Aside from the olfactory cranial nerve (CN I) and the optic nerve (CN II), the remaining cranial nerves (CN III–XII) have nuclei in the brainstem. These then have axons extending through the brainstem (the fascicle), exiting into the subarachnoid space, and on to individual paths to exit the skull. Since some cranial nerves run near each other either in the brainstem or as they exit the skull, particular combinations of cranial neuropathies can suggest a precise anatomic localization. Similarly, the combination of cranial nerve dysfunction and either long tract dysfunction (hemiparesis, hemibody sensory loss) or cerebellar dysfunction strongly suggests a brainstem lesion. When a combination of motor cranial neuropathies does not suggest a precise anatomic localization, then myasthenia gravis should be considered and, less commonly, the Miller Fisher variant of Guillain-Barré syndrome, botulism, or a subarachnoid space process. When a combination of motor and sensory cranial neuropathies does not suggest a precise anatomic localization, a subarachnoid space process is likely.
The oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nerves and the ophthalmic (V1 segment of CN V) and maxillary divisions of the trigeminal nerve (V2 segment of CN V) all run through the cavernous sinus. Pathology there can subsequently affect any combination of these nerves. After passing though the cavernous sinus, the maxillary division of the trigeminal nerve (V2 segment of CN V), which receives sensation from the upper lip to the bridge of the nose, exits the skull through the foramen rotundum. CN III, IV, VI, and the V1 segment of V (which receives sensation from the bridge of the nose to the forehead and posterior scalp) continue on through the superior orbital fissure to enter the orbital apex. Therefore, superior orbital fissure pathology can affect CN III, IV, VI, and the V1 segment of CN V, but not the V2 segment of CN V. The optic nerve travels through the optic canal to enter the orbital apex, where it is in close proximity to CN III, IV, VI, and the V1 segment of CN V. The optic nerve does not run through the superior orbital fissure or the cavernous sinus. Therefore, optic nerve involvement, such as monocular vision loss or an afferent pupillary defect, in conjunction with eye movement abnormalities from CN III, IV, or VI dysfunction, implicates orbital apex pathology.
Note that numbness from the bridge of the nose to the forehead and posterior scalp (V1 segment of CN V) may also be present with orbital apex pathology.
Neurologic findings of eye movement abnormalities or dysarthria/dysphagia in myasthenia gravis may mimic cranial neuropathies, but are actually due to the more diffuse process of impaired function at the neuromuscular junction.
The fascicle of CN VI travels ventrally in the pons exiting near the corticospinal tract. The facial nerve (CN VII) exits the ventral and lateral pons. A lesion of the ventral lateral pons will cause horizontal diplopia from an ipsilateral abduction deficit, ipsilateral facial weakness involving the forehead, and a contralateral hemiparesis (Millard-Gubler syndrome).
A dorsal lateral medullary (Wallenberg) syndrome usually involves the nucleus ambiguous, which provides motor axons to the glossopharyngeal (CN IX) and vagus (CN X) nerves, and the cranial portion of the spinal accessory (CN XI) nerve. These axons innervate most of the muscles of the palate, pharynx, and larynx, and a lesion of the nucleus ambiguous or its axons produces dysarthria and dysphagia. A dorsal lateral medullary syndrome can also involve the inferior and medial vestibular nuclei, which can lead to vertigo; the spinal nucleus of the trigeminal nerve, which mediates pinprick and temperature sensation from the ipsilateral face; and the ascending spinothalamic tract, which mediates pinprick and temperature sensation from the contralateral body. The descending sympathetic neurons can also be involved, resulting in an ipsilateral Horner syndrome.
The glossopharyngeal (CN IX), vagus (CN X), and spinal accessory (CN XI) nerves exit the skull through the jugular foramen. The hypoglossal nerve (CN XII) exits the skull through the hypoglossal canal. These four cranial nerves then run together in the retroparotid space, as does the carotid artery. In this context, the combination of dysarthria and dysphagia, weakness of contralateral head turn, and ipsilateral trapezius elevation can occur with lesions in either the jugular foramen or the retroparotid space. The presence of ipsilateral tongue weakness (which causes deviation away from the affected side) secondary to hypoglossal nerve dysfunction or a Horner syndrome from a lesion involving the ascending sympathetic neurons on the carotid artery localizes the lesion to the retroparotid space. For the reasons previously stated, a jugular foramen syndrome would be considered in a patient with dysarthria, dysphagia, weakness of contralateral head turn, and ipsilateral trapezius elevation without tongue weakness or a Horner syndrome.