What Is the Anatomy of the Cornea?
The transparent, spherical part of the eyeball is called the cornea. It is the most potent refractive element in the eye's optical system. The human cornea is avascular, acellular, moderately dry, and has a remarkable arrangement of stromal collagen lamellae to preserve its clarity. The cornea has a diameter of 11 to 12 mm horizontally and 10 to 11 mm vertically. The cornea has a standard central thickness of 520 μm and a peripheral thickness of 650 μm. Water makes up around 78 % of the corneal stroma. A healthy epithelium and a well-functioning endothelial pump determine this proportion. The cornea has a refractive index of 1.376. The cornea is divided into five layers from outer to inner surface: epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium.
What Are Some Corneal Degenerations?
Non-involutional degenerations are rare and associated with systemic or local illnesses, whereas involutional degenerations are quite common and predominantly connected to age. Here are some notable degenerations.
Band Keratopathy Degeneration:
This is due to the deposition of calcium salts in the interpalpebral area of Bowman's layer. Patients with a history of severe injury, uveitis, renal failure, interstitial keratitis, syphilis, long-term glaucoma, or juvenile rheumatoid arthritis are frequently affected. Vision frequently becomes limited to the perception of light due to the extreme density of the deposition. Therefore, treatment aims to maintain comfort and cosmesis. Chelation with EDTA (Ethylenediaminetetraacetic acid) can reduce opacification and enhance light transmission. Practitioners may be responsible for designing cosmetic lenses to maintain cosmesis. A black underprint lens can cover the scar and should match the other eye color on the upper surface for the best results.
Salzmann Nodular Degeneration:
This disorder occurs when excess hyaline or hyaline replaces Bowman's layer. It usually occurs after inflammatory or noninflammatory events like trauma. The bluish, superficial nodules may lead to mild to severe pain and visual loss based on their location and severity. Lubrication and BCLs (Bandage contact lenses) are used to treat the condition, but a corneal transplant may be necessary later. Treatments such as phototherapeutic-keratectomy (PTK) are becoming more widespread.
White Limbal Girdle of Vogt:
Peripheral thinning is a common cause of age-related corneal stromal degeneration. The White Limbal Girdle of Vogt impacts more than 50 % of people over 40. It may contain calcium deposits and is indicative of subepithelial degeneration. It has a clear zone dividing it from the limbus and only appears in the horizontal meridian.
Furrow Degeneration:
This is a benign peripheral thinning with no inflammation or ocular consequences and a lucid interval of arcus.
Terrien's Marginal Degeneration:
This gradual, painless, progressive thinning preserves the epithelium and may result in gutter vascularization. It starts in the region of the supernasal quadrant. The primary objective of treatment is to rectify the oblique astigmatism.
Mooren's Ulcer:
The condition is more ulcerative than Terrien's, spreading centrally and circumferentially with a raised, de-epithelialized leading edge. It causes moderate to severe pain and is linked to autoimmune diseases like Crohn's and a history of trauma or surgery. Recent data suggests an association with Hepatitis C. The major therapies are artificial lubricants, steroids, and mechanical protection (conjunctival resection, tarsorrhaphy, lid tape).
What Are Some Corneal Dystrophies?
Corneal dystrophies often damage only a single corneal layer. Here are some of a few notable dystrophies.
Fuch's Dystrophy:
It is an asymmetric, bilateral disorder that primarily affects women and appears in the fifth or sixth decade. Guttata are transparent vesicles secreted by the endothelium projected into the space between Descemet's membrane and the endothelium. They seem like bubbles under a microscope. However, specular reflection indicates the absence of cells. Guttata disrupts the regular pumping function of the epithelium, leading to stromal and epithelial edema, severe pain, and vision impairment. Palliative care includes the use of glaucoma medications to lessen edema, hypertonic fluids, BCLs to extract fluid, and PK when corneal opacification develops.
Macular Dystrophy:
Macular dystrophy is an autosomal-recessive disorder. It results in an accumulation of mucopolysaccharides and demonstrates alcian blue staining. It occurs in the first decade of life. Macular dystrophy develops in the superficial and core layers of the stroma and extends quickly to the limbus. It causes profound visual loss and needs a PK.
Granular Dystrophy:
This is a more prevalent autosomal dominant disease. Hyaline deposits and Masson trichrome staining identify it. Its appearance ranges from discrete patches to a powdery, diffuse presentation with clear zones. Erosions frequently breach the basement membrane and appear centrally in the anterior to mid-stroma. Pinhole effects benefit certain patients, but surgery can be required if the depositions cluster. Surgery causes the cornea to flatten and affects refractive correction. Contact lenses can be used to treat the development of anisometropia.
Lattice Dystrophy or Degeneration:
Amyloid branching filaments in lattice degeneration stain with Congo red. This exhibits birefringence while exposed to polarized light. There could be recurring erosions that leave scars that impair vision.
What Is the Treatment for Corneal Degeneration and Dystrophies?
Providing a healthy surface environment is the most effective treatment for anterior corneal problems. Often, the first course of treatment is lubricating drops. Patients with eye diseases often require instillation. Therefore, it is recommended to use non-preserved drops to minimize the possibility of cytotoxic consequences. Electrolytes such as calcium, magnesium, potassium, and sodium that help improve eye healing should also be included in artificial lubrication. Gels provide patients with long-lasting comfort, but they frequently choose the non-blurry appearance of liquids. Advanced cases of ulcerative keratitis can now be treated with several new methods, including autologous serum therapy and cyclosporine (Allergan's Restasis). Autologous serum therapy is similar to customized tear replacement in that a topical supplement is made from the patient's serum.
Conclusion:
There are many different kinds of corneal dystrophy and degeneration. Maintaining a healthy corneal surface is the best way to treat corneal problems. Practitioners must know these illnesses to provide appropriate co-management, identification, and treatment. The field of ophthalmology requires ongoing research and technological advancements to develop targeted therapies and achieve better treatment outcomes.
