As with other ocular conditions, accurate diagnosis is the key to a proper treatment plan and best visual outcome. Keratoconus is often mistaken for other eye conditions or just poor vision which cannot be corrected with glasses or contact lenses.
Measuring Corneal Curvature
The first step in detecting possible keratoconus is measuring of the curvature of the cornea with a device called a keratometer. The curvature of the cornea or any other light bending surface such as eyeglass lenses, contact lenses, binocular lenses, ect, is measured in units called diopters. Higher diopters equates to a steeper cornea – simple as that. An “average” cornea will measure about 38 to 46 diopters in steepness. Any cornea from 46 to 49 diopters is considered suspicious for keratoconus and past 50 is assumed keratoconus. This is especially true if a significant difference (more than 3 diopters) exists between the vertical and horizontal curves of the cornea which is the measure of astigmatism.
Best Correctable Vision – Possible to see 20/20?
The next step is to determine the patients best corrected spectacle (glasses) vision through a refraction in which the best combination of corrective lenses to maximize vision is calculated. Even in the presence of steep corneas, if the vision is correctable to 20/20, the degree of keratoconus or irregular cornea is minimal. A strong indicator of keratoconus is visual acuity which cannot be corrected to 20/20 or with the patient still complaining of ghost images even with the best refraction in place.
Any patient with steep corneas (over 48 diopters) and not correctable to a solid 20/20 with no other eye disease noted, should have a corneal topography mapping test performed. This test produces a topographical map of the cornea similar to that of any landscape map or elevation survey. The test is completely painless, requires no eyedrops or pupil dilation, and takes just a few seconds. In a normal cornea, the curvatures are relatively uniform across the whole mapping with curvatures under 45 diopters.
In a keratoconus cornea, the curvatures very greatly through out the cornea often showing an inferiorly decentered “cone” with curvatures usually past the 50 diopter level. The appearance of this steep cone confirms the keratoconus diagnosis. The corneal mapping is also a very valuable tool for following the progression of the keratoconus over time.
Optical Coherence Tomography (OCT)
Another recent advancement in keratoconus diagnosis is optical coherence tomography (OCT)which uses a scanning laser to image a cross section of the cornea. This allows doctors to look for the cone shaped cornea from a side view and also look for thinning of the cornea. OCT is a somewhat valuable tool but only looks through a cross section of the middle of the cornea. It usually does not pick up the cone shape of the keratoconus until it is well advanced making the corneal mapping a better diagnostic tool. However, the OCT is a fantastic device for determining the coverage of scleral contact lenses which are the best way to maximize vision in keratoconus patients.
Once the diagnosis of keratoconus has been made, the most appropriate treatment options can be determined.