OD News Articles

1st July 2002

Beware the Cone—Especially with LASIK

Keratoconus and LASIK do not mix. But keratoconus presents a challenge because early diagnosis is difficult, progression is variable, and prognosis is unpredictable. PCLI has developed a tool we routinely use to assess referred patients.

As you know, keratoconus is a non-inflammatory, self-limiting ectasia of the axial portion of the cornea. It is characterized by progressive thinning and steepening of the central cornea. Vision in most cases can be corrected with glasses and contact lenses, and patients do fairly well.

To some degree, stability of the cornea and globe is related to corneal thickness. For this reason we do not want to thin the cornea too aggressively with LASIK. This is especially important in patients with suspected keratoconus.

Following is Pacific Cataract and Laser Institute’s method for identifying early keratoconics. We have coined it the PCLI Asymmetry Test—or PAT for short. Here’s how it works.

PCLI Asymmetry Test (PAT)

1. Using topography, measure the curvature of the cornea's steepest spot. This is normally the center but if there is a steeper eccentric spot, place the topographer cursor over it and measure it there.

2. Find the mirror image spot 180 degrees from the steep spot and the same distance from the center. Measure it the same way.

3. Mark both spots on the print out, write their curvature numbers, and determine the difference between the two. 

If the difference is:

  • less than 2 diopters—cornea can be considered within normal limits.
  • between 2 and 3 diopters—proceed with caution. Patient should be steered away from corneal surgery. However, if factors such as CL history, slit lamp findings, age, family history, refractive stability, and corneal thickness are positive, LASIK may be considered. A bed of 300u after the treatment may be preferred to the usual limit of 250u.
  • more than 3 diopters—LASIK is contraindicated.

If you have any questions or would like to learn more about the fundamentals behind PAT, contact any one of our doctors.

Article Archive