Section 1: Epidemiology and aetiology
Bell's palsy is a lower motor neurone palsy of the facial nerve of idiopathic aetiology. Sir Charles Bell, the Scottish anatomist, in 1821 first described facial muscle paralysis following damage to the facial nerve.
Bell's palsy is the most common cause of facial paralysis, accounting for 70 per cent of facial palsies, when other causes have been eliminated.
It has an incidence of 11-40 per 100,000 per year, and most commonly occurs in females in their teens and twenties. The distribution is almost equal in the thirties, with a slight predominance in males over 40.
A correlation between facial palsy and pregnancy has been noted, especially during the third trimester, and in the presence of pre-eclampsia. Pregnancy is associated with a worse prognosis. Immunodeficiency due to HIV can predispose to facial palsy.
There are many reports of an association of Bell's palsy with a viral aetiology, based on clinical, experimental and post-mortem observations.
Herpes simplex virus (HSV) has been isolated from the nasopharynx of patients with Bell's palsy, and HSV autoantibodies are more prevalent in affected patients than ageand gender-matched controls.
Autopsy studies have demonstrated HSV particles in sensory ganglia of cranial nerves, such as the geniculate ganglion of the facial nerve and trigeminal ganglion of the trigeminal nerve.
HSV DNA has been identified in facial nerve endoneurial fluid and posterior auricular muscle in patients who underwent surgical decompression of the facial nerve, which was not responding to treatment.1
It is thought that latent viral reinfection of the facial nerve predisposes to Bell's palsy.
Vulnerable facial nerve
An alternative theory is small-vessel ischaemia of the facial nerve, resulting in venous stasis, reduced capillary flow, oedema and elevated intraneural pressure. Tissue damage occurs through acidosis and anoxia.2 The mechanism of primary ischaemia is unclear.
A final theory is immunological damage to the facial nerve, as evidenced by segmental demyelination of the nerve associated with lymphocytic infiltration of the perineurium.3 Immune complexes have been found in the chorda tympani branch of the facial nerve, characteristic of a viral-antibody (Type III) immunological reaction.
Regardless of the mechanism of insult, inflammation, compression and ischaemia of the facial nerve can occur as it traverses the temporal bone within its bony canal.
The bony canal is narrowest where the facial nerve exits the internal auditory meatus, and in the labyrinthine segment of the canal the nerve occupies at least 80 per cent of the cross-sectional area. For these reasons, the facial nerve is vulnerable in the presence of oedema at both these sites.
Section 2: Making the diagnosis
Bell's palsy typically presents as an acute unilateral paresis or paralysis of the face due to lower motor neurone dysfunction, with all branches of the facial nerve involved (see photos below). The onset and progression are rapid, usually in less than 48 hours. Patient with Bell's palsey below, showing paralysis of the left facial muscles at rest (Figure A) and on being asked to raise his eyebrows and smile (Figure B).
Diagnosis is clinical, and requires exclusion of less common causes of facial nerve palsy, such as acute or chronic otitis media, cholesteatoma, parotid tumour and temporal bone fracture, once otoscopy and parotid and cranial nerve examination have been performed.
Unlike Bell's palsy, a stroke will present with signs of an upper motor neurone lesion, with sparing of the forehead, due to bilateral cortical innervation of the upper facial muscles.
Other cranial nerves
Other cranial nerves in close proximity to the facial nerve in the skull base, such as the trigeminal (V), abducent (VI), vestibulocochlear (VIII), glossopharyngeal (IX) and vagus (X), may be involved, suggesting the facial weakness is the inflammatory motor component of a more extensive cranial polyneuropathy.4
There may be pain (50 per cent of cases) or numbness in the ear, face and tongue (40 per cent), hyperacusis (33 per cent), reflecting involvement of the vestibulocochlear nerve and nerve to stapedius, and dysgeusia (33 per cent), due to involvement of the chorda tympani branch of the facial nerve (subserving taste to the anterior two-thirds of the tongue), and more rarely the lingual branch of the glossopharyngeal (taste to the posterior one-third).
