In the past decade, photorefractive keratectomy with excimer laser technology has opened a new range of possibilities for the treatment of myopia and myopic astigmatism (photoastigmatic refractive keratectomy [PARK]). Studies of the results of PARK report a mean residual cylinder of approximately −0.60 diopter (D) at a follow-up of 3 to 6 months.1,2 In addition, Taylor and coauthors1 performed vector analysis to assess the change in astigmatism. They report a mean anticlockwise angle of error of 7.6 degrees ± 26.6 (SD) at 1 month, changing to a mean clockwise error of −1.1 ± 23.9 degrees at 6 months. Overall, at a follow-up of 6 months, 85% of patients having PARK were within ±1.0 D of plano refraction.1 At 1 year, 81% of patients were within ±1.0 D of the target refraction3 and at 18 months, approximately 70%.4
Despite these results, astigmatic undercorrection with PARK has been reported to present a persistent problem, limiting the predictability of the approach. Axis misalignment is a potential cause of residual astigmatism. Possible causes of axis misalignment include incorrect preoperative refraction, misalignment of the patient's head or the laser beam, cyclotorsion of the eye, or movement of the eye during laser treatment.2 Moreover, a 15-degree angle of error results in a 50% reduction in the magnitude of astigmatism corrected.5
In this study, we assessed the occurrence and degree of cyclotorsion of the eye. In our clinic's protocol for PARK treatment, patients initially have seated automated keratometry and/or corneal topography measurements with both eyes fixating. This is followed by the laser treatment in a supine position fixating with 1 eye only. We measured the astigmatism of the eye under the 2 fixation modes and/or body positions.
Subjects and Methods
The study comprised 15 normal subjects (5 men, 10 women; mean cylindrical refractive error −1.2 D). Subjects had a full orthoptic examination to rule out hidden pareses or palsies. All were seated in a dentist chair with the head fixed in a foam headrest. Subjects were asked to fixate with both eyes on a distant target at 2 meters while the right eye was measured with a handheld automated keratometer (Nidek Auto keratometer, model KM-500). The measurements were repeated in the same eye but with monocular fixation. For measurements in the supine position, the dentist chair was tilted with the head fixed and measurements were again performed under binocular and monocular fixation. Each measurement was repeated twice. There were 3 repeated keratometry measurements and a mean measurement, provided by the Nidek handheld keratometer, for each of the 4 experimental conditions.
Sixty measuring sessions consisting of 3 measurements each were used to determine the limits of agreement for the repeatability of measurements6 with the Nidek handheld keratometer. The limits of agreement indicate the 95% confidence interval for the repeatability of measurements: If a measurement is outside the limits of agreement, there is a 95% chance that the measurement indicates real change and is not due to measurement error. We compared the mean measurements with the limits of agreement and identified the cases showing real change upon changed measuring conditions.
To examine any relationship between body position and fixation mode, a multivariate analysis of variance (MANOVA) of the mean measurements was done. A P value less than 0.05 was considered statistically significant.
The limits of agreement for the repeatability of measurements in this study was 13 degrees. Two subjects (13.3%) showed more than 13 degrees of excyclotorsion (18 degrees and 31 degrees) with monocular fixation after binocular fixation in a seated position (Table 1). In a supine position, 3 subjects (20%) showed an excyclotorsion greater than 13 degrees (13 degrees, 21 degrees, and 22 degrees) with monocular fixation after binocular fixation (Table 1). Two of the subjects were the ones who showed excyclotorsion when altering fixation modes in the seated position. When the binocular measurements of the seated and supine positions were compared, none of the subjects had a torsion greater than 13 degrees. In the monocular measurements, 1 subject had an excyclotorsion of 15 degrees when changing from a seated to a supine position and 1 subject showed an incyclotorsion of 15 degrees under the same conditions.
With MANOVA analysis, there was no statistically significant relationship between fixation mode and body position.
