Contrast-enhanced MRI findings of patients with acute Bell palsy within 7 days of symptom onset: A retrospective study

Abstract

The brain magnetic resonance imaging (MRI) findings seen in patients with Bell palsy are abnormal contrast enhancement of affected facial nerves. Previous studies were conducted on a few patients, mainly those who had experienced palsy for several weeks. This study investigated the diagnostic usefulness of MRI by examining MRI findings of acute Bell palsy (within 7 days of symptom onset) in a large cohort. Among the patients with Bell palsy (idiopathic unilateral facial palsy) who visited the hospital, 123 patients who underwent contrast-enhanced MRI of the internal auditory canal within 7 days of symptom onset were reviewed retrospectively. MRI examination results were investigated along with the patient’s clinical symptoms and electrodiagnostic test results. Based on the MRI results, the frequency of abnormal contrast enhancement and contrast-enhanced areas were investigated. Of the 123 patients, 13 (11%) had normal brain MRI results, and 110 (89%) had abnormal findings. The frequency of abnormal contrast enhancement was not significantly associated with test timing (P = .56). Of the 110 patients with abnormal findings, 65 (59%) showed contrast enhancement in the labyrinthine segment and 36 (33%) in both the labyrinthine segment and geniculate ganglion. Most patients with Bell palsy who are in the acute phase showed abnormal contrast enhancement in their facial nerves, and similar findings were even observed in the examination conducted on the day of symptom onset. Brain MRI helps in the diagnosis of acute Bell palsy.

1. Introduction

Bell palsy is a peripheral facial palsy with unknown organic causes and is the most common cranial neuropathy.^[1,2] Although clinical features are the most important factors in diagnosing Bell palsy, it is helpful to identify objective facial nerve dysfunction or abnormal contrast enhancement through electrophysiological study or brain magnetic resonance imaging (MRI). In evaluating nerve conduction, the results vary significantly depending on the examination timing; in particular, many cases seem normal in the early stages.^[3] Brain MRI is helpful in the diagnosis because the affected facial nerves show abnormal contrast enhancement, but a brain imaging test is not commonly performed and is only done to exclude secondary causes if the typical course could not be identified.^[4] Therefore, although MRI findings of Bell palsy is well known, the evaluation included only a small number of patients, and there are no studies to date on acute patients within days of the onset of paralysis. Bell palsy worsens up to 3 days after onset and gradually improves over 2 to 3 months thereafter. The aim of this study was to investigate the frequency of abnormal contrast enhancement and contrast-enhanced areas by examining MRI findings of acute phase of Bell palsy (within 7 days of symptom onset) in many patients.

2. Methods

2.1. Subjects

The Department of Neurology of our hospital enrolled patients with facial palsy from November 1, 2010. Because the MRI system was changed to 3T in August 2017, we retrospectively reviewed the medical records of patients who visited the hospital with Bell palsy symptoms (acute unilateral facial palsy) from August 2017 to October 31, 2020. Patients with secondary facial palsy that can be identified via medical history, accompanying symptoms, diagnostic tests (confirming trauma, surgery, infection, and/or Ramsay Hunt syndrome), and recurrent cases were excluded. Patients who underwent contrast-enhanced MRI of the internal auditory canal (IAC) within 7 days of symptom onset were included. This study was performed with the approval of the institutional review board of our hospital (CR-23-021).

2.2. Methods

The patient’s sex, age, area of facial palsy, and time from symptom onset to diagnostic test were investigated. To exclude the secondary causes, trauma history, accompanying symptoms, and history of recurrence were considered, the accompanying neurological abnormalities were identified through neurological examination, and electrodiagnostic and brain imaging tests were performed. Facial palsy that occurred within 1 week after head trauma or dental treatment was excluded because they were thought to be related to trauma. Patients with lesions that would cause facial palsy as seen in the brain imaging test and those with painful skin lesions on the ear canal and earlobe suggesting shingles infection were also excluded. In addition, those with recurrent facial palsy were excluded.

For the electrodiagnostic test, the facial nerve conduction study and the blink reflex test were performed. In the facial nerve conduction study, it was determined that it was abnormal if the amplitude of the compound muscle action potential decreased below the normal range. In the blink reflex test, it was determined that it was abnormal if the latency period was prolonged or the waveform was lost based on the presence of R1 and R2 waveforms and the extension of the latency period. They were read by a neurologist with 17 years of experience majoring in neuromuscular diseases.

