Pathophysiology
Clinical meaning
Bell palsy is an acute, idiopathic, unilateral peripheral facial nerve (CN VII) paralysis, most likely caused by herpes simplex virus type 1 (HSV-1) reactivation in the geniculate ganglion. Understanding the detailed neuroanatomy of CN VII is essential for the NP to differentiate peripheral from central facial weakness and to identify mimics requiring urgent intervention. The facial nerve has a complex course with multiple functional components. It exits the brainstem at the cerebellopontine angle (CPA), enters the temporal bone through the internal auditory meatus, and traverses the narrow bony fallopian canal. Within this rigid canal, even mild inflammation causes nerve compression and ischemic injury. The nerve gives off three critical branches before exiting the skull: (1) the greater petrosal nerve (parasympathetic to lacrimal glands - explains eye dryness), (2) the nerve to stapedius (dampens sound - explains hyperacusis), and (3) the chorda tympani (taste from anterior 2/3 of tongue, parasympathetic to submandibular/sublingual glands). After exiting through the stylomastoid foramen, CN VII branches into temporal, zygomatic, buccal, marginal mandibular, and cervical divisions innervating all muscles of facial expression. The critical NP diagnostic...