Pterygium of the Eye

Introduction to pterygium of the eye (pterygium)

Pterygium of the eye is a triangular wedge of fibrovascular conjunctival tissue that usually begins medially on the conjunctiva and extends laterally toward the cornea. "Pterygium" refers to the shape of the tissue, which resembles an insect's wing. The plural form of pterygium is pterygia.

Pterygium of the eye is sometimes considered a trivial problem because it is unlikely to threaten visual acuity unless it approaches the visual axis. However, it can cause concern for patients because of the abnormal appearance it gives the eye and the fact that it has a tendency to be irritated, red and inflamed. Although it is a benign change, pterygium of the eye can adversely affect vision if the spread approaches or reaches the visual axis.

The worldwide prevalence of pterygium of the eye varies between 1 and 25 percent, depending on the population studied. Pterygium of the eye occurs more frequently in tropical regions, although the exact mechanisms for this are not well understood. The prevalence of pterygium of the eye is associated with chronic exposure to the sun and especially to ultraviolet light (UV), which may partly explain the geographic variation in prevalence.

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Several population-based studies have found higher rates of pterygium of the eye associated with older age, male sex, fewer years of education and an outdoor work location. In the Barbados Eye Study, a quarter of the black participants had a pterygium, a frequency 2.5 to 3 times higher than among the white participants in this study. Lower rates were associated with frequent use of sunglasses outdoors and the use of prescription glasses. One study in Australia found a higher rate of pterygia of the eye in rural areas compared with urban areas (6.7 and 1.7 percent, respectively), partly as a result of sun exposure to the eyes.

Pathogenesis of pterygium of the white of the eye

Although pterygium is classified as a degenerative disorder of the cornea, it may be considered more as a proliferative condition with several possible triggering factors. Ultraviolet light (UV), abnormal conjunctival expression of the tumor-suppressor gene p53, the presence of factors related to angiogenesis, infection with human papillomavirus [HPV] and abnormal human leukocyte antigen (HLA) expression have all been proposed as pathogenic factors. In particular, UV radiation may trigger events that produce damage to cellular DNA, to RNA and to the composition of the extracellular matrix. Hereditary factors may also contribute to the pathogenesis and therefore to the varying prevalence rates among populations.

One model for the production of epithelial cells suggests that a light-induced change in the function of limbal stem cells can explain the classic wedge shape of the pterygium, which begins at the limbus (the junction of the cornea and the sclera).

Natural course of pterygium of the eye

The natural course of pterygium of the eye is not well understood. One general observation is that a pterygium, when it is active, can grow over several months to years. Activity is marked clinically by redness and localized thickening, which probably represent active inflammation. When it is inactive (white and flat), the pterygium of the eye may remain static for decades without an increase in size or clinical significance. It is not clear how a pterygium becomes active or inactive, or whether it can be reactivated.

Clinical presentation of pterygium of the eye

The most common symptoms caused by pterygium of the eye are redness and irritation. Visual impairment is less common. In the absence of symptoms, patients may also report a change in the appearance of their eye. Sometimes they do not notice the finding at all until they are examined by an ophthalmologist.

Although they are common, the redness and irritation associated with pterygium of the eye are usually mild. Therefore, initially, a patient is unlikely to seek medical treatment. When patients do seek treatment, it is usually because there is a worsening of the symptoms of discomfort and foreign-body sensation, which impairs their daily activity.

Mild visual impairment caused by pterygium of the white of the eye can initially be ignored, although some patients have visual impairment due to a pterygium that significantly affects daily function. A pterygium extending more than a few millimeters onto the cornea can impair vision on the basis of astigmatism. Astigmatism is a refractive error in which a distorted corneal surface causes light rays entering the eye along different planes to be focused unevenly. At low power, astigmatism causes mild subjective blurring. Growth larger than 3.5 mm, and therefore more than half the pupil center in a typical cornea of 11 to 12 mm, can cause blurring of vision. Once a threshold of 45 percent of the corneal radius is crossed, or within 3.2 mm of the visual axis, the astigmatism may increase significantly. A pterygium extending more centrally may directly affect the visual axis, when the resulting opacity blocks vision.

A rare manifestation of pterygium of the eye is restricted eye movement. This occurs following multiple excision attempts when the secondary inflammation over the pterygium causes adhesion of the conjunctiva and the eyelid to which it must adhere, and prevents the eye from moving properly.

In the absence of symptoms, patients may report a growth on the surface of the clear cornea, sitting anterior to the iris. The patient or the clinician often notices the pterygium only when the pterygium appears white against the colored iris, or when there is a significant vascular component causing redness of the pterygium of the eye in contrast to the conjunctiva.

