Facial nerve disorders
Anatomy:
Facial nerve runs from pons to parotid. It is actually a mixed nerve having motor and a sensory root. The sensory component is also known as the nerve of Wrisberg and it carries secretomotor fibers to the lacrimal gland and salivary glands. This also bring fibers of taste and general sensation. Thus there are two efferents and two afferent pathways.
Components of facial nerve:
1. Special visceral efferent - This forms the motor root and supplies all the muscles derived from the second branchial arch (all the muscles of facial expression, auricular muscles, stylohyoid, posterior belly of diagastric and the stapedius
2. General visceral efferent - This component supplies secretomotor fibers to lacrimal, submandibular and sublingual glands. The smaller secretory glands in the nasal mucosa and the palate are also supplied by this component
3. Special visceral afferent - Brings taste fibers from the anterior two thirds of tongue via the chorda tympani and hard and soft palate via the greater superficial petrosal nerve. Taste is carried to the nucleus of tractus solitarius
4. General somatic afferent - This component brings general sensation from the concha, postero superior part of external canal and the ear drum. These fibres classically accounts for the presence of vesicular eruption in herpes zoster infection of the geniculate ganglion. It also brings proprioceptive sensation from the facial muscles.
Facial nerve nucleus:
Motor nucleus of the facial nerve is situated in the pons. It receives fibers from the precentral gyrus. The upper part of this nucleus is known to innervate forehead muscles and receives fibers from both the cerebral hemispheres. The lower portion of this nucleus supplies the muscles of lower face receives crossed fibers from one hemisphere. This accounts for the fact that the functions of forehead muscles are preserved in supranuclear lesions because of its bilateral innervation. Facial nucles also receives fibers from thalamus which is known to occur via alternate routes. This provides for involuntary control of facial muscles. The emotional movements of face like smiling / crying are preserved in supranuclear palsies because of these thalamic fibers.
Facial nerve course:
Motor fibers of facial nerve originates from the nucleus of 7th nerve. It hooks around the nucleus of 6th nerve and are joined by the sensory root (nerve of wrisberg). Facial nerve leaves the brain stem at the pontomedullary junction. It travels through the posterior cranial fossa and enters the internal acoustic meatus. At the lateral most part of the internal auditory meatus known as the fundus the nerve enters the canal for facial nerve. It traverses in the bony facial canal inside the temporal bone and exits out of it via the stylomastoid foramen.
The course of the facial nerve can b e divided into the following parts:
1. Intracranial part - This part likes entirely within the skull. It includes the facial nerve from the nucleus to the internal auditory canal. Its length is about 15-17 mm.
2. Intratemporal part - This portion of the facial nerve extends from the internal auditory meatus to the stylomastoid foramen. This portion of facial nerve can further be divided into:
Meatal segment - This segment is about 8-10 mm. This segment lies
entirely within the acoustic meatus.
Labyrinthine segment - This segment is about 4 mm. From fundus of
the meatus to the geniculate ganglion where the nerve makes a turn
posteriorly forming a "genu". This segment is the narrowest in
diameter (0.61-0.68 mm). Oedema or inflammation can compress the
nerve causing paralysis. This is also the shortest segment of the
facial nerve.
Tympanic also known as horizontal segment - This segment is about
11 mm long. It extends from the geniculate ganglion to just above
the pyramidal eminence. It lies above the oval window and below
the lateral semicircular canal.
Mastoid also known as vertical segment - This segment is about 13
mm long. It begins from the pyramidal process and extends up to
the stylomastoid foramen. Between the tympanic and mastoid segments lies the second genu of the nerve.
3. Extracranial part - This extends from the stylomastoid foramen as termination of its peripheral branches. This is also known as the parotid plexus or pes anserinus (goose's foot). The division actually takes place within the parotid gland. Commonly it divides into the following branches:
1. Temporal branches - Crosses the zygomatic arch to the temporal region
2. Zygomatic branches - Crosses the zygomatic arch to the orbit
3. Buccal branches - Passes forwards to below the orbit and around the mouth
4. Marginal mandibular branch - Passes forwards to the lower lip and chin
Branches of the facial nerve include:
1. Greater superficial petrosal nerve - This nerve arises from the geniculate ganglion and carries secretomotor fibres to lacrimal gland and glands of nasal mucosa and palate.
