The facial nerve is the 7th cranial nerve in the human body. It arises from 4 nuclei situated at the lower pons namely, the motor nucleus, superior salivatory nucleus(parasympathetic), lacrimatory nucleus (parasympathetic), and nucleus of Tractus solitarius (gustatory). The nerve is a mixed nerve containing the following components: (1) Branchial motor (special visceral efferent) (2) Visceral motor (general visceral efferent) (3) Special sensory (special afferent) and General sensory (general somatic afferent). The entire course of the facial nerve is presented in intracranial course passage through the temporal bone from the internal auditory meatus to the stylomastoid foramen distribution to musculature to the face & scalp. Motor and sensory roots (nervus intermedius of wrisberg ) have no epineurium and are covered by piamater and surrounded by CSF (Hence making it tensile and resistant to the slow process of stretching and compression). The motor root supplies the muscles of the face, scalp, auricle, buccinator, platysma, stapedius, stylohyoid and posterior belly of the digastric. The special sensory root is gustatory and arises from the anterior 2/3rd of the tongue via the chorda tympani nerve. It also carries the preganglionic parasympathetic (secretomotor) of submandibular and sublingual salivary glands, lacrimal glands, and the glands of nasal and palatine mucosa. The facial nerve passes through the internal auditory meatus to become the meatal portion and then the labyrinthine portion before finally entering the middle ear and exiting at the stylomastoid foramen to follow a tortuous course. The branches of distribution of the facial nerve are as follows:
Within the facial canal
(1) Greater superficial petrosal nerve
(2) Nerve to stapedius
(3) Chorda tympani
At the exit from the facial canal
(1) Post.auricular
(2) Digastric
(3) Stylohyoid
Terminal branches within the parotid gland
(1) Temporal
(2) Zygomatic
(3) Buccal
(4) Marginal Mandibular
(5) Cervical
Causes of facial nerve palsy
1) Congenital
Rare - due to congenital nuclear aplasia - Moebius’ syndrome
Myotonic dystrophy
Melkersson –Rosenthal syndrome
Congenital facial nerve palsy
2) Neurologic
Myasthenia gravis
Multiple sclerosis
Guillian-Barre syndrome
3) Neoplastic
Facial nerve tumors- Schwannoma, Neurofibroma, Neurogenic sarcoma
Paragangliomas
Meningiomas,
Vestibular Schwannoma
Parotid tumors
Temporal bone / external auditory canal tumors
4) Infections
Otitis media, mastoiditis
Bacterial causes (diptheria, tuberculosis)
Viral causes – herpes zoster, lyme disease ,mumps, infectious mononucleosis
5) Iatrogenic
Mandibular block anesthesia
Parotidectomy
Lateral skull base surgery
6) Traumatic
Temporal bone fractures
Penetrating trauma (gunshot)
Facial lacerations
High altitude palsy
7) Other causes
Toxic
Metabolic
Idiopathic - Bell’s palsy, Brainstem infarction
On clinical examination, the patient might complain of facial weakness, loss of taste sensation, reduced saliva and lacrimation and reduced sensation in the external auditory canal depending on the site of involvement of the facial nerve.
Testing of facial nerve function is done by:
ELECTRO DIAGNOSIS – testing the degree of distal axonal degeneration
TOPOGNOSIS – testing the function of accessory branches.
ORTHODOMIC CONDUCTION - The nerve is stimulated proximally and the distal muscle response recorded
ANTIDORMIC CONDUCTION – the ability of the nerve to conduct in a retrograde manner is tested
The following methods are employed to test each of the functions:
Topognosis- lacrimation, stapedial reflex, salivary flow,taste
Electrodiagnosis – NET, Maximal stimulation, EMG, ENoG
Intra-operative monitoring - determines the electrically evoked potential and mechanically evoked potential
Radiology plays an important role in the diagnosis of facial nerve disorders. MRI and CT scans predominantly form imaging modalities to detect facial nerve disorders with/without contrast. 3D reconstruction of CT and MRI provide a life-like display of larger structures within the temporal bone.
The medical line of management depends on the etiology. A wide range of medications can be used to treat facial nerve disorders.
The indications for surgical management of facial nerve palsy are
(1) Nerve palsy progressed from incomplete to complete palsy over a period of hours to days
(2) Dropping response to ENoG to less than 25% of normal or dropping steadily even after the 3rd day of the onset.
(3) Facial nerve tumor infiltration
(4) Facial nerve transection
(5) Palsy associated with CSOM (with or w/o cholesteatoma)
Surgical Modalities include
(1) Decompression
(2) Direct Nerve Repair with/without using a graft
(3) Facial Reanimation: Nerve Grafting
Nerve Crossover
Muscle transposition
Free Muscle Transfer
(4) Re-routing
The results of surgical facial repair are evaluated at 6 months to determine the final improvement.