Ocular Surface Squamous Neoplasia

Ocular surface squamous neoplasia (OSSN) is an umbrella term encompassing the full spectrum of squamous epithelial neoplasia of the conjunctiva and cornea — from mild dysplasia through carcinoma in situ to invasive squamous cell carcinoma (SCC). The term was introduced by Lee and Hirst (1995) to unify a clinically and histopathologically continuous spectrum of disease that had previously been described under multiple names (conjunctival intraepithelial neoplasia, corneal intraepithelial neoplasia, Bowen's disease of the conjunctiva, conjunctival SCC).

OSSN is the most common ocular surface tumour globally. Its incidence is strongly correlated with geographic latitude and UV irradiance, with the highest rates reported in tropical regions including sub-Saharan Africa, Australia, and Southeast Asia. In Singapore, OSSN represents a clinically significant condition given the country's extreme equatorial UV index. HIV co-infection dramatically increases incidence in sub-Saharan African populations.

OSSN typically arises at the limbus in the interpalpebral zone — the site of greatest UV exposure — and presents as a gelatinous, papillomatous, or leukoplakic conjunctival mass with characteristic dilated feeder (sentinel) vessels. Most OSSN (approximately 80%) is confined to the epithelium (intraepithelial); invasive SCC, while less common, carries risk of orbital invasion and, rarely, metastatic spread. Early diagnosis and treatment carry an excellent prognosis; delayed recognition of advanced disease risks vision-threatening and life-threatening outcomes.

The primary optometric role in OSSN is recognition, photodocumentation, and prompt referral to ophthalmology — any atypical, growing, or vascular conjunctival lesion must be referred without delay.

OSSN is multifactorial. UV radiation and human papillomavirus are the two most important etiological drivers, with immunosuppression acting as a powerful amplifier in susceptible populations.

OSSN pathogenesis follows a stepwise progression from normal conjunctival epithelium through intraepithelial dysplasia to carcinoma in situ and ultimately invasive carcinoma, driven by the accumulation of genetic mutations that disable normal cell cycle control and apoptosis.

Limbal stem cell vulnerability. The limbal stem cells (LSCs) that maintain the corneal epithelial barrier are the likely cells of origin for OSSN. These cells are highly mitotically active, long-lived, and receive the highest UV dose at the interpalpebral limbus. UV-induced DNA damage in LSCs — particularly TP53 mutations — prevents apoptotic clearance of genetically damaged cells. Accumulating TP53 mutations in limbal basal cells are detectable in the histologically normal epithelium adjacent to OSSN, suggesting a field cancerisation effect in which a broad zone of the limbal epithelium harbours subclinical mutations before a clinically detectable lesion emerges.

Intraepithelial phase (CIN I–III). As mutant clones expand, cytological atypia develops within the epithelium: nuclear enlargement, hyperchromasia, irregular nuclear contours, increased mitotic activity, and loss of normal maturation. In CIN I (mild dysplasia), atypia is confined to the lower one-third of the epithelium; CIN II (moderate dysplasia) involves the lower two-thirds; CIN III (severe dysplasia / carcinoma in situ) involves full-thickness epithelium. Crucially, the basement membrane remains intact throughout the intraepithelial phase.

Invasion. Breaching of the epithelial basement membrane marks the transition to invasive SCC. This is enabled by upregulation of matrix metalloproteinases (MMP-2, MMP-9), downregulation of E-cadherin (promoting epithelial-to-mesenchymal transition), and activation of pro-invasive signalling pathways (EGFR, PI3K/AKT, Wnt). Once invasive, tumour cells can penetrate into the substantia propria, episclera, sclera, and — in advanced cases — the orbit or nasolacrimal duct.

Corneal spread. OSSN spreads onto the cornea via intraepithelial migration rather than stromal invasion in the majority of cases. The tumour replaces the normal corneal epithelium (producing a grey, stippled, vascular surface appearance) but typically respects Bowman's layer for a prolonged period. Deep corneal stromal invasion is rare but indicates aggressive biology.

HPV oncoprotein mechanism. In HPV-associated OSSN, the viral E6 protein binds and degrades p53 via ubiquitin-proteasome pathway, while E7 protein binds and inactivates pRb, releasing E2F transcription factors that drive S-phase entry. These mechanisms synergise with UV-induced TP53 mutation to accelerate malignant progression.

OSSN is classified histopathologically by depth of epithelial involvement and invasion status, and clinically/radiologically by the TNM staging system for advanced disease.

