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Facial Palsy in the Middle Ear - Causes, Investigations to Determine Level of Injury, and Management

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I. ANATOMY OF THE FACIAL NERVE IN THE MIDDLE EAR (Intratemporal Course)

1. Labyrinthine segment (~4 mm) - shortest; from meatal foramen to geniculate ganglion; narrowest point (0.61-0.68 mm) where only pia and arachnoid invest the nerve - pathophysiologically the most vulnerable segment
2. Tympanic (horizontal) segment - runs along the medial wall of the middle ear cleft, superior to the oval window; most commonly dehiscent (~55% in some series); closely related to the stapes and footplate
3. Mastoid (vertical) segment - descends from the second genu to the stylomastoid foramen; passes posterior to the oval window niche

• Greater superficial petrosal nerve (GSPN) - from geniculate ganglion - carries parasympathetics to lacrimal gland
• Nerve to stapedius - in mastoid segment
• Chorda tympani - 5 mm above stylomastoid foramen - taste anterior 2/3 tongue, submandibular/sublingual gland secretion

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II. CAUSES OF FACIAL PALSY IN THE MIDDLE EAR

A. Infective/Inflammatory

• Incidence: 0.5% in pre-antibiotic era; now 0.005%
• Mechanism: Neuropraxia due to oedema, nerve compression, or bacterial toxic metabolites acting on a congenitally dehiscent facial canal
• Organisms: Predominantly Staphylococcus aureus; viral AOM also reported
• ~80% are partial palsies; 80% respond to ventilation tube insertion + IV antibiotics
• Most children achieve complete recovery (mean 4 months)

• Frequency: 0.16%-2.62% of COM cases
• Mechanism: Granulation tissue and/or cholesteatoma erodes the bony fallopian canal - the nerve and its vascular plexus become inflamed and axonal fibres degenerate
• Prognosis is worse in COM than in trauma, Bell's palsy, or AOM
• The tympanic segment is most vulnerable due to natural dehiscence

• Varicella-zoster virus reactivation in the geniculate ganglion
• Presents with: otalgia, vesicles on auricle/EAC, facial palsy, sensorineural hearing loss, vertigo
• Poorer prognosis than Bell's palsy (complete recovery in only ~50%)

• Pseudomonas aeruginosa infection, usually in elderly diabetics/immunocompromised
• Extends to skull base via tympanomastoid fissure to involve the facial nerve at the stylomastoid foramen

• Extension of AOM to mastoid coalescence or petrous apex
• Gradenigo's triad: otorrhoea + ipsilateral VIth nerve palsy + facial pain (CN V) - facial palsy may also occur

• Still common in developing countries
• Presents insidiously; destructive granulomatous process; facial palsy may be the presenting feature

B. Traumatic

• Complicates 7% of temporal bone fractures overall
• Transverse fractures (perpendicular to petrous axis): cross the labyrinthine/geniculate region - higher rate of facial palsy (~50%) but less common fracture type
• Longitudinal fractures (parallel to petrous axis): more common (80%); lower rate of facial palsy (~20%) - usually at the geniculate ganglion or tympanic segment
• Injury types by Sunderland classification: neuropraxia (1st degree) to complete transection (5th degree)
• Immediate complete palsy = nerve transection until proven otherwise; delayed or incomplete = oedema/neuropraxia (better prognosis)

• Middle ear over-pressure during ascent (diving, flying)
• Neuropraxia secondary to compression of vasa nervorum
• No other neurological symptoms; typically resolves with decompression
• No treatment usually required

• During myringoplasty, tympanoplasty, mastoid surgery, stapedectomy
• Immediate palsy: direct injury (transection, crush, stretch)
• Delayed palsy: oedema, local anaesthetic effect, reactivation of herpes simplex (HSV-1 reactivation - implicated in delayed palsy after uneventful middle ear surgery)

C. Neoplastic

• Glomus jugulare extends into middle ear/mastoid; causes progressive facial palsy
• Vascular mass behind tympanic membrane

• Intrinsic tumour of the nerve; insidious, progressive palsy
• Most common in geniculate region
• 30% present with facial palsy before diagnosis

• As above; can also develop as congenital middle ear cholesteatoma

• T-cell lymphoblastic lymphoma of the middle ear (reported)
• Squamous cell carcinoma of the temporal bone

