Purpose: To estimate the frequency of slipped extraocular muscle (SM) in cases of repeat strabismus surgery and to compare the results of a number of putative preoperative tests for the detection of SM, with direct intraoperative inspection and histologic confirmation.
Design: Retrospective analysis of audit records followed by prospective interventional case series.
Participants: Case records of 715 adults presenting for repeat surgical correction of horizontal strabismus. Five patients suspected of having horizontal SM were consecutively recruited for the prospective case series.
Methods: Slipped extraocular muscle frequency was determined retrospectively from surgical outcome data in 715 adult horizontal strabismus surgeries in 1 center. A separate prospective study examined 5 participants with putative SM in detail. Gaze-dependent clinical tests (squint magnitude, eye movement range, palpebral fissure widening, naked-eye saccadic velocity estimation, intraocular pressure change) and saccade main sequence parameters (infrared eye-tracking) were recorded before and 2 weeks after corrective surgery. Intraoperative diagnosis was made by direct muscle examination and confirmed by histologic examination. Length of slippage and forced duction tests were intraoperative outcome measures.
Results: Slipped extraocular muscle frequency was 10.6%. The muscles of 3 participants in the prospective series were slipped. Direct intraoperative inspection accurately identified all SM cases. All other clinical tests produced false-positive and false-negative results, although SM was suggested preoperatively by limited eye movement range. Distorted saccadic velocity profiles were significantly (P<0.0001) associated with prior strabismus surgery. Saccade main sequence parameters were not diagnostic for SM.
Conclusions: Direct intraoperative inspection can accurately diagnose SM. Preoperative diagnosis of long-standing SM was not possible. In contradiction to a previous report, SM was not reliably associated with reduced peak saccadic velocity. The association between distorted saccadic velocity profiles and previous strabismus surgery is, to our knowledge, reported here for the first time.