Bell's Palsy Affects the Facial Nerve (Cranial Nerve VII)
Bell's palsy exclusively affects the facial nerve (CN VII), which is the seventh cranial nerve responsible for controlling all muscles of facial expression on one side of the face. 1
Anatomical Course and Function of CN VII
The facial nerve is one of the most complex cranial nerves, containing multiple functional components 1:
- Branchial motor fibers: Innervate all muscles of facial expression 1
- Visceral motor fibers: Provide parasympathetic innervation to most glands of the head 1
- General sensory fibers: Supply surface sensation to a small portion of the external ear and tympanic membrane 1
- Special sensory fibers: Carry taste sensation from the anterior two-thirds of the tongue via the chorda tympani branch 1, 2
The nerve follows an extensive anatomical course 1:
- Originates in the pons (pontine segment)
- Traverses the cerebellopontine angle (cisternal segment)
- Enters the internal auditory canal (meatal segment)
- Courses through the temporal bone (labyrinthine, geniculate, tympanic, and mastoid segments)
- Exits at the stylomastoid foramen
- Travels through the parotid gland (extracranial segment)
Why Only CN VII is Affected
Bell's palsy is specifically a peripheral facial nerve palsy that occurs after the nerve exits the brainstem, typically within the narrow temporal bone canal where inflammation causes compression. 2 This peripheral location explains why the entire ipsilateral side of the face is affected, including the forehead—a key distinguishing feature from central (stroke-related) facial weakness 2.
The American Academy of Otolaryngology-Head and Neck Surgery explicitly states that Bell's palsy is a diagnosis of exclusion with no identified cause, but it definitively involves only CN VII 2.
Clinical Manifestations Related to CN VII Dysfunction
Because CN VII has multiple functional components, Bell's palsy can present with 1, 2:
- Motor symptoms: Complete ipsilateral facial muscle weakness including forehead involvement 2
- Taste disturbance: Loss or alteration of taste on the anterior two-thirds of the tongue 2
- Hyperacusis: Increased sensitivity to sound due to stapedius muscle paralysis 1, 2
- Dry eye or excessive tearing: Disrupted lacrimal gland innervation 2
- Sagging of mouth corner and eyelid: Loss of facial muscle tone 2
Important Clinical Distinction
If other cranial nerves are involved, the diagnosis is NOT Bell's palsy and requires investigation for alternative pathology such as stroke, tumor, or other central nervous system disease. 2 The presence of additional neurologic symptoms such as dizziness, dysphagia, diplopia, or other cranial nerve involvement should prompt consideration of stroke or brainstem pathology rather than Bell's palsy 2.