Pterygium

Pterygium — definition, risk factors and treatment

For suture-free pterygium removal with Prof. Mimouni call 058-644-5151 at any hour

Key points:

• Pterygium is a degenerative disease of the ocular surface associated with chronic exposure to ultraviolet light, and it is particularly common in tropical climates.
• Bare sclera excision of a pterygium should no longer be performed, because of the high recurrence rate associated with this procedure.
• Excision of a pterygium with a conjunctival autograft is the accepted technique for treating pterygium, with low recurrence rates and minimal complications.
• The most common complication of pterygium removal surgery is recurrence of the pterygium, and this usually recurs within the first year after excision.
• Recurrence rates vary with different surgical techniques, and adjuvant treatments are used in an attempt to reduce recurrence rates.
• Caution is needed with the use of adjuvant treatments to reduce pterygium recurrence, such as β irradiation or mitomycin C, since they are associated with long-term complications that can harm vision, including scleral necrosis and endophthalmitis.

Introduction to pterygium

A pterygium is a triangular, "wing-like" growth made up of conjunctival epithelium and hypertrophic sub-conjunctival connective tissue that occurs on the nasal or temporal side in the area of the palpebral fissure and grows over the cornea. A pterygium remains something of an ocular puzzle, since the full pathogenic mechanisms remain unknown.

The symptoms of pterygium range from mild irritation and dryness to reduced vision as a result of irregular astigmatism or blurring of the visual axis. The difficulty in treating this deceptively benign disease stems from our lack of understanding of the condition and its tendency to recur after surgical excision. There is an abundance of surgical and medical measures available in the treatment of pterygium.

For suture-free pterygium removal — contact Prof. Mimouni

Call 058-644-5151 at any hour

Definition and morphology

A pterygium can be defined as primary or recurrent. A recurrent pterygium is usually more aggressive and can occur several weeks to months after excision of a primary pterygium.

A pterygium is made up of a "head" at its apex, usually with a cap of blood vessels at the leading edge. The "body" represents the main part of the pterygium over the sclera and extends from the area of the canthus. A pterygium usually appears on the nasal side, in the interpalpebral fissure, but it can also appear on the temporal side. A temporal pterygium can exist alongside a nasal pterygium (a "double-headed" pterygium). The sub-conjunctival or fibrovascular component of the pterygium may vary considerably in appearance, from thin and transparent to thick and fleshy. A simple grading scale has been developed, based on slit-lamp examination and the relative transparency of the body of the pterygium, which predicts recurrence. In this grading, T1 (atrophic) denotes a pterygium in which the episcleral vessels underlying the body of the pterygium are clearly visible.

Grade T3 (fleshy) denotes a thick pterygium in which the episcleral vessels underlying the body of the pterygium are completely obscured by fibrovascular tissue.

A pterygium in which the details of the episcleral vessels are indistinctly visible or partly obscured is classified as grade T2 (intermediate).

In a randomised clinical trial comparing bare sclera excision with a conjunctival autograft, it was shown that recurrence of the pterygium is clearly related to the amount of fibrovascular tissue present in the pterygium when the bare sclera technique was used. In the conjunctival autograft group there were too few recurrences to determine significance in this form of the procedure. A fleshy pterygium had a higher tendency to recur than a more transparent pterygium; the differences in recurrence rates were highly significant both for primary pterygia and for recurrent pterygia.

The epidemiology of pterygium

Pterygium has a worldwide distribution but is more common in hot, dry climates. Pterygium generally occurs most often within the "pterygium belt" (latitude 37° north and south of the equator), with sun exposure significantly increasing the risk of developing a pterygium. A meta-analysis of 20 studies including 900 subjects in total demonstrated a pooled prevalence of pterygium of around 10%, with an increased chance of pterygium in men (2.32 times for men) and with working in the sun (1.76 times for people who frequently work outdoors). The most common age of onset appears to be in the twenties and thirties, and several studies have shown that the prevalence of pterygium rises with age.

Risk factors

Most of the risk factors appear to be mainly environmental in nature, but there are studies that have identified hereditary factors.

Ultraviolet radiation

The main environmental risk factor for the development of a pterygium is exposure to ultraviolet light. This light, absorbed in the cornea and the conjunctiva, promotes damage at the cellular level and leads to abnormal proliferation of cells.

Genetic factors

Individual case reports have described clusters of family members with pterygium, and case-control studies have shown that a family history is a risk factor, possibly even with an autosomal dominant pattern of inheritance. Even so, the lack of a clear and consistent familial link suggests that pterygium does not have a strong familial basis. More recently, genes related to DNA repair, cell proliferation, migration and angiogenesis have been shown to be associated with pterygium. This is not surprising, since disruption of cell repair and proliferation may contribute to the multifactorial pathogenesis of pterygium.

