The prevalence of pterygium has been directly related with proximity to the equator: the nearer to the equator, the greater the prevalence. Most commonly seen on either side of the equator, pterygium has been endemic in the Indian subcontinent, Southeast Asia, Hawaii, the Samoan Islands, the Middle East, Mexico, the Caribbean, Australia, Northern and Western Africa, and the sunbelt states of the United States. Cameron’s world map summarizes the prevalence rates of pterygium (14). Generally rare in the cooler climates of Europe and the northern states in the United States, its prevalence, oddly, is high among the Inuit (Eskimos).

Prevalence rates of pterygium have been as high as 22.5% on the island of Aruba (15 degrees latitude from the equator), 18% in Puerto Rico (18 degrees latitude), and 5% to 15% in Texas, Florida, California, Arizona, and New Mexico in the United States (latitude 28 to 36 degrees from the equator). The prevalence rate drops to 2% or less in geographic areas more than 40 degrees from the equator. A true pterygium is a condition found chiefly in the sunny, hot, dusty regions in the world and in people more exposed to these climatic conditions, such as fishermen, farmers, construction workers, roofers, beachgoers, and Inuit. The
Inuit are probably exposed to ultraviolet (UV) light reflected from snow.

Pterygium is most common in the elderly, but the appearance of new cases per annum is higher in the younger age group. Pterygium is uncommon in individuals younger than 20 years of age and in people wearing glasses (14). The incidence rises suddenly in the age groups between 20 and 40 years. It has been reported in three relatives in a family in whom the onset occurred when the patients were at 4, 6, and 20 years of age (15).

Interestingly, this difference in incidence and prevalence rates related to age, geography, environmental exposure, or all, does not exist in patients with pinguecula (discussed later). Both the incidence and prevalence of pinguecula increase with age.

In general, pterygium is twice as common in male as in female patients, except in Aruba, where it is equally common in both sexes.

Pterygium has a dominant inheritance with low penetrance. It is not the actual lesion that is transmitted, but rather the tendency of the eye to react in this special way to environmental factors. It is a commonly seen condition among the Hispanic population in the northern United States that left the geographic confines of Mexico and the sunbelt states of the United States many generations ago.

Pterygium is mostly a bilateral condition (Fig. 60-2). Like many other conditions, one eye may follow the other by months to years. The second eye, almost always, has a pinguecula that evolves into a pterygium. Most commonly, the nasal pterygia are seen in both eyes, but cases have been recorded in which one eye had a nasal and the second one had a temporal pterygium, double pterygia (Fig. 60-3), or other combinations.

A pinguecula is a yellowish triangular patch formed by elastotic degeneration of the connective tissue, situated in the bulbar conjunctiva on either side of the cornea. It is the combined expression of changes due to age and exposure to UV light. It affects first the nasal and then the temporal side. A pinguecula appears as a roughened, raised yellowish area in the bulbar conjunctiva in the palpebral aperture near the limbus. Gradually it develops into a triangular patch with its base abutting against the cornea. Most commonly it grows to a moderate size and then ceases growing. Occasionally, nasal and temporal pingueculae may grow along the inferior limbus to meet below the cornea.

Zehender, in 1869, introduced the idea that pinguecula was the precursor of pterygium (29). This hypothesis was reaffirmed by Fuchs, Parsons, and Alt, who proposed that proliferation of hyaline and elastic tissue in the connective tissue of the conjunctiva along with degenerative changes in its bulk progressed to invade the cornea and developed into a pterygium. Once pterygium has grown to a certain extent, it is impossible to locate the position of the supposed precursor or pinguecula, although it is supposed to be in the head of pterygium. This has not been confirmed histologically. This hypothesis may be acceptable for primary pterygium; however, it is known that pinguecula has no relationship to growth of recurrent pterygium.

Sugar (30) proposed that degenerative changes in pingueculae incite hyperplasia and hypertrophy of the elastic tissue and deposition of hyaline material that elevates the epithelium away from Bowman’s layer. This process causes changes at the limbus and adjoining cornea and leads to the laying down of connective tissue that contracts and pulls this fascial layer onto the cornea. D’Ombrain (31) was of the opinion that pinguecula and pterygium both are degenerative processes. If the pinguecula grew close to the limbus it would grow onto the cornea to form a pterygium; if it stayed away from the limbus, it would continue to stay as pinguecula.

Overall, it is accepted that pinguecula and primary pterygium are somehow related to each other (similar degenerative changes in response to similar environmental factors). However, recurrent pterygium is not preceded by pinguecula.

