The hypoglossal nerve has a single job: to supply the muscles of the tongue. Other than the palatoglossus (which is supplied by the vagus), every intrinsic and extrinsic tongue muscle receives its innervation from CN XII. The exam takes thirty seconds and is one of the most informative in neurology, because the tongue is a uniquely visible motor structure — the only muscle in the body whose bulk, contour, and fasciculations can be inspected through an open mouth in ordinary lighting. A protruded tongue is a publicly visible report on its lower motor neurons. The clinician’s task is to know what the report is telling you.

Functional Anatomy

The hypoglossal nucleus is a long, paired column in the dorsal medulla, just lateral to the midline, beneath the floor of the fourth ventricle (forming the hypoglossal trigone). Axons from the nucleus pass ventrally through the medulla and exit between the pyramid and the olive, then emerge as rootlets that coalesce into the hypoglossal nerve. The nerve exits the skull through the hypoglossal canal in the occipital bone, descends in the carotid sheath, then loops forward in the neck above the hyoid bone to reach the tongue.

Cortical input to the hypoglossal nucleus is predominantly contralateral. A unilateral cortical or corticobulbar tract lesion therefore weakens the contralateral half of the tongue. This is the opposite of the bilateral pattern seen with most other cranial nerves, and it is one reason the tongue is such a useful localizer for cortical disease.

The intrinsic muscles of the tongue change its shape; the extrinsic muscles (genioglossus, hyoglossus, styloglossus, palatoglossus) change its position. The genioglossus is the muscle whose action is most often tested. It draws the tongue forward — protruding it — and slightly downward. When both genioglossi work normally, the tongue protrudes straight ahead. When one genioglossus is weak, the tongue deviates toward the weak side. The unopposed strong-side muscle pushes the tongue across midline.

Mnemonic distinction worth fixing in mind: the tongue deviates toward the lesion; the uvula deviates away from the lesion. The tongue rule applies whether the lesion is cortical (deviation toward the contralateral lesion — that is, the tongue deviates away from the side of the hemiparesis if you frame it that way; but always toward the weak muscle) or nuclear/peripheral (deviation toward the ipsilateral lesion).

The Examination

Inspection at Rest

With the tongue resting comfortably in the floor of the mouth — not protruded — look for:

• Atrophy: unilateral or bilateral. A normal tongue has smooth, rounded contours and a pink, even surface. A denervated tongue is hollow, scalloped, and shrunken. Severe bilateral atrophy gives the tongue a wrinkled, “raisin” appearance.
• Fasciculations: small, brief, twitching contractions of individual motor units. They are best seen with the tongue at rest, not protruded — protrusion produces voluntary fine tremor that can be mistaken for fasciculations. Have the patient relax the tongue completely against the floor of the mouth for at least ten or fifteen seconds before judging.
• Spasticity at rest: a stiff, slow-moving tongue with no atrophy or fasciculations, especially in the context of pseudobulbar findings, points to bilateral upper motor neuron disease.

The distinction between fasciculations and the normal tremor of the protruded tongue is a frequent source of overcalling. Fasciculations are visible in the relaxed tongue resting in the mouth. The fine fasciculation-like movement seen when the tongue is poked out is normal voluntary tremor and means nothing.

Protrusion and Movement

Ask the patient to stick out the tongue and protrude it straight ahead. Observe for:

• Deviation — toward which side, and how marked. A subtle deviation can be enhanced by asking the patient to push the tongue forcefully against a tongue depressor.
• Range — does the tongue come out fully, or only halfway? A grossly weak tongue may stay mostly in the mouth.
• Speed and dexterity — ask the patient to move the tongue from side to side rapidly, then to push it into each cheek. Slow, stiff lateral movement suggests upper motor neuron involvement; weak, ineffective movement on one side suggests lower motor neuron involvement on that side.

Force can be tested by asking the patient to push the tongue into the inside of the cheek while you press against the cheek from outside. Asymmetric force is a sign of unilateral CN XII weakness. The tongue should also be tested for the ability to make rapid, consonant-rich speech sounds — “la, la, la, la” tests tongue tip articulation, “ka, ka, ka” tests the soft palate.

Patterns and Their Meaning

Cortical (Upper Motor Neuron) Tongue Weakness

A cortical or corticobulbar tract lesion (stroke, tumor, multiple sclerosis) produces contralateral tongue weakness with the tongue deviating toward the weak side (away from the lesion). There is no atrophy and no fasciculation — the lower motor neurons are intact. The tongue may move slowly and stiffly, especially in pseudobulbar patients. The presence of other contralateral findings — face weakness sparing the forehead, arm and leg weakness, dysphasia if the lesion is in the dominant hemisphere — confirms the cortical localization.

