Introduction
Bell’s palsy or idiopathic peripheral facial nerve palsy is the most common cause of CN VII dys-function. The facial nerve contains around 10,000 axons, of which 70% are motor nerves (special
vis-ceral efferent) that innervate muscles of the face.
The remaining fibers include general visceral effer-ent nerves that are parasympathetic nerves to the lacrimal and submandibular glands—special vis-ceral afferent nerves that represent taste from the anterior 2/3 of the ipsilateral tongue, and general somatic afferent nerves that transmit sensation from the skin of the ear pinna and external tory canal. The facial nerve travels with the audi-tory nerve in the internal audiaudi-tory canal and enters the facial canal, where it soon reaches the genicu-late ganglion containing the neuronal cell bodies for taste and ear sensation. The greater petrosal nerve, the first branch, travels to the lacrimal gland. The second branch runs to the stapedius muscle, and the third branch—the chorda tym-pani nerve—travels to the tongue. The nerve exits the facial canal at the stylomastoid foramina, where it passes through the parotid gland and spreads out to innervate 23 facial muscles (but not the masseter and lateral and medial pterygoid muscles innervated by the trigeminal nerve).
Numerous diseases cause facial palsy in adults, including trauma (facial trauma and basal skull fracture), infections (Lyme disease, otitis media, syphilis, meningitis, and mumps), tumors (parotid tumors, sarcoma, and facial nerve meningioma), and brainstem disorders (multiple sclerosis and strokes). However, almost 60% of cases are consid-ered idiopathic and due to Bell’s palsy.
Bell’s palsy occurs over 65,000 times a year, with an equal racial and sex distribution. Cases occur in all ages, but the incidence increases with age. It is rare for Bell’s palsy to be bilateral or to recur.
Pathophysiology
The pathogenesis of Bell’s palsy remains poorly understood. MRI and pathologic studies show the facial canal, especially in the tympanic and labyrinthine segments, as the site of pathology.
The nerve becomes edematous and may develop mild-to-moderate wallerian degeneration, with varying amounts of surrounding lymphocytic inflammation. The geniculate ganglion may appear normal or have inflammation. Early theo-ries suggested ischemia to the facial nerve led to nerve edema and nerve compression from the walls of the facial canal. Later, the ischemia cept was dropped and the nerve edema was con-sidered idiopathic. Recently, viral infection
CHAPTER 6—Disorders of Peripheral Nerves 63
057-066_Davis06 3/2/05 4:16 PM Page 63
theories have focused on varicella-zoster and her-pes simplex viruses as potential viruses that reacti-vate in the facial nerve or geniculate ganglion to cause nerve damage, edema, and inflammation.
While varying degrees of wallerian degeneration develop, all axons are rarely destroyed. As such, spontaneous recovery usually occurs.
Major Clinical Features
The hallmark of Bell’s palsy is the abrupt onset of painless unilateral complete or incomplete facial weakness (Figure 6-3). Since damage of the facial nerve occurs in the facial canal, other nerve branches are dysfunctional, with variable inci-dences. In 10% to 15% of patients, vesicles appear on the skin of the ipsilateral ear pinna, external auditory canal, or skin below the pinna. Varicella-zoster virus can be isolated from the vesicle, which establishes the diagnosis of herpes-zoster oticus or Ramsay Hunt syndrome. In this case, the varicella-zoster virus became latent in the geniculate gan-glion during childhood chickenpox and reactivated many years later.
Major Laboratory Findings
Remarkably few laboratory abnormalities exist in Bell’s palsy. The patient has a normal hemogram,
erythrocyte sedimentation rate, and serum elec-trolytes. The CSF is normal. If the CSF has a pleo-cytosis, the facial palsy etiology is likely due to an inflammatory or infectious process, such as vari-cella-zoster virus, Lyme disease, neurosyphillis, or sarcoidosis. Cranial MRI with gadolinium may show enhancement of the facial nerve within the facial canal. The EMG, normal for the first 3 days, shows a steady decline in activity and after 10 days, denervation potentials begin to appear. At autopsy of individuals without a history of Bell’s palsy, her-pes simplex viral DNA can frequently be detected by polymerase chain reaction in the geniculate ganglia. This suggests that the virus may become latent in that ganglion, but whether exacerbation of the latent virus produces Bell’s palsy remains controversial.
Principles of Management and Prognosis Management of the patient with Bell’s palsy is divided into treating the acute facial palsy and pre-venting complications. If there is incomplete paralysis of facial muscles, there is an excellent prognosis for full to satisfactory recovery that spontaneously occurs within 2 months. Should the facial paralysis be complete, full to satisfactory recovery spontaneously occurs in about 80% of patients over 1 to 3 months. In an effort to improve outcome, patients are often given corti-costeroids for several days, with the hypothesis that the corticosteroids will lessen facial nerve edema, reduce nerve pressure, and prevent nerve ischemia. If one believes herpes simplex virus may be the etiology, the antiviral drug acyclovir is given for a week. Observing vesicles on the ear pinna suggests another antiviral drug (famciclovir, pen-cyclovir, or high-dose acyclovir) should be given to treat the varicella-zoster viral infection.
