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Cause of facial deviation in acute stroke

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"facial paralysis"[MeSH Terms] AND "stroke"[MeSH Terms]

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central facial palsy stroke facial droop forehead sparing

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Facial deviation in acute stroke is an upper motor neuron (central) facial palsy. It comes from an interruption of the corticobulbar fibers to the facial nucleus.
Mechanism
  • The lower face muscles (below the eye) are controlled only by the contralateral motor cortex. The facial nucleus subdivision for these muscles receives crossed input only.
  • The upper face muscles (frontalis and upper orbicularis oculi) receive bilateral cortical input to the facial nuclei. A unilateral lesion therefore leaves the other hemisphere supplying the forehead. As the Neuroanatomy through Clinical Cases text puts it, upper motor neuron lesions such as stroke cause contralateral face weakness, and the forehead is spared. - Neuroanatomy through Clinical Cases, 3rd Ed.
  • The result is contralateral lower facial weakness with forehead sparing. This shows as a flattened nasolabial fold and a drooping mouth corner. The face deviates toward the unaffected side when the patient smiles. - Goldman-Cecil Medicine; Tintinalli's Emergency Medicine
Where the lesion is
  • Cortical or subcortical MCA territory infarct. This is the most common cause. It usually comes with contralateral arm and leg weakness and possibly aphasia or neglect. Tintinalli's describes MCA stroke as hemiparesis with facial plegia sparing the forehead.
  • Internal capsule or corona radiata lacunar infarct. This causes a pure motor stroke with face, arm and leg involvement.
  • Brainstem (pons) infarct above the facial nucleus. The face is weak on the side opposite the lesion. Below the facial nucleus, in the pons, a lesion gives ipsilateral lower motor neuron facial palsy. This involves the forehead, and it comes with contralateral hemiparesis, a crossed syndrome. Rosen's discusses a pontine stroke from vertebral dissection presenting this way.
Distinguishing from peripheral (Bell) palsy
  • Peripheral facial nerve lesions weaken the whole ipsilateral face, including the forehead. - Tintinalli's Emergency Medicine; Rosen's Emergency Medicine
  • Stroke spares the forehead.
  • Voluntary versus emotional movement can dissociate in stroke. Some patients show weakness on voluntary smile but not on spontaneous laughter, so test both.
Practical points
  • Facial droop is one of the FAST/BE-FAST screening signs.
  • Forehead sparing is not absolute. Pontine lesions, or a lesion at the facial nucleus itself, can give a forehead-involving pattern that mimics Bell palsy.
  • Check for associated deficits such as limb weakness, speech disturbance, gaze deviation or visual field loss. Get urgent neuroimaging.
Recent evidence check: A quick PubMed search for the last two years found only systematic reviews on rehabilitation, such as acupuncture for post-stroke central facial paralysis (PMID 41970041). None change the mechanism or localization above.
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