Epidemiology of Keratoconus
Prevalence and Incidence
Keratoconus is a relatively rare condition. The prevalence of the disease reported in the literature varied considerably. This is probably due to numerous factors, including the population studied, the tests performed, and the criteria used. Estimates range from 4 per 100,000 people to 600 per 100,000. Kennedy et al (1986) found a prevalence rate of 54.5 per 100,000 in the population of Olmstead County in Minnesota based on records from 1935 to 1982. They found an incidence (the number of new cases per year) of 2 per 100,000.
Many early studies of keratoconus reported that the prevalence of keratoconus was much highter in women than in men. However, more recent studies have found that the prevalence is higher in men or that there is no significant difference, as shown in the table below.
|Study||Males with Keratoconus||Females with Keratconus||# of people in study|
|Hall||almost equal||almost equal||288|
|Buxton||almost equal||almost equal||140|
|Totals||1.92 (ratio)||1 (ratio)|
Many practitioners who routinely see significant numbers of keratoconus patients indicate that they see few older keratoconic patients (over 55 to 60 years). This has raised the suspicion that keratoconics may have a shorter life expectancy. A study by Moodaley et al of 337 keratoconic patients aged at least 45 years found 279 living, 13 deceased and 45 untraceable. Comparing these results to actuarial life tables they did not find the death rate to be any different than the general population.
Keratoconus has been reported at birth and to develop as late as 51 years of age (Hall). The majority of patients apparently develop the disease between 12 and 20 years of age. Keratoconus usually develops in one eye first with the second eye developing at a later time, usually within a couple of years, although it may be 5 or 6 years before the second eye develops the disease. The first eye to develop the disease invariably ends up with more advanced cornea distortion. Very rarely is only one eye involved. Hall had only 8 patients out of his 288 who remained for years without the second eye becoming involved. In a large sample of keratoconic patients, Tuft et al found only 4.3% of the patients with 3 years or more of follow-up to have keratoconus in only one eye.
Patients first diagnosed with keratoconus want to know the prognosis for progression and loss of vision. Of course, doctors would like to be able to predict both the rate of progression as well as which patients will advance to severe keratoconus. The rate of progression for a particular patient is impossible to predict. Some patients advance rapidly for 6 months to a year, then stop progressing with no further change. Often there will be periods of several months with significant changes followed by months or years of no change followed by another period of rapid change. This sequence may be repeated several times. Other patients may gradually advance for periods typically up to 10 years. Pouliquen retrospectively analyzed the records of 187 patients considering the age at diagnosis and the likelihood of progression requiring corneal transplant. They concluded that there was no relationship between the age of onset and the degree of progression. For those that required transplant, the average time from diagnosis to surgery was 10 years.
Woodward found that for a group of 70 keratoconic patients (139 eyes) 16.5% required corneal transplants. Of the 23 eyes requiring grafts, 21 were grafted within 6 years of diagnosis of keratoconus and the other 2 after 10 years. The only correlation found between the baseline examination results and likelihood of requiring a graft was the corneal steepness. In this case they used the base curve of the lens required to fit the patient as this is more accurate in advanced cases than a distorted keratometer reading. If the initial contact lens fitted had a base curve of 6 mm or steeper, there was a 50% chance that a graft would eventually be required. There was no correlation between age, sex or initial visual acuity and the eventual need for a graft. Lass et al (1990) found in an evaluation of 417 patients (746 eyes) thatkeratometer readings of over 50 diopters and visual acuities of 20/50 or worse were high risk factors for eventual surgery.
Tuft et al did an extensive study of 2523 keratoconic patients. They found that 21.6% of the patients required corneal transplant with a mean duration of 8.8 years (median 7 years) from the time of diagnosis.