Numerous other theories have been proposed and refuted for the pathogenesis of pterygium. These include choline deficiency (32), inflammation (6,33, 34, 35, 36), allergic basis (34), degeneration (30,37, 38, 39), tissue angiogenesis factor (40), elastic tissue changes (41,42), and immune mechanisms (43). Theories to explain the development of pterygium on neoplastic, inflammatory, episcleritis, chlamydial, and allergic bases have not been well accepted. Both direct and reflected UVB radiation has been implicated in the causation of pterygium (14,16, 17, 18, 19, 20, 21, 22, 23). Mackenzie et al. (25), in a risk analysis study conducted in Australia, has suggested that the exposure of formative ocular tissue to UV radiation in the first 5 years of life probably either sensitizes or initiates a genetic, biochemical, or morphologic change that is reflected in later years of life by the development of a pterygium. This early exposure to UV light might initiate a tissue change, probably at the “putative” stem cells at the corneal limbus (25), that requires the later promoting effect of cumulative UV radiation to the ocular surface.

It is believed that UV radiation denatures corneal proteins and induces an antigen-antibody reaction that triggers fibrovascular proliferation (34). Some unidentified etiologic factor initiates a row of fibroblasts to advance into limbal cornea and penetrate the tissue in its superficial layers between Bowman’s layer and the basement membrane of the overlying corneal epithelium. Accumulation of these fibroblasts is thought to prepare a path for the head of the pterygium to enter the cornea, and on their way, the fibroblasts push Bowman’s membrane posteriorly. Gradually, Bowman’s membrane gets fragmented and the pterygium tissue grows through these fragmented areas into underlying stroma and becomes firmly adherent to the corneal tissue.

It remains unclear at this time if the invasion of cornea by pterygium tissue is initiated/triggered by degenerative changes in the basal cell layer or induced by solar radiation, by an immunologic response to altered basement membrane material, or by an inflammatory process with possible involvement of a type 1 hypersensitivity response to antigenic pollens and dust particles (40,43).

Barraquer (44) proposed a hypothesis that limbal elevation by a pinguecula, causing tear film disruption, desiccation of corneal tissue, and microulceration of adjoining corneal epithelium, was the etiopathologic factor that induced conjunctival fibrovascular proliferation to invade cornea. Dellen formation or microulceration of the corneal epithelium has been documented, but neither is a consistent finding in an active pterygium. This pathogenetic theory has lost ground because, contrary to this hypothesis, others have suggested that there is an accumulating iron line from the pooling of tears at the apex of the head of pterygium.

Recently, it has been proposed that pterygium is a growth disorder in which mutation of the p53 tumor suppressor
gene plays a vital role (45,46). Dushku and Reid (45) and Tan et al. (46) have demonstrated high p53 protein expression in the epithelium overlying the pterygium and have speculated on the existence of a p53 gene mutation in pterygia. The hypothesis that pterygium is a benign neoplastic lesion has been further supported by the finding of microsatellite instability and loss of heterozygosity in human pterygia, two common findings in tumorous tissues (47). This theory has not been well received because Onur et al. (48) found only 3 of 38 pterygia and Chowers et al. (49) found only half of the primary and recurrent pterygia had p53 immunoreactivity in their studies. Also, p53 gene mutation has not yet been demonstrated in pterygium. To refute further the theory that pterygium is a tumorous growth, Karukonda et al. (50) found proliferative activity to be similar among primary and recurrent pterygium and normal conjunctiva by using flow cytometry; another common finding in tumorous tissues. Chowers et al. (49) did not find a difference in p53 immunoreactivity between eyes with primary pterygium that did not recur and primary pterygium that was followed by recurrence of pterygium. In addition, proliferative activity of primary pterygia that recurred was similar to that of primary pterygia without recurrence, indicating that proliferative activity was not a reliable marker for recurrence (49). The results of an immunohistochemical study of human pterygium suggest that an immunopathogenetic mechanism is responsible for the pathogenesis of pterygium. It is postulated that this mechanism is perhaps triggered by preexisting conjunctivitis or microtrauma in combination with the patient’s predisposition (51).