D. Other

• Lyme disease (Borrelia burgdorferi) - can affect the facial nerve via middle ear/skull base spread
• Sarcoidosis (Heerfordt syndrome)
• Melkersson-Rosenthal syndrome - recurrent facial palsy + facial oedema + fissured tongue

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III. INVESTIGATIONS TO DETERMINE LEVEL OF INJURY

A. Topognostic Tests (Clinical Localisation)

Note: Topognostic testing has been largely superseded by electrodiagnostic tests and imaging because its accuracy is disappointing (especially in Bell's palsy where there is diffuse demyelination). However, it retains value in assessment and clinical MS exams.

B. Electrodiagnostic Tests (Most Important for Prognosis and Surgical Decision-Making)

• Percutaneous stimulation at stylomastoid foramen; lowest current to produce visible twitch is the threshold
• Threshold difference >3.5 mA between sides indicates significant neural degeneration
• Simple, inexpensive
• Most useful: days 3-14 in complete paralysis
• Disadvantage: subjective; cannot be used in incomplete palsy

• Stimulation increased to maximum until facial contraction plateaus
• Response graded: equal / mildly decreased / markedly decreased / no response
• Markedly decreased/no response = poor prognosis
• More reliable than NET for estimating degree of degeneration
• Most useful: days 3-14

• Gold standard electrodiagnostic test
• Bipolar stimulating electrode at stylomastoid foramen; bipolar recording electrode at nasolabial fold
• Records evoked compound muscle action potential (CMAP)
• Amplitude on paralysed side expressed as % of normal side
• "90% degeneration" = CMAP amplitude on affected side is 10% of normal side
• Prognostic thresholds:
  • <90% degeneration within 14 days: 98% chance of satisfactory recovery (no surgery needed)

  • 90% degeneration within 14 days: ~50-70% chance of incomplete recovery - consider surgical decompression

  • Completely non-stimulable: strong indication for surgery
• Fisch and Esslen demonstrated 95% degeneration (non-stimulable) = 50% unfavourable outcome
• Performed serially every 3-5 days in complete palsy
• Limitation: Not valid before day 3-5 (Wallerian degeneration incomplete); test-retest error ~20% (average multiple readings)

• Records spontaneous and voluntary muscle activity
• Detects fibrillation potentials (denervation) and polyphasic reinnervation potentials
• Fibrillation potentials appear 10-21 days after denervation
• Detection of voluntary motor unit potentials = at least partial nerve continuity = good sign
• Most valuable when ENoG shows >90% degeneration: if no voluntary motor units on needle EMG, patient may be candidate for surgical decompression
• Also useful beyond day 14 when ENoG is no longer interpretable
• Can give earliest evidence of reinnervation before clinical recovery visible

C. Imaging

• Best for bony detail - fallopian canal erosion, temporal bone fractures, cholesteatoma matrix extent
• Indicated in: trauma, CSOM/cholesteatoma, suspected bony tumour, malignancy
• Shows: fracture line, tegmen breach, labyrinthine fistula, sclerosis

• Best for soft tissue/nerve detail
• T1 with gadolinium: Enhancement of the nerve indicates inflammation/demyelination/tumour
• Idiopathic palsy: asymmetric, diffuse, intense linear enhancement of entire intratemporal nerve - most distinct at geniculate and labyrinthine segments; may persist months after recovery
• Abnormal if nodular/focal/expanding mass - suggests neoplasm
• Strategy:
  • HRCT first if trauma or CSOM suspected
  • MRI if neoplasm, inflammatory (sarcoid, Lyme, HZO) or atypical presentation
  • Both modalities are complementary

• Atypical presentation (gradual onset, twitching before weakness, other cranial nerve involvement)
• Progressive weakness
• Failure to recover by 6 months
• Preoperative planning for surgical decompression
• Suspected tumour, trauma, or CSOM

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IV. MANAGEMENT

A. General Principles - Grading

• Grade I: Normal function
• Grade II: Mild dysfunction
• Grade III: Moderate dysfunction (complete eye closure with effort)
• Grade IV: Moderately severe (incomplete eye closure)
• Grade V: Severe
• Grade VI: Total paralysis

