Which feature best differentiates central (UMN) facial palsy from peripheral (LMN) facial palsy on examination?

Which feature best differentiates central (UMN) facial palsy from peripheral (LMN) facial palsy on examination?

• A. Central palsy typically affects the forehead; peripheral palsy spares the forehead.
• B. Central palsy spares the forehead; peripheral palsy affects the entire face including the forehead.
• C. Central palsy affects the mouth only; peripheral palsy affects the eye.
• D. Central palsy affects symmetry of facial expression; peripheral palsy does not.

Answer: (D)

The key idea is forehead involvement on exam. The muscles of the upper face (forehead) get input from both sides of the brain, so a unilateral upper motor neuron lesion cannot completely paralyze the forehead; the patient can still move it, even though the lower face may be weak. In contrast, a lower motor neuron lesion affecting the facial nerve on one side knocks out all ipsilateral facial muscles, including the forehead, so the entire half of the face cannot move. So, the best differentiating feature is whether the forehead is involved. Central (UMN) palsy spares the forehead; peripheral (LMN) palsy includes forehead weakness. That’s why the description that central palsy spares the forehead and peripheral palsy affects the entire face (including the forehead) is the correct way to distinguish them.

The key idea is forehead involvement on exam. The muscles of the upper face (forehead) get input from both sides of the brain, so a unilateral upper motor neuron lesion cannot completely paralyze the forehead; the patient can still move it, even though the lower face may be weak. In contrast, a lower motor neuron lesion affecting the facial nerve on one side knocks out all ipsilateral facial muscles, including the forehead, so the entire half of the face cannot move.

So, the best differentiating feature is whether the forehead is involved. Central (UMN) palsy spares the forehead; peripheral (LMN) palsy includes forehead weakness. That’s why the description that central palsy spares the forehead and peripheral palsy affects the entire face (including the forehead) is the correct way to distinguish them.