Approach
Bell's palsy remains a clinical diagnosis of exclusion. The diagnostic history should reflect unilateral facial palsy of acute onset, affecting all branches in equal fashion, whose deficits are fully evolved within 72 hours.Unilateral pain behind the ear, mild-to-moderate otalgia, partial or complete loss of taste, reduced tolerance to sound or phonophobia, and subjective loss of sensation in the tongue and face are commonly reported.
Red flags on history or physical examination that point to an alternative diagnosis include:
Gradual onset (more than 72 hours) facial paralysis
Uneven distribution of weakness across facial zones on presentation
Frontal headache, fever, or general malaise
Bilateral involvement
Prior episode of facial palsy on the ipsilateral side
Persistence of complete flaccid paralysis at 3-4 months
Presence of other cranial or peripheral neuropathies
Presence of otological symptoms other than mild to moderate otalgia, hyperacusis, and post-auricular pain (e.g., hearing loss, vertigo).
Severe otalgia in the absence of vesicles suggests zoster sine herpete of the facial nerve.[38] The presence of general malaise, myalgia, fever, rash (erythema migrans or other), and/or frontal headache, recalled tick bite, or recent travel to a Lyme disease-endemic area should prompt work-up for Lyme disease. Recurrent episodes of Bell’s palsy are rare, and should prompt work-up for autoimmune disease, granulomatous disease, or facial schwannoma.[39] One review found that among patients previously affected by Bell's palsy, the mean incidence of recurrent Bell's palsy was 6.5%.[2] Facial palsy that is insidious in onset, recurrent in nature, or unevenly distributed across facial zones should prompt work-up for other aetiologies such as neoplasms.
History
A careful history should be taken to ascertain the following:
Onset and progression of palsy, precipitating factors (e.g., dental extraction, upper respiratory tract infection, head or facial trauma, pregnancy)
Presence of fever, general malaise, myalgia, arthralgia, headache, rash (erythema migrans or other), recalled tick-bite, or recent travel to Lyme disease-endemic region
Prior episodes of facial palsy, facial, lip, or parotid swelling, or uveitis
Otological symptoms (hearing loss, tinnitus, aural fullness, vertigo, imbalance, otalgia, otorrhoea) or prior otological surgery
Presence of other cranial neuropathies (e.g., double vision, hoarseness)
Current collagen vascular, autoimmune, granulomatous, or metabolic diseases
Use of potentially neurotoxic medications (e.g., paclitaxel)
Known prior or current malignancy or recent chemotherapy
Neck or cheek masses
Current pregnancy or recent delivery
Immunosuppression, HIV.
Examination
In addition to a focused neurological examination, a complete head and neck examination, which includes pneumatic otoscopy, tuning fork exams, and a detailed cranial nerve exam, should always be performed. Examination should ensure that parotid and/or neck masses are not present. Apart from uniformly distributed one-sided facial palsy, the physical examination should be unremarkable. Neural insult in Bell's palsy occurs at the meatal foramen deep within the temporal bone; consequently, all branches of the facial nerve are affected.
Sparing of brow function (i.e., ability to raise the eyebrow on the affected side) indicates an upper motor neuron lesion, as the dorsal division of the facial motor nucleus receives bilateral supranuclear efferent input. Uneven distribution of weakness across facial zones in the acute phase is highly suggestive of a neoplasm in the parotid or elsewhere along the course of the facial nerve and should prompt imaging studies. Examination of the parotid gland includes palpation and visual examination of the oropharynx to rule out a deep lobe parotid tumour, which may displace the tonsil medially.
The presence of vesicles in the external auditory canal or auricle is highly suggestive of varicella-zoster reactivation (herpes zoster oticus or Ramsay Hunt syndrome).
Facial synkinesis is the involuntary and abnormal synchronous movement of a facial region concomitant with reflex or voluntary movement in another facial region and may occur as a late sequela of Bell's palsy (first observed 3-6 months following onset) in up to 30% of patients.[3] The presence of synkinesis concurrent with facial weakness (a mixed picture) in the acute phase is highly suggestive of an alternative diagnosis such as a neoplastic process.
Tests
Bell's palsy is a clinical diagnosis of exclusion. When the history and physical examination are consistent with Bell's palsy, no further diagnostic studies are required, except for patients who have recently travelled to Lyme disease-endemic regions. In the latter case, Lyme serology should always be obtained.
Patients who present with near-complete or complete facial paralysis (i.e., HBS V or VI) should undergo serial electroneuronography (ENoG), also known as evoked electromyography, in order to quantify their degree of neural degeneration and determine whether they might be candidates for neural decompression. Serial ENoG should begin no sooner than 72 hours after the onset of paralysis to allow for Wallerian degeneration to occur and should continue every few days until some degree of recovery is noted or 14-21 days have passed, after which time neural decompression offers no potential benefit. Patients whose ENoG demonstrates (at any time between 72 hours and 14 days) a >90% reduction in the amplitude of the compound muscle action potential (CMAP) compared with the normal side should undergo needle EMG to confirm the absence of voluntary motor unit potentials in facial muscles. Patients meeting both these criteria should be urgently referred to neuro-otology for consideration of neural decompression.
Other investigations to consider when history and physical suggest an alternative diagnosis:
Diagnostic audiogram, including stapedial reflexes. Complete facial paralysis in the presence of an intact stapedius reflex suggests a neoplasm affecting the nerve near the stylomastoid foramen or along its course.
High-resolution (fine-cut) MRI with contrast of the posterior fossa, temporal bone, and parotid gland is indicated if neoplasm is suspected. Enhancement of the facial nerve without expansion of the fallopian canal is normally demonstrated in Bell's palsy and Ramsay Hunt syndrome. Neural enhancement may be seen for up to 1 year following onset.
High-resolution (fine-cut) CT without contrast of the temporal bone is indicated in the setting of recurrent facial palsy, abnormal pneumatic otoscopy suggestive of middle-ear disease, or facial palsy following head trauma.
Further radiological or laboratory investigations may be required, as indicated by clinical assessment.
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