Screening can prevent post-op binocular disturbances

By Dermot McGrath In Paris

CAREFUL preoperative screening of high-risk patients can help reduce the incidence of binocular vision disturbances following refractive surgery, according to Jean-Louis Arné MD.

In a prospective study carried out from December 1999 to August 2001 at Purpan Hospital, Toulouse, France, researchers found that two principal factors increased the risk of developing binocular vision problems: first, treatment for anisomyopia and second, in cases where the surgeon tried to achieve monovision.

"The findings are significant. We set out to evaluate the changes in binocular vision that can occur after refractive surgery and found that the preoperative visual status of patients must be carefully considered to avoid such complications," said Dr Arné, who presented the results of the research at the annual meeting of the French Implant and Refractive Surgery Association (SAFIR).

The study included 284 myopic and hyperopic patients. Of 178 patients with low myopia, monovision was attempted in 34 of these patients. Nine of these cases experienced postoperative asthenopic complaints and four patients developed double vision.

Of the 74 patients with myopia greater than -4 D, 17 had a difference in manifest refraction greater than 4 D between the two eyes. Postoperatively, six patients complained of "binocular discomfort" and diplopia occurred in four cases. Of the 57 other patients eight experienced an alteration of binocular vision.
Dr Arné said that the preoperative visual status of myopic patients held the key to understanding these complications.

"In near vision, the myopic patient uses his convergence less than an emmetropic person; this is particularly true for high myopes who move close to the text to increase the size of the image. Moreover, a myopic patient intolerant to contact lens experiences the prismatic effect of his spectacles. This prismatic effect lessens the effort of convergence and can hide an exophoria," he said.

Dr Arné added that symptoms of presbyopia could occur due to the disappearance of the prismatic effect after surgery. The accommodative convergence requirement of a myopic patient wearing glasses is lessened. It increases after surgery and this can lead to the occurrence of an esophoria or tropia, he said.

Reversed eye dominance
After surgery for anisomyopia, Dr Arné said that there were several factors that could affect the refractive outcome. Giving superior vision to the non-dominant eye is analogous to converting from right-handedness to left-handedness. In cases of high anisometropia, the more myopic eye had often not been fully corrected leading to a sensory deprivation. A person can lose his or her fusion ability even after 30 years.
He added that the incidence of aniseikonia could also cause problems. When the difference between the two retinal images is between one percent and three percent, the compulsion to fuse prevails and that results in subjective symptoms. When aniseikonia is higher than five percent, binocular vision becomes difficult or impossible because compensation is not possible.

Dr Arné pointed out that aniseikonia resulting from retinal image disparity is non- reversible, so it was important to anticipate the difference in the size of the retinal images induced by surgery. For this purpose, he recommended the use of graphics that demonstrate the changes that could be induced by surgery.

The possible complications arising from monovision were also not to be underestimated. Undercorrection of the non-dominant eye produces monocular blur, said Dr Arné.
"Surgically induced monovision patients will be converted from their normally bifixating status into a monofixational syndrome with permanent loss of macular binocular vision. We encountered loss of binocularity in three cases with a difference of only 1.5 D and a normal preoperative vision. Patients with a strong ocular dominance will experience asthenopic complaints when forced to use the non-preferred eye for near vision," he said.

Of the 32 hyperopic patients, 18 had a preoperative normal binocular vision and four reported postoperative binocular vision problems. Of 14 patients with a preoperative esodeviation, six experienced a reduction of the abnormalities, four were orthotropic without glasses and two were converted from esotropia to esophoria.

Dr Arné said that one case was particularly conclusive. The preoperative refraction of this patient was + 7 D and + 7.75 D. He had good visual acuity with spectacles but was intolerant of contact lens wear. Both eyes were operated on by implantation of a phakic posterior chamber lens with a three-week delay between the two surgeries. Postoperatively, the visual acuity was excellent in both eyes but the patient developed a concomitant esotropia of 25 D and experienced diplopia.

Dr Arné said that there were several possible explanations for this outcome:
"Preoperatively, the patient was orthotropic when measured with spectacles but the high hyperopic correction might have masked a small non-accommodative esotropia. A monofixational syndrome, also called primary microtropia, is another possible explanation. It is characterized by a small angle deviation or even orthotropia, combined with the inability to use both foveolas together. There is only an extramacular binocular cooperation and the three weeks delay between surgeries may have lead to its de-compensation," he reported.

Dr Arné stressed that only a careful examination of stereoscopic vision could have permitted the ophthalmologist to detect this trouble preoperatively.
"These observations must persuade the refractive surgeon to perform an examination of binocular vision before surgery, at least in cases of anisomyopia, hyperopia or when monovision is attempted in order to screen the candidates and avoid subsequent postoperative problems."

Jean-Louis Arné, MD,
Hopital Purpan,
Toulouse, France
ARNE.JL@chu-toulouse.fr

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