Comprehensive Study Notes on Corneal Anatomy, Pathology, and Keratoconus

Overview and Definition

• Definition: The cornea is the first and outermost structure of the eye.
• Optical Requirement: It must be clear to perform its role as the major part of the eye's optical system.
• Functions:
  • Optical: Light rays first meet the cornea when falling on the eye. It is a convex, transparent structure that refracts light (changes its direction as it enters the eye).
  • Barrier: The cornea fits into the surrounding sclera to form the outer coat of the eye, acting as a barrier against injury or infection.

Anatomy of the Cornea

• Layers: The cornea consists of 5 distinct layers. Its anatomy is designed to maintain the clarity necessary for its optical function.
  • Epithelium: Stratified squamous nonkeratinized cells. It includes surface cells, wing cells, and basal cells resting on a basement membrane. Dimensions: $40-50\,\mu\text{m}$.
  • Bowman’s Layer: Dimensions: $8-15\,\mu\text{m}$.
  • Corneal Stroma: Contains collagen fibers and keratocytes in a regular arrangement. Dimensions: $470-500\,\mu\text{m}$.
  • Descemet’s Membrane: Dimensions: $10-12\,\mu\text{m}$.
  • Endothelium: Contains an active pump to regulate the water content of the stroma. Dimensions: $4-6\,\mu\text{m}$.
• Physiological Characteristics:
  • Avascularity: The cornea lacks blood vessels to maintain transparency.
  • Sensation: Mediated via the trigeminal nerve.
• Cellular Structures Mentioned:
  • Desmosomes and Hemidesmosomes.
  • Tight junctions and Gap junctions.
  • Langerhans cells and Corneal nerves.

Clinical Examination

• Slit Lamp: Primary tool for examining the anterior segment.
• Anterior Segment Parameters:
  • Diameter: Normally $11 \times 12\,\text{mm}$.
  • Transparency: Normally transparent; assessed for pathologies like ulcers or opacities.
• Sensation Testing: Assessed using a cotton tip tested from the side of the patient.
• Ocular Surface Staining:
  • Fluorescein: Stains corneal and conjunctival epithelium when it is not intact, allowing dye to enter tissues.
  • Rose Bengal: Has an affinity for dead or devitalized epithelial cells that have lost or altered their mucous layer. It highlights corneal filaments and plaques. A $1\%$ solution or impregnated strip is used. It can cause intense stinging; use a small drop preceded by topical anesthesia and wash excess with saline.
  • Lissamine Green: Stains similarly to Rose Bengal but causes less irritation.
• Diagnostic Staining Patterns:
  • Interpalpebral: Common in aqueous tear deficiency.
  • Superior Conjunctival: May indicate superior limbic keratoconjunctivitis.
  • Inferior Corneal/Conjunctival: Often present in blepharitis or exposure keratopathy (lagophthalmos).

Differential Diagnosis: Red Eye, Tearing, and Photophobia

Red Eye Causes

• Adnexal: Trichiasis, distichiasis, floppy eyelid syndrome, entropion, ectropion, lagophthalmos, blepharitis, meibomitis, acne rosacea, dacryocystitis, canaliculitis.
• Conjunctival: Ophthalmia neonatorum, conjunctivitis (bacterial, viral, chemical, allergic, etc.), subconjunctival hemorrhage, inflamed pinguecula, superior limbic keratoconjunctivitis, neoplasia.
• Corneal: Infectious or inflammatory keratitis, contact lens problems, foreign body, recurrent erosion, pterygium, neurotrophic keratopathy.
• Other: Trauma, dry eye, endophthalmitis, anterior uveitis, episcleritis, scleritis, angle-closure glaucoma, carotid–cavernous fistula.

Tearing Causes

• Adults (Pain Present): Corneal abrasion, foreign body, recurrent erosion, edema, anterior uveitis, eyelash/eyelid disorders, dacryocystitis.
• Adults (Minimal/No Pain): Dry eye, blepharitis, nasolacrimal duct obstruction, punctal occlusion, ectropion, allergic conjunctivitis, crocodile tears.
• Children: Nasolacrimal duct obstruction, congenital glaucoma, foreign bodies.

Photophobia (Light Sensitivity)

• Abnormal Exam: Corneal abrasion/edema, anterior uveitis (common); posterior uveitis, scleritis, albinism, aniridia, congenital glaucoma (less common).
• Normal Exam: Migraine, meningitis, retrobulbar optic neuritis, subarachnoid hemorrhage, trigeminal neuralgia.

Comparative Diagnosis: Red Eye Conditions

Corneal Ulcers and Keratitis

• Definitions:
  • Corneal Abrasion: Loss of corneal epithelium.
  • Corneal Ulcer: Loss of corneal epithelium with stromal involvement.
  • Keratitis: Infiltration of the cornea with inflammatory cells.

Specific Infectious Types

• Bacterial Keratitis:

  • Agents: Gram-positive (Staph aureus, Staph pneumonia) and Gram-negative (Gonococci, Pseudomonas aeruginosa).
  • Symptoms: Pain, lacrimation, photophobia, blurred vision, red eye.
  • Signs: Ciliary injection, epithelial defect ($+$ fluorescein), stromal infiltrate, hypopyon, potential perforation.
  • Treatment: Empirical fortified antibiotic ED (Anti-Gm+ and Anti-Gm-). Examples: $5\%$ Cefuroxime, $1.5\%$ Gentamicin, or Fluoroquinolone monotherapy.

