Corneal, Conjunctival and Scleral Structure: Examination and Sampling

Localise the abnormal tissue

An ocular-surface case becomes easier when the lesion is assigned to a tissue layer. Epithelium, stroma, endothelium, conjunctiva, episclera and sclera have different functions, appearance and healing responses. A red eye with an epithelial defect is not managed like isolated episcleritis, and corneal oedema does not by itself identify inflammation. Relate the patient’s pain, photophobia, discharge and visual change to slit-lamp findings before choosing an investigation.

The corneal epithelium is a stratified barrier with a renewing basal population, basement membrane and attachment complexes. Limbal epithelial stem cells support its maintenance. Bowman layer is an acellular anterior stromal condensation; a deep injury can leave a permanent scar. Stromal transparency depends on collagen organisation, extracellular matrix and controlled hydration. Descemet membrane is the endothelial basement membrane. The endothelium maintains deturgescence through barrier and transport functions; reduced functional reserve produces stromal and epithelial oedema. Adult endothelial repair mainly involves enlargement and migration of surviving cells rather than reliable restoration of the original cell population.

Development, sensation and immunity

Corneal epithelium derives from surface ectoderm, whereas much anterior stromal and endothelial development involves neural-crest-derived periocular mesenchyme. Disturbed anterior segment development can affect the iris and drainage angle as well as the cornea. The conjunctiva contains non-keratinising epithelium, goblet cells and vascular connective tissue. Its mobility and mucin contribution matter in surface reconstruction. Sclera is a collagenous supporting wall; episcleral vessels lie superficial to it. Distinguishing superficial and deep vascular inflammation is more useful clinically than treating every injected eye as “conjunctivitis”.

Corneal sensory supply comes through the ophthalmic trigeminal pathway. Sensation contributes to protective reflexes and epithelial maintenance. Test it before topical anaesthetic, comparing regions and fellow eye, with a gentle cotton filament or a calibrated aesthesiometer when needed. Reduced sensation can follow herpetic disease, surgery, diabetes or proximal nerve injury. Pain and sensation are different observations: severe tissue injury can be relatively painless in a neurotrophic cornea.

The central cornea is normally avascular, and several local mechanisms modulate inflammation. This relative immune privilege is not complete isolation from immune cells or rejection. Limbal vessels, epithelial barrier defects and stromal inflammation can change antigen exposure and risk. Vascularisation in a graft recipient therefore affects prognosis, but no simple assertion that healthy corneas lack all antigen presentation is accurate.

A reproducible surface examination

Use fluorescein under an appropriate blue light/barrier filter to identify epithelial defects and assess tear breakup. Lissamine green can highlight surface damage not equivalent to a frank fluorescein-positive defect. A negative stain does not exclude stromal infection. Record lesion dimensions, depth, location, epithelial status, stromal thinning, anterior chamber response and associated lid disease. Photographs help serial comparison but do not replace a written description.

A Seidel test identifies aqueous leakage by dilution of fluorescein. A positive result requires urgent assessment of a wound or perforation. A negative result does not exclude a self-sealed open globe. Do not press on an eye with suspected rupture to provoke a positive test. Check pressure only when globe integrity permits, and use posterior assessment or ultrasound when relevant and safe.

Sampling follows the clinical question

For suspected microbial keratitis, significant central, deep, atypical or treatment-resistant lesions often merit scraping before antimicrobials when this can be done promptly. Sample the active edge and base using appropriate sterile technique, avoiding a dangerously thin area. Request stains and culture media suited to bacteria, fungi or Acanthamoeba and consider PCR for selected pathogens. A conjunctival swab is not equivalent to sampling the corneal lesion. Previous treatment can reduce yield, and a negative culture does not exclude infection.

For inflammatory or neoplastic lesions, biopsy site, depth and transport must match the requested investigation. Histology, microbiology and direct immunofluorescence may require separate specimens. Discuss an unusual sample with the laboratory before taking it; a correctly chosen test is useless if every sample is placed in the wrong preservative. Confocal microscopy can add evidence for some infections or tissue changes, but artefacts and operator interpretation limit it.

In an exam description of a paracentral white infiltrate, specify whether the epithelium is open, the stroma is thinning and the anterior chamber is inflamed. These observations determine urgency and sampling far more directly than a guessed organism from colour alone. The anatomy, immunity and examination objectives are listed in the EBO external-disease curriculum.