Cranial Nerve Disorders

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Cranial Nerve Disorders
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THIRD CRANIAL NERVE PALSIES

• Partial to complete weakness of the muscles
  innervated by the 3rd (oculomotor) nerve,
  resulting in ptosis of the lid, mydriasis, and an
  outwardly turned eye during primary gaze.
• When the patient attempts to turn the eye inward,
  it moves slowly only to the midline. Upward and
  downward gaze is compromised in the affected eye.
  When downward gaze is attempted, the superior
  oblique muscle causes the eye to rotate inward.

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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.

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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.

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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.

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FOURTH CRANIAL NERVE PALSIES

• Weakness of the muscle innervated by the 4th
  (trochlear) nerve (superior oblique muscle).
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• 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.

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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.

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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.

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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.

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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.

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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.

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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.

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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 Sjögren'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.

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FACIAL NERVE DISORDERS

• Unilateral facial weakness is a common neurologic
  sign.

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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.

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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.

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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.

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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.

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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.
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• 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.

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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.