Answer:
Idiopathic (Bell’s palsy – see below) and non-idiopathic. Several patterns of facial nerve dysfunction point to a non-idiopathic cause: simultaneous bilateral facial palsy (Guillain-Barrie, sarcoidosis, pseudobulbar palsy, syphilis, leukemia, trauma, Wegeners granulomatosis), unilateral facial weakness slowly progressing beyond 3 weeks (facial nerve neuroma, metastatic carcinoma, adenoid cystic carcinoma), slowly progressive unilateral facial weakness associated with facial hyperkinesis (cholesteatoma, facial nerve neuroma), no return of facial nerve function within six months after abrupt onset of palsy (facial nerve neuroma, adenoid cystic carcinoma, basal cell carcinoma), ipsilateral lateral rectus palsy, recurrent unilateral facial palsy (facial nerve neuroma, adenoid cystic carcinoma, meningioma).
Trauma is the second most frequent cause, usually due to temporal bone fracture or penetrating wound. Facial paralysis caused by a tumor can have a variable presentation – sudden, slowly progressive, complete or incomplete, recurrent or single episode. The tumor may be primary to the facial nerve or parotid, an extension of a cutaneous malignancy, metastatic from a distant site or of CNS origin. Several viruses have been implicated including varicella-zoster, herpes simplex and Epstein-Barr. Facial paralysis occurs in 11% of patients with Lyme disease; in 30% of cases, the paralysis is bilateral. AIDS is also an increasingly common cause of bilateral involvement. Iatrogenic injury can occur following surgery for acoustic neuroma. Even with nerve preservation, delayed facial nerve paralysis occurs in 15 to 30%; the prognosis is excellent in most cases and return of function is complete by 6 months.