontrolled cell proliferation [2]. The p53 tumor suppressor gene has been implicated and may be damaged by UV radiation-induced mutations, resulting in abnormal expression in pterygial epithelium [2]. Some have postulated that the human papillomavirus may be involved as well. Tseng et al. have speculated that a pterygium may in fact represent an area of localized limbal stem cell deÞciency [2]. As will be discussed in surgical technique, a conjunctival autograft is a safe and effective means of reducing pterygium recurrence. One type of conjunctival autograft is the limbal-conjunctival autograft in which 0.5 mm of the limbus and peripheral cornea are transplanted with the conjunctival graft and aligned limbus to limbus over the scleral defect [4]. Some authors have cited a lower recurrence rate with this technique [4], furthering the idea that a pterygium may be the

result of a limbal dysfunction. Youngson noted in 1972 that loss of limbal integrity following the removal of a pterygium was one of the principal elements in the pathology of the recurrent condition [5]. At a time when the location and function of the limbal stem cells was not elicited, Youngson noted the importance of limbal integrity in the pathology of pterygium recurrence. He postulated that loss of limbal integrity leads to the inability to determine the demarcating change in epithelial type between the cornea and conjunctiva, and that without a limbal barrier, the corneal and conjunctival epithelial cells meet on the sclera remote from the limbus and begin to form an extracorneal pterygium that then grows toward the limbus [5]. While his concept of the pathogenesis of a recurrent pterygium may not be entirely accurate, it is important to note his realization of the involvement of the limbus.

In many parts of the developing world, where ophthalmic care is not so readily accessible, severe pterygium remains a cause of corneal blindness. Patients presenting with pterygia often complain that the pterygia are cosmetically unacceptable and/or that they have a constant foreign body sensation, a gritty sensation, or a chronically red, irritated eye. All of these are acceptable reasons for excision. Sometimes patients need to be ensured that pterygia are not cancerous growths. Indeed, there are case reports of carcinoma in situ and squamous cell carcinomas that mimic the appearance of pterygia, and the surgeon must always keep this in mind and remove any suspicious lesions to send to pathology for histopathologic diagnosis.

4.3.2 Treatment of Pterygium

The deÞnitive treatment for a pterygium is surgical removal. While removal of a pterygium can seem relatively simple, the procedure can be very deceiving and recurrence rates can be high if the lesion is not removed properly. There are a variety of techniques available for surgical removal that may be employed, but each is associated with different recurrence rates and complications. Bare sclera technique, in which the pterygium is removed without associated adjunctive therapy, has a 30Ð80% recurrence rate [6]. Other procedures