Excerpt from Keratoconus

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Background

Keratoconus (KC) is a progressive, noninflammatory, bilateral (but usually asymmetrical) disease of the cornea, characterized by paraxial stromal thinning that leads to corneal surface distortion. Visual loss occurs primarily from irregular astigmatism and myopia and secondarily from corneal scarring.

Pathophysiology

All layers of the cornea are believed to be affected by KC, although the most notable features are the thinning of the corneal stroma, the ruptures in the Bowman layer, and the deposition of iron in the basal epithelial cells, forming the Fleischer ring. Breaks in and folds close to the Descemet membrane result in acute hydrops and striae, respectively.

Frequency

United States

Reported prevalence in the general population varies (50-200 per 100,000), perhaps with differences in diagnostic criteria. It is commonly an isolated ocular condition but sometimes coexists with other ocular and systemic diseases.

Commonly recognized ocular associations include vernal keratoconjunctivitis, retinitis pigmentosa, and Leber congenital amaurosis; systemic putative associations include many of the connective tissue disorders (eg, Ehlers-Danlos and Marfan syndromes), mitral valve prolapse, atopic dermatitis, and Down syndrome.

Particular risk factors include atopic history, especially ocular allergies, rigid contact lens (CL) wear, and vigorous eye rubbing.

Most KC cases appear spontaneously, although approximately 14% of them present with evidence of genetic transmission.

International

Anecdotal reports suggest an increase in prevalence in some parts of the world, Arabia, the Indian subcontinent, and New Zealand.

Mortality/Morbidity

• Because few elderly patients have been noted with KC, many wonder if KC is associated with some fatal disease. However, several recent studies provide compelling evidence that KC does not result in increased mortality.
• Advanced KC rarely may progress to corneal hydrops, the so-called acute keratoconus, wherein breaks occur in the Descemet layer, which lead to central stromal edema and potentially secondary severe corneal scarring. Patients report sudden loss of vision and some ocular discomfort in one eye but usually not much pain or conjunctival injection. Acute treatment of hydrops is palliative; many corneas flatten secondary to hydrops, and both visual acuity and CL application may improve following such events. If secondary scarring is severe, corneal transplantation (penetrating keratoplasty [PKP]) may be warranted.
• PKP is otherwise indicated when CLs and intrastromal plastic rings (Intacs) are either no longer tolerated or no longer useful in vision correction. When good CL care is available, only 10-20% of patients with KC eventually require PKP. The need for PKP increases when optimal CL care is not available, but many patients still require CL care for optimum visions following PKPs.

Sex

An equal incidence of KC occurs in males and females.

Age

• KC typically presents at puberty and progresses until the third and fourth decades of life, although it can occur or progress at any age.
• KC progresses at various rates but tends to progress more rapidly in young patients.

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