Odd-shaped corneas can be fit with soft contact lenses

Successful contact lens (CL) wear by patients with odd-shaped corneas depends on a thorough examination, careful lens fitting, and appropriate lens selection, said Thomas Quinn, OD, MS, FAAO. Dr. Quinn is in private practice in Athens, OH.

The comprehensive eye examination should include keratometry to measure the corneal curvature and corneal topography to map the cornea's surface. In patients with regular astigmatism, the flattest, steepest meridians will be 90 degrees apart. In these patients, toric soft CLs can provide significant vision improvement compared with spherical CLs, even when astigmatism is relatively low.

"In patients with irregular astigmatism, the degree of separation between major meridians will be some odd measure, not 90 degrees," Dr. Quinn explained. "This is a red flag that this patient has more than just high-level astigmatism." Irregular astigmatism generally is more effectively managed by a GP or hybrid CL rather than a soft toric.

Determine rotational sensitivity

The visual acuity achieved with soft toric CLs depends on the stability of the lens position on the cornea. Therefore, Dr. Quinn suggested determining the patient's sensitivity to lens rotation by assessing patient response to rotation of their astigmatic correcting lens in the phoropter following refraction (Becherer twist test).

"If minimal rotation elicits a blur response, that patient probably is not a good candidate for soft toric lenses," Dr. Quinn explained. "If a generous rotation is required to elicit a blur response, the patient has a good prognosis. The higher the astigmatism correction, the more important it is that rotation be minimized."

The next consideration is cylinder power. The higher the cylinder power, the greater the residual astigmatism that is induced with rotation of the lens. The higher the lens' toric power, the greater the risk that the patient will complain of vision disturbance with rotational variability.

"To minimize symptoms, select the weakest power lens possible that provides the patient with good vision," Dr. Quinn said. "Cylinder error decreases in myopic patients when the spectacle power is vertexed to the corneal plane. For hyperopic patients this increases the cylinder error."

Evaluate lens rotation

Although the ultimate goal would be for the lens to move on the cornea without rotating, some degree of rotation is acceptable as long as occurs in the same direction and the same degree because the prescription can be adjusted to compensate for it.

Corneal topography can be a factor in the degree of rotation, he noted. Astigmatism that is confined to the central cornea results in the least amount of rotation; astigmatism that extends limbus to limbus tends to cause greater degrees of rotation.

To assess the location of lens rotation, Dr. Quinn suggested using a slit lamp beam alignment technique, or simply estimating the degree of rotation. Estimating is acceptable when the degree of lens rotation is small and astigmatism is relatively low grade. Under these conditions, estimating lens rotation has been found to be accurate within 8 degrees in 95% of cases.

Rotational stability rules

"The stability of the rotation is the key to success," Dr. Quinn said, "and that depends on the patient and how the lens sits on the eye."

Stability of the rotation can be determined by asking the patient to look ahead and blink, then change his direction of gaze and blink. Following each blink, the lens marking location is assessed.

Alternatively, the digital displacement test can be used. In this test, the lens marking is assessed with a straight gaze in the first eye then that lens is manually rotated out of position under the slit lamp. The procedure is repeated in the second eye, and lens position is assessed in the first and second eyes in order. The lens returning to its original location indicates good stability.

"If it does not return, a different lens may be advisable," Dr. Quinn said.

Achieving treatment goals

Silicone hydrogel torics, the latest generation of toric CLs, are a new technology that appear to minimize rotational issues. Studies show that this material demonstrates a slower rate of dehydration. Slower dehydration, combined with design advances, means less rotation on the eye-with a side benefit of better corneal health.

With all of these issues to consider when fitting toric CLs, making adjustments along the way is not unusual to achieve optimal acuity. That's why it's important for patients to understand that fitting these CLs is as much art as science, Dr. Quinn concluded

"Let your patient know up front that this will be a process so that you can manage their expectations," he said. "That way there are no surprises."