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Diagnosis of pterygium of the eye

The diagnosis of pterygium of the eye is made by the classic clinical appearance of a wedge-shaped growth extending over the cornea. However, pterygium of the white of the eye does not always present in its classic form, and other conditions have a similar appearance.

Pterygium of the eye is usually soft, varying from almost flat, white and amorphous to thick, pink/red and fibrovascular. A pterygium is more likely to be bilateral than unilateral.

Differential diagnosis (malignant and pre-malignant lesions)

Several features help distinguish between pterygium of the eye and carcinoma (a malignant tumor) of the squamous cells of the conjunctiva, and a pre-malignant tumor (conjunctival intraepithelial neoplasia (CIN)):

• Malignant lesions are usually more vascular than pterygium of the eye and may have prominent feeder vessels.
• Malignant lesions tend more to have nodular or leukoplakic features and to have an irregular consistency.
• CIN and malignant lesions are less likely to have the triangular insect-wing shape characteristic of pterygium of the eye and may be more fan-like in appearance.
• There is a higher risk for CIN and malignant lesions to be unilateral and they often occur in axes other than the horizontal plane.

Conjunctival neoplasms that may be confused with pterygium are:

• Squamous neoplasia (OSSN) of the outer ocular surface, which includes:
• Squamous cell carcinoma (SCC)
• CIN
• Melanoma of the conjunctiva (amelanotic variant)

OSSN has been found to be associated with sun exposure, HIV/AIDS and HPV. The increased prevalence of conjunctival melanoma is similar to that of cutaneous melanoma and may be related to sunlight exposure. An ophthalmologist with appropriate expertise should be consulted to determine whether a biopsy is needed.

Differential diagnosis (benign lesions) – Pinguecula is a degenerative eye condition that is often confused with a pterygium. Pinguecula is a yellowish conjunctival lesion, slightly raised, that arises in the limbal conjunctiva. Unlike pterygium of the eye, which rises from the limbus and advances onto the cornea, pinguecula rises from the limbus and remains confined to the conjunctiva without corneal involvement. The tendency to extend over the surface of the cornea is the main distinguishing factor of a pterygium compared with pinguecula. There is usually a gap between the pinguecula and the edge of the cornea. Unlike pterygium of the eye, which is more likely to appear in the nasal conjunctiva, pinguecula can appear on the nasal side and the temporal side.

Other conditions that can mimic a pterygium include localized conjunctivitis, pseudo-pterygium (corneal pannus, a reaction to chronic blepharitis, allergy, chronic contact-lens use and more) and episcleritis. Other clinical findings for these conditions will help distinguish them from pterygium of the eye. These conditions can also cause lesions outside the horizontal axis, unlike a pterygium.

Finally, symblepharon after chemical, thermal or mechanical injury, or associated with diseases such as Stevens-Johnson syndrome, mucous membrane pemphigoid, fibrosis after surgery near the limbus and limbal dermoid, may be findings that mimic pterygium of the eye.

Treatment of pterygium of the eye

Patients with a small pterygium can be treated symptomatically for redness and irritation with tear substitutes or other eye lubricants. Treatment of patients with larger lesions that impair visual acuity or eye movement usually involves surgical excision of the pterygium. The decision to perform surgical excision also varies according to the documented growth rate and the degree of induced astigmatism.

Medical treatment – medical treatments are intended for symptomatic relief and have not been shown to stop progression or cause regression of the pterygium. Patients with pterygium of the white of the eye that does not affect vision or eye movement may be treated symptomatically with topical lubricants including drops, ointments and gels, all available without a prescription. Artificial tear substitutes are the most common lubricant for a pterygium and may help reduce symptoms. They can be given as 1-2 drops to the affected area three to four times a day. It is preferable to use preservative-free preparations; it should be noted that preservative-free preparations are more expensive.

Treatment with non-steroidal anti-inflammatory drugs (NSAIDs) and glucocorticoids may also be effective for symptomatic relief of an inflamed pterygium, but all are associated with side effects that limit their use. These agents can be used to treat redness and irritation that are not relieved by tear substitutes. The most common side effect of steroids is increased intraocular pressure. Topical NSAID drugs should be prescribed only by an ophthalmologist, since they can cause melting of the cornea with prolonged use. Topical steroids should also be prescribed only by an ophthalmologist since they can worsen an infection (if there is one) and cause glaucoma and cataract with prolonged use. Use of these drops should be avoided if the patient is not symptomatic. Any of the drugs taken chronically can lead to tachyphylaxis (tolerance) and cause "rebound" symptoms after they are discontinued.

