Facial nerve palsy is a common and often distressing presenting complaint for patients. They are often worried they have suffered a stroke. Although this is one cause of this condition, a thorough history and examination with a sound understanding of the neuroanatomy can help decipher the underlying cause and direct appropriate treatment.
When faced with a patient with a facial nerve palsy, the first question you must consider is if this is an upper or lower motor neurone lesion. An upper motor neurone lesion will classically spare the forehead muscle frontalis, and so there will be preservation of forehead wrinkles when the patient raises their eyebrows. This is not the case with a lower motor neurone lesion.
When asked, many think the following statement is true: “The forehead is spared in upper motor neurone lesions because there is bilateral innervation of frontalis”. Strictly speaking, this is wrong. There is bilateral cortical representation of frontalis. The former statement implies the left facial nerve innervates both left and right frontalis and vice versa, but this is not true. Consider Figure 1, a depiction of a patient with a right hemispheric stroke.
This patient has a right cortical defect and is asked to look up and smile. The patient cannot raise the left side of his mouth, but there is forehead sparing as evidenced by creases along the length of the forehead. The left cortex innervates the right upper and lower facial nucleus but only the upper part of the left facial nucleus. This is the explanation behind bilateral cortical representation of the forehead as both cortices can control both sides of frontalis. This cortical representation is also situated in the anterior cingulated cortex rather than the motor cortex. This explains the dissociation between voluntary and emotional facial movements in some patients – classically a patient will have a normal smile but an impairment when they are asked to show their teeth (as seen in figure 2).
If an upper motor neurone lesion is suspected, a good clinical history and examination can often localise the lesion to either the cortex or brainstem. Cortical lesions, often strokes, will have symptoms and signs in the contralateral limbs, whereas brain stem lesions may have crossed cranial nerve signs e.g. upper motor neurone facial nerve palsy with a 3rd nerve palsy suggests a midbrain lesion. At this point, further imaging is usually indicated to characterise the exact location and nature of the lesion. These options will be discussed later.
If a lower motor neurone lesion is suspected i.e. the forehead is not spared, an understanding of the facial nerve route can help locate where in the course of the facial nerve it has got damaged. Figure 3 is a schematic diagram showing the course of the facial nerve:
The origin of the facial nerve is shown in the left panel of figure 3. This is a cross section of the pons where the nucleus of the facial nerve originates (VII). Note the facial nerve actually takes a slight detour, looping around the VIth nerve nucleus before exiting the ventral surface of the pons alongside the VIIIth nerve and the nervus intermedius. This detour is important to know as nuclear facial nerve lesions are often associated with ipsilateral 6th nerve palsies.
The facial nerve, along with the VIIIth nerve and nervus intermedius occupies a space called the cerebellopontine angle before entering the facial canal via the internal auditory meatus (number 1 in the right panel fig 2). The facial nerve travels a short distance to merge with the nervus intermedius to form the geniculate ganglion (numbers 2 and 3). This ganglion is important as it supplies nerves to the lacrimal and nasal glands via the greater superficial petrosal nerve and the sphenopalatine ganglion. The main bulk of the facial nerve continues giving a small branch to stapedius (number 4). This tiny muscle contracts to dampen down the transmission of loud and potentially harmful sounds to the ear drum by reducing the oscillation of the ossicles. If this nerve is damaged, patients become sensitive to loud noises (“hyperacusis”). This is therefore a useful question to ask to localise the lesion proximal to this branch of the facial nerve. Before leaving the facial canal via the stylomastoid foramen, the chorda tympani branches from the main facial nerve to supply taste to the anterior 2/3rds of the tongue as well as salivary gland stimulation via the submaxillary ganglion. Again, asking about ageusia (loss of taste) can help localise a lesion proximal to the origin of the chorda tympani. As the facial nerve exits the stylomastoid foramen, it gives off a small sensory nerve called the posterior auricular nerve (number 5) which supplies a small patch of skin behind the ear. This may explain the ear pain some patients get in the days preceding Bell’s palsy and is certainly affected when varicella zoster virus infects the facial nerve resulting in vesicles around the ear. Finally the facial nerve travels within the parotid gland (not shown) to split into its five main motor branches serving the face – temporal, zygomatic, buccal, marginal mandibular and cervical (sometimes remembered as “To Zanzibar By Motor Car”). A parotid mass with a facial nerve palsy could indicate infiltration and therefore should be treated as a malignancy until proven otherwise.
The facial nerve functions which are clinically important can be summarised as the following:
Causes of Facial nerve palsy
The causes of facial nerve palsy may be categorised by anatomical location and pathological process. Upper motor neurone lesions may be caused by a lesion in the motor cortex, the subcortex or corticobulbar tracts. Commonly the aetiology is vascular in origin (i.e. stroke) but can include masses (e.g. neoplasm, abscess) or neurodegenerative disease (e.g. a supranuclear palsy can cause a facial apraxia even in the absence of overt muscle weakness).