• Chronic UV-B exposure — the dominant modifiable risk factor; a strong dose-response relationship with cumulative UV exposure and latitude; Singapore's equatorial position (UV index 10–14, Extreme) places the entire population at elevated baseline risk
• Male sex — consistently higher incidence in males across all studies; attributed to greater occupational outdoor UV exposure; male-to-female ratio approximately 2–3:1
• Older age — peak incidence in the sixth to seventh decade in immunocompetent populations, reflecting cumulative UV exposure; HIV-associated OSSN presents two to three decades earlier
• HIV infection — among the most powerful risk factors; 10–13-fold elevated risk; associated with larger lesions, younger age of onset, higher recurrence rates, and more aggressive behaviour; OSSN is an AIDS-defining illness in some classification systems
• HPV infection (high-risk subtypes 16, 18) — significant cofactor, especially in immunosuppressed patients and in high-prevalence regions
• Immunosuppression — organ transplant recipients, patients on systemic immunosuppressive agents, haematological malignancies; impaired immune surveillance facilitates HPV persistence and neoplastic progression
• Outdoor occupation — farmers, construction workers, fishermen; strong association with cumulative UV exposure in population-based studies
• Absence of UV-protective eyewear — independent modifiable risk factor
• Prior OSSN or ipsilateral eye surgery — local recurrence risk after inadequate excision; field cancerisation predisposes to new lesions in adjacent epithelium
• Xeroderma pigmentosum — dramatically elevated risk; multiple OSSN lesions presenting in childhood or adolescence
• Vitamin A deficiency — promotes squamous metaplasia; relevant in resource-limited settings with dietary insufficiency

OSSN is frequently asymptomatic in its early stages, making routine slit-lamp examination the primary means of detection in optometry practice. Symptomatic presentations occur as the lesion grows or becomes inflamed.

• Asymptomatic — early CIN lesions are commonly discovered incidentally during routine eye examination; patients may be entirely unaware of the lesion
• Cosmetic concern — a visible mass or white spot on the eye prompts attendance; frequently the primary presenting complaint, particularly for leukoplakic lesions visible to the patient or noticed by others
• Foreign body sensation — mild to moderate grittiness or the sensation of something in the eye, particularly with larger or elevated lesions disrupting the ocular surface
• Ocular redness — episodic or persistent conjunctival injection overlying and surrounding the lesion; often attributed by the patient to conjunctivitis, delaying diagnosis
• Epiphora — reflex tearing from ocular surface irritation; may worsen with lacrimal outflow obstruction in lesions near the lacrimal puncta
• Blurred vision — occurs when the lesion or its corneal extension involves the central visual axis; induced irregular astigmatism from corneal epithelial invasion may reduce best-corrected visual acuity; significant visual loss is a late sign
• Photophobia — secondary to corneal epithelial disruption and ocular surface inflammation
• Pain — generally not a feature of early OSSN; deep aching pain may indicate orbital invasion or secondary inflammatory changes in advanced disease
• Diplopia or proptosis — very late signs indicating orbital extension; these symptoms mandate immediate ophthalmological evaluation

• Progression to invasive SCC — the most critical complication of untreated intraepithelial OSSN; estimated 5-year cumulative risk of invasion from CIN III (carcinoma in situ) is approximately 20–30% without treatment; once invasive, the tumour can spread to the orbit, sinuses, and intracranially
• Orbital invasion — occurs in approximately 12–16% of invasive SCC; the tumour extends through the sclera into the extraocular muscles, orbital fat, and surrounding structures; associated with pain, proptosis, restricted motility, and ptosis; may require orbital exenteration (removal of globe and orbital contents)
• Regional lymph node metastasis — rare (approximately 2–8% of invasive SCC); preauricular, submandibular, and cervical lymph nodes are the primary regional drainage sites; systemic staging required for all invasive SCC
• Distant metastasis — extremely rare; reported to lung, liver, and brain in advanced cases; associated with a grave prognosis
• Corneal scarring and visual impairment — both from the disease itself (corneal epithelial replacement) and from treatment (surgical excision, cryotherapy, topical chemotherapy); central corneal involvement risks permanent visual loss
• Recurrence after treatment — local recurrence rates range from 5–56% depending on excision margin status and treatment modality; recurrent OSSN is more difficult to manage and carries higher risk of invasive transformation
• Secondary glaucoma — from tumour infiltration of the trabecular meshwork or anterior chamber in advanced cases; or from topical steroid use during post-operative management
• Limbal stem cell deficiency (LSCD) — extensive OSSN or wide surgical excision involving the limbal zone can destroy limbal stem cells, resulting in chronic corneal conjunctivalisation, vascularisation, and persistent epithelial defects
• Complications of topical chemotherapy — MMC: punctal stenosis, chronic ocular surface toxicity, scleral melt (rare); 5-FU: corneal epithelial toxicity, lacrimation, periocular skin toxicity; IFN-α2b: generally well tolerated; mild flu-like symptoms with systemic injection

OSSN has important systemic associations, most critically with HIV infection and systemic immunosuppression. The ocular lesion may be the presenting feature of an underlying systemic condition.