• Herpes Simplex Keratitis (HSV):

  • Agent: HSV Type I (rarely Type II).
  • Clinical Presentation: Epithelial keratitis (Dendritic, Punctate, Geographic), Stromal (disciform) keratitis, or keratouveitis.
  • Characteristic: Linear defect with bare stroma and terminal bulbs; decreased corneal sensation.
  • Treatment: Topical Acyclovir EO $3\%$, 5 times/day for 2 weeks.

• Herpes Zoster Ophthalmicus (HZV):

  • Agent: Varicella Zoster virus affecting the trigeminal nerve sensory nucleus.
  • Signs: Cutaneous lesions strictly unilateral (to one side of the midline); Hutchinson’s sign (vesicles on the tip of the nose indicative of naso-ciliary nerve involvement).
  • Keratitis types: Punctate, microdendritic (elevated "painted-on" appearance, minimal staining), disciform, or neurotrophic.
  • Treatment: Oral Acyclovir tablets $800\,\text{mg}$, 5 times/day for 10 days; topical Acyclovir EO $3\%$.

• Fungal Keratitis:

  • Agents: Filamentous (Fusarium) or Yeast-like (Candida).
  • Predisposing Factors: Trauma by plant object.
  • Signs: Fluffy edges, satellite lesions, ring infiltrate, hypopyon.
  • Treatment: Topical antifungal eye drops for $6-8$ weeks.

• Protozoal Keratitis (Acanthamoeba):

  • Source: Swimming pools, soil, freshwater, contaminated contact lenses/solutions.
  • Clinical Picture: Severe pain out of proportion to clinical signs.
  • Signs: Ring-shaped stromal infiltrate, radial perineuritis.
  • Treatment: Brolene ED for up to a year.

Non-Specific Treatments and Management

• Supportive Therapy: Cycloplegic ED (reduces ciliary spasm), Vitamin C and Tetracycline (promotes healing), Systemic analgesics.
• Physical Protection: Eye patching for healing; therapeutic/tectonic corneal grafts for resistant or perforating ulcers.
• Exposure Keratopathy (EK) Prevention:
  • Risk groups: ICU patients (sedated/ventilated), previous ptosis surgery, Thyroid eye disease.
  • Intervention: Lubricant ED/EO, Taping eyelids, Tarsorrhaphy in severe cases.

Tear Film Abnormalities and Vitamin A

• Causes: Dry eye disease, Xerophthalmia (Vitamin A deficiency), Keratoconjunctivitis sicca, Ocular cicatricial pemphigoid.
• Consequences: Defective vision, keratinization, opacification, neovascularization.
• Xerophthalmia: Caused by malnutrition, malabsorption, or alcoholism. Signs include Bitot spots and corneal melting.

Corneal Opacities and Dystrophies

Grading Scarring Density

1. Corneal Nebula: All iris and AC details clearly seen.
2. Corneal Macula: Iris and AC details hardly seen.
3. Corneal Leucoma: Iris and AC details not seen.

Major Corneal Dystrophies

• Epithelium: EBMD (Map-Dot-Fingerprint), Meesmann's.
• Bowman's: Reis-Buckler's.
• Stromal: Granular (white opacities), Macular (hazy opacification), Lattice (refractile branching lines).
• Endothelium: Fuchs' (Guttata with edema), PPD (Vesicular lesions).

Keratoconus

• Definition: Progressive thinning and bulging of the cornea, typically manifesting around puberty.
• Etiology: Multifactorial (Genetic, eye rubbing, UV rays, Vernal keratoconjunctivitis).
• Signs:
  • Early: Progressive irregular myopic astigmatism, scissoring of red reflex on retinoscopy.
  • Late: Apical thinning/scarring, Fleischer ring (iron deposition in epithelium), Vogt’s striae (vertical lines in posterior stroma), Munson’s sign (bulging of lower lid on down gaze).
  • Acute Hydrops: Sudden edema and pain due to a tear in Descemet’s membrane.
• Treatment: Rigid contact lenses, Corneal collagen crosslinking (using Riboflavin), Intracorneal stromal rings, Keratoplasty.

Degenerations and Other Conditions

• Arcus Senilis: Peripheral stromal lipid deposition (age-related).
• Band Keratopathy: Calcium deposits due to chronic inflammation.
• Wilson Disease: Kayser–Fleischer ring (copper dusting in peripheral Descemet membrane).
• Limbal Stem Cell Deficiency: Results from injury, surgery, or Stevens-Johnson syndrome. Leads to persistent epithelial defects and conjunctivalisation of the cornea.
• Staphyloma: Thinning of uvea/sclera causing a focal bulge; distinct from keratoconus.

Corneal Blindness and Eye Banking

• Primary Causes: Infectious agents (bacteria, virus, fungi, protozoa).
• Predisposing Factors: Trauma, contact lens use, steroid use.
• Statistics: Approximately $50,000$ corneal transplants are performed per year in the United States.
• Eye Banking: Involved in harvesting, processing, and recording corneal grafts. Advanced lamellar surgeries and bioengineered corneas are being developed to address donor shortages.

Questions & Discussion

• What is the number of corneal transplants per year worldwide? (Answer implied: US alone does $50,000$; total global need is high).
• What is the source of corneal grafts in Egypt? (Note: This is a prompting question for student research).
• At what age can keratoconus be diagnosed? As young as 4 years old.
• Which ring is associated with Wilson Disease? Kayser-Fleischer ring (copper), distinguishing it from the Fleischer ring (iron) in Keratoconus.
• All of the following are signs of keratoconus EXCEPT: Vogt’s striae, Munson’s sign, Kayser Fleisher ring, Central corneal thinning. (Answer: Kayser Fleisher ring is for Wilson disease).