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December 4, 2012
Fourth nerve palsies
Lesions of the fourth (trochlear) cranial nerve cause vertical or oblique diplopia which worsens with head tilt toward the affected eye. Diagnosis of a 4th nerve palsy can be made using the Parks–Bielschowsky three–step test: (1) determine which eye is hypertropic, (2) determine if the hypertropia worsens in left or right gaze, and (3) determine if the hypertropia worsens in right or left head tilt. In a superior oblique palsy, the hypertropia of the affected eye worsens with contralateral gaze and ipsilateral head tilt. Although the fourth nerve is most commonly injured peripherally along its intracranial course (the longest of all cranial nerves), the fourth nerve nucleus or fascicle may be implicated, resulting in central fourth nerve palsy. [The fourth nerve is unique among the cranial nerves in that all of its fibers are crossed at the peripheral nerve level. Consequently, a lesion of the fourth nerve nucleus results in a superior oblique palsy of the contralateral eye.
Submitted by Chafic Karam, MD
Disclosure: Dr. Karam serves on the editorial team for the Neurology® Resident and Fellow Section.
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