Facial nerve palsy

Causes of facial nerve palsy

Facial nerve inflammation

Facial nerve palsy may be caused by inflammation. In this case, it is called a facial nerve inflammation. Here, the peripheral nerve is inflamed due to viruses or bacteria. The possible pathogens are:

• Neurotropic viruses (viruses that infect nerve cells, e.g. HIV)
• Zoster viruses (in the context of zoster oticus)
• Borellia bacteria (neuroborreliosis)

Peripheral facial nerve palsy

Paralysis of facial muscles due to peripheral damage, i.e. an impairment of the facial nerve (nervus facialis) itself, may have various causes. In 60-75% of cases, the cause is unknown (idiopathic). The remaining 25-40% are due to a variety of causes (Finkensieper et al. 2012 ).

Possible causes of the development of peripheral facial nerve palsy:

• Idiopathic 
  The cause of the genesis is unknown here.

• Traumatic 
  The facial nerve is injured, often due to fractures of the petrous portion of the temporal bone.

• Tumours
  In most cases, the nerve is damaged by the pressure of the tumour. Various tumours may affect the nerve:
  • Tumours of the parotid gland (parotid tumours)
  • Schwannoma of the facial nerve
  • Acoustic neuroma (vestibular schwannoma)
  • Pearl tumour (cholesteatoma)
  • Tumours in the cerebellopontine angle
  • Spread of tumour cells in the meninges (neoplastic meningitis)

• Iatrogenic (caused by medical intervention)
  The damage to the facial nerve may also occur as a possible complication of surgical removal of the parotid gland or the submandibular gland.

• In the context of the following syndromes
  • Guillain-Barre syndrome (acute inflammatory disease of peripheral nerves), especially in Miller-Fisher syndrome (a variant of GBS)
  • Carey Fineman Ziter syndrome (congenital brain malformation)
  • Möbius syndrome (congenital malformation syndrome)
  • Melkersson-Rosenthal syndrome (inflammatory artery disease)
  • Heerfordt syndrome (inflammation of the parotid and lachrymal glands) 

Facial nerve palsy may also develop in the context of metabolic diseases, such as diabetes mellitus (especially in connection with arterial hypertension), and in pregnancy (especially in the last trimester) (Heckmann et al. 2017).

Depending on the cause, facial nerve palsy may also have more specific names:

• Bell's palsy in idiopathic facial nerve palsy
• Ramsay Hunt syndrome in the context of zoster oticus
• Mona Lisa syndrome during pregnancy

Central facial paresis

If central damage, i.e. a dysfunction in the brain, leads to paralysis of the facial muscles, this can also have several causes:

• stroke
• multiple sclerosis
• tumours
• etc.

A sudden paralysis of facial muscles, whether it is due to a peripheral or a central cause, should always be considered an emergency. It is necessary to seek medical advice immediately.

Symptoms of facial nerve palsy

Since the peripheral nerve is usually affected on one side, the symptoms are usually one-sided, i.e. the facial nerve palsy affects one side of the face. Bilateral facial nerve palsy occurs only in rare cases, for example, in patients with borreliosis.

The main symptom of damage to the facial nerve is the paralysis of mimetic muscles, which becomes visible as follows:

• Drooping corner of the mouth
• Drooping eyelid
• Frowning is not possible

Eye muscle paralysis often makes closing the eyelid impossible. There is a risk that the eye will dry out, which must be treated symptomatically .

Apart from the muscles, damage to the facial nerve may also affect other functions of the nerve. Impairments may manifest themselves as follows:

• Decreased tear formation
• Sensitivity to noise (hyperacusis due to damage to the stapedius muscle)
• Diminished saliva production
• Impaired taste perception (anterior two-thirds of the tongue)

Pain, numbness, or tingling of one half of the face are not caused by damage to the facial nerve, but may occasionally be observed in some lesions due to the involvement of other nerves (trigeminal nerve).

Progression & prognosis of facial nerve palsy

The duration and chances of a recovery depend greatly on the cause, nature, and extent of the facial nerve palsy.

The usually spontaneous idiopathic facial nerve palsy has a relatively good prognosis. In 71% of cases, the paralysis usually resolves itself within days or weeks, less often within months. Only about 29% of affected persons have lasting motor deficits. 16% develop synkinesias, which include, for example, simultaneous involuntary movements of the eye when closing the mouth (Peitersen 2002). First-time, one-sided facial nerve palsy also has a good prognosis, especially if the affected persons are ‘young’ (< 60 years), and the nerve is only partially damaged (neurapraxia).

If the nerve is severely damaged or completely severed (axonotmesis), as may be the case, for example, for severe fractures of the petrous bone, complete resolution (remission) of the symptoms is rather unlikely. Incomplete resolution may subsequently lead to severe asymmetries or synkinesias in the face, which are usually very stressful for the affected person.

The paralysis of muscles in the area of the mouth is not only psychologically stressful, but also limits everyday functions. The flaccid muscles cause saliva or liquids to spill out of the corner of the mouth (‘drooling’) when drinking, or cause food to remain in the cheek. The reduced strength may also affect speech. In most cases, the person concerned can no longer form sounds properly, which makes it difficult to understand them (dysarthria/speech disorder).

Symptomatic therapy and rehabilitation are particularly important for patients with severe injuries, permanent deficits, and problems with eating or speaking.

