How can you differentiate Bell's palsy from a cortical (UMN) facial weakness?

• A. Bell's palsy causes ipsilateral weakness of the entire face including the forehead.
• B. Bell's palsy spares the forehead.
• C. UMN facial weakness causes bilateral facial weakness.
• D. UMN facial weakness never affects the lower face.

Answer: (A)

Understanding how facial muscles are wired helps differentiate where the problem lies. The cortex sends signals to facial muscles with a mix of bilateral input to the upper face (forehead) and predominantly contralateral input to the lower face. That means a cortical (UMN) lesion typically weakens the lower face on the opposite side while the forehead can still move normally. A peripheral lesion, like Bell’s palsy affecting the facial nerve after it leaves the brainstem, causes weakness of all ipsilateral facial muscles, including the forehead. So seeing ipsilateral weakness that involves the forehead is the telltale sign pointing to Bell’s palsy. If the forehead moves normally but the lower face on the opposite side is weak, that pattern fits a cortical UMN lesion. The other statements don’t match the standard patterns: Bell’s palsy does involve the forehead, UMN weakness does not usually cause bilateral complete facial weakness, and UMN lesions can affect the lower face.