Facial nerve - anatomy, physiology, examination, syndromes
Anatomy
The facial nerve (CN VII) is a mixed cranial nerve with motor, sensory, parasympathetic, and special sensory (taste) functions.
Functions
- Motor
- Innervates muscles of facial expression: Frontalis. Orbicularis oculi. Orbicularis oris. Buccinator. Platysma
- Sensory: Taste from the anterior 2/3 of the tongue (via chorda tympani)
- Parasympathetic
- Stimulates: Lacrimal gland. Submandibular gland. Sublingual gland
- Reflexes
- Corneal reflex: efferent limb (blinking)
Physiology
The facial nerve is responsible for: Facial expression. Eye closure. Smiling. Frowning.Taste from anterior tongue. Autonomic (Tear and saliva production)
Examination
1. Inspection
- Observe: Facial symmetry. Nasolabial folds. Forehead wrinkles. Drooping of mouth corner
2. Motor Examination
- Ask the patient to:
- Upper Face: Raise eyebrows. Wrinkle forehead
- Eyes: Close eyes tightly
- Lower Face: Smile. Show teeth. Puff out cheeks. Whistle. Look for asymmetry or weakness.
3. Taste Examination: Test sweet, salty, sour, and bitter substances on the anterior 2/3 of the tongue (rarely performed clinically).
4. Corneal Reflex
- Afferent: CN V1. Efferent: CN VII
- Normal response: Bilateral blinking
Facial Nerve Syndromes
1. Peripheral Facial Palsy (LMN Lesion)
- Causes: Bell's palsy (most common). Infection. Trauma. Tumors
- Clinical Features: Ipsilateral paralysis of the entire half of the face. Inability to wrinkle forehead. Inability to close eye. Drooping mouth corner. Loss of nasolabial fold. Hyperacusis (sometimes). Decreased tearing/salivation. Loss of taste (anterior 2/3 tongue)
- Bell's Palsy: Acute idiopathic peripheral facial nerve paralysis.
2. Central Facial Palsy (UMN Lesion)
- Cause: Lesion above the facial nucleus (e.g., stroke).
- Clinical Features: Contralateral weakness of the lower face only. Forehead movements preserved. Eye closure preserved
- Reason: The upper facial muscles receive bilateral cortical innervation.
Other Facial Nerve Disorders
Hemifacial Spasm: Involuntary contractions of facial muscles. Usually unilateral
Facial Synkinesia: Abnormal associated facial movements after nerve regeneration.. Eye closure causes mouth movement