Which statement best differentiates an upper motor neuron facial lesion from a lower motor neuron facial lesion?

Which statement best differentiates an upper motor neuron facial lesion from a lower motor neuron facial lesion?

• A. UMN lesions spare the forehead; LMN lesions involve the entire face including the forehead
• B. UMN lesions affect all facial muscles; LMN lesions affect only lower face
• C. UMN lesions cause immediate recovery; LMN lesions cause persistent deficits
• D. UMN lesions cause flaccidity; LMN lesions cause spasticity

Answer: (A)

The distinction hinges on how facial muscles receive cortical input. The forehead muscles get input from both sides of the brain (bilateral innervation), while the muscles of the rest of the face mainly rely on contralateral corticobulbar input. So when an upper motor neuron pathway above the facial nucleus is damaged, the weakness appears on the opposite lower half of the face, and the forehead remains relatively spared because it still receives input from the undamaged hemisphere. In contrast, a lower motor neuron lesion of the facial nerve disrupts all branches on the same side, so the entire the ipsilateral face—forehead included—paralyzes, leading to inability to raise the eyebrow, close the eye, and move the mouth on that side. This explains why a lesion higher up tends to spare the forehead, while a peripheral facial nerve lesion affects the whole side of the face. The other options don’t fit this pattern: the idea that upper motor neuron lesions affect all facial muscles or that recovery is immediate doesn’t match the established distribution of cortical innervation; and the notion about flaccidity versus spasticity is not the typical way to differentiate these two types of facial palsy.

The distinction hinges on how facial muscles receive cortical input. The forehead muscles get input from both sides of the brain (bilateral innervation), while the muscles of the rest of the face mainly rely on contralateral corticobulbar input. So when an upper motor neuron pathway above the facial nucleus is damaged, the weakness appears on the opposite lower half of the face, and the forehead remains relatively spared because it still receives input from the undamaged hemisphere. In contrast, a lower motor neuron lesion of the facial nerve disrupts all branches on the same side, so the entire the ipsilateral face—forehead included—paralyzes, leading to inability to raise the eyebrow, close the eye, and move the mouth on that side.

This explains why a lesion higher up tends to spare the forehead, while a peripheral facial nerve lesion affects the whole side of the face. The other options don’t fit this pattern: the idea that upper motor neuron lesions affect all facial muscles or that recovery is immediate doesn’t match the established distribution of cortical innervation; and the notion about flaccidity versus spasticity is not the typical way to differentiate these two types of facial palsy.