Differences between an upper motor neuron and lower motor neuron facial weakness
What is the difference between an upper motor neuron (central) and lower motor neuron (peripheral) facial weakness?
If the patient with a facial droop can move the upper facial muscles (i.e., wrinkle the forehead), the lesion is supranuclear on the contralateral side.
The lesion is somewhere in the contralateral corticobulbar tracts above the facial nerve nucleus (e.g., in the crus or in the genu of the internal capsule).
If the patient cannot voluntarily move any muscle involved in facial expression (either upper or lower facial musculature), the lesion is localized to the facial nucleus or the peripheral facial nerve on the ipsilateral side.
