|
Bell's
palsy is sudden, idiopathic, unilateral peripheral 7th cranial nerve
palsy. Symptoms are hemifacial paresis of the upper and lower face.
There are no specific tests for diagnosis. Treatment may include
corticosteroids, antiviral drugs (eg, acyclovir), lubrication of
the eye, and intermittent use of an eye patch.
Etiology
Cause is unknown, but the mechanism is presumably swelling of the 7th cranial (facial) nerve due to an immune or viral disorder. Recent evidence suggests herpes simplex virus infection. The nerve is compressed, resulting in ischemia and paresis, because the nerve passes through a narrow opening (internal acoustic meatus) in the temporal bone.
The orbicularis oculi and frontalis muscles are paretic when the lesion is distal to the 7th cranial nerve nucleus (ie, peripheral) but much less so when the lesion is proximal to the nucleus (ie, central). The effects differ because the orbicularis oculi and frontalis muscles are controlled by the 7th cranial nerve nuclei (central part of the facial nerve), which receive input from both left and right hemispheres. In contrast, the lower facial muscles (below the zygomatic arch) receive input from mainly the peripheral part of the facial nerve, distal to the 7th cranial nerve nuclei, which receives input from only one hemisphere. Thus, the muscles are paretic regardless of the location of the lesion along the 7th cranial nerve.
Symptoms and Signs
Pain behind the ear often precedes facial paresis. Paresis, often with complete paralysis, develops within hours and is usually maximal within 48 to 72 h. Patients may complain of a numb or heavy feeling in the face. The affected side becomes flat and expressionless; ability to wrinkle the forehead, blink, and grimace is limited or absent. In severe cases, the palpebral fissure widens and the eye does not close, often irritating the conjunctiva and drying the cornea.
Sensory examination is normal, but the external auditory canal and a small patch behind the ear (over the mastoid) may be painful to the touch. If the nerve lesion is proximal to the geniculate ganglion, salivation, taste, and lacrimation may be impaired, and hyperacusis may be present.
Diagnosis
There are no specific diagnostic tests. Thus, Bell's palsy is a diagnosis of exclusion. It can be distinguished from a central 7th cranial nerve lesion (eg, due to hemispheric stroke or tumor), which causes weakness primarily of the lower face; patients with central lesions can usually furrow their brow and close their eyes tightly. Other disorders that cause peripheral 7th cranial nerve palsies and must be excluded include the following:
Other disorders that cause peripheral 7th cranial nerve palsy typically develop more slowly than Bell's palsy and may have other distinguishing symptoms or signs.
In Bell's palsy, MRI may show contrast enhancement of the 7th cranial nerve at or near the geniculate ganglion. However, its enhancement may reflect other pathology, such as sarcoidosis or meningeal tumor. If the paralysis progresses over weeks to months, the likelihood of tumor (eg, most commonly schwannoma) compressing the 7th cranial nerve increases. MRI can also help exclude other structural disorders causing 7th cranial nerve palsy. CT, usually negative in Bell's palsy, is done if a fracture is suspected or if MRI is not immediately available and stroke is possible. Acute and convalescent serologic tests for Lyme disease are done if patients have been in a geographic area where ticks are endemic. For all patients, a chest x-ray is taken and serum ACE is measured to check for sarcoidosis. Viral titers are not helpful.
Prognosis
The extent of nerve damage determines outcome. If some function remains, full recovery typically occurs within several months. Nerve conduction studies and electromyography predict outcome. The likelihood of complete recovery after total paralysis is 90% if nerve branches in the face retain normal excitability to supramaximal electrical stimulation and is only about 20% if electrical excitability is absent.
Regrowth of nerve fibers may be misdirected, innervating lower facial muscles with periocular fibers and vice versa. The result is contraction of unexpected muscles during voluntary facial movements (synkinesia) or crocodile tears during salivation. Chronic disuse of the facial muscles may lead to contractures.
Treatment
No treatment has proved effective for idiopathic Bell's palsy. Corticosteroids, if begun within 48 h after onset, may slightly reduce duration and degree of residual paralysis. Prednisone 60 to 80 mg po once/day is given for 1 wk, then decreased gradually over the 2nd wk. Antiviral drugs effective against herpes simplex virus (eg, valacyclovir 1 g po tid for 7 to 10 days, famciclovir 500 mg po tid for 5 to 10 days, acyclovir 400 mg po 5 times/day for 10 days) are also often given.
Corneal drying must be prevented by frequent use of natural tears, isotonic saline, or methylcellulose drops and by intermittent use of tape or a patch to help close the eye, particularly during sleep. Tarsorrhaphy is occasionally required.
Last full review/revision October 2007 by Michael Jacewicz, MD
Content last modified October 2007
| 
