In clinical practice, three distinct forms of keratoconus have been identified, each with a unique clinical presentation. Differentiating between the three forms can be helpful in counseling patients about what to expect regarding eventual progression of the disease. However, the clinical classification of keratoconus should be viewed as only a general guide. It is important to communicate to the patient that the condition can be extremely unpredictable and that its ultimate course can only be determined with time.
Puberty-onset keratoconus is by far the most common form of the condition, and, as its name indicates, it begins in early adolescence at about age 12 to16 years. This is shifting to younger age groups. In our experience on the Indian subcontinent, younger age at onset is much more common compared to the western countries. In fact various researchers from India and Middle East Asia have documented that the age of onset is earlier and the progression to severe disease is seen more often in the Asian-Indian population.
The condition is usually bilateral with one eye affected more than the other. Following its onset, there is often a dramatic progression of the condition for the next 8 to 10 years. This is typically followed by a period in which the condition seems to stabilize. Clinical experience has shown that the earlier in life the keratoconus occurs, the more severe the condition will be. Therefore, a 12 year old, exhibiting the optical and topographical manifestations of early keratoconus is more likely to have a more severe form of the condition than an individual that shows similar findings in their late 20’s.
In late-onset keratoconus, the earliest signs and symptoms begin in the late 20’s or early 30’s. Both eyes are frequently affected to a similar degree, with little or no asymmetry. This is often a more benign form of the condition, and, unlike puberty onset keratoconus, its progression is significantly less severe, rarely requiring surgical intervention in the form of a corneal transplant.
The third form of keratoconus is called “form fruste” and was first described by Amsler in 1937. It is essentially an extremely mild form of keratoconus that can occur at anytime throughout life.
The condition manifests as a central or paracentral zone of irregular astigmatism of unknown etiology. The most striking hallmark of form fruste keratoconus is its lack of progression with the condition staying stable throughout the patient’s life.