Rarely, there may be reduced lacrimation due to involvement of the greater petrosal nerve, which supplies the lacrimal gland and exits the facial nerve at the geniculate ganglion. A viral prodrome is present in 31 per cent of patients.5
Otoscopy will exclude haemotympanum and acute or chronic inflammation of the external and middle ear. Of particular note is herpes zoster oticus (Ramsay Hunt syndrome), a syndrome of acute facial palsy with otalgia and painful varicelliform cutaneous vesicles, which affects the external auditory meatus and pre-auricular skin, or soft palate.
This condition accounts for 10-15 per cent of cases of facial palsy. It is more commonly associated with other cranial neuropathies, in particular hearing loss, dysacusis and vertigo, than Bell's palsy.6 It is thought to be due to latent reactivation of varicella zoster virus in the geniculate ganglion.
The extent of facial palsy can be assessed using a grading system, such as the House-Brackmann scale (see box below), which is the preferred standard of the American Academy of Otolaryngology.7
This can provide clinical information at the time of initial presentation, which acts as a prognostic indicator and a point of comparison on follow-up.
|House-Brackmann facial nerve grading scale7|
|*reflex movement of groups of muscles that do not normally contract together, for example: blinking may occur with movement of the corner of the mouth.|
Section 3: Managing the condition
Treatment aims to reduce the disability of facial paresis, and address the inability to fully close one eye.
To prevent corneal dehydration, abrasion and ulceration, drops or lubricants can be papplied.
The lower eyelid can be elevated with tape running towards the orbital rim to improve lid closure, and a patch can be applied for sleeping.
Historically, based on the assumption that the nerve palsy is due to viral inflammation, treatment comprised steroids and antivirals.
Two RCTs compared prednisolone with and without aciclovir with placebo.8,9 Prednisolone alone resulted in a statistically higher recovery rate at three months than placebo, and also at nine months.
However, the addition of aciclovir to prednisolone compared with placebo showed no significant improvement at three months.
More recently, a meta-analysis of 18 trials involving 2,786 patients concluded that anti-virals alone are ineffective.10
Administration of antivirals plus steroids showed a possible increased benefit compared with steroids alone, but results were not significant.
The recommended regimen for prednisolone is 1mg/kg/day for five days (in divided doses) followed by a 10 day taper.
Aciclovir is of questionable value, but if used in addition to prednisolone, it is prescribed at 800mg five times daily for five days.
The antivirals used prevent viral replication by disrupting DNA polymerase. The virus is not destroyed, and this may explain why antivirals fail to have a significant benefit in treating Bell's palsy.
Referral to ENT
Refer to ENT if there is no improvement in facial palsy after three weeks. A pure tone audiogram will establish any hearing loss.
In patients with long-standing facial palsy who have been referred to ENT, transcutaneous electrical stimulation of the facial muscles can be used to preserve membrane conductivity, minimise muscle atrophy and improve facial function.
Inability to close the eye in such cases may require implantation of a gold weight in the upper lid. Alternatively, a tarsorrhaphy (suturing the upper and lower lid margins laterally) will reduce the palpebral fissure, thereby protecting the cornea.
Nerve grafting, rhytidectomy, blepharoplasty, brow lift and facial slings can improve resting muscle tone.11
Botulinum toxin can reduce tonic contractions, hemifacial spasm and synkinesis.12
Section 4: Prognosis and follow-up
Incomplete Bell's palsy at the time of first presentation is a good prognostic sign, with 99 per cent of patients making a complete recovery.
Only 75 per cent of patients with a full palsy will undergo complete resolution. Seventy-one per cent of untreated cases resolve by one year, and the remaining 29 per cent have residual paralysis, synkinesis and contracture.
Early onset of recovery increases the chance of complete resolution: 100 per cent of patients who start to recover in the first week make a full recovery, but only 61 per cent do so if recovery begins in the third week.5
If a patient's facial palsy has progressed or not started to recover three to six weeks after first presentation, referral to an ENT specialist will enable further investigation of an underlying neoplasm.