B. Eye Care (All Grades)

• Immediate and ongoing
• Artificial tears (methylcellulose) during day; lubricating ointment at night
• Taping eyelid closed at night; moisture chamber glasses
• Ophthalmology referral if corneal exposure/keratopathy develops

C. Management by Aetiology

• Admit; IV antibiotics (e.g. amoxicillin-clavulanate or cephalosporin)
• Myringotomy + ventilation tube insertion - decompresses middle ear; allows culture
• Corticosteroids: often used (no strong evidence)
• Cortical mastoidectomy: for those failing conservative management
• Facial nerve decompression: generally NOT required in AOM - prognosis good without it; consider only if fails conservative management AND ENoG shows >90% degeneration
• Outcomes: ~80% recover with ventilation tube + antibiotics; mean time to recovery 4 months

• Urgent surgical intervention is mandatory
• Cortical mastoidectomy + tympanoplasty to remove all cholesteatoma/granulation tissue
• Facial nerve exploration: required to decompress the eroded segment
• IV antibiotics pre-/perioperatively
• Prognosis is worse than other middle ear causes - axonal degeneration may be more advanced before presentation

• Incomplete/delayed onset palsy: Almost 100% achieve HB Grade I-II with conservative management; no surgical decompression required
• Complete immediate onset palsy: Requires electrodiagnostic work-up (ENoG + EMG from day 3-5)
  • If ENoG <90% degeneration: conservative management (steroids if no significant brain injury)
  • If ENoG ≥90% degeneration within 14 days AND no voluntary motor units on EMG: consider surgical decompression
  • Note: Even with 90% degeneration, 67% have good resolution without surgery; clinical discussion essential
• Surgical approach: Transmastoid for tympanic/mastoid segment; middle cranial fossa for labyrinthine/geniculate; combined approach if needed
• Nerve repair: If transected - tension-free end-to-end anastomosis preferred; if gap present, interposition cable graft (great auricular or sural nerve)
• Steroids: Effective but only if no significant associated brain injury
• 14% of cases have complete nerve transection requiring surgical repair

• Antiviral: Aciclovir (or valaciclovir) within 72 hours
• Prednisolone 1 mg/kg/day tapered over 3-4 weeks
• Combination (antiviral + steroid) superior to steroid alone
• Analgesia (severe otalgia)
• Surgical decompression: if no improvement with >90% ENoG degeneration at 14 days

• IV antipseudomonal antibiotics (piperacillin-tazobactam or ciprofloxacin)
• Prolonged treatment 6-8 weeks; CT/MRI to assess skull base extension
• Surgery (debridement) may be needed for devitalised bone

• Paraganglioma: Surgery (tympanomastoidectomy +/- infratemporal fossa approach); stereotactic radiotherapy for poor surgical candidates
• Facial nerve schwannoma:
  • HB I-II: Observe (serial imaging)
  • HB III: Consider decompression
  • HB IV or worse: Tumour resection + nerve grafting
• SCC temporal bone: Wide resection (temporal bone resection); adjuvant radiotherapy; facial palsy confers poor prognosis

D. Surgical Approaches to the Facial Nerve

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V. EXAMINATION SUMMARY - KEY POINTS FOR MS ENT

1. The tympanic segment of the facial nerve is most vulnerable in CSOM/cholesteatoma due to natural dehiscence
2. Topognostic testing localises lesion level using GSPN (lacrimation), stapedial nerve (stapedial reflex), chorda tympani (taste/salivation)
3. ENoG is the gold standard electrodiagnostic test - >90% degeneration within 14 days is the threshold for surgical consideration
4. Electrodiagnostic tests cannot be interpreted before day 3-5 (Wallerian degeneration incomplete)
5. Needle EMG is most valuable beyond day 14 and for detecting early reinnervation
6. Immediate complete palsy in trauma needs urgent work-up; delayed/incomplete palsy has excellent prognosis without surgery
7. Prognosis in COM is worse than Bell's palsy, AOM, or trauma
8. Eye care must be initiated immediately in all cases regardless of grade
9. HRCT and MRI are complementary - CT for bony detail, MRI for soft tissue/nerve enhancement
10. Tension-free nerve anastomosis is the most important factor in successful nerve repair; interposition graft (great auricular/sural nerve) for gaps

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