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KERATOCONUS

By Dany Najjar M.D.

DEFINITION:

Keratoconus is a condition in which the cornea assumes a conical shape as a result of non-inflammatory thinning of the corneal stroma.

The corneal thinning induces irregular astigmatism, myopia and protrusion leading to mild to moderate impairment in the quality of vision.

It is a progressive disorder affecting both eyes although only one eye may be affected initially.

 

EPIDEMIOLOGY:

Keratoconus has its onset at puberty and is progressive until the third to fourth decade of life when it usually arrests.

It is most commonly an isolated condition, however many diseases were reported to be associated with it, most importantly Downís syndrome, Leberís congenital amaurosis and connective tissue disorders.

Other conditions such as atopy, eye rubbing and hard contact lenses have also been reported to be highly associated with this disorder.

 

SIGNS OF KERATOCONUS:

  1. External signs:
  2. Munsonís sign: It is a V-shaped conformation of the lower lid produced by the ectatic cornea in downgaze.

    munsen.jpg (33657 bytes) Munson's sign

    Rizuttiís sign: Is a sharply focused beam of light near the nasal limbus, produced by lateral illumination of the cornea in patients with advanced keratoconus.

  3. Slit-lamp findings:
  • Stromal thinning (centrally or paracentrally, most commonly inferotemporally)
  • Posterior stress lines (Vogtís striae). These are vertical lines in the deep

stroma and Descemetís membrane that parallel the axis of the cornea.

  • Iron line (Fleischerís ring)
  • Epithelial or subepithelial scarring
  • Conical protrusion.

 

  1. Scissoring of the light reflex on retinoscopy.
  2. Keratometry findings:
  • Distortion of the mire lines
  • A documented increase in corneal curvature over time is a sensitive indicator of keratoconus.
  1. Hydrops of the cornea:

Patients with advanced disease may occasionally present with a sudden onset of visual loss associated with pain.

On slit-lamp examination, the conjunctiva may be injected and a diffuse stromal opacity is noted in the cornea. This condition referred to as hydrops is caused by breaks in Descemetís membrane with stromal imbibition of aqueous through these breaks.

The edema may persist for weeks or months, usually diminishing gradually with relief of pain and resolution of the redness and corneal edema ultimately being replaced by scarring.

 

 

PATHOLOGY:

The classical histopathologic features of keratoconus consist of a triad that includes thinning of the corneal stroma, breaks in Bowmanís layer and deposition of iron in the nasal layers of the corneal epithelium.

Depending on the stage of the disease, every layer and tissue of the cornea can however become involved in the pathological process.

Descemetís membrane is rarely affected except for breaks seen in acute hydrops.

The endothelium is usually normal.

 

MANAGEMENT:

The management of keratoconus varies depending on the state of progression of the disease. In early cases, spectacles may provide adequate visual correction but because they do not conform to the unusual shape of the cornea and the resultant induced irregular astigmatism, contact lenses provide better correction.

Contact lenses represent the treatment of choice in 90% of patients.

Corneal transplant:

Penetrating keratoplasty is the most successful surgical option for keratoconus patients who cannot tolerate contact lenses or who are not adequately rehabilitated by them.

A patient with keratoconus has an approximately 10-20% chance over his/her lifetime of needing a corneal transplant.

Patients whose best corrected spectacle visual acuity is 20/40 or worse should be offered corneal transplants instead of contact lenses.

There is a 50% chance that after a penetrating keratoplasty, patients may still need contact lenses either because of residual myopia or post-PK astigmatism.

Keratoconus (either frank or latent) is an absolute contraindication for keratorefractive surgery.


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