25th June 2023

CN XII – Hypoglossal nerve

The hypoglossal nerve arises from the medulla and travels through the hypoglossal canal, innervating the intrinsic and extrinsic muscles of the tongue. Disorders of the nerve cause problems with speech and swallowing.

Damage to the hypoglossal nerve itself (a lower motor neuron lesion) is very rare, mostly due to trauma and will cause an ipsilateral weakness of the tongue muscles, manifesting as deviation of the tongue to the ipsilateral side, due to ipsilateral tongue muscles being unable to balance the contraction of the genioglossus from the contralateral side.

Unilateral upper motor neuron lesions disrupting pathways to the hypoglossal nerve nuclei are more common and will result in contralateral weakness of the tongue, and therefore deviate to the contralateral side, as the contralateral muscles are affected and would be unable to balance the action of the unaffected side. Both unilateral and bilateral damage can occur, with bilateral damage resulting in a complete inability of the tongue to function.

Pseudobulbar palsy and bulbar palsy

Pseudobulbar palsy results from a lesion that is supranuclear to the hypoglossal nerve (i.e. an upper motor neuron lesion). Causes can be traumatic, metabolic, vascular, neoplastic, or autoimmune. In this instance, the corticobulbar tracts are most likely to be involved, and the manifestation is similar to that of a true bulbar palsy with impairment of articulation, mastication, and swallowing, but as the lesion is above the bulbar areas, it is termed ‘pseudo’ bulbar. Features include slow dysarthric speech, stiff and spastic tongue without fasciculations, dysphagia, and brisk jaw jerk. Due to the bilateral innervation of CN XII from the cortex, unilateral corticobulbar tract involvement would result in relatively preserved function.

True bulbar palsy is usually caused by a neurodegenerative disease that affects the bulbar areas, in particular, motor neuron disease with bulbar involvement such as progressive bulbar palsy, bulbar amyotrophic lateral sclerosis, and spinal and bulbar muscular atrophy (Kennedy’s disease – an X-linked recessive, CAG repeat mutation of the androgen receptor causing neurological, muscular, and endocrine loss of function.)

It is possible for other general causes of nerve disorders to have bulbar involvement and cause a bulbar palsy, e.g. a medullary brainstem stroke, or tumour specifically involving the brainstem, or neuroinflammatory conditions that can affect any nerve, e.g. Lyme disease, neurosarcoidosis, the acute bulbar palsy plus variant of Guillain Barré syndrome, but these are much rarer manifestations.