Cranial Nerve Disorders

1. Cranial Nerve Disorders

4. Causes of 3rd cranial nerve palsies The many causes of 3rd cranial nerve palsies include most major causes of CNS disease, so choice of diagnostic tests should be based on the clinical features of the palsy. Intraorbital structural lesions producing external ophthalmoplegia and ocular myopathies should be distinguished from cranial nerve disease. Exophthalmos or enophthalmos, a history of severe orbital trauma, or an obviously inflamed orbit suggests restrictive orbital disease, which may impair ocular motility. Myopathies are harder to diagnose but are suggested by a partial 3rd nerve palsy. The pupil is always spared in myopathy.

5. Causes of 3rd cranial nerve palsies Completely nonfunctional parasympathetic fibers (causing fixed dilated pupils) strongly suggest oculomotor nerve compression. The most common causes are aneurysm (especially of the posterior communicating artery), trauma, and intracranial mass lesion. Oculomotor paralysis in an increasingly unresponsive patient suggests transtentorial herniation and is a major emergency. If the pupil is completely spared but all other muscles innervated by the 3rd nerve are affected (eg, diabetic 3rd nerve paresis), the cause is likely to be an ischemic process of the oculomotor nerve or the midbrain; a demyelinating process is less likely. However, about 5% of posterior communicating artery aneurysms causing oculomotor paralysis spare the pupil.

6. Investigation Third cranial nerve palsies are most indicative of serious disease when associated with severe headache or altered consciousness. A thorough neurologic examination with CT or MRI is performed. Lumbar puncture is reserved for suspected subarachnoid hemorrhage when CT does not show blood. Cerebral angiography must be performed if aneurysm causing subarachnoid hemorrhage is strongly suspected or when the pupil is clearly affected and no head trauma serious enough to fracture the skull has occurred.

7. FOURTH CRANIAL NERVE PALSIES Weakness of the muscle innervated by the 4th (trochlear) nerve (superior oblique muscle). � These palsies are often difficult to detect because they affect vertical eye position predominantly when the eye is turned inward. The patient sees double images, one above and slightly to the side of the other. However, by tilting the head to the side opposite the palsied muscle, the patient may achieve full or almost full ocular motility without double vision.

8. FOURTH CRANIAL NERVE PALSIES There are few common identified causes of 4th cranial nerve palsies; many are idiopathic. Closed head trauma without skull fracture is a common cause of unilateral and bilateral palsies; the few cases that occur often follow motor cycle accidents. Aneurysms, tumors, and multiple sclerosis are rare causes. Evaluation of 4th nerve palsies is similar to that of 3rd nerve palsies. Usually, the diagnosis is obvious from the history and physical examination. Oculomotor exercises may help. Sometimes surgery is necessary to restore concordant vision.

9. SIXTH CRANIAL NERVE PALSIES Weakness of the muscles innervated by the 6th (abducens) nerve. The eye is turned inward; it moves outward sluggishly, reaching the midline at most. Idiopathic cases are common, although many occur in elderly or diabetic patients in whom small vessel disease may be suspected. In idiopathic cases, no other cranial nerves are involved, and improvement should occur within 2 mo.

11. Causes One identifiable cause is compression of the 6th nerve in the cavernous sinus by a tumor originating in the nasopharynx. Typically, severe pain in the head and anesthesia in the distribution of the first division of the 5th nerve also occur. Anything that causes the brain to shift may stretch the 6th nerve because of the acute angle it makes before entering Dorello's canal. Thus 6th nerve palsies may be due to large brain tumors remote from the nerve, to increased intracranial pressure, or to lumbar puncture.

12. Causes Diabetic infarction is one of the more common causes. Other causes include trauma of insufficient force to cause a basilar skull fracture, infections or tumors affecting the meninges, Wernicke's encephalopathy, aneurysm, and multiple sclerosis. In children without evidence of increased intracranial pressure, these palsies can result from respiratory infection and thus may be recurrent.

13. SIXTH CRANIAL NERVE PALSIES Diagnosing complete 6th cranial nerve palsies is easy, but determining their etiology can be more challenging. Excluding increased intracranial pressure and papilledema (by looking for retinal venous pulsations during funduscopy) is important. MRI or CT can help exclude intracranial mass lesions, hydrocephalus, and direct nerve compression by lesions in the orbit, cavernous sinus, and base of the skull. Lumbar puncture determines the CSF opening pressure and can detect leptomeningeal inflammatory, infectious, or neoplastic infiltrates entrapping the 6th nerve. A collagen vascular screen helps exclude a vasculopathic process. In many cases, 6th nerve palsies resolve once the primary disorder is treated.

14. TRIGEMINAL NEURALGIA(Tic Douloureux) A disorder of the trigeminal nerve producing bouts of excruciating, lancinating pain, lasting between seconds and 2 min, along the distribution of one or more of its sensory divisions, most often the maxillary. At surgery or autopsy, intracranial arterial and, less often, venous loops compressing the trigeminal nerve root where it enters the brain stem have been found, suggesting that the tic is a compressive neuropathy. The disorder usually affects adults, especially the elderly. Pain is often set off by touching a trigger point or by activity (eg, chewing or brushing the teeth). Although each bout of intense pain is brief, successive bouts may be incapacitating.

