From the Research
In stroke symptoms, sparing of the forehead muscles suggests a central (upper motor neuron) facial weakness rather than a peripheral facial nerve palsy. This distinction is clinically important for localizing the lesion. In central facial weakness, which typically occurs with strokes affecting the brain, the forehead muscles remain functional because they receive bilateral innervation from both hemispheres of the brain 1. This means that even if one side of the brain is damaged by a stroke, the forehead can still wrinkle normally when the patient raises their eyebrows. In contrast, peripheral facial weakness (as seen in Bell's palsy) affects all facial muscles on the involved side, including the forehead, because the facial nerve itself is damaged after it has left the brainstem. This pattern of preserved forehead movement with weakness of the lower face is sometimes called "supranuclear" facial palsy and helps clinicians differentiate a stroke from other conditions affecting facial movement 2. When evaluating a patient with facial weakness, asking them to raise their eyebrows, close their eyes tightly, and smile is an essential part of the neurological examination to make this important distinction. The course of corticofacial projections in the human brainstem, as studied by transcranial magnetic stimulation, supports the idea that the corticofacial fibers travel within the ventromedial base of the pons and cross the midline at the level of the facial nucleus 1. However, some individuals may have an 'aberrant bundle' in a paralemniscal position at the dorsal edge of the pontine base, which can affect the presentation of facial paresis. Overall, the sparing of the forehead muscles in stroke symptoms is a key clinical finding that can help guide diagnosis and treatment. Key points to consider when evaluating a patient with facial weakness include:
- The pattern of facial muscle involvement, including the forehead, eyes, and mouth
- The presence or absence of other neurological symptoms, such as weakness or numbness in the face or extremities
- The results of imaging studies, such as MRI or CT scans, to localize the lesion and determine the underlying cause of the facial weakness.