Cornea

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  • Bowman's layer:
  • Very thin acellular layer formed from collagen fibers
  • Cannot regenerate, scar forms if trauma occurs
  • Stroma:
  • Thickest layer, mainly consists of collagen and water
  • Contributes to transparency of the cornea
  • Descemet's membrane:
  • Thin acellular layer serving as a basement membrane
  • Endothelium:
  • Cuboidal single layer
  • Controls corneal hydration and transparency
  • Does not regenerate if damaged
• Anatomy of the Cornea:

  • Outermost layer in front of the colored part of the eye
  • Most sensitive organ in the body
  • Convex in shape
  • Horizontal and vertical diameters are about 11-12 mm
  • Thickness is 540 microns at the center, increasing to 700 microns at the peripheries
  • Refractive index is about 1.37
  • Avascular, receives nutrition and oxygen through diffusion
  • Corneal layers:
  • Epithelium:
  • Thickness: 50-90 microns
  • Non-keratinized stratified squamous epithelium
  • Regenerates and is the only layer that does so
  • Regeneration takes 5-7 days By stem cells in limbus (juncion betwen cornea & sclera
• Functions of the Cornea:

  • Protection of internal ocular structures
  • Refraction of light to focus on the retina
  • Corneal refractive power is about 40D, fixed in contrast with the lens
  • Vergence:
  • Behavior of light rays
  • Can be parallel, divergent, or convergent
  • Refraction:
  • Change in velocity of light passing through different media
  • Refractive power of the cornea is 40D
  • Accommodation:
  • Physiological function for near vision
  • Part of the near reflex triad
  • Adaptive mechanism to increase refractive power for near objects
  • Helm-Holtz theory:
• Contraction of ciliary muscle leads to increased refraction
• Refractive Errors:

  • Refraction is the bending of light as it enters a medium with a different refractive index
  • Most refraction occurs in the cornea and lens
  • Diopter is the unit expressing refractive power
  • Emmetropia is the ideal refraction of the human eye
  • Myopia:
  • Light focused in front of the retina
  • Causes include refractive and axial myopia
  • Presentation starts during childhood and worsens during adolescence
  • Complications include degenerative retinal changes
  • Hyperopia:
  • Light focused behind the retina
  • Causes include refractive and axial hyperopia
• • Patients at risk of closed-angle glaucoma
• Hyperopia:

  • Patients are at risk of closed angle glaucoma due to a shallow anterior chamber
  • Managed with converging lenses (+) (glasses or contact lenses) or surgery
• Presbyopia:

  • Increasing difficulty in seeing clearly when attempting close-range tasks, typically around age 40
  • Due to age-related inflexibility of the lens
  • Initial signs include accommodative lag, difficulty with accommodation in reduced light conditions, and tiring with continuous close work
  • Managed with converging lenses (+) for reading
• Astigmatism:

  • Occurs when the eye's optical system cannot produce a point focus in horizontal and vertical axes due to irregular curvature of the cornea
  • Managed with cylindrical lenses (glasses, contact lenses)
• Presentation of Ametropia:

  • Symptoms include blurry vision, headache, sore, red, watery, or tired eyes
  • Parents may observe younger children habitually rubbing their eyes, turning heads when looking at things, sitting too close to the television
  • School-aged children may present with behavioral problems or learning difficulties
  • Managed with spectacles or contact lenses
• Refractive Eye Surgeries:

  • Goals are to improve refractive state of the eye (myopia, hyperopia, astigmatism) and eliminate dependency on glasses or contact lenses
  • Laser-assisted vision correction involves excimer laser acting on the cornea to reshape it for proper light focusing on the retina
  • Different techniques for myopia, hyperopia, and astigmatism correction
• Candidates for Refractive Laser Eye Surgery:

  • Between 18-55 years of age, typically around 20 years old
  • Must have healthy eyes, stable refractive errors, not pregnant or breastfeeding, and no significant medical conditions or eye problems
• PRK (Photorefractive Keratectomy):

