What is Keratoconus? What Are the Treatment Options?

Keratoconus is a condition in which the normally spherical and transparent corneal tissue at the front of the eye bulges into a cone shape, typically towards the lower and outer parts, leading to irregular astigmatism.

Causes and Prevalence

The exact cause of keratoconus remains unknown. It occurs in approximately 54 cases per 100,000 people. The condition develops due to structural changes in the cornea, caused by an excess of destructive enzymes or a deficiency of protective enzymes. These structural alterations affect the collagen fibers and their cross-links within the corneal skeleton, leading to keratoconus. While the genetic transmission pattern is not fully established, 90% of cases are sporadic, while 10% follow an autosomal dominant inheritance pattern.

Eye Findings

Keratoconus is primarily characterized by a cone-shaped protrusion in the lower outer part of the cornea, leading to irregular astigmatism. A ring-like deposit of iron in the basal epithelial cells, known as the Fleischer ring, is often observed at the base of the cone. Other clinical signs include:

Munson’s Sign: The lower eyelid is pushed downward when the patient looks downward.
Rizzuti’s Sign: A wedge-shaped reflection is seen on the nasal limbus when a light is shone from the temporal side.
Charleaux’s Sign: A dark reflex is observed in the conical region of the cornea when using a direct ophthalmoscope in a dimly lit room.

A severe complication of keratoconus, known as acute hydrops, occurs when the Descemet membrane (one of the inner corneal layers) ruptures due to excessive stretching. This leads to fluid accumulation in the stroma, resulting in sudden pain and blurred vision. While acute hydrops may resolve spontaneously, it can leave scarring on the cornea and cause permanent visual impairment.

Diagnosis

The diagnosis of keratoconus is confirmed using corneal topography, a videokeratographic imaging system that has a 98% sensitivity and 99.5% specificity in detecting the disease. Ultrasonic or optical pachymetry may also be used to measure corneal thickness and confirm the diagnosis.

Systemic Associations

Keratoconus is found in 35% of patients with atopic conditions, where habitual eye rubbing can contribute to the development of an irregular corneal cone. Additionally, 5.5% of individuals with Down syndrome have keratoconus. Other systemic conditions associated with keratoconus include:

Ehlers-Danlos syndrome
Marfan syndrome
Crouzon syndrome
Apert syndrome
Leber’s congenital amaurosis
Retinitis pigmentosa (night blindness)
Retinopathy of prematurity
Fuchs’ dystrophy
Posterior polymorphous dystrophy

Treatment

Glasses and Contact Lenses: In the early stages, glasses can correct myopia and astigmatism. When glasses are no longer effective, rigid gas-permeable (RGP) contact lenses specifically designed for keratoconus are used.
Corneal Cross-Linking (CXL): If myopia and astigmatism are progressively worsening, corneal cross-linking (CXL) is performed to stabilize the corneal collagen and slow disease progression.
Intracorneal Ring Segments (ICRS): If there is no corneal scarring, intracorneal rings may be implanted to reduce myopia and astigmatism while stabilizing the disease.
Corneal Transplant (Keratoplasty): When other treatments are ineffective, corneal transplantation(keratoplasty) is performed.

Types of Corneal Transplant Surgery

Epikeratoplasty: A surgical option with variable visual outcomes, often yielding lower-than-expected results.
Lamellar Keratoplasty: A partial-thickness corneal transplant that reduces the risk of tissue rejection, making it a preferred method.
Penetrating Keratoplasty (PK): A full-thickness corneal transplant, which is the most commonly performed surgical procedure for keratoconus.

Outcomes of Keratoplasty

Corneal transplantation generally yields excellent visual outcomes and maintains graft transparency. However, residual astigmatism and myopia may still pose challenges. Permanent pupil dilation may occur due to nutritional deficiencies affecting the iris. While keratoconus recurrence after transplantation is rare, insufficient removal of the cone may contribute to its reappearance.

Approximately 20% of keratoplasty patients develop graft rejection, which requires prompt medical intervention to prevent transplant failure.