Pons: 6th, 7th, 8th, and Middle Cerebellar Peduncle Anatomy

From this cross-section of the pons, the proximity of the 7th and 8th fascicles can be appreciated, and a lateral inferior pontine syndrome (anterior inferior cerebellar artery territory), which could involve both of these fascicles, could cause acute prolonged vertigo accompanied by a + ipsilateral head impulse test (fascicle of 8) and ipsilateral lower motor neuron facial palsy (fascicle of 7). Although a "central" acute vestibular syndrome typically has a negative/normal head impulse test (HIT), exceptions exist where a + HIT can be seen in the setting of an acute posterior fossa stroke. Typically these "central" + head impulse tests are seen with lesions affecting the root entry zone of CN8, or those involving the vestibular nucleus or with labyrinthine ischemia. A lesion involving the middle cerebellar peduncle (MCP) itself can also result in acute prolonged vertigo, but if the 8th fascicle is spared, a normal/negative HIT is generally seen.