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Date posted: November 27, 2011

Dr P Muhammed Muneer BHMS,MD(Hom)

Anatomy of Facial Nerve
The facial nerve has a motor and sensory root, the latter being the nervus intermedius. The two roots emerge at the caudal border of the pons lateral to the recess between the inferior olive and inferior cerebellar peduncle, the motor part being medial; the vestibulocochlear nerve is lateral to the sensory root. The nervus intermedius usually cleaves at first to the vestibulocochlear rather than the facial nerve, passing to the latter as it approaches the internal acoustic meatus.

The motor root supplies: the muscles of the face, scalp and auricle, the buccinator, platysma, stapedius, stylohyoid, and the posterior belly of digastric.

The sensory root: conveys from the chorda tympani gustatory fibers from the presulcal area the tongue and, from the palatine and greater petrosal nerves, taste fibres from the soft palate. It also carries preganglionic parasympathetic innervation of the submandibular and sublingual salivary glands, lachrymal gland and glands of nasal and palatine mucosa.

Course of Facial Nerve
From their emergence from the brain, the roots pass anterolaterally with the vestibulocochlear nerve to the internal acoustic meatus; here the motor root is in an anterosuperior groove on the vestibulocochlear nerve, with the sensory root between them. At the lateral end of the meatus the nerve enters the facial canal and then descends to the stylomastoid foramen. Emerging from the foramen the nerve runs forwards in the parotid gland crosses the styloid process, retromandibular vein and external carotid artery and divides behind the neck of the mandible into branches which pierce the anteromedial surface of the parotid gland and diverge under cover of it. They form a network known as parotid plexus which supplies the facial musculature.

The Branches of Facial Nerve

In the facial canal – Nerve to stapedius
Chorda tympani

At exit from stylomastoid foramen – Posterior auricular
Digastric, posterior belly
Stylohyoid

On the face -  Temporal
Zygomatic
Buccal
Marginal Mandibular
Cervical

Applied Anatomy
Facial paralysis, often unilateral, may be due to:
(1) Supranuclear lesions in the cortico nuclear fibers from the frontal lobe, variably combined with numerous other descending fibers converging in the facial nucleus.
(2) Nuclear or infranuclear lesions involving lower motor neurons.

Supranuclear facial paralysis involving “upper motor neuron” pathways is usually a part of hemiplegia. Movements in the lower part of the face are usually more severely affected, voluntary movements being weak or absent though emotional expression is little affected. Electrical reaction of affected muscles are unaltered. Occasionally supranuclear lesions may abolish or weaken emotional movements but not voluntary movements. This dissociation shows that the supranuclear control of expressive movements is separate from the corticonuclear path for voluntary movements.

Nuclear or infranuclear lesions vary in their effects according to the lesion’s site. If the nucleus or facial pontine fibres are involved, neighbouring structures are inevitably also involved. Facial muscles are represented in cell groups within the nucleus; their degree of involvement governs the extent of paralysis, which is ipsilateral. Lesions due to adjacent damage include paralysis of the lateral rectus because of involvement of the abducent nucleus around which the facial nerve loops.

The involvement of the motor trigeminal nucleus causes paralysis of masticatory muscles.Sensory loss on the face are caused by the involvement of the principal sensory and spinal trigeminal nuclei or spinothalamic tract and paralysis of the upper and lower limbs due to corticospinal lesions. Due to proximity of facial sensory root the vestibulocochclear nerve, lesions in the posterior cranial fossa or in the internal acoustic meatus may cause loss of taste in the part of tongue with ipsilateral deafness and facial paralysis. When damage is in the temporal bone, the chorda tympani is involved and in petrous temporal fractures the vestibulocochlear nerve is involved.

Bilateral facial paralysis are caused by
(1) Bilateral infranuclear lesion’s :
• Acute idiopathic polyneuritis
• Leprosy
• Leukemia
• Syphilitic or meningococcal meningitis
• Double otitis media
• Rheumatic
• Post diphtheritic
• Bell’s Palsy
• Uveoparotid paralysis
(2) Muscle diseases:
• Myasthenia gravis/ myotonic dystrophy
• Facio-scapulo humeral dystrophy

Signs: Flattening of all normal folds, sagging of corners of mouth, fixed expression less mask like face, no voluntary movements of facial muscles. White of eye seen when patient attempts to close them. Patient talks as if he had severe stomatitis.

BELL’S PALSY
Is an acute apparently isolated LMN facial palsy for which no cause can be found.
Aetiology:
(a) Associated known clinical condition – Diabetes Mellitus, severe hypertension in last trimester of pregnancy, dental anesthesia.
(b) Causes – (i) Exposure to cold, oedema and subsequent compression of the nerve trunk within rigid fallopian canal causes circulatory disturbance, (ii) other important causes of acute facial palsy include suppurative otitis media, herpes zoster, head injury, Guillian-Barre syndrome, sarcoidosis and multiple sclerosis.

Symptoms:
sudden following exposure to chill or without any apparent precipitating cause, maximum paralysis in twenty-four hours. Post auricular pain is common and many precede paralysis by two days. There may be spontaneous loss of sense of taste, hyperacusis and watering of eyes. Sweating is less on affected side.

Signs:
Forehead cannot be wrinkled, frowning lost, eyes cannot be closed. On attempting closure eye ball turns upwards and outwards, known as Bell’s phenomenon. On showing teeth, the lips do not separate on affected side. Whistling not possible. Articulation of labial components difficult. Nasolabial fold flattened out. Angle of affected side droops with dribbling of saliva. Cheeks puffs out with expiration because of buccinator paralysis. Food collects between teeth and paralysed cheek. Fluid runs out while drinking. Base of tongue lowered.Vesicles within the external auditory meatus and ear drum in Ramsay Hunt Syndrome. Pain may precede facial weakness. Deafness may result.

Investigations
: Electromyography.
Management: Local heat.
Local treatment of muscles.
Protection of eye.

Reference
1. Gray’s anatomy
2. Harrisson’s Principles of internal medicine .15th edition
3. Last’s anatomy

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