Definition: Diseases of the twelfth cranial (hypoglossal) nerve or nuclei. The nuclei and fascicles of the nerve are located in the medulla, and the nerve exits the skull via the hypoglossal foramen and innervates the muscles of the tongue. Lower brain stem diseases, including ischemia and MOTOR NEURON DISEASES may affect the nuclei or nerve fascicles. The nerve may also be injured by diseases of the posterior fossa or skull base. Clinical manifestations include unilateral weakness of tongue musculature and lingual dysarthria, with deviation of the tongue towards the side of weakness upon attempted protrusion.