Facial Nerve Injury: Anatomy, Classification, and Management, Slides of Medicine

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DEGREE OF NERVEINJURY BY TKSTUDYBUD

COURSE OF THE FACIAL NERVE

COURSE OF THE NERVE IS DIVIDED INTO 3 PARTS:INTRACRANIAL: PONS TO INTERNAL ACOUSTIC MEATUS (15-17mm)INTRATEMPORAL: INTERNAL ACOUSTIC MEATUS TO STYLOMASTOID FORAMEN. SEGMENTS- •^ MEATAL SEGMENT - 8-10mm •^ LABYRINTHINE SEGMENT- 4mm •^ TYMPANIC OR HORIZONTAL SEGMENT- 11mm •^ MASTOID OR VERTICAL SEGMENT- 13mmEXTRACRANIAL: STYLOMASTOID FORAMEN TO TERMINATION OF ITS PERIPHERAL BRANCHES

OLDER CLASSIFCATION OF NERVE INJURIES (SEDDON)

DEGREE OF NERVE INJURY DETERMINES THE REGENERATIONOF NERVE AND ITS FUNCTION. •^ NEUROPRAXIA

- CONDUCTION BLOCK (FLOW OF

AXOPLASM THROUGH AXONS PARTIALLY OBSTRUCTED),INTRAFASCICULAR ODEMA, POSSIBLE SEGMENTALDEMYELINATION. • AXONOTMESIS

- AXON SEVERED,

WALLERIAN

DEGENERATION, ENDONEURIAL TUBE STILL INTACT. • NEUROTMESIS

- SEVERE INJURY TO THE NERVES, LOSS OF

CONTINUITY.

SUNDERLAND CLASSIFICATION (1951) CLASSIFIED INTO 5 DEGREES OF SEVERITY BASED ON ANATOMICALSTRUCTURE OF THE NERVE.

WIDELY ACCEPTED CLASSIFICATION.

ST^1 DEGREE (NEUROPRAXIA) : • PARTIAL BLOCK TO FLOW OF AXOPLASM. • NO MORPHOLOGICAL CHANGES SEEN. • RECOVERY OF FUNCTION COMPLETE. • GOOD PROGNOSIS. ND^2 DEGREE (AXONOTMESIS):

-^ LOSS OF AXONS WITH ENDONEURIAL TUBES STILL INTACT. •^ DURING RECOVERY,

AXONS WILL GROW INTO THEIR RESPECTIVE TUBES.

ETIOLOGY OF FACIAL NERVE INJURIES Trauma to the facial nerve has been described in the following categories : • Basal skull fracture (temporal bone)- most common cause of trauma,leading to facial nerve palsy, • Penetrating trauma to the extratemporal aspect of the facial nerve • Birth (forceps delivery) • Iatrogenic (for example post parotidectomy or mastoidectomy) • Barotrauma (altitude paralysis or scuba diving) • LightningST 1 3 DEGREES- SEEN IN INFLAMMATORY AND VIRAL DISORDERSTHTH^4 , 5

DEGREES- SEEN IN SURGICAL OR ACCIDENTAL TRAUMA, NEOPLASMS.

NERVE FUNCTION TESTING

These tests are best at least 2 to14 days after injury for Wallerian degeneration of axons to takeplace.They are only necessary in cases of paralysis. It is not used in cases of paresis as the nerveis intact.The two types of tests are: •^

Electroneurography or evoked electromyography (ENOG or evoked EMG) - measuresevoked muscle action potential using skin electrodes. Nerve injury is a percentage offunction relative to the normal side.

-^

Electromyography (EMG) - measures voluntary muscle response with electrodes in thetarget muscle detecting action potentials during muscle contraction with a functioningnerve. The above tests can help classify the nerve injury according to the Sunderland classification.

SURGICAL MANAGEMENT Nerve damage secondary to impingement (the nerve isintact) • Decompression of epineural sheath in a proximal to distalfashion. Nerve transection (Sunderland 5th degree of nerve injury) • Either primary neurorrhaphy using fine suture or fibrin glue oran interposition nerve graft when a primary anastomosis is notpossible • Interpositional nerve graft is common with the greater auricularnerve.