May 16, 2004
Topgraphic tips in Keratoconus
Review of Ophthalmology april 2004
Dr. Holladay uses four criteria to help differentiate keratoconus from lens warpage in cases where there is a “hot spot” on topography in the inferior cornea. The more criteria the suspect fulfills, the higher his suspicion that it’s keratoconus.
The first is a steep cornea. “If it’s keratoconus, the mean keratometry reading is usually greater than 46 D . The second is the location of the hot spot. “In keratoconus, the cone is rarely exactly at 6 o’clock—it’s usually at 6:30 or 5:30, that is, slightly to either side of 6 o’clock . The third criterion is the appearance of the hot spot. In a cone, it is almost circular, but with contact lens warpage the shape looks like a smile or a horizontal ellipse . The fourth parameter is the thickness of the hot spot compared to a symmetrical point above.For example, if the hot spot is 3.5 mm inferior at 5 o’clock, then you’d measure at 3.5 mm superior at 11 o’clock, normally, the inferior cornea is no more than 20 µm thinner than the corresponding value of the point above, especially in the afternoon after the patient’s been awake for several hours. Since the inferior cornea dries out more in the exposure zone during the day, it’s between 10 and 25 µm thinner as the day progresses. If the hot spot is more than 30 µm thinner than the corresponding superior point, then this is another vote for a cone.
Dr. Holladay adds that, in the majority of cases, all four parameters point to the same diagnosis, but occasionally one or two may be different. In these cases, the comparison of the thickness of the “hot spot” to the corresponding area above is key.
Posted by mehdi khanlari at May 16, 2004 07:36 AM