Bell’s Palsy – causes, symptoms, diagnosis, treatment, pathology


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 right
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. Regardless of the cause,
when the facial nerve isn’t able to conduct the brain’s signals, the result is that
there’s weakness or paralysis of the facial muscles. Now, it’s important to distinguish Bell’s
palsy from other causes of facial palsy like a stroke. If the underlying problem is in
the brain or brainstem before the upper motor neurons cross the midline, it’s called an
upper motor neuron lesion. This causes paralysis of only the lower half of the face on the
contralateral side as the lesion, since the upper half of the face is still receiving
some information from the ipsilateral motor cortex. However, if there’s a lower motor
neuron lesion like in Bell’s palsy where the facial nerve is damaged, information from
the contralateral and ipsilateral motor cortex is lost for the upper face, as well as information
from the contralateral motor cortex for the lower face. This results in the paralysis
of all the muscles on the side of the affected nerve. The main symptoms of a Bell’s palsy can
be seen by looking at a person’s face. There’s an absence of the nasolabial fold, which is
the skin fold that runs from the side of the nose to the corner of the mouth. There’s
also drooping of the eyelid and drooping of the mouth. In some people, there’s also
dryness of the affected eye or mouth because the facial nerve innervates the lacrimal,
submandibular, and sublingual glands. Sometimes there’s also hypersensitivity to loud noises
and a loss of taste sensation on the anterior ⅔ of the tongue. The diagnosis of a Bell’s palsy is based
on identifying that the problem is with the facial nerve and not finding an alternative
explanation like a stroke or brain tumor. Bell’s palsy affects each person differently.
Most people recover within 6 months after the onset, but some people develop permanent
facial weakness or paralysis. Treatment isn’t needed in all cases of Bell’s palsy, but
in some severe cases, corticosteroids can help reduce the nerve inflammation and speed
up the recovery. All right, as a quick recap. Bell’s palsy
occurs when there’s a disruption of the facial nerve from an unknown cause. The most
common symptoms are weakness or paralysis of both upper and lower facial muscles on
one side of the face due to the loss of all lower motor neurons. Bell’s palsy is treated
with corticosteroids, but in most cases, symptoms usually subside on their own.

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