Bell Palsy Workup

Updated: May 04, 2021
  • Author: Danette C Taylor, DO, MS, FACN; Chief Editor: Selim R Benbadis, MD  more...
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Approach Considerations

Determining whether facial nerve paralysis is peripheral or central is a key step in the diagnosis. A lesion involving the central motor neurons above the level of the facial nucleus in the pons causes weakness of the lower face alone. Thorough history taking and examination, including the ears, nose, throat, and cranial nerves, must be performed. (See Presentation.)

The minimum diagnostic criteria include paralysis or paresis of all muscle groups on one side of the face, sudden onset, and absence of central nervous system (CNS) disease. Note that the diagnosis of IFP can be made only after other causes of acute peripheral palsy have been excluded. (See DDx.)

If the clinical findings are doubtful or if paralysis lasts longer than 6–8 weeks, further investigations, including gadolinium-enhanced magnetic resonance imaging (MRI) of the temporal bones and pons, should be considered. [34]  Electrodiagnostic tests (eg, stapedius reflex test, evoked facial nerve electromyography [EMG], audiography) may help to improve the accuracy of prognosis in difficult cases. (See Workup.)

No specific diagnostic tests are available for Bell palsy, though the following may be useful for identifying or excluding other disorders:

  • Rapid plasma reagin (RPR) and/or venereal disease research laboratory (VDRL) test or fluorescent treponemal antibody absorption (FTA-ABS) test

  • HIV screening by means of enzyme-linked immunosorbent assay (ELISA) and/or Western blot

  • Complete blood cell (CBC) count

  • Erythrocyte sedimentation rate measurement

  • Thyroid function studies

  • Serum glucose level

  • Cerebrospinal fluid analysis

In addition to the above tests, blood glucose or hemoglobin A1c levels may be obtained to determine if the patient has undiagnosed diabetes. Persons with diabetes have a 29% higher risk of being affected by Bell palsy than do persons without diabetes.

Serum titers for herpes simplex virus (HSV) may be obtained. However, this test 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 (immunoglobulin M [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 for M pneumoniae, though only 2 of those who tested positive had respiratory symptoms. [35]




Imaging Studies for Bell Palsy

Imaging considerations

The latest guidelines (2103) from the American Academy of Otolaryngology-Head and Neck Surgery Foundation do not recommend diagnostic imaging and routine lab testing for patients with new-onset disease. [36]  

If the history and physical examination lead to a diagnosis of Bell palsy, then immediate imaging is not necessary, 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.

CT scanning and MRI

Imaging with CT scanning or 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.

MRI is useful as a means of excluding other pathologies as the cause of paralysis and is preferred for imaging the cerebellopontine angle.

MRI in patients with Bell palsy may show enhancement of the seventh cranial nerve (facial nerve) at or near the geniculate ganglion. Alternatively, MRI may demonstrate 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. [37] Increased signal intensity of the premeatal or intratemporal segment of the facial nerve after administration of gadolinium has been noted in patients with Bell palsy. [38, 39] Attempts to correlate outcome with quantification of signal intensity increases have yielded contradictory results.


Conduction Testing and EMG

Useful tests for evaluation of the function of the facial nerve include nerve conduction testing and EMG. Nerve conduction velocities and EMG produce a graphic readout of the electrical currents, displayed by stimulating the facial nerve and recording the excitability of the facial muscles it supplies. These tests may aid in assessing the outcome of a patient who has persistent and severe Bell palsy.

This testing is not part of the acute workup; the tests are most useful when performed 3–10 days after the onset of paralysis. Note that most electromyographic/nerve conduction studies do not show an abnormality for 3 weeks following a peripheral nerve injury.

Comparison to the contralateral side helps to demonstrate the extent of nerve injury and has prognostic implications. 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 is 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. [40] Blink reflexes can be used to measure conduction across the involved segment, but they are commonly absent in Bell palsy.



Electroneurography is a physiologic 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 the onset of paralysis, some studies suggest that 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 the 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. Patients who have 90% degeneration by 5 days have a worse prognosis than those with 90% degeneration at 14 days.

Electrodiagnostic testing is not recommended in patients with incomplete facial paralysis; however, it may be offered to those patients with complete facial paralysis. [36]


Brainstem Auditory Evoked Response and Audiometry

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 the BAER of 17 patients with Bell palsy and found no evidence of retrocochlear lesions of the auditory system in any of the patients. [41] In 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. [42]

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

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


Histologic Findings

In a review of 12 autopsy cases of patients with Bell palsy, most cases showed inflammatory changes around the mastoid cells and walls of the arteries. [43] 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.


Other Tests

Salivary flow test

Salivary flow also may be tested. The physician places a small catheter into 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.

Schirmer blotting test

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.

Nerve excitability test

The nerve excitability test determines the threshold of the electrical stimulus needed to produce visible muscle twitching. This test is technically challenging and has very limited clinical availability.