Facial Nerve Palsy

The VIIth cranial (facial) nerve is largely motor in function (some sensory fibres from external acoustic meatus, fibres controlling salivation and taste fibres from the anterior tongue in the chorda tympani branch). It also supplies the stapedius (so a complete nerve lesion will alter auditory acuity on the affected side). From the facial nerve nucleus in the brainstem, fibres loop around the VI nucleus before leaving the pons medial to VIII and passing through the internal acoustic meatus. It passes through the petrous temporal in the facial canal, widens to form the geniculate ganglion (taste and salivation) on the medial side of the middle ear whence it turns sharply (and the chorda tympani leaves), to emerge through the stylomastoid foramen to supply all the muscles of facial expression including platysma.

Weakness of the muscles of facial expression and eye closure. Face sags and is drawn across to opposite side on smiling. Voluntary eye closure may not be possible and can produce damage to conjunctiva and cornea.

• In partial paralysis, lower face is generally more affected.
• In severe cases, there is often demonstrable loss of taste over front of tongue and intolerance to high pitched or loud noises. May cause mild dysarthria and difficulty with eating.

The most common system used for describing the degree of paralysis is the House-Brackmann scale, where 1 is normal power and 6 is total paralysis.

It is important to identify whether the patient has an upper motor neurone (UMN) or lower motor neurone (LMN) lesion to assist in identifying cause.

• In a LMN lesion the patient can't wrinkle their forehead - the final common pathway to the muscles is destroyed. Lesion must be either in the pons, or outside the brainstem (posterior fossa, bony canal, middle ear or outside skull).
• In an UMN lesion, the upper facial muscles are partially spared because of alternative pathways in the brainstem i.e. the patient can wrinkle their forehead (unless bilateral lesion) and the sagging of the face seen with lower motor neurone palsies is not as prominent. There appear to be different pathways for voluntary and emotional movement.

CVA's usually weaken voluntary movement often sparing involuntary movements (e.g. spontaneous smiling). The much rarer selective loss of emotional movement is called mimic paralysis and is usually due to a frontal or thalamic lesion.

• If bilateral particularly consider Guillain-Barre or Lyme disease.
• If recurrent particularly consider lymphoma, sarcoidosis, and Lyme disease.
• In children particularly consider Lyme and middle ear disease.

Acute LMN palsy

Acute LMN palsy can present at any age but most frequently seen at 20-50 years affecting both sexes equally. Incidence is around 30 cases per 100,000 per year, slightly higher in pregnant women (45 per 100,000).² There is usually a rapid onset of unilateral facial paralysis. Aching pain below ear or in mastoid area is also common and may suggests middle ear or herpetic cause if severe. There may be hyperacusis, and patients with lesions proximal to the geniculate ganglion may be unable to produce tears and have loss of taste.

Bell's palsy

Originally described by Sir Charles Bell in 1821. Incidence 20/100,000 between 10-40 years, but 59/100,000 over 65 years.
In the past no cause was found in the majority of cases of LMN facial nerve palsy, and these were labelled as idiopathic (i.e. Bell's Palsy).
Recent work suggests that a large number of these cases may be due to herpetic viral infection - particularly Herpes Simplex type 1, or Varicella (herpes) Zoster which clearly may have implications for management.^1,2,7

Ramsay-Hunt syndrome

LMN facial nerve palsy specifically due to Varicella (herpes) zoster. Pain is often a prominent feature and vesicles are seen in the ipsilateral ear, on the hard palate, and/or on the anterior two thirds of the tongue. It can include deafness and vertigo, and other cranial nerves can be affected. When the rash is absent it is known as zoster sine herpete.

• Serology - lyme, herpes and zoster (paired samples 4-6 weeks apart). It may not influence management, but may reveal aetiology.
• Check blood pressure in children with Bell's palsy (2 case reports of aortic coarctation presenting with facial nerve palsy and hypertension).⁸
• The following tests are rarely done but combined with a good understanding of the neuroanatomy can determine the level of the palsy:
  • Schirmer tear test (reveals reduced flow of tears on the side of a palsy affecting the greater palatine nerve).
  • Stapedial reflex (an audiological test absent if stapedius muscle is affected).
  • Electrodiagnostic studies (generally a research tool) reveal no changes in involved facial muscles for the first three days, but a steady decline of electrical activity often occurs over the next week, and will identify the 15% with axonal degeneration.

Ideally this should be a multidisciplinary approach, encompassing ophthalmologists, Ear, Nose, and Throat surgeons, plastic surgeons and psychologists.⁹

General measures

• Reassurance - the majority of cases resolve spontaneously - see prognosis.
• Eye care - ophthalmologists play an important role in preventing irreversible blindness from corneal exposure. This may be successfully achieved by using lubricating drops hourly and eye ointment at night ±eye patch. Botulinum toxin or surgery (upper lid weighting or tarsorraphy) may also be required temporarily.⁹ After the cornea has been protected, but recovery is thought to be unlikely, longer term management of eyelid and facial reanimation may be arranged.

Bell's palsy management

• Drug treatment is still controversial and intradepartmental variation exists.
  • Steroids - Most now give 7-10 days of prednisolone (1mg/kg/day - adult 60-80 mg/day) as early as possible (ideally within 72 hours), either alone or with antivirals because this study showed steroids result in more complete recovery rates at 3 and 9 months.¹⁰
  • Antivirals - There is less evidence for the use of antivirals in the absence of any viral vesicles.¹¹ The general conclusion of an earlier Cochrane analysis was that the use of steroids and aciclovir was neither safe nor evidence based,¹² although a recent multicentre, randomised, placebo-controlled trial of valacyclovir and prednisolone treatment showed that the rate of patient recovery among those treated with valacyclovir was significantly better than the rate among those treated with prednisolone alone.¹³ Further research is required for definitive answers. A current Scottish primary care trial hopes to provide them.¹⁴
    
    In the meantime, a reasonable course of action is to use steroids in early cases, always use aciclovir for Ramsay-Hunt syndrome and consider referral or discuss all non-straightforward cases with neurology or ENT department, who can offer appropriate follow up.
• Surgery - Surgical transmastoid decompression of the facial nerve in severe cases is being investigated but cannot currently be recommended.^2,15 Where nerve fails to regenerate, cosmetic surgery to elevate mouth or anastomosis of hypoglossal nerve to the facial nerve may help.

71% of untreated patients with idiopathic non progressive Bell's palsy recover completely (84% have near-normal function) usually within a few weeks.¹⁶

Poor prognostic features:

• Complete palsy or severe degeneration (electrophysiology)
• No signs of recovery by three weeks
• Age >60
• Severe pain
• Ramsay Hunt syndrome (herpes zoster virus)
• Associated with either hypertension, diabetes, or pregnancy

Those with axonal degeneration may not show any re-innervation for three months, and recovery may be partial or not at all.
Following this synkinesis is often seen e.g. blinking causes angle of mouth to contract. Also aberrant parasympathetic re-innervation may cause symptoms such as gustatory lacrimation ('crocodile tears'). Symptoms can be helped by subcutaneous or intramuscular injections of botulinum toxin.¹