Trochlear Nerve Cranial Nerve IV

Cranial nerve IV is unique in that it exits from the brain stem dorsally and crosses to the other side before encircling the brain stem on the way to the cavernous sinus. This anatomy renders it particularly susceptible to trauma in which forces are brought to bear on the dorsal midbrain. This situation usually occurs in the setting of very severe head trauma in which the brain stem is forced downward and angulated backward by a sudden shift of supratentorial structures. The course of cranial nerve IV in the subarachnoid space is relatively protected from compressing lesions by the adjacent free edge of the tentorium. Within the

Figure 9-5 View of the posterior orbit showing the origins of the extraocular muscles and their relationships to the optic and ocular motor neReproduced with permission from Miller NR: Walsh and Hoyt's Clinical Neuro-Ophthalmology, Vol 2, 4th ed. Baltimore, Williams & Wilkins, 1985.)

cavernous sinus, cranial nerve IV can be found in the deep layer of the lateral wall inferior to cranial nerve III (see Fig 9-4 ). Cranial nerve IV enters the orbit through the superior orbital fissure superior to cranial nerve III but outside the annulus of Zinn (see Fig.: 9-5 ). It lies in the superior orbit, crosses over the superior rectus muscle, and innervates the superior oblique muscle. M

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