Cornea,uvea and sclera

Cornea Part 1

The Shape of the Cornea

  • Aspherical

  • Diameter: 11.5 mm to 12 mm

  • Refractive index: 1.37

  • Power: 43D to 45D

  • Thickness: 500-600 microns (0.5 to 0.6 mm)

  • Thickness at the limbus: 1 mm

  • Average thickness: 540 microns

Structure of Cornea

  • Upper Layer - Epithelial Layer

    • Single layer of columnar cells known as basal cells.

    • Two to three layers of wing cells above.

    • Uppermost layers are stratified squamous epithelium, non-keratinized.

    • Microvilli present on the epithelial surface for tear film attachment.

    • Basal cells attach to the basement membrane via hemidesmosomes.

    • Corneal dystrophy involves problems with hemidesmosomes.

  • Second Layer - Bowman's Membrane

    • Part of the stroma; acellular and does not regenerate.

  • Third Layer - Stroma

    • Thickest layer (90% of corneal thickness).

    • Composed of 200-300 layers of Type-1 collagen lamellae.

    • Contains ground substance: Glycosaminoglycans (GAGs) including keratin sulphate and chondroitin sulphate.

    • Also includes fibroblasts/keratocytes.

  • Fourth Layer - Dua's Layer

    • Acellular and strongest layer.

  • Descemet's Membrane

    • Composed of two parts: banded and non-banded.

    • Banded part laid down in the uterus; non-banded is secreted by endothelial cells.

    • Breaks in membrane cause Haab's striae and Vogt's striae.

    • Schwalbe's line appears on gonioscopy.

  • Endothelium

    • A single layer of polygonal cells which does not regenerate.

    • Maintains dehydrated corneal state; critical number of endothelial cells required.

Important Information

  • The cornea is the most densely innervated tissue in the body.

  • Sensory Supply: via the ophthalmic division of the trigeminal nerve (nasociliary nerve → long posterior ciliary nerve).

  • Neurotrophic keratitis: Results from fifth nerve lesion, affecting epithelial health.

  • Neuroparalytic keratitis: Due to seventh nerve palsy; leads to exposure keratitis.

Physiology of Cornea

  • Avascular except for limbal blood vessels.

  • Dehydrated to maintain clarity and health.

  • Nutrition: from aqueous humour.

Pump and Barrier Functions

  • Na+/K+ ATPase Pump: pumps water out of the cornea.

  • Endothelium functions:

    • Barrier against water entry.

    • Dysfunction leads to corneal edema.

  • Normal endothelial cell count: 2500-3000 cells/mm² in adults and 3500-4000 cells/mm² in children.

  • Damage results in pleomorphism and polymegathism.

  • <500 cells/mm² can lead to stromal edema, epithelial edema, and bullous keratopathy.

Transparency of Cornea

  • Factors: Regular arrangement of epithelial cells, regular arrangement of stromal lamellae, <half wavelength distance between lamellae, maintained by GAGs.

  • Endothelium maintains dehydration via combined pump and barrier functions.

Investigation Related to Cornea

  • Keratometry: Measures central optical zone and curvature.

  • Keratoscopy/Placido's disc: For assessing corneal surface.

Cornea Part 2

Keratoplasty

  • Keratoplasty: Replacement of diseased cornea with donor cornea.

  • Timing: Ideally within 6 hours of death; safe extraction from 6-24 hours.

Contraindications for Corneal Donation

  • Absolute contraindications: Unknown cause of death, systemic infections (e.g., HIV, tuberculosis), intraocular tumors.

  • Relative contraindications: History of intraocular surgery, certain systemic conditions.

Tests before Keratoplasty

  • Examine all three layers of donor cornea for health.

Types of Keratoplasty

  • Penetrating Keratoplasty (PK): Full thickness replacement.

  • Lamellar Keratoplasty (LK): Partial thickness replacement.

    • Types based on layer involvement:

      1. Anterior Lamellar (SALK and DALK).

      2. Deep Endothelial (DSEK/DSAEK).

Keratoprosthesis

  • Artificial Cornea Replacement: After one or two failed transplants (Boston keratoprosthesis and osteo-odonto keratoprosthesis).

Corneal Degeneration Types

  • Arcus Senilis: Age-related lipid deposits.

  • Band-Shaped Keratopathy (BSK): Calcium deposition in cornea.

  • Vortex Keratopathy: Drug deposition in whorl-like pattern.

Corneal Opacities

  • Types: Nebular, macular, and leucomatous opacities based on the extent of stroma involvement.

  • Management: Optical iridectomy, keratoplasty, or corneal tattooing.

Sclera

  • Sclera: Tough white outer layer of the eye forming 5/6 of the outer coat.

  • Layers of Sclera:

    1. Episclera: Vascularized connective tissue.

    2. Sclera Proper: Dense, avascular tissue.

    3. Lamina Fusca: Innermost layer with melanocytes.

Uveitis

  • Definition: Inflammation of uveal tissue.

  • Classification: Anterior, intermediate, posterior, and pan-uveitis.

  • Etiologies: Infectious, non-infectious, masquerade syndrome, etc.

  • Management: Focus on topical and systemic treatment based on type and cause.