Bell palsy

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Introduction0:00–0:34

Bell palsy, named after the Scottish doctor Charles Bell, who first described the condition, is the most common type of facial nerve palsy.
Facial palsy occurs when there's damage to the facial nerve, also known as the seventh cranial nerve, which results in weakness and paralysis on one side of the face.
This can happen due to various conditions, including stroke or tumors, but if the underlying cause remains unknown, we call it Bell palsy.
Remember when you were a kid and felt crushed because your family was watching a movie instead of your favorite cartoon?
That grumpy face you made, that's your facial nerve telling your muscles what to do. The ultimate control of facial muscles comes from the left and right upper motor neurons in the primary motor cortex of the precentral gyrus.

Physiology0:34–3:31

First, let's talk about the lower right half of the face. The upper motor neuron from the left side of the brain travels down to the brain stem, crosses over the midline, and connects to a lower motor neuron in the facial nucleus.
From here, the lower motor neuron sends signals through the right facial nerve to the lower right half of the face. So when it comes to the lower part of your face, each side of your brain handles the opposite lower side.
But the upper half of the face is like, nah, I want to backup and signals from both sides of the brain. In this case, the upper motor neuron from the left side of the brain extends to the lower motor neuron on the opposite side, but at the same time, the upper motor neuron from the right side sends back up to the lower motor neuron on that same side.
So when it comes to the upper half of the face, both sides of your brain team up to get the job done. Now let's focus on the facial nerve, which emerges from the pontomedullary junction of the brain stem.
Next, it enters the petrus part of the temporal bone where it travels through a narrow Z-shaped canal called the facial canal.
After passing through the canal, it exits the skull through a small opening in the temporal bone called the stylo mastoid foramen.
From here, the facial nerve passes through the parotid gland where it branches off to innervate various facial muscles which are responsible for facial expressions.
The facial nerve also controls the submandibular and sublingual glands which produce saliva, as well as the lacrimal gland, which creates tears.
It also carries taste sensations from the front 2/3 of your tongue and innervates the stapedius muscle, which helps reduce the vibrations of the stapes bone against the oval window, protecting your hearing from loud noises.
Now Bell palsy is also known as idiopathic facial nerve palsy because the underlying cause is unknown. However, we know that it involves inflammation of the nerve within the facial canal.
As the nerve becomes inflamed and swells, it has little room to expand. This increases pressure on the small blood vessels supplying the nerve, eventually reducing blood flow and causing nerve ischemia and dysfunction.
And since small blood vessels are crucial in nourishing the nerve, remember that conditions affecting small blood vessels like uncontrolled diabetes and hypertension can increase the risk of developing Bell palsy.
In some individuals, herpes simplex virus type one can reactivate from its dormant state in the geniculate ganglion and travel along the facial nerve, leading to inflammation.
This helps explain why this virus is often found in the endoneal fluid around the affected nerve. Now, in Bell palsy, signals from both upper motor neurons are still reaching lower motor neurons, but on the affected side, as the signal travels along the facial nerve through the facial canal, it gets blocked.

Pathology3:31–4:50

In other words, it never reaches facial muscles, causing complete weakness in both the lower and upper halves of the face.
When the lower part loses innervation, the corner of the mouth droops, which can lead to drooling. If the person smiles, the smile looks uneven, and the skin fold running from the side of the nose to the corner of the mouth disappears.
This is called the loss of the nasolabial fold. Finally, puffing out the cheeks is almost impossible, so simple things like blowing up a balloon for a birthday party become a real struggle.
On the flip side, when the upper half loses innervation, the forehead on that side loses its wrinkles, and the person cannot raise their eyebrow.
The muscles around the eye become weak, making it difficult to fully close the eye. When they try, the eye moves upward, which is known as the bell phenomenon.
And since they can't blink properly, the eye doesn't get the moisture it needs, which can lead to dryness and even corneal ulcers.
And the story doesn't end with just muscle weakness, since the facial nerve also controls the submandibular and sublingual glands.
Individuals with Bell palsy may experience a dry mouth. On top of that, losing sensation from the front 2/3 of the tongue means no taste of their favorite vanilla ice cream.

Signs and symptoms4:50–5:24

And finally, when the stapedius muscle stops working properly, everyday sounds that usually feel fine can suddenly seem unbearably loud.
This is known as hyperacusis. Keep in mind that all these symptoms often start with pain around the ear and usually develop quickly, often within just a few hours.
Now, to diagnose Bell palsy, the first step is to rule out central causes of facial palsy. The main clue is the forehead.
If a person can still wrinkle the forehead on the weak side, it's likely central facial palsy. For example, a tumor, stroke, or multiple sclerosis can affect the upper motor neuron on one side of the brain, stopping it from sending signals to the facial nucleus on the opposite side.

Diagnosis5:24–5:36

Treatment5:36–5:56

However, the upper motor neuron on the unaffected side still sends backup signals to both facial nuclei, including the one on the affected side of the face.
This allows the forehead muscles to move. But if your patient can't wrinkle the forehead on the weak side, diagnose peripheral facial palsy.
In this case, both upper motor neurons are sending signals to the facial nuclei, but the signal from the facial nucleus on the affected side is not reaching the facial muscles.

Review5:56–7:45

Next, rule out potential causes of peripheral facial palsy. First, acoustic neuroma is a benign tumor of Schwann cells of the vestibulocochlear nerve.
It usually develops in the pontomedullary angle, close to the facial nerve. As it grows, it compresses the facial nerve, causing facial palsy.
On top of weakness, individuals may experience hearing loss and balance issues. Next, mumps infection can trigger parotitis or inflammation of the parotid gland.
The swollen gland can press on the facial nerve as it passes through, leading to facial palsy. Finally, Lyme disease caused by the bacterium Borrelia bergdorferi can result in unilateral or bilateral facial palsy.
In this case, look for a characteristic rash called erythema migraines and ask about possible tick exposure. Once you rule out all these potential causes, you can diagnose Bell palsy.
Treatment primarily relies on corticosteroids to reduce nerve inflammation and sometimes antiviral medications to fight the herpes simplex virus.
All right, as a quick recap, Bell palsy is also known as idiopathic facial nerve palsy because the exact cause remains unknown.
However, we know that it involves inflammation of the nerve within the facial canal. And as the nerve swells, it has little room to expand, which increases pressure on the small blood vessels that nourish it.
This pressure reduces blood flow, causing nerve ischemia and dysfunction. Common symptoms include an inability to wrinkle the forehead or close the eye, loss of the nasolabial fold, and drooping of the mouth on the affected side.