Bell palsy

Bell palsy, also known as idiopathic peripheral facial paralysis, is characterized by rapid onset facial nerve paralysis, often with resolution in 6-8 weeks, without an identifiable etiology. As there are numerous causes of facial nerve palsy, many acute in onset, it is a diagnosis of exclusion supported by a typical presentation. 

Epidemiology

The peak age of presentation is between 15-50 years with ~25 cases per 100,000 per year. There is no gender predominance ¹⁰.

Risk factors

• pregnancy

• diabetes mellitus

• increasing age

• hypothyroidism ¹⁰

Clinical presentation 

The classical presentation is rapid onset unilateral facial paralysis in a lower motor neuron pattern whereby both upper and lower facial muscles are affected (cf. central cause e.g. stroke where unilateral facial paralysis is in an upper motor neuron pattern and classically only affects the lower facial muscles, specifically sparing the frontalis muscle) ¹². Onset can be as quick as over a few hours, and generally paralysis reaches its peak within 72 hours ¹². 

Associated signs and symptoms include ¹²:

• ipsilateral hyperacusis

• ipsilateral dysguesia, particularly the anterior two-thirds

• pain behind the ipsilateral ear or neck, often the earliest symptom

• lack of automatic-voluntary dissociation

• impaired corneal reflex

In chronic stages, facial synkinesis can develop, whereby there is involuntary facial movement that occurs during voluntary movement of a different facial muscle group ¹⁶. For example, there may be eyelid closure upon opening of the mouth (Marin-Amat syndrome) ^16,17.

Pathology

The etiology of Bell palsy is yet to be elucidated, and thus the diagnosis remains one of exclusion after other etiologies for facial palsy have been considered ^12,13. Prominent theories include reactivation of a virus with herpes simplex virus type 1 (HSV-1) being the strongest pathogenic candidate, an immune-mediated phenomenon, or a microvascular issue ^12,13. Anatomical predisposing factors, such as narrowing of the path of the facial nerve through the internal auditory canal, have also been suggested as contributory ¹³.

Bell's palsy can be a plausible non-serious adverse effect of COVID-19 vaccination ¹⁵.

Radiographic features

There is a limited role for imaging of patients with Bell palsy. MRI should be considered for the following patients:

• decompressive surgery is being contemplated

• atypical signs and symptoms suggest another diagnosis may be possible:

  • slow progressive palsy

  • spasm preceding palsy

  • multiple cranial nerves involved

  • recurrent palsy

  • no recovery after 6-8 weeks

MRI

In Bell palsy, long segments of the facial nerve enhance in a uniformly linear fashion, more intensely than the contralateral non-affected side. However, enhancement of the nerve is not seen in all patients, reported variably between 57-100%. It is also essential to appreciate the normal pattern of facial nerve enhancement, that include the geniculate ganglion and mastoid segments.

The facial nerve on either side of the geniculate ganglion is most frequently involved, from the distal internal auditory canal to the distal tympanic segment. The mastoid and extratemporal segments are less frequently involved.

Importantly, nodularity should raise suspicion of a neoplastic cause and a careful search for remote areas of leptomeningeal enhancement performed.  

Treatment and prognosis

Corticosteroids are the mainstay of treatment, to be started within 72 hours of symptom onset ¹². Additionally, eye care should also be sought, with use of eye drops (artificial tears) and ointments, and taping down eyes shut, to protect the cornea ¹². There is no role for empiric antiviral treatment ¹². For facial synkinesis, therapies such as botulinum toxin A injections can be administered ¹⁶.

Prognosis is generally good, with 70-90% of patients making a full recovery, especially if corticosteroids were used ¹². Recurrent Bell palsy is reported to occur in 4-7% of patients ¹⁴, with multiple recurrences rare. Recurrence can be ipsilateral or contralateral to the original episode. The mean time to recurrence is ~10 years (range 7.8-11.2 years), although ~50% experience recurrence within 5 years ¹¹.

Complications

Complications of incomplete recovery from Bell palsy include ¹²:

• cosmetic, psychological, and social sequelae

• facial synkinesis

• risk of corneal ulceration (without eye care)

History and etymology

It is named after Sir Charles Bell (1774-1842), a Scottish anatomist, surgeon, and physiologist, who described it in 1821 ³.

Differential diagnosis

• care should be taken not to mistake normal facial nerve enhancement on MRI for that seen in Bell palsy

  • focal enhancement in the most lateral aspect of the internal auditory meatus is probably the most useful feature and has been proposed as a marker of severity and prognosis

• Ramsay Hunt syndrome

• facial nerve hemangioma

• perineural spread of tumor

• Guillain-Barré syndrome

• neurosarcoidosis

• lymphoma

• Lyme disease

Practical points

• facial nerve enhancement may persist by up to one year beyond clinical improvement