How to Test the 12th Cranial Nerve (Hypoglossal Nerve)
Ask the patient to protrude their tongue and observe for deviation toward the side of any lesion—this is the primary and most important clinical sign of hypoglossal nerve dysfunction. 1
Primary Examination Steps
Tongue Protrusion Test
- Instruct the patient to stick out their tongue as far as possible and observe the direction of movement 1
- In a healthy individual, the tongue protrudes in the midline without deviation to either side 1
- With hypoglossal nerve palsy, the tongue deviates toward the side of the lesion due to unopposed action of the contralateral genioglossus muscle 2, 1
- This occurs because the hypoglossal nerve provides somatic motor innervation to all intrinsic tongue muscles and three extrinsic tongue muscles (styloglossus, hyoglossus, and genioglossus), except the palatoglossus 1, 3
Inspection for Atrophy
- Look for unilateral wasting or fatty infiltration of the tongue, which appears as decreased bulk on the affected side 1
- Visible atrophy is a sign of chronic denervation and indicates a lower motor neuron lesion 1, 4
- Fasciculations (visible muscle twitching) may be present with lower motor neuron lesions but are absent with upper motor neuron lesions 1
Speech Assessment
Dysarthria Evaluation
- Listen for dysarthria, particularly difficulty with lingual consonants (such as "t," "d," "l," "n"), which indicates impaired tongue motor function 1
- Dysarthria is a key clinical manifestation of hypoglossal nerve dysfunction and is characterized by difficulty with speech articulation 2, 1
- Normal articulation without dysarthria is expected in healthy individuals 1
Distinguishing Upper vs. Lower Motor Neuron Lesions
Lower Motor Neuron (LMN) Lesions
- Tongue deviates toward the side of the lesion 1
- Visible atrophy and fasciculations are present 1
- Most commonly caused by neoplasms involving the hypoglossal canal or extracranial segment 2, 1
- Nuclear lesions are typically accompanied by additional brainstem signs 1
Upper Motor Neuron (UMN) Lesions
- Tongue deviates away from the lesion side 1
- No atrophy or fasciculations are present 1
- Usually associated with other signs of corticobulbar tract involvement 1
Critical Pitfalls to Avoid
Common Examination Errors
- Do not confuse tongue deviation with facial asymmetry from facial nerve (CN VII) palsy, which affects facial muscles but does not affect tongue protrusion 1
- Remember that lesions at the jugular foramen or brainstem may affect multiple lower cranial nerves (CN IX-XII) simultaneously, creating complex clinical pictures 1
- Always assess for associated signs such as other cranial nerve involvement (CN IX, X, XI) to localize the lesion accurately 1
When to Consider Imaging
- Any confirmed hypoglossal nerve deficit warrants imaging evaluation 2
- MRI is the best imaging modality to evaluate patients with clinical signs of hypoglossal nerve palsy, using contrast-enhanced sequences of the orbits, face, and neck with thin-cut high-resolution techniques through the posterior fossa and hypoglossal canal 2, 3
- Most hypoglossal nerve palsies are due to neoplasm involving the hypoglossal canal, though vascular lesions (including carotid dissection), inflammatory processes, infections, and trauma are also important causes 2, 3, 5