Facial Nerve Palsy: Anatomy and Function

Facial Nerve Palsy Overview

• Facial nerve palsy involves dysfunction of the facial nerve, leading to unilateral facial weakness.
• The condition affects the muscles responsible for facial expression on one side of the face.

Facial Nerve Pathway

• The facial nerve exits the brainstem at the cerebellopontine angle.
• It travels through the temporal bone and parotid gland.
• The nerve divides into five branches: temporal, zygomatic, buccal, marginal mandibular, and cervical.

Function of the Facial Nerve

• Motor: Controls facial expression, stapedius muscle, posterior digastric, stylohyoid, and platysma muscles.
• Sensory: Provides taste sensation from the anterior 2/3 of the tongue.
• Parasympathetic: Supplies submandibular, sublingual salivary glands, and the lacrimal gland (tear production).

Upper vs. Lower Motor Neuron Lesions

• Differentiating between upper motor neuron (UMN) and lower motor neuron (LMN) lesions is crucial.
• New onset UMN facial nerve palsy requires immediate stroke management.
• LMN facial nerve palsy can generally be managed less urgently.
• Forehead muscle innervation differs: each side receives UMN input from both sides of the brain, but LMN input from only one side.
• UMN Lesion: Forehead is spared; patient can move their forehead on the affected side.
• LMN Lesion: Forehead is not spared; patient cannot move their forehead on the affected side.
• If a patient can raise both eyebrows, it suggests an UMN lesion. If not, it suggests an LMN lesion.

Upper Motor Neuron Lesions

• Unilateral UMN lesions can occur due to strokes or tumors.
• Bilateral UMN lesions are rare but may occur in pseudobulbar palsy and motor neuron disease.

Bell's Palsy

• Bell's palsy is an idiopathic condition causing unilateral LMN facial nerve palsy.
• Most patients fully recover over weeks, but it may take up to 12 months.
• About one-third of patients may have some residual weakness.
• Treatment within 72 hours of symptom onset may include prednisolone (either 50mg for 10 days or 60mg for 5 days followed by a 5-day reducing regimen).
• Lubricating eye drops are needed to prevent eye dryness and damage.
• Urgent ophthalmology review is required if eye pain indicates exposure keratopathy.
• The eye can be taped shut at night to prevent damage.

Ramsey Hunt Syndrome

• Ramsey Hunt syndrome is caused by the varicella-zoster virus (VZV).
• It presents as a unilateral LMN facial nerve palsy.
• Patients typically have a painful, vesicular rash in the ear canal, pinna, and around the ear.
• Vesicles (fluid-filled blisters) can extend to the anterior 2/3 of the tongue and hard palate.
• Treatment involves acyclovir and prednisolone, along with lubricating eye drops.
• Key indicator in exam questions is facial nerve palsy with a vesicular rash around the ear.

Other Causes of Lower Motor Neuron Facial Nerve Palsy

• Infections: Otitis media, otitis externa, HIV, and Lyme disease.
• Systemic Diseases: Diabetes, sarcoidosis, leukemia, multiple sclerosis, and Guillain-Barré syndrome.
• Tumors: Acoustic neuroma, parotid gland tumor, and cholesteatoma.
• Trauma: Direct nerve trauma, surgery, and base of skull fractures.