2. Nerve to stapedius - It arises at the level of second genu and supplies the stapedius muscle
3. Chorda tympani nerve - This branch of facial nerve arises from the middle of vertical segment and passes between the incus and neck of the malleus. It leaves the tympanic cavity through petrotympanic fissure. It also carries secretomtor fibers to submandibular and sublingual glands and brings taste from anterior two thirds of the tongue.
4. Communicating branch - This branch of facial nerve joins the auricular branch of vagus and supplies the concha, retroauricular groove, posterior meatus and the outer surface of ear drum.
5. Posterior auricular nerve - This branch supplies muscles of pinna, occipital belly of occipitofrontalis and communicates with auricular branch of vagus.
6. Muscular branches - This supplies the stylohyoid, and posterior belly of digastric.
7. Peripheral branches - The nerve trunk after crossing the styloid process forms two divisions i.e. a upper temporofacial and lower cervicofacial, which further divide into smaller branches. These branches include temporal, zygomatic, buccal, mandibular and cervical.
Blood supply:
Facial nerve derives its blood supply from the following vessels:
1. Anterior inferior cerebellar artery - supplies the nerve at the CP angle
2. Labyrinthine artery - This is a branch of anterior inferior cerebellar artery supplies the nerve in the internal auditory canal
3. Superficial petrosal artery - This is a branch of middle meningeal artery which supplies geniculate ganglion and the adjacent region
4. Stylomastoid artery a branch of posterior auricular artery supplies the mastoid and tympanic segments of facial nerve. All these arteries form an external plexus which is in the epineurium and feeds a deeper intraneural internal plexus.
Surgical landmarks of facial nerve for mastoid and middle ear surgeries:
These landmarks are useful in identifying the course of the nerve during mastoid surgeries. The following are some of the reliable landmarks that could be used by the surgeon:
1. Processes cocheariformis - This helps in the identification of geniculate ganglion which lies anterior to it. The tympanic segment of the nerve starts at this level
2. Oval window and horizontal canal - The facial nerve runs above the oval window and helow the horizontal semicircular canal
3. Short process of incus - Facial nerve lies medial to the short process of incus at the level of aditus
4. Pyramid - Facial nerve runs behind the pyramid and the posterior tympanic sulcus
5. Tympanomastoid suture - The vertical / mastoid segment of facial nerve runs behind this suture line
6. Digastric ridge - The facial nerve exits the mastoid at the anterior end of digastric ridge
5. Cervical branch which runs forwards over the suprahyoid muscle and supplies the platysma muscle.
Landmarks for identification of facial nerve during parotid surgery:
1. Pointer cartilage - The facial nerve lies 1 cm deep and slightly anterior and inferior to the pointer. Cartilage pointer is actually a sharp triangular piece of cartilage of the pinna that points to the facial nerve
2. Tympanomastoid suture - Facial nerve lies 6-8 mm deep to this suture line
3. Styloid process - The facial nerve crosses lateral to the styloid process
4. Posterior belly of digastric - On tracing the posterior belly of digastric muscle backwards along its upper border to its attachment to the digastric groove, the facial nerve could be found to lie between it and the styloid process
Anomalies involving the facial nerve:
1. Dehiscence:
This is actually the most common anomaly. Normally facial neve lies sheathed by bony covering forming the bony canal. Dehiscence (absence of this bony covering over the nerve) is the common anomaly. This abnormality is commonly seen in the tympanic segment over the oval window. Sometimes bony covering over geniculate ganglion may be dehiscent. The nerve could also be dehiscent in the retrofacial mastoid air cell area. Dehiscent nerve is prone to injury during surgery. It can also be easily affected during infections.
2. Prolapse of the facial nerve:
The dehiscent facial nerve could prolapse over the stapes and could make stapes surgery or ossicular reconstruction pretty difficult
3. Facial nerve hump:
The facial nervbe could make a hump posteriorly near the lateral canal making it vulnerable to injury during mastoid surgery.
