Answer: Excellent question. Central (CNS, e.g. stroke) causes of Facial Nerve (Cranial Nerve VII) palsy spares the forehead (the patient can still wrinkle their forehead) however, peripheral (outside of the CNS) causes of Facial Nerve Palsy (such as Bell’s palsy) do not: the patient is unable to use their frontalis muscle to wrinkle their forehead. Note however, that Bell’s Palsy ONLY affects the Facial Nerve. Therefore, if there are other neurologic deficits, other causes, including stroke should be entertained.
Unfortunately, it is sometimes difficult to confidently state that a particular neurologic presentation is due to a cranial nerve palsy (e.g. Bell’s palsy) versus a more sinister cause (e.g. stroke). If there is any doubt whatsoever, the safest option would be transport to a stroke center.