Pterygium

A pterygium (from the Greek pterygion, meaning wing) is a fibrovascular proliferation of subconjunctival tissue that grows across the limbus to invade the corneal surface. Unlike a pinguecula — which remains confined to the conjunctiva — a pterygium is characterised by the progressive destruction of Bowman's layer and superficial corneal stroma as the leading fibrovascular pannus advances centrally. This corneal invasion distinguishes pterygium as a potentially vision-threatening condition requiring active monitoring and, in selected cases, surgical intervention.

Pterygia are predominantly nasal (approximately 90% of cases), likely due to the limbal focusing effect of UV radiation described by Coroneo (1993), which concentrates UV light on the nasal limbal area via peripheral corneal optics. The condition may be unilateral or bilateral, with bilateral occurrence in up to 30–40% of cases in high UV-exposure populations.

Globally, pterygium prevalence ranges from 1% in temperate climates to over 22% in some tropical and subtropical populations. In Singapore, population-based studies (Tanjong Pagar Survey, Singapore Malay Eye Study, Singapore Indian Eye Study) report prevalence rates of 7–10% in adults, with significantly higher rates in outdoor workers and individuals with prolonged sun exposure. Singapore's equatorial location and extreme UV index make pterygium one of the most commonly encountered anterior segment conditions in local optometry practice.

Pterygium is a multifactorial condition, with ultraviolet radiation as the dominant and best-established etiological driver. The condition represents a complex interplay of environmental, genetic, cellular, and molecular factors.

Pterygium pathogenesis is best understood as a UV-initiated, stem cell–mediated fibrovascular proliferative process, with overlapping features of wound healing, neoplasia, and chronic inflammation.

Limbal stem cell damage and conjunctivalisation. The corneal epithelium is maintained by limbal stem cells (LSCs) located in the Palisades of Vogt at the corneoscleral junction. Repetitive UV-B irradiation generates reactive oxygen species (ROS) that cause strand breaks in LSC DNA, triggering p53-dependent and p53-independent apoptosis pathways. When p53 is mutated (as is common in pterygium), damaged LSCs survive and undergo transformation, losing their barrier function. Conjunctival fibroblasts and epithelium then migrate centrally, replacing corneal epithelium — a process termed "conjunctivalisation."

Bowman's layer dissolution. The advancing pterygium head is preceded by a zone of fibrovascular tissue that secretes MMPs — particularly MMP-1 (interstitial collagenase) and MMP-9 (gelatinase B) — which dissolve the collagen IV scaffold of Bowman's layer. This irreversible destruction explains why even successful pterygium excision leaves a permanent Bowman's layer defect and potential scarring at the excision site.

Angiogenesis and fibrovascular stroma. VEGF overexpression, driven by UV-induced HIF-1α activation, stimulates robust neovascularisation within the pterygium body. These feeder vessels supply the growing fibrovascular mass and confer the characteristic reddish appearance of active pterygia. Subepithelial fibroblasts produce disordered collagen (types I, III, and V) interspersed with elastotic fibres, forming the fleshy stroma of the mature lesion.

Corneal flattening and astigmatism induction. As the fibrovascular mass anchors to the corneal stroma, the mechanical pull of the pterygium along its axis flattens the corneal curvature in the axis of the pterygium and steepens the perpendicular axis, producing characteristic with-the-rule or against-the-rule astigmatism. The degree of induced astigmatism correlates with the length of corneal invasion and the bulk of the pterygium body.

Recurrence biology. Post-surgical recurrence is driven by residual fibrovascular stem cells in the episcleral tissue and Tenon's capsule. Residual VEGF, TGF-β, and inflammatory cytokine signalling stimulates renewed fibroblast migration and angiogenesis at the surgical site. This is the mechanistic basis for adjunctive use of antimetabolites (mitomycin C, 5-fluorouracil) and anti-VEGF agents in high-recurrence cases.

Multiple classification systems have been developed. The two most widely adopted are the Tan grading system (based on pterygium translucency and underlying episcleral vessel visibility) and the Johnston morphological classification (based on tissue texture). Both are clinically applicable and predict recurrence risk.