4. Bifurcation / Trifucation of facial nerve:
In this anomaly the facial nerve in its vertical part divides into two / three branches, each branch occupying a separate bony canal and exits through individual foramen.
5. Bifurcation and enclosing stapes :
In this type the facial nerve divides proximal to the oval window. Obe branch passing above and the other one below, uniting later.
6. Course between round and oval windows:
In this abnormality, the nerve crosses the middle ear passing between oval and round windows. Anomolies are more common in congenitally malformed ears. This fact should be borne in mind while performing surgery in patients with congenitally malformed ears.
Severity of nerve injury can be graded using the following mechanism:
1. Neuropraxia - This is a conduction block where flow of axoplasm through the axons is partially obstructed.
2. Axonotmesis - Injury to axons
3. Neurotmesis - Injury to nerve
Sunderland classification:
First degree paralysis:
Partial block to flow of axoplasm; no morphological changes could be seen. Recovery of function is complete (neuropraxia).
Second degree paralysis:
Loss of azons, but endoneural tubes remain intact. During recovery, axons will grow into their respective tubes and the result is good (axonotemesis).
Third degree paralysis:
In this there is injury to endoneurium. During recovery phase, axons of one tube can grow into another. Synkinesis can occur (neurotmesis).
Fourth degree paralysis:
There is injury to perineurium in addition to all of the above. Scarring will impair regeneration of fibres (partial transection).
Fifth degree paralysis:
There is injury to epineurium seen in this type in addition to all the facts stated above (complete transection).
The first three degrees are seen in viral inflammatory disorders, while the fourth and fifth degrees are seen in surgical / accidental trauma to then nerve or in neoplasms.
House Brackmann's classification of facial nerve function grading:
This grading system is used to characterize the degree of facial nerve paralysis. In this grading system, grade I is assigned to normal function, while grade VI would represent complete paralysis. the intermediate grades usually vary according to function at rest and with effort.
Grade I - Normal facial function in all areas
Grade II (mild dysfunction) -
Gross - Sight weakness which is oticeable on close inspection
May have slight synkinesis
At rest, normal symmetry and tone
Motion - Forehead - moderate to good function
Eye - Complete closure with minimal effort
Mouth - Slight asymmetry
Grade III (Moderate dysfunction)
Gross - Obvious but not disfiguring difference between sides
Noticeable (not severe) synkinesis, contracture or hemifacial spasm
Motion - Forehead - Slight to moderate movement
Eye - Complete closure with effort
Mouth - Slightly weak with maximum effort
Grade IV (Moderately severe dysfunction)
Gross - Obvious weakness / disfiguring asymmetry
At rest, normal symmetry and tone
Motion - Forehead - None
Eye - Incomplete closure
Mouth - Asymmetrical with maximum effort
Grade V (Severe dysfunction)
Gross - Only barely perceptible motion
At rest, asymmetry
Motion - Forehead - None
Eye - Incomplete closure
Mouth - Slight movement
Grade VI (Total paralysis)
No movement at all
Causes of facial paralysis:
Causes for facial nerve paralysis could be due to:
Central causes:
1. Brain abscess 2. Pontine gliomas 3. Poliomyelitis 4. Multiple sclerosis
Peripheral causes:
Peripheral lesion could involve the nerve in its intracranial, intratemporal or extratemporal portions. Peripheral lesions are common than central ones. Nearly 2/3 of facial paralysis of them are peripheral lesions and of idiopathic in nature.
Intracranial portion of facial nerve:
1. Acoustic neuroma
2. Meningioma
3. Congenital cholesteatoma
4. Metastatic carcinoma
5. Meningitis
Intratemporal part of facial nerve:
Idiopathic causes
Bell's palsy
Melkersson Rosenthal syndrome
Infections
ASOM
CSOM
Ramsay Hunt syndrome
Malignant otitis externa
Trauma
Surgical - Mastoidectomy and stapedectomy
Accidents - Fracture temporal bone
Neoplasms
Malignancies of external & middle ear
Glomus jugulare tumor
Facial nerve neuroma
Metastasis of temporal bone (breast cancer, bronchus and
prostrate)
Extracranial portion of facial nerve
Malignancy parotid
Surgery of parotid
Accidental injury in parotid region
Neonatal facial injury (forceps delivery)
Systemic diseases
Diabetes mellitus
Hypothyroidism
Uraemia
Polyarteritis nodosa
Wegener's granulomatosis
Sarcoidosis
Leprosy
Leukemia
Demyelinating diseases
Electrodiagnostic tests:
Electrodiagnositc tests are very useful in differentiating neuropraxia from degeneration of the nerve. These tests are also of immense prognostic value and could indicate the time for surgical decompression of the nerve.
