Describe the idiopathic type.
Idiopathic (most common cause of peripheral facial nerve palsy): Acute idiopathic peripheral facial palsy is also known as Bell palsy.
List secondary causes.
Trauma (e.g., temporal bone fracture)
Herpes zoster (Ramsay Hunt syndrome)
Borreliosis (Lyme disease)
Malignant otitis externa
Tumors (parotid gland tumors, acoustic neuroma)
Sarcoidosis (Heerfordt syndrome)
Describe the pathophysiology.
The muscles responsible for eyelid and forehead movements are innervated by fibers from both sides.
Central facial palsy: There is a unilateral upper motor neuron lesion between the cortex and nuclei in the pons (corticobulbar tract), but the muscles of the eyelids and forehead are still supplied by input from the other side, so function is preserved.
Peripheral facial palsy: There is a unilateral lower motor neuron lesion between the nuclei and muscles, which results in the paralysis of the ipsilateral eyelid and forehead muscles because no other input reaches these muscles.
The lower facial muscles are only innervated by fibers from the contralateral hemisphere (via ipsilateral nuclei and the ipsilateral peripheral nerve), so they are paralyzed in both central and peripheral facial palsy.