• Cumulative UV exposure — the strongest modifiable risk factor; a clear dose-response relationship exists; UV-B in particular is most damaging. Singapore's equatorial UV index (typically 10–14, classified as Extreme) drives high local pterygium prevalence
• Geographic latitude — within 37° of the equator ("pterygium belt"); prevalence drops sharply at higher latitudes. Singapore, at 1.3°N, lies squarely within the high-risk zone
• Outdoor occupation and recreational exposure — farmers, fishermen, military personnel, construction workers, surfers; a dose-response relationship with years of outdoor work has been demonstrated in multiple Singapore cohort studies
• Male sex — consistently higher prevalence in males in most epidemiological studies; largely attributable to greater occupational outdoor UV exposure rather than a biological sex difference per se
• Advancing age — prevalence increases with age due to cumulative UV exposure; peak prevalence in the fourth to sixth decades; rare below age 20
• Dry and dusty environments — chronic ocular surface desiccation and particulate exposure promote limbal microtrauma and inflammatory activation
• Absence of UV-protective eyewear — not wearing sunglasses or wide-brimmed hats is an independent modifiable risk factor; UV400 wrap-around eyewear is protective
• Ethnicity — some studies report higher rates in Asian and African populations compared to European populations within the same geographic region, suggesting possible genetic susceptibility differences
• Genetic susceptibility — familial occurrence is documented; VEGF promoter polymorphisms and HLA associations have been reported in selected populations
• Previous pterygium excision — prior excision is the strongest risk factor for recurrent pterygium; recurrent disease is more aggressive and has higher subsequent recurrence rates
• Smoking — some evidence suggests ROS from cigarette smoke compounds UV-induced limbal damage

• Asymptomatic (common in early grades) — small atrophic pterygia may be entirely symptom-free and discovered incidentally on routine examination
• Cosmetic concern — the visible reddish triangular mass on the white of the eye is a frequent presenting complaint, particularly in younger patients
• Ocular surface irritation — foreign body sensation, grittiness, burning, and mild discomfort from chronic ocular surface irregularity; exacerbated by dry or dusty environments and air conditioning — all highly relevant to Singapore's indoor climate
• Redness — localised injection over the pterygium body, particularly during inflammatory flares or periods of increased UV or environmental exposure
• Epiphora (tearing) — reflex tearing from ocular surface irritation
• Blurred or distorted vision — progressive visual impairment from induced irregular astigmatism; becomes clinically significant as the pterygium head encroaches beyond 2–3 mm onto the cornea. Spectacle or contact lens correction may partially compensate but cannot correct irregular astigmatism
• Visual axis obstruction — in advanced (Grade 3–4) pterygia reaching or crossing the pupil margin; direct mechanical obstruction of the visual axis causes significant vision loss
• Contact lens intolerance — the elevated, irregular tissue makes both soft and rigid lens fitting problematic; induced astigmatism may render previously successful lens wearers intolerant
• Restricted ocular motility — in large or adherent pterygia; patient may complain of difficulty looking toward the pterygium or, rarely, of diplopia

• Progressive corneal invasion and visual axis encroachment — the most serious natural history complication; once the pterygium head crosses the pupil margin (Grade 3–4), vision is directly threatened by mechanical axis obstruction and dense irregular astigmatism
• Irregular corneal astigmatism — the primary cause of visual impairment in pterygium; induced by mechanical traction on the corneal stroma; may not be fully correctable with spectacles or standard soft contact lenses; hard contact lenses or scleral lenses may improve acuity but do not address the underlying progression
• Permanent Bowman's layer scarring — corneal haze and subepithelial scarring at the pterygium head site persists even after successful excision; may leave permanent irregular astigmatism and suboptimal best-corrected visual acuity
• Ocular motility restriction and diplopia — fibrous adhesion of the pterygium to the scleral and episcleral tissue in advanced or recurrent cases; may require strabismus surgery in addition to pterygium excision
• Dry eye exacerbation — the elevated pterygium mass disrupts the precorneal tear film, worsening pre-existing dry eye and amplifying ocular surface symptoms
• Contact lens fitting failure — progressive irregular astigmatism and conjunctival tissue distortion make contact lens wear increasingly difficult
• Recurrence after excision — the most common surgical complication; recurrence rates vary widely (5–80%) depending on surgical technique; recurrent pterygia are typically more vascular, fibrous, and invasive than the primary lesion, with increasing risk with each subsequent excision
• Post-surgical complications — dellen formation at graft edges, conjunctival graft displacement or necrosis, scleral or corneal melt (particularly with overuse of MMC), pyogenic granuloma formation, symblepharon (conjunctival adhesion), and infectious keratoscleritis
• Interference with LASIK / refractive surgery candidacy — induced irregular astigmatism and corneal distortion compromise corneal topography assessment and contraindicate keratorefractive surgery; pterygium must be excised and the cornea allowed to stabilise (minimum 3–6 months) before any refractive procedure

Pterygium is primarily a localised ocular surface condition without direct systemic disease associations of the same nature as scleritis or episcleritis. However, several systemic and epidemiological relationships are clinically relevant.