Recurrent facial nerve paralysis can occur in up to 12 per cent of patients. Patients with a recurrence are 2.5 times more likely to have a positive family history.13 Ipsilateral recurrence is an indication for referral to ENT, again to exclude a tumour. Contralateral recurrence is usually benign.
The prognosis for Ramsay Hunt syndrome is worse, with persistent facial weakness in 30-50 per cent of patients, and full recovery in only 10 per cent after total loss of function without treatment.14
Investigating nerve function
An ENT specialist may arrange evoked electromyography (EEMG) or electroneuronography (ENoG) to investigate facial nerve function in a case of complete paralysis which fails to improve.
Such tests are, however, time-consuming, expensive and only of use soon after onset of paralysis, usually before the patient has been referred to hospital.
ENoG delivers a stimulus to the facial nerve as it passes through the stylomastoid foramen, and measures an evoked compound muscle action potential. Its prognostic potential is greatest when used from three days to three weeks after onset of paralysis.
EEMG records post-synaptic potentials with electrodes in the nasolabial fold, and is most reliable during the onset of denervation, i.e. two to three weeks from onset of paralysis.
The earlier the EEMG response drops to less than 10 per cent of normal, the worse the prognosis.
Surgical decompression of the facial nerve can be offered in the third week if EEMG amplitudes are less than 10 per cent of the normal side, but long-term trials are awaited.
1. Murakami S, Mizobuchi M, Nakashiro Y et al. Bell palsy and herpes simplex virus: identification of viral DNA in endoneurial fluid and muscle. Ann Intern Med 1996; 124: 27-30.
2. Takeda T, Takeda S, Kozakura K et al. An animal model of ischemic facial palsy. Eur Arch Otorhinolaryngol 1994; S418-9.
3. Wakisaka H, Hato N, Honda N et al. Demyelination associated with HSV-1-induced facial paralysis. Exp Neurol 2002; 178: 68-79.
4. McKeever P, Proctor B, Proud G. Cranial nerve lesions in Bell's palsy. Otolaryngol Head Neck Surg 1987; 97: 326-7.
5. Peitersen E. Bell's palsy: the spontaneous course of 2,500 peripheral facial nerve palsies of different etiologies. Acta Otolaryngol Suppl 2002; 4-30.
6. Devriese PP, Moesker WH. The natural history of facial paralysis in herpes zoster. Clin Otolaryngol Allied Sci 1988; 13: 289-98.
7. House JW, Brackmann DE. Facial nerve grading system. Otolaryngol Head Neck Surg 1985; 93: 146-7.
8. Sullivan FM, Swan IR, Donnan PT et al. Early treatment with prednisolone or acyclovir in Bell's palsy. N Engl J Med 2007; 357: 1598-607.
9. Engstrom M, Berg T, Stjernquist-Desatnik A et al. Prednisolone and valaciclovir in Bell's palsy: a randomised, double-blind, placebo-controlled, multicentre trial. Lancet Neurol 2008; 7: 993-1000.
10. de Almeida JR, Al Khabori M, Guyatt GH et al. Combined corticosteroid and antiviral treatment for Bell palsy: a systematic review and meta-analysis. JAMA 2009; 302: 985-93.
11. Kartush JM, Linstrom CJ, McCann PM et al. Early gold weight eyelid implantation for facial paralysis. Otolaryngol Head Neck Surg 1990; 103: 1016-23.
12. Laskawi R, Damenz W, Roggenkamper P et al. Botulinum toxin treatment in patients with facial synkinesis. Eur Arch Otorhinolaryngol 1994; S195-9.
13. Pitts DB, Adour KK, Hilsinger RL, Jr. Recurrent Bell's palsy: analysis of 140 patients. Laryngoscope 1988; 98: 535-40.
14. Robillard RB, Hilsinger RL, Jr., Adour KK. Ramsay Hunt facial paralysis: clinical analyses of 185 patients. Otolaryngol Head Neck Surg 1986; 95: 292-7.
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