For the brain imaging test, the IAC-view brain MRI (3T; GE, Milwaukee, WI) was used, and the contrast-enhanced test was performed via intravenous injection of Gadolinium (a bolus of 0.2 cm³/kg of body weight). Abnormal facial nerve enhancement was characterized by marked asymmetric enhancement of the facial nerve in contrast to the contralateral nerve (Fig. 1). The contrast-enhanced area was divided into the meatal segment, labyrinthine segment, geniculate ganglion, and tympanic segment for evaluation. The brain imaging test results were read by a radiologist with 20 years of experience majoring in neuroimaging.

Figure 1.

Contrast-enhanced axial T1-weighted magnetic resonance image shows abnormal enhancement of the labyrinthine segment (arrow) and geniculate ganglion (arrowhead). No enhancement was seen in the meatal segment (brace) and cisternal segment.

Chi-square test was used to analyze the relationship between the timing of MRI and the presence of abnormal enhancement. We also assessed the association between the MRI results and electrodiagnostic test. A P value < .05 was considered to be significant. The statistical analyses were performed using SPSS 25.0 (IBM Corp., Armonk, NY, USA) for Windows.

3. Results

A total of 312 patients visited the hospital with acute unilateral facial palsy, of whom 31 were excluded because they did not undergo examinations after their one-time visit to the hospital and were not followed up. Of the 281 patients, 13 were excluded due to suspicion of secondary causes, and 123 out of the remaining 268 patients who underwent IAC-view MRI within seven days were included (Fig. 2). All patients were Korean.

Figure 2.

Flow diagram of patients’ selection.

Of the 123 patients, there were 64 men and 59 women, with a mean age of 57.2 ± 13.8 (range, 21–87) years (Table 1). There were 68 patients with right facial palsy and 55 with left facial palsy. In total, 81 patients underwent the electrodiagnostic test, with a mean test timing of 10 days after symptom onset (10.4 ± 12.9). There was 1 patient with normal results, 47 patients who were abnormal only in the blink reflex test, and 33 who were abnormal in both the facial nerve conduction study and the blink reflex test.

Table 1.

Demographic characteristics of the patients and clinical characteristics of facial palsy.

Age, years (mean ± SD)	57.2 ± 13.8 (range, 21–87)
Sex (n)
Women	59 (48%)
Men	64 (52%)
Affected side (n)
Right	68 (55%)
Left	55 (45%)
Contrast-enhanced MRI (n)	123
Timing of MRI, days (mean ± SD)	1.7 ± 1.8
Result of MRI
Abnormal	110 (89%)
Normal	13 (11%)
EDX (n)	81
Timing of EDX, days (mean ± SD)	10.4 ± 12.9
Result of facial NCS (n)
Normal	48 (59%)
Abnormal	33 (41%)
Result of blink reflex (n)
Normal	1 (1%)
Abnormal	80 (99%)

EDX = electrodiagnosis, MRI = magnetic resonance image, NCS = nerve conduction study, SD = standard deviation.

The brain MRI results were normal in 13 (11%) patients and abnormal in 110 (89%). Of the 123 patients, 34 (28%) underwent the test within 24 hours after symptom onset; 41 (33%), on the second day; 14 (9%), on the third day; 16 (13%), on the fourth day; and the remaining 18 (15%), between the fifth and seventh days. No significant association was found between the frequency of abnormal contrast enhancement and the timing of the test (P = .56, Fig. 3). Out of the 110 patients who had abnormal findings, 65 (59%) showed contrast enhancement in the labyrinthine segment and 36 (33%) demonstrated the same in both labyrinthine segment and geniculate ganglion (Fig. 4). MRI results were not associated with electrodiagnostic test including facial nerve conduction study and blink reflex test (P = .30 and P = .11, respectively).

Figure 3.

Facial nerve enhancement according to timing of MRI. The frequency of abnormal contrast enhancement is not affected by the timing of the test (P = .56). MRI = magnetic resonance imaging.

Figure 4.