Pterygium is a clinical diagnosis based on characteristic slit-lamp findings. Ancillary investigations are important for surgical planning, monitoring progression, and excluding atypical or malignant mimics.

The prognosis of pterygium is generally good with appropriate monitoring and timely surgical intervention, but strongly depends on pterygium grade, surgical technique, and patient compliance with UV protective measures post-operatively.

• Vision preservation — the vast majority of patients retain good visual acuity if the pterygium is identified and surgically excised before reaching the visual axis. Vision loss from direct axis obstruction is largely preventable with early referral
• Astigmatism outcome — induced astigmatism partially or fully resolves after excision; the degree of recovery depends on the extent of Bowman's layer destruction and the duration of corneal involvement. Significant irregular astigmatism and corneal scarring from advanced pterygium may persist despite successful excision
• Recurrence risk — the dominant prognostic variable post-operatively. Conjunctival autograft reduces recurrence to 5–15% compared to 30–80% with bare sclera excision. Recurrent pterygia carry higher recurrence risk with each subsequent surgery and are more difficult to manage. Younger age, T3 morphology, and nasal location predict higher recurrence
• Photoprotection compliance — the single most modifiable post-operative prognostic factor; consistent UV400 sunglasses wear significantly reduces recurrence risk and contralateral eye progression. Patient education is fundamental
• Corneal scarring — permanent Bowman's layer scarring at the excision site is universal after significant corneal invasion; the severity ranges from mild haze to dense leucoma affecting BCVA. Excimer laser phototherapeutic keratectomy (PTK) may improve corneal clarity in selected cases post-excision
• Refractive surgery candidacy — after pterygium excision, corneal topography should be repeated at a minimum of 3–6 months (some surgeons advocate 6–12 months) to confirm stabilisation of the corneal surface before any keratorefractive procedure is planned
• Bilateral progression — patients with a unilateral pterygium should be monitored for contralateral pterygium or pinguecula progression, as the same UV exposure risk applies to both eyes

• Measure the head at every visit. Document the distance from the pterygium head to the corneal centre using the slit-lamp reticule or AS-OCT at each examination. A change of ≥0.5 mm per year is significant and warrants timely ophthalmological referral.
• Corneal topography beats clinical estimation. Induced astigmatism begins before the pterygium head is visibly distorting the cornea. Topography detects this earlier than subjective refraction, enabling more timely surgical referral and reducing the risk of irreversible Bowman's layer scarring.
• Pseudopterygium probe test. When uncertain whether a limbal lesion is a true pterygium or pseudopterygium, attempt to pass a blunt probe or spatula under the lesion head. If it slides freely, the diagnosis is pseudopterygium; if firmly adherent, it is a true pterygium.
• Stocker's line indicates slow growth. The presence of a brown iron line at the pterygium head is a reassuring sign of a relatively quiescent lesion; its absence — particularly in a young patient — suggests active growth warranting more frequent monitoring.
• Refer before the visual axis is threatened. The optimal window for surgical referral is when the head is at 2–3 mm from the corneal centre: early enough to avoid significant Bowman's scarring, late enough to avoid operating on a lesion that may not progress. Waiting until the visual axis is covered risks permanent corneal scarring.
• UV protection counselling is the most important non-surgical intervention. Both for preventing progression in primary pterygium and reducing recurrence post-operatively. UV400 wrap-around sunglasses reduce oblique UV exposure most effectively — standard flat-frame glasses are less protective due to peripheral UV entry.
• Singapore context: watch the young outdoor worker. Pterygium in a patient under 40 in Singapore is not rare given extreme UV exposure. Younger patients tend to have more aggressive pterygia with higher recurrence rates. Early monitoring and UV protection advice should begin at the first presentation.
• Post-excision topography must stabilise before LASIK. Advise patients seeking refractive surgery after pterygium excision that corneal topography must be documented as stable for at least 3–6 months post-operatively before keratorefractive surgery can be considered. Residual irregular astigmatism post-excision may preclude or modify LASIK candidacy.