Pearls and oy-sters: central fourth nerve palsies.

CLINICAL PEARLS Lesions of the fourth (trochlear) cranial nerve cause vertical or oblique diplopia by impairing the ability of the superior oblique muscle to intort and depress the eye. This binocular diplopia worsens in downgaze and lateral gaze away from the affected eye. Because intorsion is necessary to maintain fusion in ocular counter-roll, this diplopia also worsens with head tilt toward the affected eye. Diagnosis of a superior oblique palsy can be made using the Parks-Bielschowsky 3-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. Alternate cover, cover-uncover, andMaddox rod testing can be helpful examination techniques. 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 a central fourth nerve palsy. The fourth nerve nucleus is located within the midbrain adjacent to periaqueductal gray matter and dorsal to the medial longitudinal fasciculus (MLF) at the level of the inferior colliculus. After leaving the nucleus, the fascicles of the fourth nerve decussate in the anterior medullary velum at the roof of the aqueduct of Sylvius then exit the brainstem dorsally (figure 1). 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. While isolated central fourth nerve palsies have been reported, lesions of the fourth nerve nuclei or fascicles typically also affect adjacent brainstem structures. A central fourth nerve palsy, therefore, should be suspected whenever a fourth nerve palsy is accompanied by brainstem signs, and the location of the lesion should be presumed to lie within the dorsal midbrain contralateral to the affected eye. Central lesions within the dorsal midbrain may also result in bilateral fourth nerve palsies. Clinical features suggestive of bilateral fourth nerve palsies include right hypertropia in left gaze, left hypertropia in right gaze, and alternating hypertropia with head tilt to either side (i.e., right hypertropia with right tilt and left hypertropia with left head tilt).