USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 46-year-old man comes to the emergency department because of the sudden onset of inability to move the right side of his face. Physical examination shows drooping of the corner of the mouth. Motor examination shows difficulty smiling and puffing out the cheeks; no abnormalities are noted in raising the eyebrows or wrinkling the forehead. Which of the following is the most likely cause of these findings?
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.
Peripheral nerves that emerge from the brain and brainstem are called cranial nerves, and there are a total of 12 pairs of cranial nerves.
The seventh cranial nerve, the facial nerve, emerges from the brainstem, and then enters the temporal bone where it travels through a narrow, Z-shaped canal, called the facial canal.
The facial nerve exits the skull through a tiny hole called the stylomastoid foramen.
From there, the facial nerve branches off to different facial muscles that help with facial expression, like the ones you use while whistling to your favorite song.
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.
The facial nerve also innervates the sublingual and submandibular glands, which secrete saliva, the lacrimal gland which produces tears, and mucous membranes of the nose, mouth, and nasopharynx.
In the ear, it innervates the stapedius muscle which dampens the vibration of the stapes, a small bone that help transmit vibrations from the eardrum; this protects you from loud noises.
The facial nerve also carries sensory information about taste from the anterior ⅔ of the tongue. So if you lick an ice cream cone - that’s the facial nerve registering the flavor!
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.