15. TRIGEMINAL NEURALGIA(Tic Douloureux) Differential diagnosis includes neoplasm, vascular malformation of the brain stem, a vascular insult, and multiple sclerosis (especially in a younger patient). Postherpetic pain is differentiated by its typical antecedent rash, scarring, and predilection for the ophthalmic division. Trigeminal neuropathy may occur in Sj�gren's syndrome or RA, but with a sensory deficit that is often perioral and nasal. Migraine may produce atypical facial pain, with normal examination results, but the pain is more prolonged and is burning or throbbing.

16. FACIAL NERVE DISORDERS Unilateral facial weakness is a common neurologic sign.

18. Bell's Palsy Unilateral facial paralysis of sudden onset and unknown cause. The mechanism presumably involves swelling of the nerve due to immune or viral disease, with ischemia and compression of the facial nerve in the narrow confines of its course through the temporal bone. Pain behind the ear may precede facial weakness. Weakness develops within hours, sometimes to complete paralysis. The affected side becomes flat and expressionless, but patients may complain instead about the seemingly twisted intact side. In severe cases, the palpebral fissure widens, and the eye does not close. The patient may complain of a numb or heavy feeling in the face, but no sensory loss is demonstrable. A proximal lesion may affect salivation, taste, and lacrimation and may cause hyperacusis.

19. Diagnosis� Weakness of the entire half of the face distinguishes Bell's palsy from supranuclear lesions (eg, stroke, cerebral tumor), in which the weakness is partial, affecting the frontalis and orbicularis oculi less than the muscles in the lower part of the face. Bell's palsy must be differentiated from unilateral facial weakness due to other disorders of the facial nerve or its nucleus, chiefly geniculate herpes (Ramsay Hunt's syndrome), middle ear or mastoid infections, sarcoidosis, Lyme disease, petrous bone fractures, carcinomatous or leukemic nerve invasion, chronic meningeal infections, and cerebellopontine angle or glomus jugulare tumors. Skull x-rays and CT and MRI scans are obtained when the diagnosis is in doubt. MRI may show contrast enhancement of the facial nerve, but CT and skull x-rays are typically negative. However, they may show a fracture line, bony erosion due to infection or neoplasm, or internal auditory canal expansion due to a cerebellopontine angle tumor. CT and MRI scans may show the contrast-enhancing mass of angle or glomus tumors. Blood tests for Lyme disease help diagnose it. A chest x-ray and serum ACE are used to detect sarcoidosis, a common cause of facial nerve paralysis in blacks.

20. Prognosis and Treatment The extent of nerve damage determines outcome; nerve conduction studies and electromyography are useful. Complete recovery within several months invariably follows acute partial paralysis. The likelihood of complete recovery after total paralysis is 90% if the nerve branches in the face retain normal excitability to supramaximal electrical stimulation but is only about 20% if electrical excitability is absent. Misdirected regrowth of nerve fibers may innervate lower facial muscles with periocular fibers and vice versa, resulting in contraction of unexpected muscles during voluntary facial movements (synkinesia) or "crocodile tears" during salivation. Facial muscle contractures may follow chronic weakness. Measures must be taken to prevent corneal drying. They include frequent use of natural tears, isotonic saline and methylcellulose drops, and strips of skin tape to help close the eye. Supportive measures, such as temporary patching, may suffice to protect the exposed eye; tarsorrhaphy may be needed when palpebral fissure persists.

21. Corticosteroids Some studies suggest that corticosteroids (eg, prednisone 60 to 80 mg/day po begun 24 to 48 h after onset and given for 1 wk, then decreased gradually over the 2nd wk) help modestly reduce residual paralysis and expedite recovery. Mild electrical stimulation of the nerve and massage of the facial muscles have no proven benefit. Hypoglossal-facial nerve anastomosis may partially restore facial function if none has returned in 6 to 12 mo but results in difficulty in eating and speaking, so its role is limited.

22. GLOSSOPHARYNGEAL NEURALGIA A rare syndrome characterized by recurrent attacks of severe pain in the posterior pharynx, tonsils, back of the tongue, and middle ear. The cause is unknown, and no pathologic change can be found (except rarely, when due to a tumor in the cerebellopontine angle or the neck). Men are more commonly affected, usually after age 40. � As in trigeminal neuralgia, intermittent attacks of brief, severe, excruciating pain occur paroxysmally, either spontaneously or precipitated by movement (eg, chewing, swallowing, talking, sneezing). The pain, lasting seconds to a few minutes, usually begins in the tonsillar region or at the base of the tongue and may radiate to the ipsilateral ear. The pain is strictly unilateral. In 1 to 2% of patients, increased vagus nerve activity causes cardiac sinus arrest with syncope. Attacks may be separated by long intervals.

23. Diagnosis and Treatment Location of the pain, precipitation of an attack by swallowing or by touching the tonsils with an applicator, and temporary elimination of pain with lidocaine applied locally to the throat (after which the pain cannot be evoked by stimulation) distinguish glossopharyngeal neuralgia from trigeminal neuralgia of the mandibular division. Tonsillar, pharyngeal, and cerebellopontine angle tumors and metastatic lesions in the anterior cervical triangle must be ruled out by brain imaging. Carbamazepine is the drug of choice. Phenytoin, baclofen, or amitriptyline in doses as for trigeminal neuralgia (see above ) or trazodone 150 to 400 mg/day in 3 divided doses may be added if necessary. If they are ineffective, cocainization of the pharynx may provide temporary relief, and surgery may be necessary. When pain is restricted to the pharynx, the nerve in the neck may be avulsed; it must be sectioned intracranially if pain is widespread.