Sudden onset facial palsy is a medical emergency and requires urgent assessment by a physician to rule out potentially life threatening conditions.
Cerebrovascular accident, also called stroke, can cause facial weakness. There are two primary types of stroke that will cause facial weakness. The first is a cortical stroke, which is the more common kind of a stroke when people lose the ability to move an arm, a leg, or both, because of bleeding or a blood clot that deprives the motor part of the cerebral cortex from proper blood flow. When someone has a cortical stroke, often they lose the ability to voluntarily move the face, but when they involuntarily smile, just laughing at something, the smile function can be preserved. This phenomenon occurs because the pathways from which we generate involuntary spontaneous expressions are different than ones from which we generate voluntary expressions, and only the voluntary expressions are inhibited after a motor cortex stroke.
The other common kind of stroke that affects facial movement is called a brainstem stroke. A brainstem stroke deprives the origin of the facial nerve, a structure called the facial motor nucleus, of its blood supply. This causes the facial nerve to malfunction. Brainstem strokes also often involve other cranial nerves, so at the same time as one loses facial movement, one might also lose the ability to move the eye properly, or to see or hear properly on that same side. Strokes in this region and in the cerebellum can also result in dizziness and extreme balance difficulty. If somebody has a brainstem stroke, both their voluntary and their involuntary smiling abilities are lost, and in order to bring back movement to the face, surgical interventions are required.