Further risk factors

Chronic irritation or inflammation occurring in the limbal area or the peripheral cornea has been proposed as a risk factor for pterygium. This theory suggests that chronic inflammation brings about focal limbal stem cell deficiency and the subsequent development of a pterygium. There are also those who argue that there is an angiogenic factor present that promotes the growth of a pterygium. Recently it has been demonstrated that levels of vascular endothelial growth factor are higher in pterygium. There are studies that have also shown a link between the papilloma virus and pterygium.

The pathogenesis of pterygium

Is it a degenerative or a proliferative disease?

Pterygium has long been considered a chronic degenerative condition, and this is based on histological examination. This disease is classically described as "elastotic degeneration": pterygium tissue is characterised by abnormal sub-epithelial tissue containing altered collagen fibres that stain positively with elastic stains. There are also clinical and aetiological features of pterygium that point to a proliferative disorder, not very different from benign tumours, and a link has been identified between pterygium and the p53 oncogene. It has likewise been shown that ultraviolet light, the main environmental risk factor in pterygium, causes mutations of the kind seen in solar keratosis, Bowen's disease and skin carcinomas. Furthermore, pterygium has a strong tendency to recur aggressively after surgical excision, and adjuvant treatment with antimetabolites reduces its recurrence. It should be noted that the prevalence of conjunctival epithelial neoplasia in pterygium is about 1.8%.

Limbal stem cell deficiency and pathological changes in the epithelium

The classic signs, or clinical hallmarks, of limbal stem cell deficiency include growth of conjunctiva over the cornea, growth of new blood vessels, chronic inflammation and destruction of the basement membrane. These signs are clearly present in pterygium, and so many researchers today have suggested that it is a manifestation of dysfunction or local deficiency of stem cells, possibly as a result of destruction of stem cells related to ultraviolet light.

Pterygium tissue demonstrates intrinsic pathologies of DNA repair as a result of ultraviolet light irradiation, expressed as loss of heterozygosity in cells. Studies have shown that the basal epithelial layers of primary and recurrent pterygia display over-expression of the tumour suppressor gene p53, an oncogene that acts as a transcription factor to activate or repress the expression of genes controlling growth, thereby explaining the classic histological finding of epithelial hyperplasia seen in pterygium. Abnormal apoptosis has also been found to occur in the epithelium of primary and recurrent pterygia. Studies of pterygium have also shown increased fibroblast activity with abnormal TGF-β signalling and over-expression of connective tissue metalloproteinases.

Treatment options for pterygium

There are several treatment options for pterygium, all of them based on the principle of excising the lesion combined with various methods of avoiding recurrence of the pterygium.

The best known treatments are bare sclera excision; excision with simple closure of the conjunctiva or transposition of the conjunctiva; bare sclera excision combined with anti-proliferative treatments; and excision of the lesion with reconstruction of the ocular surface (for example pterygium excision combined with a conjunctival autograft, which is the accepted and most advanced treatment today).

It is also possible to perform pterygium removal surgery with a laser.

Possible complications after pterygium removal surgery

Pterygium removal combined with a conjunctival autograft is generally considered a safe procedure, since serious sight-threatening complications do not usually develop. Early complications after surgery include graft oedema, haemorrhage in the graft, graft retraction or suture breakage, inversion of the graft and necrosis of the graft. Conjunctival granulomas can also form shortly after surgery. Because of excessive inflammation and local irritation occurring at the site of exposed Tenon's tissue, granulomas may appear at the graft harvest site, in the recipient bed adjacent to the conjunctival graft, or as a suture granuloma.

Late complications after surgery include epithelial inclusion cysts, conjunctival scarring or fibrosis at the donor site, and raised intraocular pressure caused by prolonged use of steroids as part of the treatment after surgery.

Reasons for recurrence after a conjunctival autograft

The morphological appearance of pterygium recurrence after a conjunctival autograft points to a number of factors that may lead to graft failure. Recurrence of the pterygium may occur at the upper or lower margins of the conjunctival graft, which suggests that pterygium tissue was inadequately excised, or that the size of the graft was insufficient. A thick graft including Tenon's tissue may lead to retraction of the graft and prolapse of the edge in the early postoperative period, causing exposure of bare sclera at the margins of the graft, which then becomes the site of recurrence. Early breakage of sutures will also lead to local graft retraction and local recurrence at that site. Even so, it should be noted that in certain cases a thin conjunctival graft that is properly aligned and sutured may subsequently develop a recurrent pterygium in the limbal area, which suggests that some factor in the cornea or the limbus may be responsible for the recurrence in this case.

What is a pterygium?