OSSN is primarily a clinical diagnosis based on slit-lamp biomicroscopy, supported by ancillary imaging. Definitive diagnosis requires histopathological examination of excised tissue. Clinical suspicion sufficient to prompt urgent referral is the critical contribution of the optometrist.

Prognosis is excellent for intraepithelial OSSN treated adequately. For invasive SCC, prognosis depends on tumour size, depth of invasion, margin status at excision, and the presence of orbital or nodal involvement.

• Intraepithelial OSSN (CIN I–III) — excellent prognosis with complete excision or topical chemotherapy; recurrence rates of 5–15% with adequate treatment and clear margins; no metastatic risk while confined to epithelium; eye and vision preserved in the majority
• Invasive SCC — localised — recurrence rates of 15–40% depending on surgical technique and margin status; globe preservation achievable in the majority; risk of corneal scarring affecting BCVA in central lesions
• Invasive SCC — with orbital involvement — significantly worse prognosis; orbital exenteration required in approximately 12–16% of invasive cases; 5-year survival approximately 50–60% in this subgroup
• Metastatic disease — extremely poor prognosis; median survival from metastasis is less than 12 months in most published series
• HIV-associated OSSN — higher recurrence rates despite adequate treatment (20–50%); improved prognosis with antiretroviral therapy (ART) and immune reconstitution; ART itself may cause partial OSSN regression via restored immune surveillance
• Recurrence monitoring — all treated OSSN patients require lifelong slit-lamp surveillance; most recurrences occur within 2 years of treatment but late recurrences beyond 5 years are documented; 3-monthly review for 2 years then 6-monthly thereafter is a widely adopted protocol
• Second primary tumours — field cancerisation means patients with OSSN have elevated risk of new primary OSSN in the same or contralateral eye; vigilant bilateral monitoring is mandatory

• Any limbal lesion with sentinel vessels requires urgent referral. Feeder vessels running directly toward a conjunctival mass are the single most clinically important sign of OSSN. No benign lesion (pterygium, pinguecula, nevus) has this pattern. Do not observe and review — refer the same day or within the same week.
• OSSN is commonly misdiagnosed as pterygium. The interpalpebral limbal location and occasional fibrovascular appearance can closely mimic pterygium. Key differentiators: OSSN has an elevated, lobulated, or leukoplakic surface; prominent sentinel vessels; and on AS-OCT, thickened hyperreflective epithelium with an abrupt edge — none of which characterise pterygium.
• AS-OCT is the most valuable optometric diagnostic tool for OSSN. The characteristic thickened hyperreflective epithelium with abrupt transition to normal epithelium at the lesion edge is highly specific for OSSN. Where AS-OCT is available, perform it before referral — it strengthens the referral communication and helps the ophthalmologist plan surgery.
• Ask about HIV status sensitively. OSSN in a patient under 50, with a large or aggressive lesion, or with bilateral disease should prompt a sensitive discussion about HIV risk factors and recommendation for testing. Early HIV diagnosis and ART initiation dramatically improve both systemic and ocular prognosis.
• Corneal involvement does not mean the disease is advanced. OSSN frequently extends onto the corneal epithelium (intraepithelially), which appears as grey, stippled, or vascular corneal surface change. This is usually still intraepithelial (pre-invasive) and is entirely treatable. The presence of corneal extension does not indicate deep stromal invasion unless seen on AS-OCT.
• Leukoplakia does not mean keratinised = benign. Surface keratinisation (white plaque) does not predict histological grade — leukoplakic lesions can be CIN I, CIN III, or invasive SCC. All leukoplakic conjunctival lesions at the limbus require biopsy.
• Monitor the contralateral eye. Field cancerisation from chronic UV exposure means the fellow eye may harbour subclinical dysplastic changes. Examine both eyes systematically at every visit.
• Counsel on UV protection as part of every OSSN encounter. UV photoprotection (UV400 wrap-around sunglasses, broad-brimmed hats) remains the most important preventive measure for reducing new primary OSSN formation and recurrence post-treatment in Singapore's extreme UV environment.