contact lens case reports

Visual Rehabilitation for Post-Refractive Surgery Patients

BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDR�, FCLSA
DECEMBER 1999

E.W. is a 36-year-old male who underwent bilateral LASIK in January 1999. Post-operatively, his undercorrected visual acuities were OD 20/50 and OS 20/20. Manifest refraction was OD +2.00 sph.

Because he was symptomatic of his overcorrected right eye, it was decided that a soft lens would be the most appropriate means to manage the induced anisometropia.

We fit E.W. with a variety of different soft contact lenses, all of which provided inconsistent visual acuity. He continued to be symptomatic despite multiple adjustments in lens parameters.

The Post-LASIK Corneal Shape

Since the late 1800s, the human cornea has been described as essentially spherical in the center (3mm to 4mm) and aspheric in the paracentral and peripheral zones. This aspheric configuration offers clear retinal imagery when the pupil diameter is greater than 4.0mm (Fig. 1). If the cornea was completely spherical and the pupil was large, the peripheral rays would be refracted anteriorly to the central rays (Fig. 2). The excess peripheral power created by a spherical surface results in an undesirable blurred image known as a spherical aberration.

Following refractive surgery, a complex relationship exists between visual acuity, defocus, diffraction and optical aberrations. The normal pre-operative cornea has a positive aspheric shape factor of +0.10 to +0.30, while the post-LASIK cornea has a shape factor of -0.50 to -3.50, with "0" being spherical. A lack of post-operative asphericity can create aberrations in a number of patients, especially those with larger pupils.

Aspheric Soft Lenses

The Specialty Choice A.B. lens (Specialty Ultravision) and the Frequency 55 aspheric lens (CooperVision) incorporate anterior aspheric optics throughout the optical portion of the lens to aid in the correction of spherical aberration.

Cameras often use aspheric lenses to decrease spherical aberrations, and aspheric condensing lenses are commonly used with indirect ophthalmoscopes to produce clearer retinal viewing.

We eventually fit E.W. with an anterior aspheric soft lens and he immediately reported a dramatic improvement in both distance and near acuity.

These aspheric lens designs clearly offer a number of advantages for patients who are experiencing visual acuity problems following refractive surgery.

Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University.

Mark Andr� is director of contact lens services at the Oregon Health Sciences University