Bell palsy

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.

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 brainstem, 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 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 backup to the lower motor neuron on the 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 brainstem. Next, it enters the petrous 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 stylomastoid 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 two-thirds 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 1 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 endoneurial 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. 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.