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P.C. Knox, S.I. Chen, I.B. Marsh, L.M. Gannon; Extraocular Muscle Surgery, Muscle Slippage and their Effects on Saccadic Eye Movements . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1967.
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Purpose: Slippage of an extraocular muscle, where the muscle slips back within its fascial sheath, is a potentially important and under-diagnosed complication of surgery involving the extraocular muscles. We investigated a number of putative tests for the detection of slipped muscles (SM) and quantitatively examined saccadic eye movements in four adult patients who had previously undergone surgery and had a longstanding, suspected SM. Methods: Four adult patients were investigated before and two-weeks after corrective surgery. After a full clinical examination (which included measurement of palpebral fissure dimension and differential IOP), infrared oculography was used to record saccadic eye movements in response to targets presented at 2°, 5°, 10° and 15° to the left and right of fixation. Eyetracker output was digitised with 12 bit precision at 1kHz and written to computer hard disk for offline analysis in which we measured the main sequence parameters, and scored distortions observed in saccade velocity profiles. Intraoperatively, a forced duction test, direct visualisation and measurement, and histopathology were all used to establish the presence or absence of SM. Results: Intraoperative examination confirmed that in two of the four patients, one of the extraocular muscles had slipped within its fascial sheath. IOP measurement and palpebral fissure measurements performed preoperatively did not accurately predict these findings. Saccade peak velocity measurements were difficult to calibrate across all patients and eyes. There was limited evidence of muscle slippage causing reductions in peak saccade velocities and increases in saccade duration in the direction of the line of action of the slipped muscle compared with the opposite direction. We also observed distortions in saccade velocity profiles consisting of double velocity peaks and unexpected decelerations and accelerations. Distortion scores were significantly higher for eyes in which there had been previous surgery (p<0.001). Conclusions: A number of suggested preoperative tests failed to accurately detect the presence of SM. In our patients, all of whom had longstanding muscle slippage, even saccade peak velocity measurements were of limited value. A clearer finding was the distortion of saccade velocity profiles. It remains to be seen whether these are due to muscle surgery alone, or develop in response to compensation for the effects of surgery and/or SM. However, velocity profile distortions may be a useful indicator of muscle damage.
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