We found an excyclotorsion greater than 13 degrees in 13% to 20% of the subjects when fixating with 1 eye instead of both. Cyclovergence has been found to occur as a component of cyclofusion.7 Motor cyclofusion of up to 6 to 8 degrees has been measured,7 whereas sensory cyclofusion has been presumed to reach a maximum amplitude of about 8 degrees.8 The total amplitude of cyclofusion, therefore, can be as high as 15 degrees.8
In our study, 2 subjects consistently had more than 15 degrees of excyclotorsion in seated and supine positions. If they had been eligible for PARK, the cyclotorsion could have resulted in significant undercorrection. Vajpayee and coauthors4 evaluated the efficacy of an in situ axis alignment for the treatment of myopic astigmatism. In a prospective trial, patients, positioned beneath the laser system, were asked to rotate an axis-alignment dial into sharpest focus with the eye to be treated. In 85% of the eyes, the axis of cylinder was different from that calculated by routine refraction. In 10% of the eyes, there was more than a 10-degree difference from the manifest subjective refraction. At 6 months, the refractive outcome was not significantly different from that in a control group, although there was some indication that the group treated with the axis-alignment system had a higher percentage of patients with a visual acuity of 6/6 or better. However, this study was performed under monocular viewing conditions. We speculate that if binocular and monocular viewing conditions had been compared, the results might have been different.
The results of the measured axes of cylinder in the various body positions was much less consistent. When subjects moved from a seated to a supine position under binocular viewing conditions, we found no significant torsion. Under monocular viewing conditions, 1 subject had excyclotorsion and 1 had incyclotorsion. Smith and Talamo9 found that body position had no influence on ocular cyclotorsion. They used Maddox double-rod testing to assess ocular torsion in the seated and supine patient. However, all their observations were performed under binocular viewing conditions, which might have precluded the notice of torsional movements under monocular viewing conditions.
The Nidek handheld automated keratometer was used for all measurements in this study. To validate the Nidek instrument itself, its readings were compared with those of a table-mounted Zeiss keratometer.10 There were no statistically significant differences in keratometry readings between the 2 instruments, although the Nidek keratometer had more axis variability.10 However, the range of axis measurements using the Nidek keratometer was less than 10 degrees in 80% of patients and less than 20 degrees in all patients.10 We found a 95% confidence interval for the repeatability of measurements of 13 degrees and used this number as the cutoff point in this study. If we had used the above-mentioned 20 degrees as a cutoff point, 7% to 13% of our subjects would have shown excyclotorsion as they proceeded from binocular to monocular fixation. Recently, video image-processing devices have been described for photographic documentation of ocular torsion. By using the iris striations as landmarks, these devices may allow more accurate assessment of the amount of cyclotorsion.11
The number of subjects in our study was small, yet there was an indication that cyclovergence is an essential part of the fusion mechanism in individuals. Because the total amplitude of cyclofusion can be as much as 15 degrees, it can be appreciated why pure torsional diplopia in patients is scarce.8 In addition, it has been reported that voluntary torsion can be trained, with amplitudes up to 30 degrees.12
Caution should be taken when photoastigmatic refractive keratectomy, either as PARK or under a stromal flap as laser in situ keratomileusis, is planned in individuals who show significant torsion under monocular viewing conditions. Individuals at risk are those with dissociated vertical deviation, ocular motor pareses (trochlear nerve palsy), torsional and/or latent nystagmus, latent cyclophoria, or vestibular diseases. Keratometry or corneal topography measurements are recommended under binocular and monocular viewing conditions to identify individuals with significant monocular torsion. Significant torsion can be anticipated when a discrepancy between the subjective monocular refraction and the binocular keratometry readings is noticed in the axis of astigmatism In addition, full orthoptic examination should be considered. During the procedure, attention should be addressed to possible tilting of the patient's head, unintentional rotation of the microscope, or distortion of the globe by a lid retractor.9 Moreover, surgeons may consider marking the axis of astigmatism before performing PARK to ensure correct alignment of the laser beam. Meticulous attention should help to limit residual astigmatism in photoastigmatic refractive keratectomy.
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