Lesion sites determined via brain MRI. Sixty-nine patients show abnormal enhancement in only one segment, the meatal segment in 2 (2%), labyrinthine segment in 65 (59%), and geniculate ganglion in 2 (2%). Forty patients have enhancing site over 2 segments, both labyrinthine and geniculate ganglion in 36 (33%) and both meatal and labyrinthine segment in 4 (4%). One patient shows diffuse enhancement from meatal segment to geniculate ganglion. MRI = magnetic resonance imaging.

4. Discussion

Bell palsy is an idiopathic disease invading the seventh cranial nerve, and detailed medical history taking of clinical features and accompanying symptoms and a physical examination to identify other otologic and neurological abnormalities are the most important for diagnosis. In general, Bell palsy is considered first if there is acute peripheral facial palsy and no other neurological symptoms.^[1,2] However, hemifacial muscle palsy, including the forehead and muscles around the eyes, does not always indicate peripheral palsy,^[5] and it is sometimes difficult to differentiate based on clinical findings alone. The blink reflex test, facial nerve conduction study, and facial electromyography are performed as additional diagnostic tests,^[1] and brain MRI also shows abnormal findings, which is helpful in diagnosis.

Although the facial nerve conduction study and electromyography are commonly performed, the results are usually normal when performed early because abnormal findings take time to show after symptom onset.^[3] Thus, results from these examinations for diagnostic purposes in the early onset stages would be inaccurate. Since it takes approximately 1 week to show Wallerian degeneration, nerve conduction study results obtained after this period help determine the patient’s prognosis. Therefore, nerve conduction study has value in determining the prognosis rather than the diagnosis in Bell palsy.^[6] Compared to the nerve conduction study, the blink reflex is abnormal even at a relatively early stage and more sensitive in revealing facial nerve abnormality.^[7]

When treating patients with acute facial nerve palsy, imaging tests are not performed on all patients due to the low frequency of severe neurological disorders. It is common to perform imaging tests to determine the cause, such as brain lesions, only when the clinical features are nonspecific or a normal recovery process is not detected.^[4] However, the brain MRI can be performed immediately, unlike electrodiagnostic test, when a patient visits the hospital in the evening or the emergency room on the weekend. Furthermore, abnormal contrast-enhanced findings in the affected facial nerve was commonly observed with brain MRI (up to 100%).^[8–11] The mainly contrast-enhanced areas are known as the geniculation ganglion and labyrinthine segment.^[12] Characteristic imaging findings and involved areas of patients with Bell palsy are well known, but it has the limitation of targeting only a small number of patients. This study shows good diagnostic performance of brain MRI in a large number of patients. In addition, a previous report involved patients several weeks to months after symptom onset. Although the patients visited the hospital within 2 to 3 days after recognizing their symptoms, the acute imaging findings were not studied. In this study, 89% of the patients who underwent the examination within one week showed abnormal contrast-enhanced findings, and 88% of the patients who underwent the examination within 24 hours showed abnormal findings.

There have been several studies about the prognostic value of the MRI in Bell palsy. In this regard, inconsistent findings were observed. A study demonstrated that contrast enhancement of the paralytic facial nerve can be a radiological sign indicating a prolonged recovery.^[13] However, another study reported contrast enhancement of the facial nerve had no effect on the severity and prognosis of Bell palsy.^[14]

This study was based on the registration ledger of facial nerve palsy, but there was a limitation in that detailed data were obtained by reviewing the medical records retrospectively. In addition, since the results were from a single tertiary care hospital and all participants were Korean, it may not be generalizable. However, this study is meaningful in that the MRI findings were studied in a large scale of patients with acute Bell palsy.

In conclusion, most patients with acute Bell palsy in the present study showed abnormal contrast enhancement, and similar findings were found in the examination within 24 hours. Brain MRI is useful in the diagnosis of acute Bell palsy.

Acknowledgments

The author would like to offer special thanks to professor Sang Gyu Kwak for his assistance with statistics.

Author contributions

Conceptualization: Jung Im Seok.

Data curation: Jung Im Seok, Jae Han Park, Jung A Park, Youngrok Do.

Methodology: Jung Im Seok, Jae Han Park.

Supervision: Jung Im Seok.

Writing – original draft: Jung Im Seok.

Writing – review & editing: Jung Im Seok.