Diagnosis: facial nerve palsy

Facial nerve palsy is usually diagnosed by a neurologist. In some cases, it may also be necessary to consult an otorhinolaryngologist.

The physician begins with a physical examination, which should give information on the form and cause of the facial nerve palsy. Of all the possible symptoms of facial nerve palsy, the function of facial muscles is the easiest to check clinically. The affected person may be told to perform the following tasks before the diagnosis is made (Hacke 2016): 

• Frown
• Open/close your eyes
• Wrinkle your nose
• Mouth/lips: pucker your lips, smile, whistle, puff out your cheeks (including on one side)

Bell's phenomenon is observed if the affected person cannot close his or her eye completely. This means that the upward movement of the eyeball (and thus the white of the eye) is visible, because the eyelid is not completely closed. This movement is a physiological defensive reflex of the eye to protect the cornea (which only occurs in 75% of the population).

Severe pareses can be detected even before mobility is tested. The initial visual examination (inspection) reveals eyelids that have a different width, a drooping eye and corner of the mouth, and asymmetrical facial wrinkles.

Facial nerve palsy is diagnosed by visual and clinical examination, followed by electrophysiological examination, transcranial magnetic stimulation, lumbar puncture, and, rarely, imaging of the head.

The physician has further diagnostic options for a more detailed diagnosis:

• Electrophysiological examination
  (Electroneurography of the distal facial nerve/electromyography of mimetic muscles)
  It clearly indicates whether the facial nerve is damaged and the muscles are weak.
  
• Transcranial magnetic stimulation (at the exit from the brain stem)
  Above all, it may indicate a peripheral problem in the early phase.
  
• Lumbar puncture:
  Lumbar puncture is performed to obtain a sample of the cerebrospinal fluid. If certain pathogens are suspected, the CSF diagnostics can provide more information.
• Head imaging (cranial CT or MRI)
  It is usually not required. It is only performed in cases of suspected central damage (stroke, tumour, etc.).

Therapy for facial nerve palsy

If the cause is known, the focus is on the treatment of the underlying disease. If the cause is unknown, as is the case with the idiopathic facial nerve palsy, standard pharmacological treatment involves the administration of glucocorticoids and/or antiviral medications.

In some cases, however, symptomatic therapy may be necessary, for example, when the eyelid does not close properly. Artificial tears, eye ointment, and/or a protective eye dressing are used to protect the eye from drying out. In the event of persistent problems with eyelid closure, it may sometimes be necessary to apply gold or platinum weights to the eyelid.

Botulinum toxin (Botox for short) is used in some cases for the symptomatic treatment of synkinesias (simultaneous involuntary movement of the eye when moving the mouth). It must be re-injected every 3-4 months to achieve lasting results.

In the case of severe and persistent pareses, microsurgical techniques are sometimes applied for the reconstruction of the facial nerve, e.g.

• using the intact nerve of the opposite side (cross-face facial nerve suture)
• or by connecting to other nerves (hypoglossal-facial jump nerve anastomosis).

The free (dynamic) muscle transfer of the forehead muscle (frontalis muscle) is also an option. Since research in this area has not yet been exhausted, there are increasingly other options for surgical treatment.

Treatment of facial nerve palsy

Therapy for facial muscle paralysis is recommended in the case of central pareses, long-term peripheral pareses, and as a complementary therapy alongside the pharmacological treatment. Exercises in speech therapy, physiotherapy, and/or electrical stimulation serve to restore functions, improve movement disorders (dyskinesia), strengthen the muscles, and increase self-efficacy for psychological reasons.

Speech therapy, physiotherapy & occupational therapy

Exercises to train mimetic muscles are usually performed by speech therapists, but they may also be performed by physiotherapists and occupational therapists. The exercises may be performed when emotions are expressed (especially laughing), and/or in front of a mirror. Examples of exercises to train mimetic muscles:

• Look angry/knit your eyebrows
• Look astonished/raise your eyebrows

• Close your eyes completely (gently/tightly) or partially
• Open your eyes wide
• Blink 

• Raise/wrinkle your nose
• Open your nostrils wide
• Lower your nose/narrow your nostrils 

• Smile (with your mouth closed/open)
• Show your teeth
• Purse your lips (whistle/pucker your lips)
• Puff out your cheeks
• etc.

Various exercises for the forehead, eyes, nose, and/or mouth are selected individually for each patient and serve to improve functions (Pereira et al. 2011). There is evidence for this, especially in chronic cases with a moderate paresis (Teixeira et al. 2011). For patients with severe paresis, starting exercise therapy early leads to positive effects with regards to the severity and duration of regression (Nicastri et al. 2013).

Electrotherapy & biofeedback

In the case of peripheral pareses, the focus is on treatment with special currents (low frequency, long pulse duration). When the nerve is completely severed, especially after surgical procedures, the focus is on electrotherapy to protect the muscles from degradation (atrophy) and from abnormalities of the connective tissue.

The misconception that the stimulation hinders reinnervation (restoration of nerve function) has been refuted in recent studies (Carraro 2018). Initial studies even demonstrate that low-frequency electrical stimulation promotes innervation (Gordon et al. 2016).

In the case of central paralyses of facial muscles, functional electrical stimulation (EMG-triggered multi-channel electrostimulation), biofeedback, or a combination of both is used. Exercises cover one or two muscles (1-channel or 2-channel) and serve to restore functions or prevent involuntary movements.