I am often asked what a pterygium is. A pterygium (also known as surfer's eye) is a disease of the ocular surface characterised mainly by a white or pinkish, wing- or wedge-shaped growth of limbal tissue from the conjunctiva over the adjacent cornea. In most cases a pterygium grows from the inner corner of the eye (the one closest to the nose), but growth from the outer corner, or from both sides of the eye at the same time, is sometimes possible. It is not malignant tissue and there is no invasion of the interior of the eye or of any other part of the body. The appearance of a pterygium can range from a delicate transparent area with a few blood vessels, in the early stages, to a thick opaque growth that blocks vision in the later stages.

A pterygium is made up of three distinct parts: the cap, the head and the body/tail. The cap, or leading edge, is a flat area on the cornea consisting mainly of fibroblasts that invade and destroy Bowman's membrane. The head is a vascular area located behind the cap and firmly attached to the cornea. The body/tail is the mobile area of the conjunctiva, which can easily be separated from the underlying tissue.

Histologically, the sub-epithelial tissue shows senile elastosis (basophilic degeneration) of the substantia propria with abnormal collagen fibres. There is breakdown of Bowman's membrane, followed by invasion of the corneal surface. A distinctive feature of the epithelial cells of a pterygium is the evidence of the presence of various types of matrix metalloproteinases that are absent from the normal cells of the conjunctiva or the cornea.

The condition usually requires no treatment at all unless it is causing a genuine disturbance of vision or represents severe discomfort. Even when no treatment is given, it is advisable to be followed up periodically by an ophthalmologist to make sure the growth is not causing vision problems.

The exact cause of pterygium is still not known. Many theories have been proposed about possible causes of pterygium, including the activation of specialised growth cells called fibroblasts by radiation; choline deficiency (choline serves as a precursor for a number of important compounds in the cells of living organisms); an inflammatory disorder; a vascular problem causing uncontrolled growth of blood vessels; immune system disorders; disorders of the tear film; and even a possible viral link.

Risk factors for pterygium

The risk factors for pterygium can be associated with the following conditions:

• Sun exposure — the condition is more common in people who spend a lot of time outdoors in hot weather, with excessive exposure to the ultraviolet radiation in sunlight considered to be the cause.
• A dusty or sandy environment — long-term exposure to dry, windswept places is considered a factor that may contribute to the development and worsening of the condition.
• Age — the risk rises with age.
• A pterygium may be caused by a combination of several of the factors mentioned above.

What is a pterygium — symptoms

In its early stages a pterygium is usually asymptomatic; however, when it is inflamed it may cause itching, a sensation as though there is a foreign body (something rough) inside the eye, increased watering and burning. In later stages the pterygium can grow over the iris and the pupil and cause blurring and disturbance of vision.

Diagnosing a pterygium

A pterygium can usually be diagnosed from its distinctive appearance and symptoms. Even so, in the early stages there is a possibility of confusion with similar growths affecting the conjunctiva, such as a pinguecula. If a pterygium is diagnosed, a thorough eye examination should be carried out in order to assess its effect on vision and to rule out less common diagnoses that can cause a growth in the eye.

What is a pterygium — treatment?

Treatment of a pterygium is initially limited to monitoring over time to see whether it is extending over the visual axis. Symptoms of irritation, foreign-body sensation and watering can be relieved with over-the-counter anti-allergy drops, lubricating eye drops, artificial tears, anti-inflammatory preparations and ointment.

Surgery is the only way to remove a pterygium. If eye drops and ointments do not provide sufficient relief, surgery may be recommended to remove the pterygium. Surgery is also recommended when the condition is causing impairment of vision. In such a case it is advisable to have the surgery before the pterygium grows over the cornea, since the operation can then scar the cornea and cause permanent vision problems. Pterygium removal is sometimes also done for cosmetic reasons. During the operation the pterygium is carefully removed and a piece of conjunctiva is taken from under the eyelid and grafted onto the area where the pterygium was. The operation is performed under local anaesthesia and lasts about 30 minutes.

In certain cases a pterygium can return after it has been removed surgically, and so surgical intervention should be considered carefully. The eye may also feel dry and irritated after surgery. Medication can be prescribed to provide relief and to reduce the risk of the pterygium growing back.

Preventing pterygium

Where possible, exposure to environmental factors that could cause a pterygium should be avoided, by using sunglasses or a hat to protect the eyes from sunlight, wind and dust. If you wear glasses, they should, like sunglasses, provide protection against the sun's ultraviolet (UV) rays. If you already have a pterygium, limiting exposure to sunlight, wind, dust, pollen and smoke can slow its growth.

Prof. Michael Mimouni is an ophthalmologist specialising in pterygium, with many years of experience in the diagnosis, treatment and surgery of pterygium of the eye using the most advanced methods.