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Differentiating stroke from Bell’s Palsy

Bell’s Palsy is when there is damage to cranial nerve 7 (also known as the facial nerve) causing weakness or paralysis of the muscles of the face. Bell’s Palsy specifically refers to when the cause of the paralysis is unknown, although viral causes are common. Facial nerve palsy includes Bell’s Palsy as well as known secondary causes of facial nerve damage.

What is the corticobulbar tract?

  • Nervous system tract that supplies muscles of the head and neck
  • Arise from the primary motor cortex and synapse on motor nuclei of the cranial nerves
  • Lower motor neurons (LMN) from arising from the cranial nerve nuclei carry motor signals to the facial muscles
  • If there is a unilateral upper motor neuron (UMN) lesion that is innervating a cranial nerve nuclei, it usually doesn’t result in any symptoms, because each cranial nerve nuclei is bilaterally innervated from both the left and right motor areas of the brain (think of it like a back-up innervation)
  • EXCEPTION: the only exception to this rule involves half of facial nerve innervation and the hypoglossal nerve
    • Upper motor neurons for the hypoglossal nerve receive unilateral innervation from the contralateral motor area of the brain
    • The facial nerve nucleus can be split into an upper portion (LMN emerging from the upper portion of this nucleus innervate the muscles of upper face, i.e. forehead) and lower portion (LMN emerging from the lower portion of this nucleus innervate the muscles of the face below the forehead)
      • The upper portion of the facial nerve nucleus receives bilateral innervation (from the left and right motor areas of the brain)
      • The lower portion of the facial nerve nucleus receives only contralateral innervation
  • Here is a video with a good diagram and explanation, as a companion piece to this explanation! https://youtu.be/w8SHT83HPRg
  • Here is a link to an excellent summarizing diagram! https://www.ebmconsult.com/articles/anatomy-stroke-vs-bells-palsy

How do we distinguish Bell’s Palsy from a stroke?

  • Check for FOREHEAD SPARING
  • If there is an upper motor neuron lesion like in a stroke, i.e. lesion of the nerve before it reaches the cranial nerve nuclei, it will only affect the lower half of the face on the contralateral side (left UMN lesion causing right sided facial drooping, but forehead will not be affected)
    • Why? – because as we discussed, the upper portion of the facial nerve nucleus (that innervates the upper portion of the face) receives bilateral innervation, whereas the lower portion of the facial nerve nucleus only receives contralateral innervation
  • In Bell’s Palsy, there is a LMN lesion because just the facial nerve is damaged, resulting in facial muscle paralysis on the ipsilateral side which involves the forehead as well
  • That’s a lot of words so let’s summarize
    • Bell’s Palsy – complete unilateral upper and lower facial paralysis
    • Stroke – contralateral lower face paralysis (forehead is spared)

Prognosis and Treatment

  • Good prognosis as most people recover within 3 weeks to 6 months although some people have persistent weakness
  • Treat with corticosteroids (prednisone 60 to 80 mg/day for 1 week) and adjuvant anti-viral therapy