Detecting strabismus before
refractive surgery

History

Strabismologists are becoming increasingly aware of a strabismic entity, primary microtropia, which is found in up to 5% of first-generation relatives of persons with infantile esotropia. Most individuals with primary microtropia escape detection because their minute misalignment is missed and their binocularity is nearly normal. Visual system stresses, like some of the ones caused by KRS, place these surgical candidates at a higher risk of developing postoperative problems, Burke said.

"The first part of the medical history is to determine whether there has ever been a strabismic problem. If so, what type of therapy was undertaken? Make sure you ask whether anyone related to them has strabismus, especially infantile esotropia," he said.

Searching for clues

Intermittent diplopia, fuzzy vision (with or without ghost images), and asthenopia with the possibility of convergence or accommodative insufficiency are red flags for potential strabismic concerns. Burke outlined the tests that should be included in the preoperative exam.

A check for prisms in the patient's glasses is needed. Careful manifest and cycloplegic refractions should also be performed. "Under manifest conditions in the myope, you must determine the minimum minus power necessary for their threshold maximum visual acuity. In the hyperope, both the minimum and the maximum plus power for maximum visual acuity must be measured," he said.

When performing the cycloplegic refraction, the minimum minus power in the myope must be determined and the "push plus to blur" power in the hyperope. Near vision must also be evaluated. "To do this, use the smallest target to check accommodative ability at the usual one-third meter distance," he said.

In addition, the muscle-balance exam should include the cover test. Cover and uncover one eye while observing the non-occluded eye for movement to detect a tropic misalignment. Then a deliberate alternate cover test should be performed. "This means a slow, continuous, back-and-forth covering of one eye, then the other eye, changing eyes at least five to six times, to break up fusion and uncover a phoria," he said.

Routine stereopsis and fusion-potential tests are recommended, as well as a check for unusual head positions and measurements in the primary, straight-face position.

Using the history and physical exam, Burke defined three strabismic risk levels associated with KRS.

Minimal risk

Minimal risk, or a no-risk patient, is the typical KRS candidate. Burke said that this patient is a myope who has not had any problems in glasses or contacts; who has no history of strabismus, diplopia, or other asthenopic complaints; and has no prisms in the glasses. For the minimal risk patient, the muscle-balance exam reveals a trivial phoria, and the patient possesses excellent binocularity. The manifest and cycloplegic refractions match within 0.5 D.

"Any patient who deviates from these parameters is at risk of developing strabismic and binocular function problems," he said.

Moderate risk

The moderate risk patient has prisms in his or her glasses. The prism's significance should be determined as a precaution. "First, neutralize or eliminate the prism. Then, recheck the muscle-balance tests and the binocular functions," he said.

Burke classifies all hyperopes at the moderate-risk level because of the frequent undercorrection of KRS and the regressions that appear to be more common in this subgroup. The undercorrected hyperope may develop accommodative esotropia and complain about premature presbyopia, therefore, the full hyperopic refractive error should be known before surgery.

David B. Granet? MD

Monovision is a moderate risk. "Many studies have shown that the threshold for loss of binocularity is a difference of only 1.25 D. The intraocular blur begins to cause suppression, resulting in stress that may break down fusional reserves, resulting in the development of a new strabismus," Burke said.

Research by Marshall Parks, MD, clinical professor, George Washington University, Washington, D.C., indicates that up to one-third of the surgically induced monovision patients will be converted from their normally bifixating status into a monofixational syndrome, with permanent loss of macular binocular vision. The shutdown of the bifixational reflex is suspected to occur in about 3 months, Parks said.

Burke estimates that 98% of the population has moderate to strong ocular dominance, which mean that many monovision patients will experience asthenopic complaints when forced to use the nonpreferred eye for some tasks. "In those patients who are already functioning in a monofixational situation from previous eye muscle surgery ... [as well as the] hidden primary microtropia patient, the change to monovision may precipitate frank double vision, by the loss of sensory adaptation or the loss of macular suppression in their nondominant eye. This may cause fixation-switch diplopia when [they are] forced to use their nondominant eye," he said.

Patients considering monovision should have a contact lens simulation of at least 3 weeks, with full muscle-balance testing and binocular functions reevaluated at the end of that time. "The best rule to follow for monovision is to keep the difference between the eyes to the absolute minimum," he said.

High risk

A small portion of patients, such as those with constant tropias, fall into the high-risk category and should receive a strabismic evaluation from a specialist.

Other high-risk patients include those who complain of intermittent double vision, ghost images, or visual confusion or stress (they may have a hidden strabismus), convergence insufficiency, or the infrequent problem of accommodative insufficiency not associated with presbyopia.

"When the strabismologist examines the patient preoperatively, the eye-muscle problem has a chance to be corrected, or, at the very least, the extent of the risk of KRS may be more completely understood by the patient and the surgeon," Burke said.

Anisometropia of 4 D-plus places this patient at high risk, because of aniseikonia. When the corrected retinal image sizes are unequal, visual stress may impair motor fusion, resulting in the development of a new strabismus. Fixational-anomaly angle kappa is also an area of risk.

Proper screening

David B. Granet? MD? associate professor at the University of California, San Diego, agrees that it is important to screen KRS patients for strabismic risks to avoid potential problems.

Granet noted that his guidelines for examination of patients before KRS and his risk evaluation are similar to Burke's. He said, "During the cycloplegic refraction, although it is faster and easier on the patient to do the refraction with tropicamide, a full cycloplegic agent like Cyclogyl [cyclopentolate HCl, Alcon] should be used to avoid a problem."

Strabismus evaluation before refractive surgery would be a noncovered service for Medicare, unless a prior procedure had induced the refractive condition.

When performing strabismus evaluations on patients who would be covered by Medicare, remember the differences in requirements between the evaluation/
management codes and eye codes. E/M codes mandate measuring alignment in primary gaze, whereas the eye codes mandate only "basic sensorimotor examination." My interpretation of the E/M requirement would be an alternate cover test with prism measurements, along with a near points of convergence (NPC) and versions, whereas "EOM (extraocular muscle) full" might suffice for the eye codes; however, it is recommended that the E/M minimums be used. In general, one should address primary gaze alignment, NPC, and rotations (versions). The "Fly" (a test for gross steropsis) usually suffices for gross fusion status.