Lower motor neurone lesions can be categorised anatomically to a few key areas in the facial nerve course:
1) Nuclear: vascular, inflammatory and occasionally infiltrative pathology can affect the facial nerve nucleus or intrapontine fascicle. It is usually associated with an ipsilateral 6th nerve palsy and may also affect the descending corticospinal tracts causing a contralateral limb weakness – the Millard Gubler syndrome.
2) Cerebellopontine anglesyndrome: the cerebellopontine angle is a triangular recess bordered by the pons, cerebellar peduncle and petrous temporal bone laterally. Its contents include the Vth nerve superiorly, the IXth and Xth nerve inferiorly and the VIIth and VIIIth nerve in between. One of the first signs of this syndrome is the loss of corneal reflex on the ipsilateral side. It is usually caused by an acoustic neuroma but can also be caused by meningiomas, metastases, cholesteotomas or even aneurysms.
3) Facial canal: the proximal part of the canal (called the labyrinthine portion) is particularly prone to ischaemia and compression due to its relatively poor blood supply and small calibre. Transverse fractures of the temporal bone are a common cause of facial nerve injury. Infection such as malignant otitis externa or suppurative otitis media can spread to the skull base leading to facial nerve palsy. Varicella zoster infection affecting the geniculate ganglion can lead to a facial nerve palsy with vesicles around the ear. This is called Ramsay Hunt syndrome. Neoplastic processes including metastases or nasopharyngeal carcinoma can also affect the facial nerve here. Finally, it is thought that damage to the nerve in the facial canal causes Bell’s palsy.
4) Parotid region: a parotid mass with a facial nerve palsy should be treated with a high index of suspicion for malignancy. Inflammatory parotitis from infection or granulomatous conditions e.g. sarcoidosis can cause facial nerve lesions. Other aetiologies distal to the stylomastoid foramen include iatrogenic e.g. complication of carotid endarterectomy or temporal artery biopsy.
Examination technique for when a patient presents with facial nerve palsy:
A full cranial and peripheral nerve examination should be performed. Once you decide whether the lesion is upper motor neurone or lower motor neurone, the examination should be tailored to find more clues as to the location of the lesion and potential complications of facial nerve palsy.
Looking for the location of the lesion:
For upper motor neurone lesions, evidence of forehead sparing is usually sufficient. An examination of the other cranial nerves should be performed to see if this is a lesion in the corticobulbar tracts, in the brainstem or higher up. The limbs should be examined for increased tone, hyperreflexia, pyramidal weakness and upward going plantars.
For lower motor neurone lesions, answering the following questions may help to localise the lesion:
Looking for complications of facial nerve palsy:
The main complications include drying of the eyes, particularly at night where the eyes cannot stay shut. This may lead to an exposure keratitis. Some patients with persistent facial nerve palsy may have their eyelid surgically manipulated to correct for this – an operation called tarsorrhaphy. Patients can also suffer from epiphora – excessive lacrimation due to orbicularis oculi muscle weakness which impairs the pumping action of the canaliculi. Drooling may occur around the mouth and drinking may also be difficult.
Bilateral facial nerve palsy
This is an unusual occurrence and usually prompts a diagnostic hunt for the underlying cause. The top three causes of this condition include:
Other causes of bilateral facial nerve palsy may also be categorised by the anatomical location of the lesion:
There are also some rare conditions which cause facial nerve palsies including:
In essence,Bell’s palsy is an acute lower motor neurone facial nerve palsy which was previously thought to be of unknown cause but this opinion is changing (see pathology). Provided is a summary of the disease characteristics:
1) Incidence: 20/100,000 per year (1)
2) Age: may occur in childhood but risk increases with age.
3) Sex: equally affected
4) Geography: no correlation with location
5) Predisposing factors: weak associations with diabetes (2) and hypertension, risk is three times greater during pregnancy, particularly during the 3rd trimester or in the first postpartum week (3)
6) Pathology: Detection of herpes simplex virus type I DNA in endoneurial fluid is present in most patients (4) . Both primary infection and reactivation of latent infection have been implicated.
7) Clinical features:
9) Differential diagnosis:
1) Peitersen E. The natural history of Bell's palsy. Am J Otol. 1982;4(2):107.
2) Mountain RE, Murray JA, Quaba A, Maynard C. The Edinburgh facial palsy clinic: a review of three years' activity. J R Coll Surg Edinb. 1994;39(5):275
3) Hilsinger RL Jr, Adour KK, Doty HE Idiopathic facial paralysis, pregnancy, and the menstrual cycle. Ann Otol Rhinol Laryngol. 1975;84(4 Pt 1):433
4) Murakami S, Mizobuchi M, Nakashiro Y, Doi T, Hato N, Yanagihara N. Bell palsy and herpes simplex virus: identification of viral DNA in endoneurial fluid and muscle.Ann Intern Med. 1996;124(1 Pt 1):27.
5) Hashisaki GT. Medical management of Bell's palsy. Compr Ther. 1997;23(11):715.
6) Esslen E. Investigations on the localization and pathogenesis of meato-labyrinthine facial palsies. In: The Acute Facial Palsies, Esslen E (Ed), Springer-Verlag, Berlin 1977. p.41.
7) Gilden DH. Clinical practice. Bell's Palsy. N Engl J Med. 2004;351(13):1323.
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