CRANIAL NERVE III PALSY
Lesions of the oculomotor nerve can involve the nucleus in the midbrain, or nerve fascicles within the ventral midbrain, subarachnoid space, cavernous sinus, superior orbital fissure, or orbit. As an example, in the setting of an isolated right nuclear third nerve lesion (see Fig. 9-7 (Figure Not Available) ), there is complete disruption of the outflow of that nerve. Additionally, fibers from the left subnuclei for the superior rectus (contralateral outflow) and levator palpebrae (bilateral outflow) also are affected as they pass through the right-sided lesion. The combination of nuclear and fascicular damage results in partial bilateral ptosis and failure of elevation of both eyes. The ptosis contralateral to the lesion is usually incomplete because the levator receives some nondecussating neuronal fibers from the normal side of the caudal central nucleus. More ventral lesions in the brain stem only cause disruption of the fibers emerging from the ipsilateral third cranial nerve nucleus. The clinical characteristics of these fascicular third nerve lesions are the same as those of any lesion distal to the nucleus, and only the
functions of that ipsilateral nerve are affected. Localizing the lesion along the fascicular portion of the nerve depends on the presence of associated neurological features. Nuclear lesions are caused primarily by small infarcts secondary to occlusion of the medial penetrating vessels from the basilar artery (see Chapter22 ), '571 or rarely by small hemorrhages. Other intramedullary lesions associated with cranial nerve III dysfunction include masses, including neoplastic (lymphoma, metastatic), inflammatory (sarcoidosis), and infectious (tuberculosis, fungal) etiologies.
Cranial nerve III palsy with concomitant involvement of other CNS structures leads to a constellation of findings that have been given various eponyms and usually are seen as stroke syndromes (see ChapteL.2.2 ). Aneurysms of the posterior communicating artery are an important cause of third cranial nerve lesions from compression of the nerve outside the CNS. Compression can occur as the nerve passes near the vessel, at its junction with the supraclinoid portion of the internal carotid artery near the cavernous sinus. '41] Cranial nerve III palsy from an enlarging or bleeding saccular aneurysm of the posterior communicating artery is a neurological emergency, because the aneurysm is unstable and a fatal subarachnoid hemorrhage may occur. Clinical findings generally include third nerve innervated extraocular muscle dysfunction with pupillary sphincter paralysis. The latter dysfunction results from the compression of superficial parts of the nerve where the parasympathetic fibers are located.
In contrast, third nerve lesions from diabetes or hypertension '421 , y typically spare pupillary function. These disorders are associated with small vessel arteriosclerosis and affect deeper parts of the third nerve. Involvement of the pupillary sphincter is more variable in inflammatory and neoplastic compressive disorders, but typically there is some degree of dysfunction. When attempting to determine whether a pupil-sparing cranial nerve III palsy is due to a medical or nonaneurysmal cause, knowledge of the degree of extraocular muscle dysfunction relative to the degree of internal ophthalmoplegia can be helpful. '41 A medical or ischemic cranial nerve III palsy is most likely when there is a total or near-total external ophthalmoplegia and complete or near-complete sparing of the pupillary sphincter. With incomplete external involvement, pupillary function is much less reliable as an indicator of whether the problem is caused by aneurysm or intrinsic ischemia.
In the setting of major head trauma, the dorsal midbrain and both fourth nerves are impacted in the niche of the tentorium cerebelli, and both nerves tend to be contused together. Because of bilateral injury to the ascending reticular formation, the patient is usually unconscious for a protracted period of time after the injury, following which there are complaints of vertical double vision. On examination, a relative elevation of the right eye (right hypertropia) in left gaze and relative elevation of the left eye (left hypertropia) in right gaze can be found. This reversal of vertical deviation indicates bilateral fourth cranial nerve palsies. Unilateral fourth nerve palsy sometimes follows minor head trauma, and there is reason to believe that many of these cases represent decompensation of longstanding or congenital fourth nerve palsy that was not symptomatic. The mechanism of this occurrence is unclear, but it is important to ask patients with this disorder to bring in childhood photographs to search for head tilt that would indicate a congenital condition.
Fourth cranial nerve palsies can also occur secondary to intraneuronal microvascular disease in elderly patients and are often associated with diabetes or longstanding hypertension, just as with ischemic third or sixth cranial nerve palsies. The diplopia with ischemic fourth nerve involvement tends to improve over a period of several weeks or months following onset. Other causes of fourth nerve dysfunction include tumors in the region of the midbrain tectum and cavernous sinus, superior orbital fissure, and orbital disorders (see the section entitled Cavernous Sinus Syndrome).
