Refractive surgery can lead to strabismic complications, study shows

A history of strabismus or hyperopia may place a patient at high risk for developing diplopia.

Issue: December 1, 2001

EAST MELBOURNE, Australia — Surgeons need to be aware of the potential for strabismic complications of LASIK, according to a surgeon here.

Lionel Kowal, FRANZCO, in an article published in Clinical and Experimental Ophthalmology, provides minimum screening techniques for discovering at-risk patients and classifies patients into groups according to their potential risk for strabismic complications.

Screening

According to Dr. Kowal, the surgeon should record the current spectacle strength and check for any prism component in the patient’s prescription.

Refraction should be performed for near and distance vision. Myopic patients should be given the weakest minus correction that allows threshold acuity. Hyperopic patients should have both absolute hyperopia and manifest hyperopia measured. Near acuity should be assessed using threshold near cards. This should be followed by a cover test for distance and near. If no deviation is seen, then an alternate cover test should be done to detect any phoria.

The last step is cycloplegic refraction. According to the report, myopic patients should be checked to see if with less minus sphere the patient can still reach threshold. Hyperopic patients should be checked to see how much extra plus can be added without blurring.

Low and moderate risk

According to Dr. Kowal, patients with little or no risk for strabismic complications have myopia, no history of strabismus or diplopia, no prisms in their spectacles, phorias absent or minimal under an alternate cover test and current spectacle correction and repeat refraction and cyclopentolate refraction all within 0.5 D of sphere.

“In medium-risk patients, the refractive surgeon needs to simulate the refractive result with contact lenses and then demonstrate a safe range of motor fusion,” he said.

Hyperopic patients, planned monovision and habitually overcorrected myopic patients are at moderate risk for strabismic complications.

Those with moderate risk should be tested for motor fusion for both distance and near fixation.

Dr. Kowal said the surgeon should incorporate a cover test and an alternate cover test as part of the routine refractive surgery evaluation. He further stated that a horizontal prism bar should become routine refractive surgery evaluation equipment, used for a 1- to 2-minute evaluation in medium-risk patients to detect those who can safely have refractive surgery.

High risk

Dr. Kowal said a history of strabismus or hyperopia with reduced motor fusion are major warning signs that a patient is at high risk for diplopia or strabismic complications following LASIK.

He said high risk patients should not have refractive surgery unless they are assessed by a strabismologist or by an orthoptist who practices in classical orthoptics.

“One problem with high-risk patients like those who already have strabismus is the presence of features that indicate the possibility of clinically important deterioration with time independent of refractive surgery,” he said.

For example, he said that small-angle exotropia with marked superior oblique overaction is likely to deteriorate, or a shallow suppression scotoma as demonstrated with a Bagolini filter bar may lighten and the patient may get spontaneous diplopia.

“If these patients deteriorate, it is likely the refractive surgery will be blamed even if the underlying strabismus is the main or only culprit,” Dr. Kowal said.

Dr. Kowal also said that hyperopic patients can be at high risk.

“Too many refractive surgeons think of hyperopia as the mirror image of myopia, and it is not. Clinical hyperopia can be quite volatile. The refractive surgeon must understand the detection and significance of the different subtypes of hyperopia — absolute, manifest, latent, total and facultative — and understand which requires treatment and what under- or overtreatment is likely to do in the short, medium and long term to all the different subtypes and on patient function and comfort,” he said.

Decentration

In a case report that Dr. Kowal co-authored, also published in Clinical and Experimental Ophthalmology, decentration of a LASIK treatment of approximately –20 D of sphere produced diplopia and, because of some corneal flap striae, some loss of best corrected vision.

In the case, a patient with high myopia developed vertical binocular diplopia after decentered LASIK with associated decompensation of existing exophoria into an exotropia.

According to the report, decentration had the effect of grinding a large vertical prism onto the patient’s cornea. The existing exophoria decompensated into an exotropia as a result of the new vertical imbalance.

The patient reported the onset of diplopia after a few weeks. Dr. Kowal said that had the patient been questioned properly after undergoing LASIK, his diplopia might have been recognized immediately after surgery.

Dr. Kowal said that a long lead-up to symptoms of even a few years is feasible if an outcome of surgery is reduced motor fusion that then slowly deteriorates even further.

In the case reported, a hard contact lens on the right eye eliminated the patient’s diplopia, which, the authors stated, strongly suggests that the corneal disorder acquired following LASIK was the main and probably only cause of the patient’s diplopia.

For Your Information:

Lionel Kowal, FRANZCO, can be reached at Locked Bag No. 8, East Melbourne, 8002, Australia; (61) 3-9639-1500; fax: (61) 3-9662-3964; e-mail: strabism@netspace.net.au.

Reference:

Yap E, Kowal L. Diplopia as a complication of laser in situ keratomileusis surgery. Clin Experiment Ophthal. 2001;29:268-271.

Kowal L. Refractive surgery and diplopia. Clin Experiment Ophthal. 2000;28:344-346.