A 15-year-old boy presents to your office with a 1-day history of unilateral facial weakness. On exam, he is unable to fully close his right eye. He also cannot raise his right eyebrow. The nasolabial fold on the right side is less prominent compared with the left side. His extraocular movement is full bilaterally, and he has normal pupillary responses. Strength in the arms and legs is full bilaterally.

Where do you localize the lesion?

A. Motor cortex

B. Facial nerve

C. Neuromuscular junction

D. Facial muscles

Explanation: The correct answer is “B.” Due to bi-hemispheric innervation of facial nerve nuclei, a lesion above the facial nerve nucleus (as in stroke) produces facial weakness below the forehead. In the case of facial nerve palsy, the entire face is weak on the affected side, including the forehead. It is therefore very important to assess the presence or absence of forehead weakness when evaluating a patient with facial weakness. (See figure below). When you diagnose facial nerve palsy or Bell palsy in Lyme-endemic regions, testing for Lyme disease is indicated if season is right, as Lyme disease can cause facial nerve palsy (unilateral or bilateral). Idiopathic facial nerve palsy or Bell palsy generally has a good prognosis with minimal residual deficits. However, eye care needs to be discussed to prevent corneal injury. Eye ointment, eye drops, and occasionally an eye patch are recommended. Oral steroids are often prescribed.

Left facial nerve injury in a 2-day-old infant. (Reproduced with permission from Cunningham FG, Leveno KJ, Bloom SL, et al, eds. Williams Obstetrics. 24th ed. New York, NY: McGraw-Hill Education; 2014, Fig. 33-3.)

Source: Peterson AR and Wood KE. Pediatrics Examination and Board Review. New York, NY: McGraw-Hill Education; 2017.