Minimal nerve excitability test:
In this test the nerve is stimulated at steadily increasing intensity till the facial twitch is just noticeable. This is compared with that of the normal side. In conduction block there is no difference between the normal and paralyzed side.
In injuries associated with degeneration nerve excitability is lost gradually. If the difference between the sides exceed 3.5 m amp the test should be considered as positive for degeneration. It should be noted that degeneration of nerve fibers cannot be detected before 48-72 hours of its commencement.
Maximal stimulation test:
This is similar to the minimal nerve excitability test, but instead of measuring the minimal nerve excitability threshold the current level which elicits maximal facial movement is determined and is compared with that of the opposite side. Reponse is visually graded as equal, decreased or absent. Reduced or absent response with maximal stimulation indicates degeneration and in these cases recovery is incomplete.
Electroneuronography (ENoG):
This is a type of evoked electromyography. The facial nerve is stimulated at the level of stylomastoid foramen and the compound muscle action potentials are picked up by surface electrodes and recorded. Supramaximal stimulation is used to obtain maximal action potentials. The responses of action potentials of the paralyzed side are compared with that of the normal side. Percentage of degenererating fibers can be calculated. Faster the rate of degeneration within the first two weeks poorer is the prognosis.
Electromyography:
This test assesses the motor activity of facial muscles by direct insertion of needle electrodes into the orbicularis oculi and orbicularis oris muscles. Recordings are made during rest and voluntary contraction of muscle. In normal resting muscle, biphasic or triphasic potentials are seen every 30-50 ms. In denervated muscle spontaneous involuntary action potentials known as fibrillation potentials are seen. These potentials appear within 14-21 days after denervation of the muscle. This test is useful in planning reanimation procedures. Preservation of these potentials after 1 year of injury indicates that reinnervation is taking place. Electroneuronography and electromyography are complementary and helps in prognosis of these cases. It also helps in decision making regarding the procedure for reanimation.
Localization of lesions involving the facial nerve:
Central lesions causing facial paralysis can be caused by cerebrovascular accidents (hemorrhage, thrombosis or embolism), tumor or abscess. These lesions cause paralysis only in the lower half of face on the contralateral side. Forehead movements are retained due to bilateral innervation of frontalis muscle. Involuntary emotional movements and the tone of the facial muscles are also retained.
Peripheral lesions causing facial paralysis involve all muscles of the face, and on the involved side these muscles are paralysed. These patients are unable to frown, close the eye, purse the lips or whistle.
Lesions at the level of nucleus can be identified by associated paralysis of 6th cranial nerve.
CP angle lesions can be identified by the presence of vestibular and auditory defects. Other cranial nerves like 5th, 9th, 10th and 11th cranial nerves could also be affected.
Level of lesions of facial nerve from the internal acoustic meatus up to the stylomastoid foramen can be localised by topognostic tests. Lesions involving the facial nerve outside the temporal bone, i.e. in the parotid area affects only the motor functions of the nerve. Facial nerve paralysis could be incomplete as some branches of the nerve could survive carcinoma involvement.
Topognostic tests for lesions involving facial nerve in its intratemporal portion:
Schirmer test:
This test is basically prepared to quantitate tear production. This test is performed by placing strips of white filter paper at the junction of the middle and lateral thirds of the lower eyelids after administration of a topical anesthetic agent. The tear production is measured with the eyes closed. Produced tears will wet the filter paper. The length of the filter paper which becomes wet is assessed at the end of 5 minutes. Normal test result is between 10mm and 30 mm of wet filter paper. Normally it should not exceed 30 mm. A value of more than 30 mm is considered to be epiphora. A value of less than 10 mm is considered to be dry eye (hyposecretion).
