USMLE® Step 2 style questions USMLE
A 45-year-old man comes to the office to evaluate right-sided facial heaviness for the past 10 hours. He also reports decreased tear production on the right side and the inability to tolerate loud sounds that started simultaneously. Medical history is remarkable for a right-sided parotid gland adenoma which was surgically resected one month ago. The patient has no hearing loss, ringing sensation in the ears, or difficulty walking. Vitals are within normal limits. Physical examination findings are shown in the image below.
Examination of the right auditory canal and auricle is noncontributory. Which of the following is the most likely cause of the findings seen in this patient?
Bell’s palsy, named after the surgeon Charles Bell who first described it, is when there’s weakness or paralysis of the muscles on one side of the face, caused by damage to the seventh cranial nerve, which is the facial nerve.
The underlying cause of cranial nerve damage is idiopathic which means it’s unknown, so when there’s facial nerve a paralysis from a known cause like a stroke, a tumor, or trauma, it’s not considered a Bell’s palsy.
George Clooney had this disorder for nine months when he was a teenager.
Broadly speaking, the nervous system has two parts: the central nervous system, which consists of the brain, brainstem, and spinal cord, and the peripheral nervous system, which consists of all of the nerves that fan out from the central nervous system.
The facial nerve exits the skull through a tiny hole called the stylomastoid foramen.
Ultimately, control of each side of the face comes from a region of the brain called the motor cortex.
For example, let’s start with the lower half of the right side of the face. An upper motor neuron extends down from the left motor cortex, goes across the midline in the brainstem to the right side, and then meets with a right lower motor neuron which hitches a ride on the right facial nerve.
For the upper half of the right side of the face, things begin similarly. There’s another upper motor neuron that extends down from another region of the left motor cortex, also goes across the midline in the brainstem to the right side, and meets with another left lower motor neuron which also hitches a ride on the left facial nerve.
The one huge difference is that there’s another upper motor neuron that extends down from a region in the right motor cortex, and stays on the ipsilateral or same side to meet with same the lower motor neuron.
In other words, there are two upper motor neurons, one from each side of the brain, giving input to one lower motor neuron.
The left half of the face is similarly innervated. So that means that each facial nerve contains motor information for the lower face coming from the contralateral motor cortex, and motor information for the upper face coming from both motor cortices.
Bell’s palsy occurs when the facial nerve gets damaged, and although the precise cause is unknown, it’s often associated with viral infections like herpes simplex virus, Epstein-Barr virus, and varicella-zoster virus, as well as the bacteria Borrelia burgdorferi which causes lyme disease.
- "Clinical Practice Guideline" Otolaryngology–Head and Neck Surgery (2013)
- "Modern management of facial palsy: a review of current literature" British Journal of General Practice (2013)
- "The neurologist’s dilemma: A comprehensive clinical review of Bell’s palsy, with emphasis on current management trends" Medical Science Monitor (2014)
- "Bell Palsy and Herpes Simplex Virus: Identification of Viral DNA in Endoneurial Fluid and Muscle" Annals of Internal Medicine (1996)