decussate in the anterior medullary velum (the roof of the aqueduct) just caudal to the inferior colliculus. It emerges for the dorsal surface of the lower midbrain as one or more rootlets that leave the midbrain contralateral to their nucleus of origin.66 The cisternal segment of the trochlear nerve extends anteriorly over the lateral surface of the brainstem. It lies adjacent to the free edge of the tentorium cerebellum and passes between the posterior cerebral and the superior cerebellar arteries. Because of its small caliber and hidden location under the tentorial edge, the cisternal portion of the nerve can be easily injured during neurosurgical procedures to treat tumors or aneurysms when the surgery involves manipulation of the tentorial edge.554 It then travels along the free edge of the tentorium to pierce the dura along the lateral aspect of the clivus to enter the cavernous sinus. The trochlear nerve lies just inferior to the oculomotor nerve in the lateral wall of the cavernous sinus. It enters the orbit through the superior orbital fissure but remains outside the annulus of Zinn along with the lacrimal and frontal nerves. In the orbit, it runs anteriorly and medially beneath the superior periorbita to cross over the superior rectus muscle as a single fascicle just before it enters into the superior nasal portion of the superior oblique muscle.138

Clinical Features

Head Posture

Ambulatory children who develop acute fourth nerve palsies are frequently noted by their parents or teachers to have adopted a head tilt (which is almost always to the side opposite the palsied eye). Kraft et al299 found an incidence of compensatory head posture in 71.2% of 139 patients with superior oblique paresis. Similarly, children with congenital trochlear nerve palsy often tilt their heads, but because they do it from infancy, it is more readily overlooked. In some infants, the head tilt is heralded by a body tilt that begins within the first month of life, indicating that the baby learns to use gravity to passively tilt the head in a compensatory fashion.79 Some turn their heads away from the side of the palsied muscle to eliminate an incomitant hypertropia, while others maintain a combined head tilt and head turn.

Some children with congenital or acquired trochlear nerve palsy are brought to medical attention because of vertical diplopia associated with a hypertropia of the affected eye. On testing of versions, the hypertropia is found to decrease in horizontal gaze toward the affected eye and to increase in horizontal gaze away from the affected eye due to the increasing vertical action of the oblique muscles in adduction. The primary position hypertropia is often greater at distance than near and increases more at near when a prism adaptation test is performed to correct the vertical deviation.411 The three-step

test shows an increase in the vertical deviation on head tilt toward the side of the affected superior oblique muscle and a decrease in the vertical deviation when tilting away from the affected side. Orbital MR imaging may show the superior oblique tendon to be present, hypoplastic (in congenital cases), atrophic (in acquired cases), or absent. MR imaging and tendon anomaly associated with congenital trochlear nerve palsy (Fig. 6.9).96,482,484 Sato et al found a greater reduction in superior oblique muscle volume in congenital (65.8% reduction) than in acquired cases (45.3% reduction),484 and found an absence of the superior oblique tendon on MR imaging to be predictive of a larger primary position vertical deviation.482 However, Chan and Demer96 found the clinical findings to be indistinguishable in children with present and absent tendons. Kono and Demer295 found the superior oblique muscle contralateral to the palsied eye to be hypertrophied relative to normal controls. Kushner313 has attributed this phenomenon to the innervational effects of a chronic head tilt toward the side of the normal eye, which provides tonic innervation to the normal superior oblique muscle. Enlargement and increased contractility of the contralateral inferior rectus muscle have also been described.255 However, MR imaging shows no evidence of inferior oblique muscle hypertrophy.295

Even in acute cases of trochlear nerve palsy, when there has been insufficient time for an inferior oblique contracture

Fig. 6.9  Congenital left superior oblique palsy. Coronal MR image shows selective hypoplasia of left superior oblique muscle. With permission from Brodsky and Karlsson79