SEVENTH CRANIAL NERVE

SEVENTH CRANIAL NERVE

(Facial Nerve)

APPLIED ANATOMY

Facial nerve nucleus is situated in the lowermost and ventral most part of pons. Facial nerve fibres originate from it and then course backwards and hook round the nucleus of 6th cranial nerve, thereby an eminence called facial eminence is formed on the floor of the fourth ventricle. The fibres then Come out from the substance of the pons at the cerebellopontine angle and then course along the base of the brain together with 8th cranial nerve and pars intermedius of Wrisberg (which courses in the opposite direction along with it upto geniculate ganglion and contains the taste fibres). The nerve then enters the petrous part of the temporal bone along with the 8th cranial nerve which now follows a separate course. The facial nerve then enters another bony canal called facial canal at the bend of which there is a ganglion called geniculate ganglion from where the pars intermedius of Wrisberg starts. From this ganglion three petrosal nerves are derived. The nerve then courses downwards and gives a branch to stapedius muscle and only 2-4 mm above the stylomastoid foramen gives rise to chorda tympani nerve.

The nerve then comes out through the stylomastoid foramen, gives four branches and then divides into two main divisions inside the substance of the parotid gland. The upper one is called temporofacial division and the lower one is called cervicofacial division which supply the individual muscles on the face.

FACIAL PARALYSIS

Facial paralysis can be conveniently divided into upper motor neurone or supranuclear paralysis and lower motor neurone paralysis. The latter is again divided into nuclear and infranuclear types of paralysis.

Upper motor neurone type of Paralysis

Site: It may occur anywhere from the cortex upto just above the facial nerve nucleus in the pons but the commonest site is the internal capsule.

Causes: Cerebrovascular accidents (commonest being thrombosis) involving lenticulostriate branch of middle cerebral artery.

Effect:

(i) Paralysis of the lower half of the face opposite to the side of lesion. (The upper half escapes because it is controlled from both sided pyramidal tracts).

(ii) Hemiplegia opposite to the side of lesion but on the same side as facial paralysis.

(iii) Wasting of the affected muscle is absent.

(iv) Reaction of degeneration does not occur in facial muscles.

(v) If hemi opia is present it indicates a hemisphere lesion.

During emotional movement, the angle of the mouth is not deviated as the fibres controlling emotional movements after being derived from frontal lobe travel along a different pathway. Therefore, deviation of the face during emotional movements may occur without any deviation during voluntary movement when the supranuclear pathway for emotional movements from frontal lobe to facial nucleus is affected. This is called Mimic facial palsy.

Lower motor neurone Paralysis

Effect varies according to the site of lesion as given below. Reaction of degeneration and wasting are constantly present.

Nuclear type of Paralysis

Site: Pons.

Causes: Vascular lesions, trauma, tumour, encephalitis, syringobulbia, chronic motor neurone disease, disseminated sclerosis, bulbar poliomyelitis etc.

Effect:

(i) Paralysis of one-half of the face on the same side as lesion.

(ii) 6th nerve palsy on the same side as lesion.

(iii) Hemiplegia on the opposite side.

(iv) Paralysis of the lower Jaw.

This is a form of crossed paralysis and is called Millard-Gubler’s syndrome.

When the lesion is acute as in pontine haemorrhage some other features are seen in addition to above. These are:

(v) Pin point pupil.

(vi) Deep coma.

(vii) Hyperpyrexia.

(viii) Horner’s syndrome.

(ix) Reversed conjugate ocular deviation etc.

Infranuclear type of Paralysis

(a) Site: Cerebello-pontine angle.

Causes: Cerebellar tumour, auditory nerve tumour, trauma.

Effects:

(i) Paralysis of one-half of the face on the same side as lesion.

(ii) Perceptive type of deafness.

(iii) Cerebellar syndrome.

(iv) Loss of taste sensation on the anterior two-thirds of tongue on the same side. Loss of corneal reflex due to involvement of ophthalmic division of Trigeminal nerve.

(b) Site: Base of the brain.

Causes: Auditory nerve tumour, fracture base of skul1, basal meningitis, etc.

Effects

(i) Paralysis of one-half of the face on the same side as lesion.

(ii) Perceptive type of deafness.

(c) Site: Entry in the facial canal.

Causes: Chronic suppurative otitis media, post mastoidectomy, osteomyelitis, fracture, secondary deposits etc.

Effects

(i) Paralysis of one-half of the face on the same side as lesion.

(ii) Loss of taste sensation on the anterior two-thirds of the tongue on the same side.

(iii) Hyperacusis (increased sensation to ordinary auditory stimuli) on the affected side.

(iv) Dizziness at times, loss of balance.

(d) Site: Geniculate ganglion.

Cause: It is due to herpetic affection of the geniculate ganglion.

Effects:

Onset-Acute with severe pain in the ear.

(i) Paralysis of one-half of the face on the same side as lesion.

(ii) Loss of taste sensation on the anterior two-thirds of the tongue on the same side.

(iii) Hyperacusis.

(iv) Fever which may come with slight chill.

(v) Aches and pains all over the body.

(vi) Appearance of herpetic rashes on the base of the concha, posterior half of the tympanic membrane and posterior wall of the external auditory meatus, occasionally in tongue and pharynx.

(vii) Complete recovery is unusual.

All these constitue Ramsay-Hunt’s syndrome or Geniculate Herpes.

(e) Site: At the level of Nerve to stapedius.

Causes: As mentioned in (c).

Effect:

(i) Paralysis of one-half of the face on the same side as lesion.

(ii) Hyperacusis.

(iii) Loss of taste sensation on the anterior (i) two-thirds of the tongue on the same side.

(f) Site: At the level of chorda tympani nerve.

Causes: As mentioned in (c)

Effect:

(i) Paralysis of one-half of the face on the same side as lesion.

(ii) Loss of taste sensation on the anterior two-thirds of the tongue on the same side.

(g) Site: Stylomastoid foramen (Lowest end of facial canal).

Causes: It occurs after exposure to chill and cold when inflammation of the nerve sheath or periosteum of the bony canal develops. The nerve is compressed and paralysis results.

Effects: Paralysis of one-half of the face on the same side as lesion.

This is called Bell’s palsy and is the commonest of all infranuclear types of facial palsy.

(h) Site: Extreme periphery of the facial nerve.

Causes: Peripheral neuritis form various causes, parotid, tumour, operation on parotid, birth injury by forceps delivery etc.

Effect: Paralysis of one-half of the face on the same side of the lesion but it depends on the involved fibres.

CAUSES OF BILATERAL FOCIAL PALSY

1. Leprosy.

2. Acute infective polyneuritis.

3. Birth injury by forceps.

4. Basal meningitis.

5. Fracture base of skull.

6. Syphilitic osteosclerosis.

7. Uveoparotid syndrome (Sarcoidosis).

8. Bilateral otitis media.

9. Leukaemia.

10. Encephalitis.

11. Bilateral Bell’s Palsy.

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