Cornea, External Disease and Anterior Segment
Core concepts
Know the layered anatomy and what each layer contributes to disease: epithelium (tight junctions, limbal stem cells), Bowman layer, stroma (collagen lamellae and keratocytes, governs scarring and oedema), Descemet membrane, endothelium (pump without regeneration). Master the corneal oedema pathway (endothelial failure → stromal hydration → folds then bullae), the difference between epithelial and stromal defects, and the logic of tectonic vs optical keratoplasty.
External disease questions reward pattern recognition: blepharitis (anterior vs posterior), dry eye (aqueous deficiency vs evaporative/MGD), and the red-flag infections. Learn dendritic ulcer (HSV, debridement not steroid), suppurative ring infiltrate (bacterial or fungal keratitis), and the contact-lens-associated Acanthamoeba picture (severe pain, radial keratoneuritis).
Traps
- Confusing a healing epithelial defect (smooth, grey) with an active ulcer.
- Steroid before excluding HSV or fungal infection.
- Assuming a hypopyon means bacterial endophthalmitis; sterile hypopyon occurs in severe keratitis and uveitis.
- Missing neurotrophic keratopathy when sensation is not tested.
Rule of thumb
Any corneal problem: test sensation, stain with fluorescein, assess the endothelium (specular/guttae), and ask about contact lenses and prior surgery before committing to a diagnosis.
Worked example
A soft-contact-lens wearer with severe pain out of proportion to signs, a ring infiltrate, and radial perineural lines. The pattern points to Acanthamoeba; confirm with confocal microscopy or culture on non-nutrient agar with E. coli, start biguanide/ diamidine therapy, and avoid steroids until the organism is controlled.