Primary oblique muscle overaction - The brain throws a wild pitch

Background: Sensorimotor and orbital anatomical mechanisms have been invoke d to explain primary oblique muscle overaction. Methods: Review of primitive visuo-vestibular reflexes and neuroanatomical pathways corresponding to vestibulo-ocular reflexes, and correlation with k nown clinical abnormalities in patients with primary oblique muscle overact ion.. Results: Bilateral superior oblique muscle overaction, which corresponds to a backward pitch in lateral-eyed animals, can occur when structural lesion s involving the brainstem or cerebellum increase central otolithic input to the extraocular muscle subnuclei that modulate downward extraocular muscle tonus. Bilateral inferior oblique overaction, which corresponds to a forwa rd pitch in lateral-eyed animals, may result from visual disinhibition of c entral vestibular pathways to the extraocular muscle subnuclei that modulat e upward extraocular muscle tonus. Conclusions: Primary oblique muscle overaction recapitulates the torsional eye movements that occur in lateral-eyed animals during body movements or d irectional luminance shifts in the pitch plane, These primitive ocular moto r reflexes become manifest in humans when early-onset strabismus or structu ral lesions within the posterior fossa alter central vestibular tone in the pitch plane.