Ophthalmology/Diseases of Cornea

Introduction to Cornea

The cornea is a transparent, avascular, watch-glass like structure. It forms anterior 1/6th of the outer fibrous coat of eyeball.

Functions

1. Major refracting medium
2. Protects intraocular contents

Applied Physiology

Corneal Transparency

It is due to the following factors:

1. Peculiar arrangement of corneal lamellae (Lattice Theory of Maurice)
2. Avascularity
3. Relative state of dehydration maintained by barrier effects of epithelium and endothelium and active bicarbonate pump of endothelium

Source of Nutrients

Solutes enter the cornea by:

• Simple diffusion through aqueous humor
• Active transport through aqueous humor
• Diffusion from perilimbal capillaries

Oxygen directly enters from the air via an active process of epithelium

Congenital Anomalies

Megalocornea

When the horizontal diameter of cornea is of adult size at birth or 13mm or greater after the age of 2 years.

Microcornea

When the horizontal diameter is less than 10mm since birth.

Cornea Plana

Bilaterally the cornea is comparatively flat in this condition.

Congenital Cloudy Cornea

Cloudiness in the cornea since birth due to generally

1. Sclerocornea
2. Tears in Descemet's membrane
3. Ulcer
4. Metabolic conditions
5. Posterior Corneal Defect
6. Endothelial dystrophy
7. Dermoids

Inflammatory Conditions

The inflammation of cornea is called keratitis.

Morphological Classification

A. Ulcerative Keratitis (corneal ulcer)

1. Depending on location
   1. Central
   2. Peripheral
2. Depending on purulence
   1. Purulent Corneal Ulcer (suppurative)
   2. Non-Purulent Corneal Ulcer (non-suppurative)
3. Depending on hypopoyon
   1. Simple Corneal Ulcer
   2. Hypopyon Corneal Ulcer
4. Depending on Depth
   1. Superficial
   2. Deep
   3. Impending perforation
   4. Perforated
5. Depending on Slough fornmation
   1. Non-Sloughing Corneal Ulcer
   2. Sloughing Corneal Ulcer

B. Non-Ulcerative Keratitis

1. Superficial keratitis
   1. Diffuse superficial keratitis
   2. Superficial Punctate Keratitis (SPK)
2. Deep Keratitis
   1. Non-suppurative
      1. Interstitial
      2. Disciform
      3. Sclerosing
      4. Keratitis Profunda
   2. Suppurative
      1. Central Corneal
      2. Posterior Corneal

Etiological Classification

1. Infective keratitis
   1. Bacterial
   2. Viral
   3. Fungal
   4. Chlamydial
   5. Protozoal
   6. Spirochaetal
2. Allergic Keratitis
   1. Phlyctenular keratitis
   2. Vernal keratitis
   3. Atopic keratitis
3. Trophic Keratitis
   1. Exposure keratitis
   2. Neuroparalytic keratitis
   3. Keratomalacia
   4. Atheromatous Ulcer
4. Associated with diseases of skin and mucous membrane
5. Associated with systemic collagen vascular disorders
6. Traumatic keratitis
7. Idiopathic keratitis
   1. Mooren's Corneal Ulcer
   2. Superior limbic keratoconjunctivitis
   3. Thygseon's superficial punctate keratitis

Degenerative Disorders

It happens when normal corneal cells undergo some degenerative changes due to age or some pathological conditions.

Classification

A.Etiological Classification

1. Age related degenerations
   1. Arcus senilitis
   2. Vogt's white limbal girdle
   3. Hassal-Henle bodies
   4. Mosaic degeneration
2. Pathological degenerations
   1. Fatty degeneration
   2. Amyloidosis
   3. Calcific degeneration
   4. Salzmann's nodular degeneration
   5. Furrow degeneration
   6. Spheroidal degeneration
   7. Pellucid marginal degeneration
   8. Terrien's marginal degeneration
   9. Mooren's ulcer

Corneal Dystrophies

These are inherited disorders in which the cells have some inborn defects due to which cornea becomes hazy with the passage of time. There is no associated systemic pathology.

Classification

1. Anterior Dystrophies
   1. Epithelial basement membrane dystrophy
   2. Reis-Buckler's Dystrophy
   3. Meesman's Dystrophy
   4. Recurrent Corneal Erosion Syndrome
   5. Stocker-Holt Dystrophy

1. Stromal Dystrophies
   1. Lattice Dystrophy
   2. Crystalline Dystrophy (Schnyder's)
   3. Granular Dystrophy (Groenouw's type I)
   4. Macular Dystrophy (Groenouw's type II)

1. Posterior Dystrophies
   1. Cornea Gutta
   2. Fuch's late hereditary endothelial dystrophy
   3. Posterior polymorphous dystrophy (of Schlichting)
   4. Congenital hereditary endothelial dystrophy (CHED)

Ectatic Conditions

• Keratconus: is a non-inflammatory, mostly bilateral, ectatic condition of cornea in it's axial part.

• Keratoglobus: It is a familial and hereditary bilateral congenital disorder characterized by thinning and hemispherical protrusion of the entire cornea.

• Keratoconus Posterior: Extremely rare condition. Cone-like bulging of posterior surface of cornea occurs.

Abnormalities in transparency of cornea

Any condition that alters corneal transparency leads to loss of vision. The most common causes of loss of corneal transparency are:

1. Corneal Oedema
2. Drying of Cornea
3. Depositions on Cornea
4. Inflammations of Cornea
5. Corneal Degenerations
6. Dystrophies of cornea
7. Vascularization of cornea
8. Scarring of Cornea

Corneal Opacities

This term is typically used for the loss of transarency of cornea due to scarring.

Causes:

1. Developemental Anomalies
2. Following birth trauma
3. Healed Corneal wounds
4. Healed Corneal Ulcers

Corneal Oedema

Normal water content of cornea is 78%. If it becomes greater than this, central thickness increases and transparency reduces.

Causes:

1. Raised IOP
2. Endothelial damage
3. Epithelial damage

Vasclarization of Cornea

Normally the cornea has only small capillary loops in the periphery for about 1mm. It can be invaded by vessels leading to irritation and decreased corneal transpaency.