  • 1st generation of refractive laser eye surgeries
  • Involves complete removal of the epithelial layer, with a healing duration of 5-7 days and full recovery in 1 month
• LASIK (Laser Assisted In Situ Keratomileusis):

  • Most commonly performed refractive surgery
  • Involves creating a flap in the cornea, reshaping the stroma with an excimer laser, and replacing the flap for healing
  • Recovery time is 24-48 hours
• Eyedrops for Post-op Complications:

  • Topical antibiotics to prevent infection, topical steroids for inflammation, topical NSAIDs for pain, lubricating drops for dryness
• Refractive Lens Exchange Surgery:

  • Involves placing a synthetic lens (IOL) between the iris and the natural lens to correct refractive errors
  • Suitable for patients not eligible for laser eye surgery or with extreme refractive errors or presbyopia
• Keratoconus:

  • Characterized by progressive bilateral central thinning of the cornea, changing it from a dome shape to a conical shape
  • Etiology theories include enzyme abnormality, connective tissue abnormality, and genetic factors
  • Presentation includes blurred and distorted vision, myopia, eye rubbing, diplopia, photophobia, and astigmatism
• Diagnosis of Keratoconus:

  • Corneal topography and pachymetry are used for mapping and measuring corneal abnormalities
• Corneal topography is an anon-invasive medical imaging technique for mapping the anterior curvature of the cornea
• Pachymetry is a medical device used to measure the thickness of the cornea
• Goal of treatment for corneal conditions: To improve vision and stop progression
• Stabilize disease by Corneal collagen cross-linking (CXL) using riboflavin drops to photosensitize the eye followed by exposure to UVA light
• Keratoconus can be treated in early stages with glasses or rigid contact lenses to restore optics of the eye
• Corneal ring segments result in a flatter cornea and clearer vision, typically combined with glasses or rigid lenses for effectiveness
• Keratoplasty, either penetrating or deep anterior lamellar keratoplasty (DALK), may be required for severe cases
• Bacterial Keratitis is the most common type and can be caused by various bacteria including Staph Aureus, Pseudomonas, and Coliforms
• Symptoms of Bacterial Keratitis include rapid and progressive pain, eye redness, foreign body sensation, and decreased vision
• Treatment for Bacterial Keratitis includes topical broad-spectrum antibiotics, therapeutic mydriasis, and corneal transplantation in severe cases
• Herpes simplex keratitis is caused by reactivated herpes simplex virus and can lead to corneal scarring and vision loss if untreated
• Symptoms of Herpes simplex keratitis include ocular irritation, pain, redness, photophobia, blurry vision, and vesicular skin rash
• Treatment for Herpes simplex keratitis includes topical and oral antivirals, ulcer dendritic debridement, and avoiding topical steroids
• Herpes zoster ophthalmicus is a reactivation of herpes zoster virus affecting the ophthalmic division of the trigeminal ganglia
• Symptoms of Herpes zoster ophthalmicus include pain, burning, itching, impaired vision, and vesicular eruption in the innervation area of the ophthalmic nerve
• Fungal Keratitis is a sight-threatening infection of the cornea caused by fungi such as candida, aspergillus, and fusarium
• Symptoms of Fungal Keratitis include blurred vision, redness, tearing, pain, and corneal infiltrate
• Diagnosis of Fungal Keratitis involves fluorescein visualization, corneal scraping for staining and culture
• Treatment for Fungal Keratitis includes topical and systemic antifungals such as amphotericin B and natamycin
• Acanthamoeba keratitis is rare and vision-threatening, often seen in contact lens wearers with poor hygiene practices
• Symptoms of Acanthamoeba keratitis include blurry vision, redness, pain, photophobia, tearing, and corneal infiltrates
• Diagnosis of Acanthamoeba keratitis involves slit lamp examination, fluorescein staining, and corneal scrapings for staining and culture