Evaluation Guidelines

Neuroimaging. Radiographical studies are helpful in determining the site of lesion and should be obtained whenever the etiology is unclear, when the paralysis is recurrent, progressive, or atypical, or if there was trauma ( . Tab]eJJ..-2 )■ If there is evidence of cerebellopontine angle or internal auditory canal pathology (e.g., facial weakness with ipsilateral sensorineural hearing loss, tinnitus, and/ or vestibular abnormalities), magnetic resonance imaging (MRI) with gadolinium enhancement should be obtained. MRI can also demonstrate intracranial, temporal bone, base of skull, and facial soft tissue lesions. High-resolution computed tomography (CT) is useful in defining the bony anatomy of the facial canal. CT readily demonstrates fractures (face, mandible, temporal bone) and bony abnormalities (congenital anomalies, osteopetrosis). It helps differentiate between intratemporal facial nerve neuromas (smooth upward displacement of bone overlying the geniculate with expansion of the tympanic segment) and hemangiomas (irregular bony erosion around the geniculate). In addition, CT can show the extent of bone erosion with lesions like cholesteatoma, glomus tumors, carcinoma, and skull base osteomyelitis.

TABLE 11-2 --

USEFUL TESTS IN THE EVALUATION OF D

SORDERS OF CRAN

IAL NERVE Vll

Neuroimaging

Electrophysiology

Fluid and

Tissue

Analysis

Neuropsychology Tests

Other Tests

Supranuclear facial paresis

MRI, CT for infarction, tumor

EEG: abnormal wave patterns in seizures

CSF: increased RBC

Mental status impairment

NA

Nuclear lesions: Millard-Gubler and Foville syndromes

MRI for infarction, tumor

CSF: increased RBC

NA

NA

Cerebellopontine angle lesion

MRI for tumor

ABR: delay in wave V or increased interaural difference

NA

NA

Audiogram: sensorineural hearing loss ENG: vestibular weakness

Facial canal lesion

MRI, CT for tumor, fractures, inflammation

NA

NA

Audiogram: sensorineural or conductive hearing loss; acoustic reflex: absent; electrogustometry: reduced; salivary flow: reduced; Schirmer's test: reduced

Stylomastoid foramen syndrome

MRI for tumor, fracture

NA

NA

Hemifacial spasm

MRI for vascular loop compression

NA

NA

Postparalytic movements and synkinesis

NA

NA

NA

NA

Myokymia

MRI for pontine tumor, MS plaques

EMG: spontaneous activity at 30-70 per second

CSF: elevated Ig G and gamma globulin

NA

NA

MRI, magnetie resonance imaging; CT, computed tomography; EEG, electroencephalogram; ABR, auditory brain stem response; CSF, cerebrospinal fluid; RBC, red blood cell; ENG, electronystagmography; MS, multiple sclerosis; EMG, electromyography; NA, not applicable.

Electrophysiology. Electroencephalography may help distinguish supranuclear causes of facial nerve dysfunction, especially in the instances of suspected epilepsy with a post-seizure paresis, called Todd's paresis. Nerve conduction studies should be performed in patients with a unilateral complete paralysis. In testing the involved side, if the threshold is 3.5 mA greater than the contralateral ("normal") side, there is a poor prognosis for spontaneous recovery. This test, however, is not absolutely reliable, and even when there is complete transection of the facial nerve, it conducts impulses distal to the injury for up to 72 hours (until wallerian degeneration occurs).^] Electroneurography can be used to determine the peak-to-peak amplitude of the evoked compound action potential of the involved side compared with the other side. Because the amplitude is considered proportional to the number of functioning motor fibers, if there is a 90 percent or greater reduction in the amplitude of the affected side (which represents more than 90 percent degeneration of the nerve), the prognosis is poor.

Muscle action potentials generated by voluntary and spontaneous activity can be measured through electromyography. By placing needles within the facial musculature, electrical silence can be seen in acute paralysis, severe muscle wasting, hypoplasia of the nerve or muscle, or normally innervated muscle in a resting state. Voluntary potentials are present with voluntary contraction. Polyphasic potentials appear during nerve regeneration from 4 to 6 weeks after onset of the paralysis. Fibrillation potentials represent degeneration of the motor nerve; they are usually seen 10 to 14 days after nerve injury. Electromyography is helpful in patients with long-standing facial paralysis and in infants with congenital facial paralysis, but it is not very useful with acute paralysis.