There are studies that have examined vascular endothelial growth factor inhibitors to block the angiogenesis responsible for the formation of the pterygium. Small case series have found that intraconjunctival injections of bevacizumab, but not ranibizumab, help reduce the size of the pterygium. It is not clear whether these inhibitors improve symptoms or affect the long-term prognosis.

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Follow-up of the pterygium varies according to the degree of inflammation, growth and proximity to the visual axis. If there is no visual impairment, restriction in eye movement or involvement of the pupil, or other signs/symptoms, follow-up at intervals of 6 to 12 months is reasonable.

Surgery – surgery usually involves excision of the pterygium of the eye, often with additional medical and surgical treatments to help lower recurrence rates. Excision usually corrects astigmatism caused by pterygium of the eye and visual impairment from opacity of the visual axis. However, recurrence of pterygium of the eye is still possible.

Indications – surgery to excise a pterygium:

• Induced astigmatism causing visual impairment
• Opacity in the visual axis
• Documented growth that threatens to affect the visual axis through astigmatism or opacity
• Restriction in eye movement
• Significant cosmetic effect or recurrent irritation

Surgical approach – surgical excision is technically simple. The excision procedure usually lasts about half an hour and is done on an outpatient basis under local anesthesia, with or without intravenous sedation (IV).

Simple "bare sclera" excision is associated with high recurrence rates, and therefore excision should be combined with adjunctive measures. These include conjunctival autografts, topical or subconjunctival medications, or irradiation. A meta-analysis of 24 trials that included 1815 eyes found that the most effective adjunctive treatment for preventing postoperative recurrence was a combination of conjunctival autograft and cyclosporine eye drops. A 2019 report on an international survey among cornea specialists found that excision with an autograft or conjunctiva-to-conjunctiva graft is the preferred technique, with 61 percent using this technique and preferring fibrin glue over the use of sutures.

Follow-up of pterygium of the white of the eye

The ophthalmologist usually follows the patient closely after surgery to make sure there are no complications and that there is normal healing while documenting outcomes. The progression of the pterygium can be documented photographically or by means of drawn documentation and measurement of its size. Topographic mapping of the cornea can also be performed for follow-up.

Pain after surgery due to surgical peeling over the surface of the cornea may require opioids for several days after surgery, although discontinuing use of the bare-sclera technique and progressing to more advanced techniques such as the use of autografts with the use of fibrin glue instead of sutures reduce the likelihood of postoperative pain requiring opioids. Sensitivity to light (photophobia) usually lasts several days. Discomfort and sensitivity to light limit the patient's activity in the early period after surgery. Patients are usually prescribed topical antibiotics after surgery for several days or weeks, and topical glucocorticoids for several weeks. Patients should be followed by the ophthalmologist as long as they are taking glucocorticoids because of possible side effects, including infection and secondary glaucoma.

Potential complications

Intraoperative complications are very rare for the removal of pterygium of the eye because the procedure is extraocular; however, complications include inadvertent eye perforation or damage to an extraocular muscle, as well as endophthalmitis, which may occur as a result of unintentional perforation of the eyeball.

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A corneal ulcer is a rare complication associated with the use of adjuncts that have an anti-metabolic effect such as mitomycin or 5FU, which are spared when an autograft is performed.

Repeated excisions tend to lead to corneal scarring, irregular astigmatism, fibrotic restriction of extraocular movements leading to diplopia, and the formation of symblepharon (adhesion of the eyelid to the surface of the eyeball). All of these can contribute to an abnormal appearance of the globe and can interfere with visual function. Therefore, it is advisable to perform the surgery with a physician who specializes in pterygium of the eye and uses advanced methods to reduce the chances of recurrence (use of an autograft with biological glue).

Recurrence of pterygium of the eye

The recurrence rates of simple bare-sclera surgical excision are high, ranging between 30 and 80 percent of cases, and rates as high as 88 percent are even reported in certain subpopulations. Recurrence and recurrent abnormalities may cause disruption of the ocular surface and the following complications. The recurrence rate decreases drastically when an autologous conjunctival graft with glue is used. When there is a recurrence, it is usually seen four months after surgery. If the pterygium has not recurred after a year, it is unlikely to recur.

Prevention of pterygium of the eye

The effectiveness of preventive measures for pterygium of the eye is not known. Exposure to ultraviolet light (UV) is an important risk factor for the development of a pterygium. Several population-based studies indicate that the use of sunglasses and hats may help prevent a primary pterygium of the eye. It has not yet been determined whether a specific measure reduces progression after the onset of the pterygium, primary or recurrent. Sun protection by means of a hat and/or UV-blocking glasses are sensible approaches in the absence of proof.