Frequently the patient will have facial weakness such that he or she cannot fully close the eyelid, exposing the cornea to abrasions and drying. After applying ointment, these patients should tape their eyelid closed while sleeping. Some patients have diminished tearing in the involved eye and require frequent application of liquid tears. A few patients will have aberrant regeneration of the facial nerve during recovery, leading to synkineses (unintentional facial movements accompanying volitional facial movements).
64 FUNDAMENTALS OF NEUROLOGIC DISEASE
Right Bell's Palsy
Right Peripheral 7th Nerve Paralysis
Figure 6-3 Bell’s palsy.
057-066_Davis06 3/2/05 4:16 PM Page 64
RECOMMENDED READING
British Medical Research Council. Aids to the Examination of the Peripheral Nervous System.
4th ed. Philadelphia: W. B. Saunders; 2000.
(Superb booklet that outlines how to test each muscle, describes areas of sensation for all periph-eral nerves, and easily can be kept in the physi-cian’s bag.)
Green DA, Stevens MJ, Feldman EL. Diabetic neu-ropathy: scope of the syndrome. Am J Med 1999;107(suppl):2S–7S. (This journal supple-ment has a series of useful articles on diabetic neuropathy.)
Hughes RAC. Peripheral neuropathy. BMJ 2002;324:466–469. (General review of causes
and workup for all patients with peripheral neuropathy.)
Katz JN, Simmons SP. Carpal tunnel syndrome. N Engl J Med 2002;346:1807–1812. (Excellent review.)
Marenda SA, Olsson JE. The evaluation of facial paralysis. Otolaryngol Clin N Amer 1997;30:
669–682. (Reviews causes of facial paralysis, including Bell’s palsy, clinical findings, laboratory tests, and natural history.)
Sweeney CJ, Gilden DH. Ramsay Hunt Syndrome.
J Neurol Neurosurg Psychiatry 2001;71:149–154.
(Reviews facial paralysis due to reactivation of varicella-zoster virus.)
CHAPTER 6—Disorders of Peripheral Nerves 65
057-066_Davis06 3/2/05 4:16 PM Page 65
This page intentionally left blank
Overview
For many years, the spinal cord was conceived as a conduit that carried impulses from the brain to the trunk and limbs and vice versa. We now know that spinal cord functions are not solely passive, but rather modulate or generate many afferent and efferent pathways. For example, endorphin-con-taining neurons in the dorsal horn actively modu-late afferent peripheral pain fiber impulses, resulting in diminishment or enhancement of per-ceived pain. Important aspects of normal walking appear to be generated from clusters of motor neurons located in the lower thoracic and upper lumbar spinal cord. Rapid limb withdrawal from a painful stimulus and deep tendon reflexes do not involve the cortex but result from local circuitry in the cord.
The spinal cord, which is about the diameter of a thumb, extends caudally from the medulla to the first or second lumbar vertebra in adults and slightly lower in infants (Figure 7-1). From L2 to S2 the central vertebral canal is composed of nerve roots, ending in the cauda equina. The absence of spinal cord below L2 is the reason why a lumbar puncture can be safely performed in the lower lumbar area.
Spinal cord dysfunction results from traumatic, ischemic, nutritional, malignant, or degenerative conditions. Diseases affecting the spinal cord usu-ally cause three clinical pictures. Two involve spinal cord parenchyma and one involves spinal cord roots. The first is degenerative with loss of specific spinal cord elements, as seen in amy-otrophic lateral sclerosis (ALS) and subacute com-bined degeneration (vitamin B12 deficiency). The second is from a lesion at one level of the spinal cord as seen in back or neck trauma, cervical myelopathy from a central protruding interverte-bral disk, or acute transverse myelitis. The third is from compression of exiting spinal cord nerve roots, producing a radiculopathy (sensory and motor dysfunction of a single dermatome/
myotome) due to focal lesions such as posterolat-eral prolapse of a vertebral disk or a neurofibroma compressing a spinal cord root.
Clinical signs depend on the level of the spinal cord damage and whether the damage involves part or all of the cord. Thus to understand the clin-ical signs produced by lesions in spinal cord parenchyma, one must know the differences between upper motor neuron and lower motor neuron dysfunction (Figure 7-2) and anatomic location and function of key spinal cord tracts (Figure 7-3 and Tables 1 and 2).
67