Aetiology

• There is increasing evidence to suggest that the main cause of Bell's palsy is latent herpes viruses (herpes simplex virus type 1 and herpes zoster virus), which are reactivated from cranial nerve ganglia (1,2).
• There is also a correlation between facial palsy and COVID-19 (3) and some studies have suggested a possible association between Bell’s palsy and COVID-19 vaccination although more studies are needed to confirm or disprove this (4)
• herpes zoster virus has traditionally been associated with Ramsay Hunt syndrome (typical cutaneous vesicles and cochleovestibular dysfunction)
  • however vesiculation may not necessarily appear (zoster sine herpete) or may be delayed in up to half of patients
  • preherpetic neuralgia (dermatomal pain and dysaesthesia before vesiculation) may be the only clinical indicator that herpes zoster virus is involved
  • almost a third of facial palsies previously diagnosed as idiopathic are thought to be due to zoster sine herpete
• In most patients, the condition occurs in isolation. Less commonly, it may be associated with: (1)
  • diabetes mellitus
  • a viral polyneuropathy with inflammatory demyelination
  • severe hypertension
  • last trimester of pregnancy
  • dental anaesthesia
  • the common cold

Reference:

1. Glass GE, Tzafetta K. Bell's palsy: a summary of current evidence and referral algorithm. Fam Pract. 2014 Dec;31(6):631-42

2. Alshami S, Hodge D, Bradley E, The epidemiology and surgical outcomes of facial nerve palsy in a population-based cohort. Invest. Ophthalmol. Vis. Sci. 2017 June;58(8):3848.

3. Gupta S, Jawanda MK. Surge of Bell's palsy in the era of COVID-19: systematic review. Eur J Neurol. 2022 Aug;29(8):2526-43.

4. Garg RK, Paliwal VK. Spectrum of neurological complications following COVID-19 vaccination. Neurol Sci. 2022 Jan;43(1):3-40