Facial nerve lesion

Motor fibers ascend within the pons and form a sharp bend (“internal genu”) around the sixth nerve (abducens) nucleus, forming a visible bump in the floor of the 4th ventricle (facial colliculus). The seventh nerve exits from the brainstem at the pontomedullary junction where it may be involved in CPA tumors. It enters the supero-anterior portion of the internal auditory canal. The geniculate ganglion (“external genu”) is located within the temporal bone. The first branch from the ganglion is the greater superficial petrosal nerve (GSPN), which passes to the pterygopalatine ganglion and innervates the nasal and palatine mucosa and the lacrimal gland of the eye; lesions proximal to this point produce a dry eye. The next branch is the branch to the stapedius muscle; lesions proximal to this point produce hyperacusis. Next, the chorda tympani joins the facial nerve bringing taste sensation from the anterior two–thirds of the tongue. Basal skull fractures may injure the nerve just proximal to this point. Traveling with the chorda tympani are fibers to the submandibular and sublingual glands. The facial nerve exits the skull at the stylomastoid foramen. It then enters the parotid gland, where it splits into the following branches to the facial muscles (cranial to caudal): temporal, zygomatic, buccal, mandibular, and cervical. Lesions within the parotid gland (e.g. parotid tumors) may involve some branches but spare others.