Stapedial reflex:
This reflex is lost in lesions involving intratemporal portion of the facial nerve (above the level of the nerve to stapedius). This can be tested by tympanometry.
Taste test:
Taste can be measured by a pinch of salt or sugar solution placed on one side of the protruded tongue, or by electrogustometry. Impairment of taste indicates lesion above the chorda tympani nerve.
Submandibular salivary flow test:
This test measures the function of chorda tympani nerve. Polythene tubes are passed into both wharton ducts and drops of saliva is counted during one minute period. Decreased salivation indicates injury to facial nerve above the level of chorda.
Complications of facial paralysis:
Incomplete recovery - Facial asymmetry persists. Eye cannot be closed resulting in epiphora. A weak oral sphincter causes drooling and difficulty in taking food.
Exposure keratitis - When eye cannot be closed tear film from the cornea evaporates causing dryness, exposure keratitis and corneal ulcer. This condition worsens in the presence of decreased lacrimation which is a feature of facial paralysis. This condition can be prevented by use of artificial tears (methylcellulose eye drops) every 1-2 hours. Temporary tarsorrhapy is also indicated. Eye closure can be improved by using gold weight implant sutured to the tarsal plate deep to levator palpebrae muscle.
Synkinesis - This occurs when the patient wishes to close the eye, the corner of the mouth also twitches. This is due to cross innervation of fibers.
Tics and spasms - This condition is caused as a result of faulty regeneration of fibers. Involuntary movements are seen on the affected side of the face.
Contractures - This is due to fibrosis of atrophied muscles or fixed contraction of a group of muscles. They affect movements of face but facial symmetry at rest is good.
Crocodile tears - There is unilateral lacrimation when the patient mastigates. This is due to faulty regeneration of parasympathetic fibers which supply the lacrimal glands instead of salivary glands. This condition can be treated by sectioning of greater superficial petrosal nerve or tympanic neurectomy.
Frey's syndrome (gustatory sweating) - In this condition, there is flusihing and sweating of skin over parotid area during mastication. This results from parotid surgery.
Hyperkinetic disorders of facial nerve:
Hemifacial spasm - This is characterised by repeated, uncontrollable twitchings of facial muscles on one side. There are two types of hemifacial spasm.
Essential (idiopathic) the cause is unknown. This condition can be treated by selective sectioning of the branches of facial nerve in the parotid or by puncturing the facial nerve with a needle in its tympanic segment. Botulium toxin injection can be used in the affected muscle. It blocks the neuromuscular transmission by preventing release of acetylcholine.
Secondary hemifacial spasm - This is caused by acoustic neuroma, congenital cholesteatoma, or glomus tumor. Majority of hemifacial spasm is caused by irritation of the nerve because of the presence of vascular loop at the CP angle. Microvascular decompression through posterior fossa craniotomy has produced high success rates.
Blepharospasm - In this condition twitchings and spasm are limited to orbicularis oculi muscles on both sides. The eyes are closed due to muscle spasms causing functional blindness. The cause is uncertain but could be due to lesions involving basal ganglia. Botulinum A toxin can be injected into the periorbital muscles would provide relief for 3-6 months. Injections can be repeated if needed.
Surgical management of facial nerve paralysis:
1. Decompression - The facial nerve may be compressed by oedema. Removing its bony covering and by excising its sheath would allow space for its expansion and would provide relief.
2. END to END anastomosis - This can be done if the gap between the severed ends of the nerve is just a few millimeters and suturing of the cut ends without undue tension is possible.
3. Nerve graft (cable graft) - When the gap between the severed ends of the nerve cannot be closed by end to end anastomosis, then nerve grafting can be performed. Greater auricular nerve, or lateral cutaneous nerve of the thigh, or sural nerve can be used for this purpose.
4. Hypoglossal facial anatomosis - In this procedure hypoglossal nerve could be anastomosed to the severed end of the facial nerve. This procedure improves facial muscle tone and also permits some movements of facial musculature. The tongue muscles atrophy on the same side.