Nuclear (Lower Motor Neuron) Tongue Weakness

A brainstem nuclear lesion produces ipsilateral tongue weakness with atrophy, fasciculations, and protrusion toward the affected side. The nucleus sits in the dorsal medulla, and adjacent structures are often involved. The most informative associated finding is contralateral hemiparesis from involvement of the adjacent pyramid — the syndrome of medial medullary infarction (sometimes called the Dejerine medial medullary syndrome). In its full form, this includes ipsilateral tongue weakness with atrophy and fasciculations, contralateral hemiparesis sparing the face, and contralateral loss of dorsal column sensation. It is uncommon but classical.

Other nuclear causes include syringobulbia (especially the extension of an upper cervical syrinx into the medulla), brainstem tumor, multiple sclerosis plaque, and bulbar motor neuron disease. Bilateral nuclear involvement is the picture in advanced ALS and produces marked tongue atrophy, prominent fasciculations, and severe bulbar dysarthria.

Peripheral CN XII Lesions

Lesions of the nerve after it exits the skull are less common but recognizable. The most frequent location is in the neck, where the nerve may be compressed by a carotid dissection (especially extending up to the skull base), by a parapharyngeal mass, or by a retropharyngeal abscess. Internal carotid artery dissection is a classical cause: a young patient with neck pain, ipsilateral Horner syndrome (sympathetic chain), and an ipsilateral tongue deviation has CN XII compression from an expanding dissected vessel until proven otherwise.

Iatrogenic injury during carotid endarterectomy is the other recognized peripheral cause. Patients note new dysarthria postoperatively and on exam have ipsilateral tongue deviation. Most cases recover, but the loss can be persistent.

Skull base masses involving the hypoglossal canal — most often metastases, less often schwannomas — can produce isolated CN XII palsy or combinations with other lower cranial nerves.

Bilateral Hypoglossal Involvement

Bilateral tongue weakness has a different differential. The most common cause by far is motor neuron disease (ALS with bulbar features), in which the tongue progressively atrophies, fasciculates conspicuously, and becomes immobile. The combination of bilateral tongue atrophy, fasciculations, brisk gag and jaw reflexes, and limb features of upper and lower motor neuron disease defines bulbar-onset ALS. Other causes of bilateral tongue weakness include progressive bulbar palsy (a variant of ALS), Kennedy disease (X-linked spinobulbar muscular atrophy, with characteristic perioral fasciculations and androgen insensitivity), and rare syndromes of inflammatory or paraneoplastic bulbar dysfunction.

🔍 Did You Know?

A small subset of patients with internal carotid artery dissection present primarily with a hypoglossal palsy — ipsilateral tongue deviation, atrophy if the injury is chronic, and dysarthria — sometimes with a partial Horner syndrome. The hypoglossal nerve courses near the upper portion of the dissected vessel, and an expanding hematoma compresses it. Neck pain and a tongue that deviates to one side in an otherwise well young person should prompt immediate vascular imaging.

Pitfalls and Pearls

• The tongue deviates toward the lesion. This is the unifying rule, regardless of whether the lesion is cortical, nuclear, or peripheral. (The cortical lesion deviates the tongue toward the weak side — the contralateral side, which is therefore “toward” the cortically-mediated weakness, not “toward” the side of the brain that was injured. The clinically useful framing is always: which muscle is weak? The tongue deviates toward that side.)
• Fasciculations are seen in the relaxed tongue, not the protruded one. Voluntary fine tremor of the protruded tongue is normal and not pathological.
• No atrophy + no fasciculation + deviation = cortical lesion. Atrophy or fasciculation in a deviating tongue = lower motor neuron lesion somewhere along the pathway.
• Bilateral tongue atrophy with fasciculations is ALS until proven otherwise. Look at the rest of the body: brisk reflexes in some areas, wasted weakness in others, no sensory findings.
• Hypoglossal nerve palsy in a young person with neck pain is carotid dissection until proven otherwise.
• The tongue is a fast localizer for dominant hemispheric strokes. A patient with new aphasia and a tongue deviating to the side of weakness has a left hemispheric stroke. The bedside picture is virtually complete.
• Dysarthria with a “thick” tongue and no other localizing signs needs careful inspection: bulk, fasciculations, lateral movement. Subtle bulbar motor neuron disease often presents with vague dysarthria and is missed unless the tongue is scrutinized.

References

1. Campbell WW. DeJong’s The Neurologic Examination. 7th ed. Philadelphia: Lippincott Williams & Wilkins; 2013. Chapter 20.
2. Keane JR. Twelfth-nerve palsy: analysis of 100 cases. Arch Neurol. 1996;53(6):561-566.
3. Mokri B, Silbert PL, Schievink WI, Piepgras DG. Cranial nerve palsy in spontaneous dissection of the extracranial internal carotid artery. Neurology. 1996;46(2):356-359.
4. Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Philadelphia: Wolters Kluwer; 2017.
5. Brooks BR, Miller RG, Swash M, Munsat TL; World Federation of Neurology Research Group on Motor Neuron Diseases. El Escorial revisited: revised criteria for the diagnosis of amyotrophic lateral sclerosis. Amyotroph Lateral Scler Other Motor Neuron Disord. 2000;1(5):293-299.