| Bell's Palsy Etiology and Incidence Bell's palsy, an idiopathic paralysis of the facial muscles innervated by the seventh cranial nerve, is considered responsible for 80% of all facial mononeuropathies. The exact etiology of Bell's palsy is unknown, although serologic and DNA evidence suggest that herpesvirus infection may be responsible for some cases. Ischemia with subsequent edema of the facial nerve and adjacent structures has also been invoked to explain facial palsy in patients with certain vascular risk factors. The incidence of Bell's palsy increases with age, and is slightly more common in the winter. It is often associated with certain conditions such as pregnancy, diabetes, and hypothyroidism. In younger patients, it is most common among women, but this gender distribution reverses after age 50. Clinical Presentation and Course The distinction among Bell's palsy and other facial paralyses usually isn't difficult. In Bell's palsy, onset is acute, with maximal deficit developing within a few hours. The motor deficit is almost always unilateral, and in two-thirds of cases it may be accompanied by pain in or behind the ear. Fever, tinnitus, and mild hearing diminution may also be present during the first few hours. Symptoms include facial muscle paresis, facial asymmetry, and drooling. The palpebral fissure appears widened, the forehead is smooth, and the nasolabial fold is flattened. Bell's phenomenon (the normal upward deviation of the eye with lid closure) is exaggerated because of weakness of the orbicularis oculi. The corneal reflex may be decreased on the involved side. Lacrimation is only rarely defective, and depending on the level of the injury, loss or perversion of taste on the anterior two-thirds of the tongue or hyperacusis may occur and cause altered taste, decreased tearing, decreased salivation, or altered sensitivity to sound. These symptoms may fluctuate during the first few days following onset. Both the upper and lower parts of the face are affected. This feature distinguishes Bell's palsy from a central supranuclear lesion. Differential Diagnosis Other causes of facial paralysis involve injury to the facial nerve and include bacterial infection (usually from a source in the ear), herpes zoster, diabetes mellitus, sarcoidosis, Guillain-Barr� syndrome, tumor (acoustic neuroma, pontine glioma, neurofibroma, cholesteatoma, parotid gland tumor, meningeal carcinomatosis), trauma (fracture of the temporal bone), and Lyme disease. Bilateral facial paralysis raises a different set of diagnostic possibilities, including Guillain-Barr� syndrome (with or without HIV infection), sarcoidosis, and Lyme disease. Diseases of the neuromuscular junction such as myasthenia gravis or botulism must also be considered in the presence of bilateral facial nerve dysfunction. Evaluation The patient found to have a mononeuropathy of the seventh cranial nerve should first be examined for evidence of the underlying cause. The patient should be checked for zosteriform lesions on the tympanic membrane, in the external auditory canal, and behind the ear. The skin is examined for the characteristic truncal erythematous lesion of early Lyme disease and for neurofibromas. The tympanic membrane should also be checked for cholesteatoma and evidence of otitis media. The jaw is examined for tenderness and trauma to the temporal bone. The lymph nodes should be palpated for enlargement, and the chest should be auscultated for signs of interstitial involvement suggestive of sarcoidosis. A neurologic examination completes the assessment, with a focus on the detection of additional neurologic deficits. Laboratory studies are of no value in the detection of the condition itself. However, laboratory analysis may be useful in detecting underlying conditions, such as diabetes, that can increase risk for Bell's palsy. Testing for Lyme disease is also important in the evaluation, because the identification of this disease has important consequences for management. Computed tomography or magnetic resonance imaging studies are necessary only when a posterior fossa mass is suspected as the cause of the seventh nerve palsy. Lumbar puncture is indicated if inflammation, granuloma, or malignancy is a consideration. In patients with atypical or persistent facial palsy, gadolinium-enhanced magnetic resonance imaging can help differentiate Bell's palsy from other causes. Electromyography may be used to predict recovery, but it is not needed for diagnosis and is most informative when at least 3 weeks have elapsed after the onset of facial paralysis. Management In 75% to 90% of cases, patients recover without treatment, most within 3 weeks. The prognosis can be assessed by electromyographic testing of the involved muscles at least 72 hours after the clinical nadir. Patients with evidence of extensive axonal degeneration have a poorer prognosis. Those with partial or complete preservation of the compound muscle action potential amplitude have anatomic continuity of the facial nerve, partial axonal preservation, and a better prognosis. Electromyography is indicated only for patients with severe clinical involvement that hasn't improved 7 to 10 days after onset. Of greatest importance during the acute stage of Bell's palsy is the prevention of injury to the cornea, which is exposed by weakness of the orbicularis muscle. When the lid is weak, methylcellulose drops should be prescribed for use twice a day and at bedtime. In addition, the lid may need to be taped shut at night. If the patient has eye pain, visual impairment, or other ocular symptoms, corneal abrasion should be suspected. In this case, the patient should be referred for ophthalmic consultation, and slit-lamp examination with fluorescein is indicated. Although treatment usually isn't necessary, if paralysis is severe and the patient is seen within a few days of onset, a short course of corticosteroid therapy may increase the chances for maximal recovery. One study reported full facial recovery in 88% of a group treated with prednisone and in 64% of an untreated control group. Another study found a decrease in the frequency of chronic autonomic dysfunction (from 10% to 1%) when prednisone was administered early in the course of illness. Nevertheless, steroids should always be used cautiously, because most patients will recover fully without them. Patients with Lyme disease as the underlying cause of their condition should not receive steroids, because steroids can exacerbate the situation by compromising immune function. On the basis of serologic and DNA evidence that herpesvirus infection may be the cause of many cases of facial paralysis, some authorities now recommend institution of appropriate antiviral therapy (e.g., a course of acyclovir) at the onset of symptoms of Bell's palsy. Initial studies comparing prednisone therapy with and without antiviral therapy show a modest benefit from the addition of antiviral therapy. However, more data is needed to define the role of antivirals in the treatment of Bell's palsy. Outcome Although most patients recover, poor outcomes in Bell's palsy can result if damaged nerve fibers do not regenerate normally. Lacrimation during eating ("crocodile tears") appears when fibers regrow and connect with lacrimal ducts instead of salivary glands. Abnormal movements (facial synkinesis) may occur if regenerating motor fibers innervate inappropriate muscles. In those patients who do not achieve an acceptable recovery, autografting with a hypoglossal-to-facial anastomosis may provide reasonable cosmetic results and afford lasting protection of the eye. Patients in this category should be referred to an otolaryngologist or a neurosurgeon for follow-up care. |