Pterigium

01Pathophysiology

Pterygium arises from chronic ultraviolet-B radiation damaging the limbal stem cells at the corneoscleral junction. These stem cells normally act as a barrier preventing conjunctival epithelium from migrating onto the corneal surface. When this barrier is disrupted, conjunctival tissue proliferates across the limbus and invades the cornea as a fibrovascular sheet.

At the molecular level, UV exposure induces p53 tumor suppressor gene mutations in the limbal epithelium and upregulates matrix metalloproteinases (MMPs), which degrade the corneal Bowman layer and allow tissue invasion. Simultaneously, pro-angiogenic factors such as vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) drive the prominent blood vessel ingrowth that gives the lesion its characteristic fleshy, vascularized appearance.

The hallmark histological finding is elastotic degeneration of subepithelial collagen, where normal collagen fibers are replaced by amorphous, basophilic elastotic material. This is identical to the actinic damage seen in sun-exposed skin. The overlying epithelium may show varying degrees of dysplasia, which is why severely atypical lesions must be distinguished from neoplasia.

The strong nasal predilection is explained by the Coroneo effect: UV light entering from the temporal side is refracted and internally focused by the cornea onto the nasal limbus, concentrating the radiation dose at that location. This is why the nasal side receives roughly 20 times more UV exposure than the temporal side, and why pterygium overwhelmingly originates nasally.

Clinically, the fibrovascular tissue growing onto the cornea mechanically distorts the corneal curvature, which produces irregular astigmatism and explains why patients with advancing pterygium complain of progressively blurred vision even before the lesion physically covers the pupil.

02Classification and Clinical Manifestation

Morphological Classification (Tan Classification)

Classification by Extent of Corneal Involvement

Clinical Features Worth Noting

03Diagnostic Workup

Pterygium is fundamentally a clinical diagnosis. A slit-lamp examination revealing the characteristic wing-shaped fibrovascular growth arising from the nasal interpalpebral conjunctiva and extending across the limbus onto the cornea is diagnostic. No laboratory or imaging tests are required for routine cases.

Corneal topography becomes important when surgical planning is being considered. It documents the degree of induced astigmatism before excision and serves as a baseline for postoperative comparison. The typical finding is corneal flattening along the axis of the pterygium with overall irregularity.

The critical diagnostic question on examinations is not confirming pterygium itself but rather distinguishing it from look-alikes, especially ocular surface squamous neoplasia (OSSN). When a lesion appears atypical (gelatinous surface, leukoplakia, feeder vessels with a papillomatous texture, rapid growth, or location outside the typical nasal interpalpebral zone), excisional biopsy with histopathological examination becomes the most accurate test. The classic histological finding in pterygium is elastotic (actinic) degeneration of the substantia propria with an overlying conjunctival epithelium containing goblet cells. If dysplasia or carcinoma in situ is found, the diagnosis shifts to OSSN.

A practical clinical test to distinguish pterygium from pseudopterygium (a common exam distractor) is the probe test: a blunt probe can be passed beneath a pseudopterygium at the limbus because it is not truly fused to the underlying tissue, whereas in true pterygium the tissue is firmly adherent to the limbus and cornea.

04Management and Treatment

Conservative Management

For patients with a small, stable, asymptomatic pterygium not threatening the visual axis, the mainstay is UV protection and lubrication. Patients should be counseled to wear wraparound UV-blocking sunglasses and use preservative-free artificial tears as needed for irritation. These patients are monitored periodically (every 6 to 12 months) with slit-lamp examination and, if needed, serial photography to document progression.

When a pterygium flares with redness and irritation, a brief course of topical NSAIDs or a mild topical corticosteroid can be prescribed. Prolonged corticosteroid use is avoided due to the risks of glaucoma and cataract formation.

Surgical Management

Surgery is the definitive treatment and is indicated when the pterygium encroaches within 3 to 4 mm of the corneal center (approaching the visual axis), causes visually significant astigmatism, restricts ocular motility, or is a cosmetic concern refractory to conservative management.

The most tested concept regarding pterygium surgery is the surgical technique and its relationship to recurrence:

The bare sclera technique (simple excision without grafting) carries a recurrence rate of 30% to 80% and is considered inferior. This technique should not be selected on examinations as the best answer.

Excision with conjunctival autograft is the current standard of care. The pterygium is excised, and a free graft of conjunctiva harvested from the superonasal or superotemporal bulbar conjunctiva is placed over the defect. This reduces recurrence to approximately 5% to 15%. Fibrin glue is preferred over sutures for securing the graft because it reduces operative time, postoperative discomfort, and may further lower recurrence.

Mitomycin C (MMC) is an alkylating agent used as an adjunct in cases with high recurrence risk (recurrent pterygia, young patients, fleshy/T3 morphology). It is applied intraoperatively at a concentration of 0.02% to 0.04% for 1 to 3 minutes on the bare sclera before graft placement. Complications of MMC include scleral thinning, delayed epithelial healing, and rarely scleral necrosis, so its use requires careful patient selection.

Amniotic membrane transplantation is an alternative to conjunctival autograft, especially when conjunctival tissue must be preserved (e.g., in patients who may need future glaucoma filtering surgery). Recurrence rates are slightly higher than with conjunctival autograft but still far better than bare sclera excision.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap with pterygium questions is confusing it with a pinguecula. Both are UV-related degenerative lesions of the conjunctiva, and both favor the nasal interpalpebral zone. The discriminating feature that examination writers rely on is whether the lesion crosses the limbus onto the cornea. If it does, it is a pterygium. If it stays confined to the conjunctiva, it is a pinguecula. Students who fail to look for this detail in the vignette will choose the wrong answer.

The second common trap involves the pseudopterygium. The vignette will describe a patient with a history of ocular trauma, chemical burn, or chronic inflammation (such as Stevens-Johnson syndrome) who now has tissue growing onto the cornea. The test-writer expects you to recognize that this is a pseudopterygium, not a true pterygium. The giveaway is the clinical history of prior ocular surface injury, and the fact that a probe can pass beneath the tissue at the limbus.

A third trap tests whether you know the correct surgical technique. If the question asks for the "next best step" for a pterygium approaching the visual axis, the answer is excision with conjunctival autograft, not simple excision alone. Choosing bare sclera excision is the expected wrong answer because it carries an unacceptably high recurrence rate.

Finally, remember that the Stocker line (iron deposition at the leading edge) indicates a slowly progressive lesion, similar in concept to other corneal iron lines (Hudson-Stahli line, Ferry line near filtering blebs, Fleischer ring in keratoconus). If you see "iron line at the leading edge of a corneal growth" in a vignette, think pterygium.

The core competency being tested with pterygium is pattern recognition of a classic clinical presentation (nasal wing-shaped growth in a UV-exposed patient), the ability to distinguish it from its closest mimics using a single discriminating feature, and knowledge of the appropriate surgical approach when intervention is indicated.