Hypoglossal Nerve Disorders


Hypoglossal Nerve Disorders

Hypoglossal Nerve: The hypoglossal nerve is the twelfth cranial nerve, and innervates all the extrinsic and intrinsic muscles of the tongue, except for the palatoglossus which is innervated by the vagus nerve.[a] It is a nerve with a solely motor function. The nerve arises from the hypoglossal nucleus in the brain stem as a number of small rootlets, passes through the hypoglossal canal and down through the neck, and eventually passes up again over the tongue muscles it supplies into the tongue. There are two hypoglossal nerves in the body: one on the left, and one on the right.

The hypoglossal nerve arises from the hypoglossal nucleus in the medulla oblongata of the brainstem. It then passes laterally across the posterior cranial fossa, within the subarachnoid space. The nerve exits the cranium via the hypoglossal canal.

Now extracranial, the nerve receives a branch of the cervical plexus that conducts fibres from C1/C2 spinal nerve roots. These fibres do not combine with the hypoglossal nerve – they merely travel within its sheath.

The nerve is involved in controlling tongue movements required for speech and swallowing, including sticking out the tongue and moving it from side to side. Damage to the nerve or the neural pathways which control it can affect the ability of the tongue to move and its appearance, with the most common sources of damage being injury from trauma or surgery, and motor neuron disease. The first recorded description of the nerve is by Herophilos in the third century BC.

Hypoglossal Nerve Disorders

Hypoglossal Nerve Function

The hypoglossal nerve arises from the hypoglossal nucleus. This is found the full length of the medulla, close to the midline. The roots of the hypoglossal nerve itself arise between the olive and the pyramid. These roots then unite to form two bundles which then pass through the hypoglossal canal (in the occipital bone) by piercing the dura mater. It is at this point that the two bundles unite to form the full hypoglossal nerve as a single entity.

After leaving the hypoglossal canal, the hypoglossal nerve receives twigs from C1 and C2 containing general somatic motor fibers, as well as some general sensory fibers from the C2 ganglion. It is these fibers that go on further in the neck to supply the strap muscles.

The hypoglossal nerve then continues its journey to the tongue by passing posterior to the internal carotid artery and the glossopharyngeal and vagus nerves. It then passes inferiorly sandwiched between the internal jugular vein and the internal carotid artery. It then loops forward over the occipital artery receiving some fibers from the pharyngeal plexus at that point. As it descends further, it passes over several arteries—the external and internal carotid and lingual arteries. It lies on the hyoglossus inferior to the lingual nerve and submandibular duct then passes inferior to the mylohyoid and digastric. From the hypoglossal nerve are several important branches, which will be discussed later—the meningeal, thyrohyoid, and muscular branches as well as a component to the superior root of the ansa cervicalis.

The hypoglossal nerve is mainly a somatic efferent (motor) nerve to innervate the tongue musculature. The nerve also contains some sympathetic postganglionic fibers from the cervical ganglia, which innervates tongue vessels and some small glands in the oral mucosa.

The hypoglossal nerve consists of four branches: the meningeal, descending, thyrothyroid, and muscular. However, only the muscular branch is considered part of the real hypoglossal nerve originating from the hypoglossal nucleus. Other branches originate from spinal nerves (mainly C1/C2) or the cervical ganglia. The meningeal branch arises from C1 and C2 nerves to the posterior cranial fossa. The descending branch carries information from C1 and then C2/C3 to innervate the neck muscles (omohyoid, sternohyoid, and sternothyroid). The thyrothyroid branches innervate the thyrohyoid muscle of the neck. The descending and thyrohyoid branches mostly originate from the cervical plexus.

The muscular/lingual branch is a general somatic efferent (GSE) branch and innervates all the muscles of the tongue (both intrinsic and extrinsic), except for the palatoglossus, which receives motor innervation from the vagal nerve (CN X). The muscular branch controls most actions of the tongue including, protruding, retracting, depressing the tongue, and changing the tongue’s shape.

Damage to the hypoglossal nerve causes paralysis of the tongue. Usually, one side of the tongue is affected, and when the person sticks out his or her tongue, it deviates or points toward the side that is damaged.

Geniohyoid muscle
Thyrohyoid muscle

The path of the nerve starts in the hypoglossal nucleus of the brainstem, which is located near the midline of the medulla oblongata, and then travels between the carotid artery and jugular vein to end up on the underside of the tongue.

