History, Signs, and Symptoms of Keratoconus
One of the first indications an eye-care practitioner has that patients may be developing keratoconus is that they are never satisfied with their spectacle prescription. They may report seeing a number of practitioners and having many pairs of spectacles. They may also report many changes in their spectacle prescription over a relatively short amount of time, such as 12-18 months, and may even know that there has been significant change in their astigmatism correction
Keratoconus affects both eyes, though it usually develops first in one eye, and this eye often continues to be worse than the second eye. Symptoms normally start between 16 and 25 years of age. Patients may report that their eyes itch, that they have allergies, and they often rub their eyes with more than the usual force.
In addition to a decrease in visual acuity, patients with keratoconus often complain of visual discomfort similar to a patient with uncorrected astigmatism. They will report "squinting" in order to see better. They are often more sensitive to light than the normal patient. They report flare or halos around lights, particularly when driving at night. Many will even avoid night driving. Ghost images, seeing double with one eye covered (monocular diplopia), and multiple images are common complaints. Due to the distortion, there may be many areas of the cornea with different refracting powers creating multiple confusing images.
There may be a family history of keratoconus, but this is relatively rare. Keratoconus is more common in patients with atopic dermatitis, connective tissue disorders, retinitis pigmentosa, and Down syndrome.
One of the first signs of keratoconus is a change in the astigmatic spectacle correction of patients in the 16-25 year-old age range. It is unusual to have non-keratoconic patients change 0.75 to 1.50 diopters (units of spectacle power) over a period of a few months in this age range. Likewise it is unusual to have a significant change in cylinder axis. Therefore, when the practitioner sees such changes, especially when accompanied by complaints of dissatisfaction with recent glasses, keratoconus must be considered. The astigmatic change may also be accompanied by signicant changes in near-sighted correction.
The eye-care practitioner can also use some standard office equipment to assist in the diagnosis of keratoconus. One such instrument is the retinoscope, which uses the red reflex seen when a light is shone directly into the pupil to objectively determine a spectacle prescription. The red reflex looks much like the "red eye" phenomenon in photographs. The size, extent, and location of the cone can be qualitatively determined with retinoscopy.
Another useful instrument is the keratometer. This instrument uses the reflection of circles, called mires, from two small points projected approximately three millimeters apart on the central cornea to measure the steepness of the cornea. If the cone is located in this central part of the cornea, the mires will seem distorted, not perfectly round. However, the cone may be located above, below, or to the side of the central cornea. Since a cone gives the cornea a very steep shape, the reading may be off the scale of the instrument. The eye-care practitioner must take into account not only readings from the instruments, but the patient's complaints and other signs.
Videokeratography is another technique which can be used to monitor changes in the cornea. It involves using a computerized instrument called a corneal topographer, which creates a topographic map. This is much like the topographic maps created of land. It shows how steep the cornea is and how quickly the steepness changes by using color-coded printouts. Advantages of corneal topographers are that they allow you to see a cone, even if it is off-center. Changes in the cornea over time are easy to quantify. However, these instruments do have their limitations and are not always accurate.
The biomicroscope, commonly called the 'slit lamp', is a special microscope fitted with chin- and headrests which allows the eye-care practitioner to view the eye under high magnification. The slit lamp also contains special colored filters which can enhance the doctor's view. The biomicroscopic examination of the cornea is important. However, the keratoconus has to be relatively advanced for biomicroscopic signs to be evident.