Idiopathic Lower Motor Neurone Facial Palsy
Acute, unilateral lower motor neuron (LMN) facial nerve (CN VII) weakness of unknown cause. Suspected viral-mediated inflammation (e.g. HSV or VZV reactivation).
Facial Nerve Anatomy & Function
- Motor: Muscles of facial expression, stapedius, posterior digastric, and stylohyoid.
- Parasympathetic: Lacrimal gland, submandibular and sublingual glands.
- Sensory: Taste (anterior two-thirds of tongue), somatic sensation from posterior external auditory canal and tympanic membrane.
Risk Factors
- Pregnancy, especially third trimester
- Pre-eclampsia
- Obesity
- Diabetes mellitus
- Hypertension
- Recent URTI or viral illness
Clinical Features
Onset
- Rapid onset over hours (not stroke-like); typically unilateral.
Facial Signs
- Inability to raise eyebrow or wrinkle forehead (LMN pattern)
- Loss of nasolabial fold, drooping of mouth
- Inability to purse lips, whistle, or puff cheeks
- Excessive drooling
- Altered taste sensation (anterior 2/3 tongue)
Eye Involvement
- Lagophthalmos (incomplete eyelid closure)
- Dry eye due to impaired lacrimation
- Epiphora (excessive tearing from poor eyelid tone)
- Corneal exposure → risk of keratitis, ulceration
Ear
- Hyperacusis (due to stapedius weakness)
Upper vs Lower Motor Neuron Facial Palsy
| Feature | LMN (e.g. Bell’s palsy) | UMN (e.g. stroke) |
|---|
| Forehead involvement | Yes | No (forehead spared) |
| Laterality | Unilateral | Often contralateral |
| Other CNS signs | Absent | May be present |
Red Flags (Suggest Alternate Diagnoses)
- Progressive or recurrent symptoms
- Bilateral facial palsy
- Systemic symptoms (fever, rash, arthralgia)
- Diplopia, dysphagia, vertigo, or numbness
- History of malignancy, head trauma, tick bite
- Parotid or cutaneous facial masses
Differential Diagnosis
Peripheral Causes
- Ramsay Hunt Syndrome (HZV oticus): Ear or oropharyngeal vesicles, severe pain, CN VIII involvement
- Otitis media: Conductive hearing loss, ear pain
- Lyme disease: Tick exposure, rash, bilateral palsy
- Sarcoidosis: Recurrent or bilateral palsy, parotid enlargement
- Guillain–Barré syndrome: Bilateral, ascending weakness
- HIV infection: Consider in bilateral or recurrent palsy
- Tumours: Parotid, temporal bone, cerebellopontine angle
Central Causes
- Stroke: Forehead sparing, acute onset with other deficits
- Multiple sclerosis: Painless, young adult, spontaneous recovery
- Brain tumours/metastases: Gradual onset, raised ICP signs
Clinical Assessment
- Differentiate LMN vs UMN
- Examine all cranial nerves
- Inspect ear canal and tympanic membrane (look for vesicles)
- Oropharyngeal exam for parotid mass (tonsillar asymmetry)
- Neurological exam: cerebellar signs, brainstem features
- Skin: look for rash (erythema migrans – Lyme, zoster)
Management
✅ First-line Therapy
- Corticosteroids: Start within 72 hours
- eTG: Prednisone 1 mg/kg (max 75 mg) orally daily for 5 days; taper over next 5 days if desired
- Eye protection:
- Artificial tears and ointment
- Sunglasses for protection
- Eyelid taping at night
- Refer to ophthalmology if exposure keratitis
⚠️ Antivirals
- Valaciclovir 1000 mg TDS x 7 days may have benefit when combined with steroids
- Indicated in: Suspected Ramsay Hunt syndrome or severe cases
- Not routinely used for isolated Bell’s palsy
⚕️ Supportive Care
- Referral:
- No recovery at 3 months
- Atypical or progressive features
- Ophthalmologic complications
- Suspected malignancy or CNS involvement
- Psychosocial:
- Facial asymmetry can impact self-esteem and mental health
- Offer psychological support or counselling as needed
Prognosis
- 70% show improvement within 3 weeks
- Complete recovery in 3–6 months in most
- Worse prognosis if: complete paralysis at onset, age >60, hyperacusis, taste loss, Ramsay Hunt syndrome
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