Strabismus and Diplopia Following Refractive Surgery
Rudolph S. Wagner, MD
Multiple factors are involved in the relationship between strabismus and refractive error. Ophthalmologists are familiar with the dynamics of hypermetropia producing accommodative esotropia. However, few are cognizant of the complex factors leading to newly acquired diplopia in adult patients, even those patients with a history of previous strabismus.1 With refractive surgery now commonplace, recognition of patients with strabismus or with compromised binocular vision prior to correction of their refractive error has become increasingly important. In this tutorial, the factors predisposing to strabismus and diplopia following refractive surgery are presented using case presentations. A protocol that is useful in the evaluation of patients preoperatively is also described.
In addition to the manipulation of accommodative factors causing a decompensation of strabismus, loss of binocularity resulting from surgical monovision can occur.2 The interruption of fusion that occurs following monocular surgery or following delay in surgery between the eyes, which results in a disparity of sensory input, occurs less frequently.3
In patients with a history of strabismus, diplopia occurs following refractive surgery from other causes, including technical complications with the procedure itself,4 which include corneal scarring, creation of too small ablation zone, and a decentered treatment zone. A change in the astigmatic axis or power rarely produces diplopia.
Case 1
A 40-year-old woman underwent bilateral LASIK to correct 6 D of myopia in the right eye and 5.75 D of myopia in the left eye. She had a small left exotropia and hypertropia preoperatively, and suppressed her left eye while wearing contact lenses. Three days following LASIK, her refraction was +2.25 – 1.25 × 50º in the right eye and +0.50 in the left eye. She had a history of strabismus surgery at age 2 and exotropia at age 9.
Three months after surgery, she had an esotropia of 2 Δ D and a right hyperphoria of 10 Δ D at distance fixation and a left esotropia of 14 Δ D at near fixation. She complained of diplopia and had a diplopic response on Worth 4 dot testing. At 7 months following surgery, she was wearing a contact lens (+1.50 – 1.00 × 40° to eliminate her diplopia and strabismus. She eventually required additional LASIK to eliminate the hyperopia in her right eye, which eliminated her symptoms.
Analysis: The patient had an overcorrection of the myopia in her dominant eye which resulted in an accommodative esotropia. No longer in her suppression scotoma, she experienced diplopia. She preferred additional LASIK to wearing a contact lens since this was her original objective.
Case 2
A 47-year-old woman presented with exotropia immediately following bilateral LASIK to correct hyperopia in each eye. She had a history of strabismus surgery to correct a right esotropia as a child and again 12 years prior to the LASIK to correct a left exotropia. Four months following LASIK, she noticed increasing exotropia and diplopia. At the time of her presentation, her uncorrected visual acuity was 20/25 in the right eye and 20/40 in the left eye. Her manifest refraction was +0.25 in the right eye and +1.00 in the left eye. She had a left exotropia of 30 Δ D for both distance and near fixation. She underwent a 5-mm resection of the left medial rectus muscle and an 8-mm adjustable recession of the left medial rectus muscle. This corrected her alignment and eliminated her symptoms.
Analysis: The patient admitted that prior to having LASIK, she did not frequently use hyperopic correction and was not bothered by presbyopia. She was most likely using accommodative convergence to control her exotropia. Immediately following LASIK, her exotropia became manifest as she no longer needed to accommodate.
These cases clearly illustrate how refractive surgery can modify strabismus, even in patients with minimal deviations. Both patients were managed successfully, one with strabismus surgery and one with additional refractive surgery. It is important to elicit any history of strabismus, amblyopia, or other disturbances of binocular vision in patients considering refractive surgery. It is also important to perform an ocular motility exam, including sensory testing to recognize patients with potential problems following refractive surgery.
Case 3
The following case is an example of co-management.
A 38-year-old man with a history of a right esotropia and mild amblyopia was referred for management of his strabismus. He was wearing +3.25 – 0.50 ×180º in the right eye and +3.25 – 0.25×180º in the left eye. He saw 20/30 in the right eye and 20/20 in the left with his present correction. He wanted refractive surgery and the refractive surgeon referred him for management of his strabismus. Without correction, he had a right esotropia of 35 Δ D. With correction, he measured a 10 Δ D right esotropia at distance and a 14 Δ D right esotropia at near fixation—the maximum prescription. The decision was made to have the refractive surgery first. Following LASIK, his vision was 20/30 in the right eye and 20/20 in the left eye without correction. He had a 16 Δ D right esotropia. A 6-mm recession of the right medial rectus muscle was performed using an adjustable suture. Postoperatively, at 1 week, he had a 2 Δ D esotropia and has remained stable for over 8 months.
Analysis: This case illustrates comanagement at its best. Strabismus surgery was performed after refractive surgery. The patient understood the decision making process and was pleased with the results.
Prior to performing refractive surgery, the ophthalmologist should perform an evaluation of the strabismus and binocularity status of the prospective candidate. In certain situations, as in the third case presented in this tutorial, an evaluation by a strabismologist prior to refractive surgery would seem prudent (Table 1).
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Table 1.
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In all cases, a thorough history should be obtained with the purpose of identifying prior strabismus, diplopia, amblyopia or other disruptions of binocularity. Patients should be questioned regarding the use of prisms in glasses, orthoptic therapy or optometric eye exercises and occlusion therapy. Previous experience with monovision should be described.
The patient’s glasses should be read for the presence of prism or unusual refractive correction. Refractive status should be evaluated both with manifest and cycloplegic refraction. Kushner suggests that for patients with myopia, the least minus correction for threshold acuity be targeted. In patients with hyperopia, the least plus correction for threshold acuity (absolute hyperopia) should be demonstrated and the most plus correction accepted for threshold acuity be targeted. The difference between the maximum plus correction and cycloplegic refraxtion (latent hyperopia) should be determined.5
A cover/uncover and alternate cover test must be performed, looking for any strabismus. This should be performed at both distance and near fixation, preferably in the cardinal gaze positions particularly if any deviation in the primary gaze position is detected. Versions and ductions of the extraocular muscles should be evaluated.
Sensory testing should include a test of stereoacuity (Titmus, Random Dot E) and a test of fusion such as the Worth 4 Dot test at both distance and near fixation.
Determination of the dominant eye, perhaps with contact lenses for monovision should be performed, which is particularly important when monovision is to be incorporated into refractive surgical correction. In addition, contact lens trial correction of anticipated refractive surgical correction can be used to predict postoperative alignment and potential for diplopia.4
Unexpected strabismus or diplopia can usually be avoided or at least anticipated with the proper preoperative evaluation as described in this tutorial. In cases of an untoward result, successful management either with strabismus surgery or additional refractive surgery can frequently be accomplished.