Approach to facial palsy: Clinical sciences

Facial palsy refers to weakness or paralysis of the muscles that control facial expression caused by injury anywhere along the motor pathway that innervates these muscles. One of the stops in this motor pathway is cranial nerve 7, also known as the facial nerve. Now, based on the location of the injury, facial palsy can be classified as central, which occurs due to an upper motor neuron lesion; or peripheral, which occurs as a result of a lower motor neuron lesion.

Now, here’s a high-yield fact to keep in mind! The facial nerve motor nucleus is located in the pons and receives input from upper motor neurons in the primary motor cortex.

It’s important to note that upper motor neuron innervation to the facial muscles is bilateral for the upper part of the face and contralateral for the lower part of the face.

In addition to muscles, the facial nerve innervates other structures along its path, including taste receptors of the anterior two-thirds of the tongue, sensory receptors to the external auditory canal and pinna of the ear, as well as the stapedius muscle of the inner ear. Finally, don’t forget that the facial nerve carries parasympathetic fibers to lacrimal glands and all salivary glands except the parotid.

Now, if your patient presents with facial droop, start with a focused history and physical exam. As a result of damage to the motor pathway, your patient will lose control of the muscles of facial expression, so they will typically report a relatively sudden onset of symptoms, such as drooling out of one side of the mouth and slurred speech. Sometimes, taste receptors of the anterior two-thirds of the tongue may lose function, causing impaired taste. Next, if sensory receptors to the external auditory canal and pinna of the ear are involved, signal dysfunction might cause pain behind the ear. Furthermore, the stapedius muscle might stop working, leading to discomfort when your patient is exposed to normal sound levels, which is also known as hyperacusis. Finally, loss of parasympathetic innervation to the lacrimal glands and the salivary glands can lead to decreased tearing and salivation.

Additionally, due to weakness or paralysis of the muscles of facial expression, the physical exam will typically reveal impaired eye closure, loss of the nasolabial fold on the affected side due to loss of underlying muscle tone, and an asymmetric smile.

With these findings, you can diagnose facial palsy, so your next step is to assess the type. The key is in the physical exam. If the patient can lift the eyebrow on the affected side, this means the forehead is spared from the deficit because the ipsilateral motor cortex is still providing innervation. However, since there’s no contralateral input, all structures below the forehead will be affected! This indicates a central facial palsy, or an upper motor neuron facial palsy, so your next step is to assess the underlying cause.

First, order a brain MRI which is going to help you differentiate between the most common causes of central facial palsy, which include stroke, demyelination, tumors, and abscesses. Let’s start with a stroke! In this case, history will typically reveal risk factors such as diabetes mellitus, hypertension, and tobacco use, and your patient will report a sudden onset of facial palsy in combination with arm and leg weakness. If the brain MRI shows an area of restricted diffusion in the pons, which is consistent with acute ischemia, diagnose a stroke as a cause of central facial palsy.

Next up is demyelination as a cause of facial palsy! These individuals will typically report a prior episode of optic neuritis! In this case, the brain MRI will often demonstrate periventricular, finger-like projections of ovoid lesions, also known as Dawson fingers. This finding is suggestive of multiple sclerosis, so diagnose central facial palsy due to demyelination.

Moving on to brain tumors! In these patients, history will reveal risk factors, such as tobacco use, as well as a history of chest mass suggestive of lung cancer. If the brain MRI shows one or multiple contrast-enhancing masses, diagnose a brain tumor as a cause of facial palsy!

Finally, let’s take a look at brain abscesses. In addition to facial palsy, these patients will typically report fever and high-risk behavior, including intravenous substance use! Next, if the brain MRI shows a smooth peripheral rim enhancing mass with central cystic changes, diagnose a brain abscess as a cause of facial palsy!

Now, let’s take a look at individuals who are unable to lift the eyebrow on the affected side, which indicates forehead involvement! In this case, even though the nucleus is receiving input from the ipsilateral and contralateral motor cortex, there is a lesion at the level of the nucleus or distal to the nucleus, which impacts all structures on the affected side, including the forehead! So, if your patient is unable to lift the eyebrow, diagnose peripheral facial palsy and be sure to assess the underlying causes, which often include Guillain-Barré syndrome, Lyme disease, Ramsay Hunt syndrome, and Bell palsy.