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Adam L. Prickett, Wallace Chamon, Kelly M. Bui, Joelle Hallak, Pejman Bakhtiari, Dimitri T. Azar; A Novel Approach To Determine Theoretical Head Tilt Effect On Ocular Cyclotorsion Measurements During Laser Refractive Surgery. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5570.
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To determine the degree and pattern of cyclotorsion in paired eyes undergoing Laser refractive surgery between the time of wavefront measurement and laser surgery while accounting for theoretical head tilt.
Direction and degree of cyclotorsion were measured in 110 eyes of 55 patients undergoing bilateral sequential Laser refractive surgery with the VISX STAR S4 excimer laser system. Preoperatively, the WaveScan Wavefront system was used to record an iris image of each eye with the patient seated. This image was then compared to the image obtained prior to laser ablation, with the patient in supine position, under the laser. Presumed head tilt was determined on an individual patient basis by calculating the average cyclotorsion of both eyes. This approach assumes that if both eyes of the same patient rotated the same amount in the same direction (clockwise, CW or counter-clockwise, CCW) the amount of cyclotorsion measured was entirely due to head tilt. Adjusted cyclotorsion was calculated by subtracting the presumed head tilt for each eye of the same patient.
For the unadjusted data, 41.8% percent of patients had bilateral excyclotorsion, while only 10.9% had bilateral incyclotorsion. The number of patients with comitant rotations was 30.9% CCW and 16.4% CW. After adjusting for presumed head tilt on an individual basis, 70.9% of patients had bilateral excyclotorsion, while only 25.5% had bilateral incyclotorsion. No relative movement was observed in 3.6%. The average presumed head tilt was found to be 0.83 degrees CCW (Standard Deviation: 2.6) and ranged from 6.85 CCW to 5.25 CW.
In this preliminary study, after adjusting for presumed head tilt (comitant eye rotations), a large majority of patients presented bilateral excyclotorsion between seated and supine positioning. The physiologic basis for this observation is unclear, but may be related to decreased relative tone of the incyclotorting muscles relative to the excyclotorting muscles in the supine position, or to the placement of a lid speculum causing slight excyclotorsion. Given that primary eye positioning is difficult to guarantee, bilateral cyclotorsion measurements for unilateral refractive surgery is recommended, as well as for toric IOL implantation. Additional analysis using a larger pool of data is needed.
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