A pterygium appears as a triangular, fleshy growth almost invariably found on the nasal side slightly below the horizontal meridian (Fig. 60-1). Usually both eyes are affected on the nasal side, although unequally. Almost always it starts over the area of a preexisting pinguecula. The first
change is the appearance of gray, circumscribed opacities in the cornea near the limbus. The conjunctiva opposite these opacities shows shrinkage that is apparent by its tenseness and a displacement of the semilunar fold. As the conjunctiva encroaches on the cornea, it is preceded by the appearance of the same gray infiltrates in this tissue, at first as small islands, which gradually fuse. When fully developed, the head of the pterygium looks triangular with a blunt apex. Central to the apex are the small irregular opacities that can be seen with the slit lamp at the level of Bowman’s layer. More central to these changes, Stocker’s line may be seen, a pigmented line at the level of this membrane containing iron deposits (55). Toward the limbus, the conjunctival fold runs backward to the sclera in a tightly drawn triangular wing. At the limbal area it is referred to as the neck of the pterygium, and the fleshy mass spreading out into a fan shape on sclera is the body of the pterygium. The upper and lower borders of the body are folded; a probe can be slipped under the folds for a very short distance, and not through the full thickness because the area of adhesion is always smaller than its total breadth. These folds merge imperceptibly into the bulbar conjunctiva at the base of the pterygium and usually have considerable tension, which is manifested by the straight course of the vessels, the numerous folds, and the gross displacement of the plica semilunaris.

Nasal pterygium is by far the most common, representing 60% of total pterygia (Fig. 60-1).

The next most common is the pterygium growing from the temporal side onto the clear cornea, representing approximately 20% of total pterygia (Fig. 60-3).

Nasal and temporal pterygia presenting in the same eye are called double pterygia (Fig. 60-3) and represent approximately 20% of total pterygia.

Pterygia in both eyes are called bilateral pterygia; pterygia are usually bilateral (Fig. 60-2). The presence of a nasal pterygium in one eye and a temporal pterygium in the other eye, or double pterygia in one eye and a nasal or temporal pterygium in the second eye, is common (Fig. 60-2).

Regrowth of the pterygium after primary excision is called recurrent pterygium. Pathologically, recurrent pterygium differs from primary pterygium; it is fibrovascular tissue growing onto the cornea without elastotic degeneration. It involves the underlying sclera, episclera, and Tenon’s capsule and grows onto corneal stroma, where it is very firmly adherent to the underlying tissues (Fig. 60-4). Recurrent
pterygium can cause restriction of ocular motility because of scar tissue or involvement of the horizontal rectus muscle sheaths.

A pseudopterygium is the result of an inflammatory process wherein a fold of inflamed conjunctiva becomes adherent to denuded cornea near the limbus or more centrally. Marginal corneal ulceration, traumatic epithelial defects, or peripheral corneal degenerations can involve the chemotic conjunctival fold in the healing process, dragging conjunctiva across the cornea. Pseudopterygium does not respect the interpalpebral location and can be seen at any meridian on the eye. It makes a bridge of tissue growing from conjunctiva onto the corneal denuded area over limbus; a probe can be passed under its full breadth (6). It is usually unilateral and does not tend to regrow after primary breaking of the adhesions.

Because the etiology and pathogenesis of pterygium growth are unclear, the role of prophylaxis is also uncertain. Theoretically, preventing or limiting exposure to UV light might be of some help. A change of environment might not be feasible or practical. Protection of the eyes by regular spectacles, dark sunglasses, or hats when exposed to the sun or irritating environmental conditions could certainly do some good, as suggested by Cameron (14), Rosenthal et al. (58,59), and Mackenzie et al. (25). Cameron (14) found in Australia that the rate of incidence of pterygium was reduced from an average of 15% to 3% among those who had worn glasses constantly from before the age of 15 years. Mackenzie et al. (25) found that the normal population that did not wear regular spectacles was at a threefold higher risk for development of pterygium compared with the population that wore regular spectacles. The population that wore dark sunglasses when exposed to the sun was five-fold better protected than the general population against pterygium growth. This association has also been found with wearing a hat in the sun. Mackenzie et al. (25) has recommended avoidance of exposure to UV light, especially in the first 5 years of life.

Since the time of Susruta, approximately 1000 years B.C., medical treatment has been tried and found unsatisfactory. In the 20th century, local applications of solid choline chloride (32) or of topical steroids (6), or subconjunctival injection of hyaluronidase (6) have proved of little value (6).

The only effective treatment for pterygium is surgery. However, none of the surgical procedures is perfect and
universally accepted because of high recurrence rates. Since the time of Susruta, mankind has tried almost every imaginable procedure to remove pterygia with the hope that there would be no recurrence. Even in ancient times the problems of pterygium management were recognized. Susruta wrote, “Any remnant of the pterygium should be removed with a scarifying ointment to prevent recurrence” (3). According to Rich et al. (60), “to manage pterygia, we can incise, excise, bury, transplant, graft, freeze, burn, cauterize, diathermize, divulse, evulse, chemically assault, irradiate or simply leave them to fate.” The many therapeutic options available to manage pterygia imply that no single one is completely effective. Almost all (91.6%) recurrences appear by 360 days after surgery. It has been suggested that 1 year is the optimal follow-up time to identify recurrence of pterygium (61).