By using electrodes to record the blink reflex after percutaneous stimulation of the supraorbital nerve, a trigeminofacial reflex can be tested. Measuring conduction of the reflex arc between CN V (afferent) and CN VII (efferent) tests both central and peripheral segments of the facial nerve. Currently, the trigeminofacial reflex is the only direct test able to measure intracranial pathology of the facial nerve. y

Auditory brain stem response can be further used to evaluate brain stem anatomy when a retrocochlear or nuclear cause of facial nerve dysfunction is considered. Delay in the absolute latency of ipsilateral wave V or an interaural difference in wave V of more than 0.2 ms suggests retro-cochlear involvement of CN VIII. This observation helps localize the etiology of facial palsy as within or proximal to the internal auditory canal (e.g., acoustic or facial nerve neuroma).

Fluid and Tissue Analysis. Basic tests to obtain include complete blood count with differential (for infectious mononucleosis, leukemia, inflammation), chemistry panel (for systemic diseases), fluorescent treponemal antibody titer (for syphilis), and erythrocyte sedimentation rate/antinuclear antibody test/ rheumatoid factor (for collagen vascular disease). Depending upon the clinical presentation, other tests may be helpful, including thyroid function tests, glucose tolerance test, angiotensin-converting enzyme levels (sarcoidosis), antibody titers (Lyme disease, Epstein-Barr virus, herpes zoster virus, human immunodeficiency virus), monospot test/heterophile antibody titers (mononucleosis), serum heavy metal/toxin levels (lead, arsenic, mercury, carbon monoxide), urine analysis (porphyria, diabetes, sarcoidosis), and stool analysis (Clostridium botulinum toxin).

Muscle biopsies are helpful in determining the existence of a mitochondrial myopathy. In addition, if a facial reanimation procedure is considered in patients with longstanding facial paralysis or in infants with congenital facial paralysis, facial muscle biopsy should be performed to determine whether viable muscle fibers are present. If the musculature has degenerated and has been replaced by fibrosis, any procedure attempting to bring only neural input to the area will fail. y In patients requiring a tissue diagnosis, biopsies of skin lesions, subcutaneous nodules, enlarged lymph nodes, or salivary gland tissue may reveal disorders such as sarcoidosis, parotid gland tumor, or vasculitis (e.g., temporal arteritis or periarteritis nodosa).

Cerebrospinal Fluid. If the possibility of central nervous system infection or malignancy exists, a lumbar puncture should be performed. After noting the opening pressure and qualities of the fluid, samples should be sent for cell count, chemistry levels, microbiological staining and culture, antigen titers, and cytology testing. Intracranial hypertension, meningitis (bacterial, fungal, mycobacterial, viral, carcinomatous, aseptic), and malignant processes such as metastasis (lung, breast, melanoma), leukemia, lymphoma, and other primary intracranial neoplastic processes can be detected. In addition, the diagnosis of other etiologies can be supported, like Guillain-Barre(c) syndrome (increased protein level), multiple sclerosis (elevated IgG and gamma globulin levels), syphilis (positive serology result), and subarachnoid or intracerebral hemorrhage (red blood cells).

Other Tests. Audiometry should be performed in all patients with facial palsy. Because CN VII is intimately related to CN VIII in the cerebellopontine angle and internal auditory canal and because the facial canal courses by the cochlea and through the middle ear, many causes of palsy have an associated hearing loss. The type of loss (sensorineural versus conductive) as well as the extent (mild to profound) are essential factors for localization and planning of treatment. Another useful audiometrical test is the acoustic reflex test, which measures the contraction of the stapedius muscle in response to auditory stimuli. The result can localize the site of lesion as proximal or distal to the branch to the stapedius (mastoid segment of the facial nerve). Tests of vestibular function, like electronystagmography, may be helpful in differentiating central from peripheral lesions in patients who have accompanying vertigo.

By evaluating taste or salivation, the status of the chorda tympani nerve can be determined. Based upon impairment, lesions can be localized as proximal or distal to this branch of the mastoid segment. For example, electrogustometry can measure the taste threshold in response to a small electric current applied to the anterior tongue. The current elicits a metallic or bitter taste, and by controlling the amount of current, the threshold for gustation can be obtained. An elevated threshold suggests involvement of the chorda tympani. Salivary flow can be analyzed by cannulating Wharton's duct. The salivary production is measured for 5 minutes, and a reduction of 25 percent between

sides signifies chorda tympani involvement. In addition, salivary flow can be evaluated indirectly by checking the pH of the saliva, because the pH rises with increased salivation. A pH of 6.1 or less is considered to be significant for reduced flow. y

When certain systemic disorders are suspected to be the etiology of the facial palsy, chest radiography should be obtained. Based upon characteristic radiographic findings, the diagnosis of sarcoidosis, tuberculosis, lymphoma, or lung cancer can be supported.

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