The greater petrosal nerve arises from the geniculate ganglion of the facial nerve in the petrous temporal bone and carries preganglionic parasympathetic fibres from the superior salivatory nucleus. It exits the petrous apex through the hiatus of the facial canal, traverses the middle cranial fossa on the anterior petrous surface, passes through the foramen lacerum, and unites with the deep petrosal nerve (sympathetic) to form the nerve of the pterygoid canal (Vidian nerve), which reaches the pterygopalatine ganglion to supply secretomotor fibres to the lacrimal gland and nasal glands.
The greater petrosal nerve is damaged in fractures of the petrous temporal bone at the geniculate ganglion level, producing dry eye (from lacrimal gland denervation), loss of taste from the palate (as it carries taste fibres from the palate via the pterygopalatine ganglion), and reduced nasal secretion. Geniculate ganglion herpes zoster (Ramsay Hunt syndrome) involves this nerve and the facial nerve, producing facial palsy with ear vesicles and severe otalgia. The nerve is visualised during middle fossa approaches to the internal auditory canal.
Longitudinal fracture of the petrous bone through the geniculate ganglion damages the greater petrosal nerve, producing ipsilateral dry eye from lacrimal denervation, reduced nasal secretion, and loss of palatal taste, in addition to facial nerve injury from direct ganglion trauma.
Aberrant regeneration of greater petrosal nerve fibres after facial nerve injury reroutes secretomotor fibres to the lacrimal gland instead of the salivary glands, causing reflex lacrimation during eating rather than salivation, a pathognomonic sequela of geniculate ganglion or proximal facial nerve injury.
