Describe the course of the facial nerve and its applied anatomy in ear disease.

Course of the Facial Nerve

• It is a mixed nerve with four components: motor to the muscles of facial expression, stapedius, stylohyoid and the posterior belly of digastric; secretomotor (parasympathetic) to the lacrimal, submandibular, sublingual and nasal glands; special sensory (taste) from the anterior two-thirds of the tongue; and general sensory from the concha and a small area behind the ear
• The sensory and secretomotor fibres travel as the nervus intermedius, a separate root between the motor root and the eighth nerve

Topodiagnosis and Applied Aspects

• The branches allow the level OF A lesion TO BE deduced from the signs, and this is the classical application — each branch lost places the lesion above its origin:
• Lacrimation reduced (Schirmer test) — lesion at or above the geniculate ganglion, since the greater superficial petrosal nerve leaves there
• Stapedial reflex absent, with hyperacusis — lesion above the nerve to stapedius
• Taste lost and salivary flow reduced — lesion above the chorda tympani
• All of these intact with facial weakness — lesion below the chorda, in the mastoid segment or extratemporal
• The most important clinical distinction IS upper versus lower motor neurone, and it turns on the forehead: the upper face has bilateral cortical representation, so an upper motor neurone (supranuclear) lesion spares the forehead and the patient can still wrinkle the brow and close the eye; a lower motor neurone lesion involves the whole half of the face
• Two refinements are worth knowing: an upper motor neurone lesion often shows emotional sparing, the face moving normally on spontaneous laughter though not to command, because emotional pathways are separate; and it is accompanied by other neurological signs
• Causes relevant to ENT — bell’S palsy, much the commonest, a diagnosis of exclusion; RAMSAY hunt syndrome (herpes zoster oticus, with vesicles in the concha and canal, severe pain and a worse prognosis); acute and chronic suppurative otitis media, and cholesteatoma, in which facial palsy is an indication for urgent surgery; malignant otitis externa; temporal bone fracture; iatrogenic injury at mastoid or parotid surgery; parotid malignancy, in which facial palsy strongly suggests malignancy rather than a benign tumour; and acoustic neuroma
• Temporal bone fractures — a longitudinal fracture (80%) more often causes conductive loss with delayed, incomplete palsy which recovers; a transverse fracture causes sensorineural loss and immediate, complete palsy, implying transection, which warrants exploration
• The timing distinction is what matters: immediate and complete palsy suggests transection and needs surgical consideration; delayed or incomplete palsy suggests oedema and is managed conservatively
• Grading is by the house-brackmann scale from I (normal) to VI (total paralysis), and should be recorded at every visit so that progress can be judged
• The eye IS the urgent responsibility in any facial palsy: lubricants, ointment and taping at night, and tarsorrhaphy where the cornea is threatened — particularly if corneal sensation is also impaired
• The labyrinthine segment is the narrowest, which is why oedema compresses the nerve there and why decompression targets it
• The tympanic segment is dehiscent in ten to fifteen per cent, and is the segment most often injured in middle ear surgery
• Acoustic neuroma spares the facial nerve until late despite stretching it, which is a diagnostic peculiarity worth knowing