Bell Palsy Workup

In many cases, the history and physical examination lead to the diagnosis of Bell palsy. If the clinical findings are doubtful or if paralysis lasts longer than 6-8 weeks, further investigations should be considered.^[2]

No specific diagnostic tests are available for Bell palsy, though the following may be useful:

• Rapid plasma reagin (RPR) and/or venereal disease research laboratory (VDRL) test or fluorescent treponemal antibody absorption (FTA-ABS) test
• Human immunodeficiency virus (HIV) screening by means of enzyme-linked immunosorbent assay (ELISA) and/or Western blot
• Complete blood cell count
• Determination of the erythrocyte sedimentation rate
• Thyroid function studies
• Serum glucose level
• Cerebrospinal fluid analysis

If the history and physical examination lead to a diagnosis of Bell palsy, then immediate imaging is not necessary. Imaging is not required because most patients with Bell palsy improve within 8-10 weeks. If the paralysis does not improve or worsens, imaging may be useful. If the patient has a palpable parotid mass, imaging may be necessary.

Blood glucose or hemoglobin A_1c levels may be obtained to determine if the patient has undiagnosed diabetes.

Serum titers for herpes simplex virus may be obtained, but this is usually not helpful owing to the ubiquitous nature of this virus.

Antineutrophil cytoplasmic antibody (cANCA) levels are indicated if applicable to exclude Wegener granulomatosis.

In areas where Lyme disease is endemic, serum titers (IgM and IgG) for Borrelia burgdorferi should be obtained.

Serum titers (IgM and IgA) for Mycoplasma pneumoniae may be obtained. A study in Germany measured titers in patients with Bell palsy and found that several patients had elevated titers to M pneumoniae, and only 2 of those who tested positive had respiratory symptoms.^[28]

Radiological evaluation by computed tomographic (CT) scanning and other methods is indicated if there are other associated physical findings or if the paresis is progressive and unremitting. CT scanning demonstrates the architecture of the temporal bone and may be used if some other pathology is suspected.

Magnetic resonance imaging (MRI) of patients with Bell palsy may show enhancement of the seventh cranial nerve (facial nerve) at, or near, the geniculate ganglion. However, if the paralysis progresses over weeks, the possibility is high of a neoplasm compressing the facial nerve.

Tumors that compress or involve the facial nerve include schwannoma (most common), hemangioma, meningioma, and sclerosing hemangioma. Perform gadolinium-enhanced MRI when findings are atypical or when the facial nerve paralysis appears central to rule out a tumor or vascular compression.^[29]

Little correlation between the enhancement of the facial nerve and the clinical outcome has been noted. However, a recent analysis of early MRIs with gadolinium of the intratemporal facial nerve demonstrated the ability to predict the long-term outcome of the facial paralysis; these findings (increased signal intensity in the internal auditory canal after administration of gadolinium) correlated favorably with those of electrodiagnostic testing. Thus, MRI is useful as a means of excluding other pathologies as the cause of paralysis. MRI is preferred for imaging the cerebellopontine angle.

The stethoscope loudness test may be used to assess the functioning of the stapedius muscle. The patient wears the stethoscope, and the activated tuning fork is placed at the bell of the stethoscope. The loud sound will lateralize to the side of the paralyzed stapedius muscle.

Useful tests for evaluation of the function of the facial nerve include nerve conduction testing and electromyography (EMG). These tests may aid in assessing the outcome of a patient who has persistent and severe Bell palsy. They are most useful when performed 3-10 days after the onset of paralysis. Do note that most electromyographic studies/nerve conduction studies do not show an abnormality for 3 weeks following a peripheral nerve injury. EMG and nerve conduction velocities produce a graphic readout of the electrical currents displayed by stimulating the facial nerve and recording the excitability of the facial muscles it supplies.

Comparison to the contralateral side helps determine the extent of nerve injury and has prognostic implications. This is not part of the acute workup. Nerve conduction responses are abnormal if a difference of 50% in amplitude between the paralyzed and normal side is detected; a difference of 90% between the 2 sides suggests a poorer prognosis. May et al demonstrated that prognosis may be favorable if the motor amplitude of the affected side was greater than 25% of that of the normal side. An incomplete recovery was observed in patients whose results demonstrated less than 25% amplitude on the paralyzed side.^[30] Blink reflexes can be used to measure conduction across the involved segment, but they are commonly absent in Bell palsy.

Electroneurography is a physiological test that uses EMG to objectively measure the difference between potentials generated by the facial musculature on both sides of the face in response to a supramaximal electrical stimulation of the facial nerve. Because all electrodiagnostic testing is performed on the nerve distal to the proposed site of injury, sufficient time is needed for wallerian degeneration to occur, usually 48-72 hours. Testing should begin 3 days from the onset of complete paralysis.

Electrodiagnostic testing measures the facial nerve degeneration indirectly. If a patient does not reach 90% degeneration within the first 3 weeks of onset of paralysis, some studies suggest the prognosis is excellent, with over 80-100% of the patients recovering with excellent function. The patients who reach over 90% degeneration within the first 3 weeks of onset of paralysis have a much more guarded prognosis, with only 50% having good recovery of facial motion. The rate of degeneration also predicts the prognosis. Those who have 90% degeneration by 5 days have a worse prognosis than those with 90% degeneration at 14 days.

Brainstem auditory evoked response (BAER) may be obtained in patients with peripheral facial nerve lesions and other neurologic involvement. This test measures the transmission of response through the brainstem and is effective in detecting, notably, retrocochlear lesions. Hendrix and Melnick evaluated BAER of 17 patients with Bell palsy. They found no evidence of retrocochlear lesions of the auditory system in any of their patients with Bell palsy.^[31] In another study by Shannon et al, BAER was recorded in 27 patients with Bell palsy; only 6 patients had prolonged brainstem transmission but normal auditory function.^[32] These studies were small and do not support routine use of BAER in patients with Bell palsy. However, when a patient presents with multiple cranial neuropathies (eg, of the seventh and eighth cranial nerves), BAER may be useful.

If hearing loss is suspected, audiography and auditory evoked potentials (AEPs) should be pursued once an underlying structural lesion has been excluded. Typically, the hearing threshold is not affected by Bell palsy. Impedance testing may reveal an absent or diminished stapedial reflex because of paresis of the stapedial branch of the facial nerve.

Blepharokymographic analysis, a high-speed eyelid motion-analysis system, has been recently used to evaluate movement of the eyelids. Computerized-based analysis may prove helpful in diagnosing Bell palsy, predicting prognosis, and evaluating response to therapeutic measures such as a gold weight placement (used in cases in which spontaneous recovery has been limited).

Salivary flow also may be tested. The physician places a small catheter into both the paralyzed and normal submandibular glands. The patient is then asked to suck on a lemon, and the salivary flow is compared between the 2 sides. The normal side is the control.

The nerve excitability test determines the threshold of the electrical stimulus needed to produce visible muscle twitching.

The Schirmer blotting test may be used to assess tearing function. The use of benzene will stimulate the nasolacrimal reflex, and the degree of tearing can be compared between the paralyzed and normal sides.

A review of 12 autopsy cases of patients with Bell palsy was summarized in Peter Dyck’s Peripheral Neuropathy^[33] . This review stated that most cases showed inflammatory changes around the mastoid cells and walls of the arteries. The most common site of involvement was the geniculate ganglion. Surgical findings described constriction of the nerve at the stylomastoid foramen with swelling of the nerve itself. Microscopic findings showed an inflammatory reaction with infiltration of macrophages on the nerve.