Howard Ying
Johns Hopkins

Superior oblique muscle palsy or paresis (SOP) is classically defined by the ocular movement abnormality consisting of hypertropia of the affected eye in straight ahead gaze, with greater hypertropia in adduction and ipsilateral head tilt (Parks’ 3-step test).  Emerging lines of evidence, however, suggest that this triad of ocular movement abnormalities may not always represent a clear-cut neuromuscular deficit, but may also result from supranuclear neural or other extramuscular anatomic mechanisms.  It is well known that skew deviation of vestibular, brainstem, or cerebellar origin may present similarly but shows fundus intorsion of the hypertropic eye whereas SOP shows fundus extorsion of the affected or hypertropic eye.  Other “SOP imposters” have been discovered by careful examination of the orbit during surgery.  Anatomic orbital anomalies such as excess tendon laxity, trochlear pulley heterotopy, and anomalous tendon insertions may mimic true SOP, and, if so, should be directly addressed in the surgical correction.  Recent imaging studies which show no discernable anatomic orbital anomalies and normal superior oblique muscle size and contractility suggest the existence of other “SOP imposters.”  In order to test these mechanisms, we have developed protocols to examine 3-axis eye movements in three groups: 1) in normal subjects after cyclovertical adaptation paradigms, 2) in patients with “SOP,” and 3) in primates with acquired true SOP.  From these studies, we found that physiologic adaptation can mimic SOP in normal subjects, underscoring the importance of neural mechanisms. We also further expanded the ocular motor ‘signature’ of acquired SOP, which may encompass adaptive responses of the other cyclovertical effectors.  With better understanding of the mechanical and adaptive neural mechanisms that underlie this ocular movement abnormality, we may explain the heterogeneity of clinical findings, reduced response rate to standard surgery, and propose mechanism-directed solutions to this enigmatic disorder.
 
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