Laser in situ keratomileusis (LASIK) and excimer laser photorefractive keratectomy (PRK) are widely used for the correction of myopia.1–4 Binocular problems after PRK and LASIK have been reported.5–8 There are also studies that demonstrate improvements in binocular visual function after successful refractive surgery. We report a patient with intermittent left exotropia who initially exhibited successful alignment and improvement in stereoacuity after achieving emmetropia in both eyes following LASIK. However, the alignment was not preserved due to myopic regression in the right eye.
A 44-year-old white woman had uneventful LASIK at our clinic for the treatment of high myopia and anisometropia. The refractive error was −11.00 −0.50 × 130 in the right eye and −13.50 −1.50 × 145 in the left eye. Preoperatively, the best spectacle-corrected visual acuity (BSCVA) was 20/40 and 20/32, respectively.
An orthoptic examination showed free motility in all directions. Cover testing elicited an intermittent exotropia of 35 prism diopters (PD) at distance in the left eye that could be corrected with spectacles. She exhibited good near stereo acuity, which was tested with the TNO random-dot test, but also left suppression in the Worth 4-dot test at distance (6 m) preoperatively (Table 1). A fundoscopic examination showed a regular macula and myopic degeneration of the peripheral retina in both eyes. The central corneal thicknesses were 604 μm and 605 μm.
Laser in situ keratomileusis was performed using the Hansatome® microkeratome (Bausch & Lomb) and the SVS Apex Plus® excimer laser (Summit Technology, Inc.). The 160 μm base plate was used to cut a corneal flap 160 μm thick. Using the SVS Apex Plus excimer laser with 120 mJ/cm2 fluence and a 10 Hz repetition rate, a spherical ablation of −11.00 D was performed by the LASIK software in the right eye. An ablation of −13.00 D −1.50 × 145 was performed using toric PRK with pretreatment software by MO 1 disc in the left eye during the same surgical session.
On the first postoperative day, both corneas were clear and successful alignment was achieved with refractions of +1.25 in the right eye and +1.00 +0.75 × 70 in the left eye. A week after the LASIK procedure, examination showed exophoria at near and distance with +0.75 in the right eye and +0.50 +0.50 × 70 in the left eye. The uncorrected visual acuity (UCVA) was 20/50 in both eyes.
When the patient was seen 2 months later, she presented central fusion in the Worth 4-dot test at distance (6 m) and the UCVA was 20/40 in the left eye (+0.25 × 65). The UCVA remained 20/50 in the right eye, and the refraction was −0.75. Six months after LASIK, the patient complained that right-eye vision was not as good as that in the left eye and described symptoms of “burning and watering eyes” followed by “doubling and blurring” of print after reading for several minutes. She said that sometimes she felt she had a squint in the right eye. The examination revealed regression in the right eye with a refraction of −2.75 −0.25 × 70. There was no refractive or topographic evidence of iatrogenic keratectasia, and the videokeratographic evaluation showed regular ablation patterns in both eyes. The UCVA had decreased to 20/63 in the right eye but remained 20/40 in the left eye with a stable refraction. Right exotropia of 25 PD at distance was observed with the prism cover test. The deviation decreased to 15 PD with full correction. Exophoria was present at near fixation with or without glasses. The patient refused to have an enhancement procedure in the right eye or to have a strabismus procedure and preferred to use eyeglasses.
Laser in situ keratomileusis is an effective procedure for the correction of myopia and myopic anisometropia.8,9 Although there are some reports of binocular problems after LASIK,5–7 improvements in BSCVA and optical quality have also been reported.1 The exoangle is primarily a motor innervational disorder established during infancy and generally kept latent by sensorimotor innervation of binocular vision. The interplay of the motor etiology and the sensorimotor compensation allows a spectrum of possible clinical manifestations of intermittent exotropia.10 Once the tropic aspect of this divergent deviation occurs, it usually recurs in a pattern unique to the patient throughout his or her life. The varied recurrence pattern of the exotropia is the feature that distinguishes 1 case from another. The patient we report is a distinctive case as she had left intermittent exodeviation because the anisometropic refractive error was not fully corrected during childhood. Although LASIK treatment effectively improved the patient's visual quality and sustained fusion for some time, the reduction in visual performance aggravated the tropic phase in the opposite eye. The tropic phase of intermittent exotropia is provoked by many factors. Since binocular vision is the essential compensatory sensorimotor function, anything that disturbs this mechanism may cause exotropia to replace exophoria.
Although the myopic regression rate after LASIK is more prominent with higher refractive errors,11 in our case the regression occurred in the eye with the lower refractive error and the better fixation. Unfortunately, the outcome of a refractive procedure in each eye can vary significantly. In this case, the lack of correlation between the 2 eyes may be related to the use of different treatment nomograms.
This case emphasizes the importance of a similar optical quality in each eye to maintain the compensatory sensorimotor factor. Relatively low degrees of anisometropia that have not been fully corrected will likely lead to poor optical quality and perhaps aniseikonia, which can disrupt fusion.7 As observed in this case, in patients whose sensorimotor compensation for the divergent deviation deteriorates to this extent, the last stage of their intermittent exotropia is usually characterized by maintenance of the phoric phase for near only.
In conclusion, we consider LASIK an effective option for maintaining binocular visual quality in myopic anisometropic patients. Binocular visual quality plays a compulsory function in keeping intermittent exotropia latent by sensorimotor innervation. However, caution is indicated in myopic eyes worse than −12.00 D as there is a higher regression rate, which may disrupt the compensatory system.12
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