Hypoglossal Nerve Cadaver

Hypoglossal nerve injuries can range from subtle deficits to sequelae with a significant impact on quality of life that can affect swallowing and airway functions. Although the potential for traumatic and surgical injury has been noted for a long time, the past several decades have witnessed several advances in surgical management for this injury that serve to minimize its effects. Nonetheless, injury is serious, and early recognition is essential for initiating appropriate management. Although varying by etiology, hypoglossal nerve palsy generally has a poor prognosis, particularly if the nerve has been transected and is not immediately repaired. One retrospective analysis of 1000 patients reported that only 15% of patients experienced significant recovery (Keane, 1996).

Located in the medulla next to the fourth ventricle, the paired hypoglossal nuclei have bilateral innervation (Loh, Maya, & Go, 2002). Roughly 10-15 rootlets proceed from these nuclei, course superior to the vertebral artery, and leave the skull base through the hypoglossal canal (Loh et al., 2002). The internal carotid artery and jugular bulb are lateral to the hypoglossal nerve in this location, and after leaving the hypoglossal canal, the nerve transits via the nasopharyngeal carotid space (Loh et al., 2002). At this level, the glossopharyngeal and spinal accessory nerves are closely associated with the hypoglossal nerve up until the mastoid tip, where the hypoglossal nerve approaches the mylohyoid and hyoglossus muscles, lying between these two muscles in the sublingual region. Throughout this course, the hypoglossal nerve communicates with numerous nerves. This includes anastomoses with the sympathetic trunk, pharyngeal nerves, glossopharyngeal nerve, and the vagus nerve (Shoja et al., 2014).

The motor functions of the instrinsic tongue muscles, as well as the genioglossus, hyoglossus, and styloglossus muscles, are all innervated by the hypoglossal nerve. These muscles are essential for swallowing and speech. To test nerve function, the physical examiner has a patient protrude his or her tongue. In infranuclear, or lower motor neuron lesions, the tongue deviates toward the injured side, as the contralateral genioglossus is acting unopposed. In contrast, supranuclear (upper motor neuron) lesions result in deviation away from the lesion. In addition to causing deviation toward the lesion, an injury involving a hypoglossal nucleus may result in tongue atrophy and fasciculation.

As with other nerve injuries, MRI is ideal for assessing lesions close to the medulla. Moving distally in evaluating lesions in the hypoglossal canal, a CT scan also has utility in assessing for bony lesions (Loh et al., 2002). Skull base fractures, skull base tumors (such as glomus jugulare tumors), and nasopharyngeal neoplasms may cause injury in this area (King et al., 1999). Moving distally into the carotid and sublingual spaces, malignancies are most likely to cause nerve disruption in the former, while malignancies and infectious processes may occur in the latter (Keane, 1996). Although uncommon, carotid artery dissections may also cause hypoglossal palsies (Lieschke, Davis, Tress, & Ebeling, 1988; Spitzer, Mull, & Topper, 2001).

Hypoglossal Nerve Stimulation

Start by inspecting the tongue as it rests in the patient’s mouth. Unilateral weakness or paralysis can be strongly suspected if the tongue is curled in a gentle arabesque. The tip of the tongue will point to the normal side due to unopposed normal tone in that half of the tongue. Look for atrophy and fasciculations.

Test the genioglossus by asking the patient to protrude the tongue. With unilateral weakness or paralysis, the tongue will point to the affected side due to unopposed action of the normal muscle.

The nuclei are dorsal and medial in the medulla. Supranuclear innervation is predominantly from the contralateral cortex and descends in the corticobulbar tract. The fibers leave the medulla and pass through the hypoglossal canal. Peripherally the nerve supplies the intrinsic muscles of the tongue, the genioglossus (tongue protrusion), the hypoglossus, and the styloglossus.

Supranuclear lesions usually produce a transient mild weakness of the contralateral side of the tongue. There is considerable individual variation, however, and on occasion a supranuclear lesion can produce what appears to be a nuclear lesion, due to the degree of the weakness. The transient nature is usually the clue.

Nuclear lesions produce atrophy, weakness, or paralysis and fasciculations. Amyotrophic lateral sclerosis and polio are two causes. Syringobulbia, infarction, and intraspinal tumors are other etiologies. Nuclear lesions are often bilateral because of the closeness of the two nuclei in the medulla.

Hypoglossal nerve involvement after the individual fibers exit from the medulla is seen with many of the lesions that affect the spinal accessory nerve. Tumors can compress the nerve in the hypoglossal canal and in the jugular foramen. Basilar meningitis, due to granulomatous infection or carcinoma, can produce hypoglossal lesions. Unilateral twelfth nerve palsy has been reported as one of the more common cranial mononeuropathies due to metastases.