The abducens nucleus can be the site of a destructive lesion, particularly small vessel infarction or small focal hemorrhages. In these settings, ipsilateral paralysis of conjugate horizontal gaze results. In addition, peripheral cranial nerve VII palsy also occurs because the fascicles of nerve VII wrap over the superior aspect of the cranial nerve VI nucleus, forming a small bulge in the adjacent floor of the fourth ventricle (see Chapter11 ). Nuclear cranial nerve VI lesions are also associated with more severely affected abduction of the ipsilateral eye than adduction of the contralateral eye, resulting in an esotropia and asymmetrical duction deficits. This phenomenon may be difficult to distinguish from a combined nuclear and fascicular lesion or separate lesions at the different sites. As with the third and fourth cranial nerves, fascicular abducens nerve palsies can occur secondary to microvascular lesions in hypertensive and diabetic patients, in which case the abduction deficit tends to improve over a 3- to 6-month period. Conditions that can cause sixth cranial nerve dysfunction include demyelination, neoplasia, inflammatory and infiltrating lesions of the cavernous sinuses, and leptomeninges (carcinomatous meningitis, chronic or acute infectious meningitis). Because of its long intercranial course along the bony ridges of the calvarium, the sixth cranial nerve is also susceptible to stretching and distortion more than other cranial nerves are. This phenomenon is characterized by failure of abduction in one or both eyes accompanying lesions remote from the sixth cranial nerve or the lateral rectus. This falsely localizing sixth nerve paresis occurs most commonly in the setting of raised intracranial pressure and as a transient phenomenon following lumbar puncture. This has been postulated to occur as a result of a transient shift of the brain stem secondary to changes in CSF pressure gradients. This unique susceptibility to small brain stem displacement is thought to result from the nerve's being fixed on one end at its emergence from the pons and at the other end, at Dorello's canal in the petrous tip.
INTERNUCLEAR OPHTHALMOPLEGIA AND THE MEDIAL LONGITUDINAL FASCICULUS SYNDROMES
Lesions affecting the MLF cause failure of the adducting eye to move, whereas the abducting eye deviates laterally
to its full extent but has accompanying nystagmus during conjugate deviation. The medial rectus adducts without difficulty during convergence, distinguishing the paresis from a third nerve or muscle lesion.
This striking pattern of disconjugate eye movements is referred to as INO because the lesion disconnects the sixth and the third cranial nerve nuclei by causing failure of neural conduction in the internuclear pathway, the MLF. The peculiar monocular nystagmus can be either transitory (1 or 2 beats) or sustained. When the degree of dysfunction in the MLF is mild, the adducting eye may deviate fully but cannot attain the proper velocity during a saccadic refixation. In this case, there is a noticeable dissociation between the two eyes during the saccade, because the abducting eye completes the movement earlier than the adducting eye. To observe this phenomenon, it is necessary for the examiner to watch both of the patient's eyes simultaneously. This is best accomplished by looking at the bridge of the patient's nose from a distance of about 2 meters, so both of the patient's eyes can be clearly seen and differences can be appreciated in trajectory as the patient makes saccades to the right and left.
The clinical importance of diagnosing the MLF syndrome is its exquisite localizing value for lesions deep in the substance of the brain stem tegmentum. This general area in the brain stem contains the ascending reticular activating system, which is necessary for consciousness, along with several adjacent cranial nerve nuclei, and various ascending and descending sensory and cerebellar pathways. Therefore, the isolated occurrence of an MLF syndrome in an alert individual without other brain stem signs or symptoms suggests the presence of a highly discrete lesion. In the adult, this is caused by either small demyelinating plaques of multiple sclerosis, by small infarctions due to small vessel disease, and very occasionally can be encountered in the setting of head trauma. In children, the MLF syndrome can be the first sign of a brain stem glioma.
Differentiating between multiple sclerosis (see Ch.a.píe.L48 ) and small vessel infarctions (see Chapter^ and Chapter45. ) can be problematic, because both tend to evolve acutely or subacutely and resolve over a period of days or weeks. Patients with multiple sclerosis are typically younger than 40 years old, whereas most individuals with infarctions are older than 60 years. Bilateral MLF lesions indicate multiple sclerosis, because a plaque may occur in the midline, whereas vascular lesions are often limited to one side by the margins of the vascular territory.
Patients with myasthenia gravis (see Chapter50 ) can present with failure of adduction in one eye with dissociated nystagmus of the other eye. The dissociation between medical rectus function on lateral gaze compared with near convergence is, however, not typical of myasthenia gravis. Nonetheless, any patient presenting with the motility pattern that characterizes a pure MLF lesion, either unilateral or bilateral, without other signs suggestive of demyelinating or ischemic brain stem diseases, should have an edrophonium chloride (Tensilon) test as part of the workup (see ChapterJ 5 ).
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