Distinguishing LMN from UMN Hypoglossal Nerve Lesions
Lower motor neuron (LMN) lesions of the hypoglossal nerve cause tongue deviation toward the side of the lesion with visible atrophy and fasciculations, while upper motor neuron (UMN) lesions cause deviation away from the lesion side without atrophy or fasciculations.
Key Clinical Differences
LMN Lesions (Nuclear or Infranuclear)
Tongue Deviation Pattern:
- The tongue deviates toward the side of the lesion upon protrusion 1, 2
- This occurs because the weakened ipsilateral muscles cannot push the tongue forward effectively 3
Muscle Changes:
- Visible atrophy (wasting) of the tongue on the affected side, appearing as decreased bulk 3
- Fasciculations (visible muscle twitching) present in the affected hemitongue 4
- Fatty infiltration may develop with chronic denervation 3
Associated Features:
- Dysarthria with difficulty articulating lingual consonants 1, 3
- Nuclear lesions typically accompanied by additional brainstem signs 1
- Most commonly caused by neoplasms involving the hypoglossal canal or extracranial segment 1
UMN Lesions (Supranuclear)
Tongue Deviation Pattern:
- The tongue deviates away from (contralateral to) the side of the lesion upon protrusion 4
- This pattern is rare because bilateral cortical innervation of the hypoglossal nucleus usually prevents clinically apparent weakness from unilateral UMN lesions 4
Muscle Changes:
Associated Features:
- May present with dysarthria but without visible muscle wasting 4
- Often accompanied by other UMN signs (spasticity, hyperreflexia) in other body regions
- Typically seen with pontine or higher lesions 4
Clinical Examination Algorithm
Step 1: Observe tongue at rest
- Look for unilateral atrophy or fasciculations → suggests LMN lesion 3
- Normal bulk bilaterally → consider UMN lesion if weakness present
Step 2: Ask patient to protrude tongue
- Deviation toward weak side + atrophy/fasciculations → LMN lesion 1, 3
- Deviation away from lesion side + no atrophy → UMN lesion 4
Step 3: Assess for associated signs
- Check for other cranial nerve involvement (CN IX, X, XI) suggesting skull base pathology 1
- Examine for UMN signs in limbs (spasticity, hyperreflexia) if UMN lesion suspected
Important Caveats
Bilateral cortical innervation: Unilateral UMN lesions rarely produce clinically apparent hypoglossal weakness because both motor cortices innervate the hypoglossal nucleus 4. When UMN signs do appear, they typically indicate bilateral involvement or specific pontine tegmental lesions 4.
Avoid confusion with facial nerve palsy: Tongue deviation differs from facial asymmetry caused by facial nerve (CN VII) palsy, which affects facial muscles but not tongue protrusion 3.
Combined syndromes: Lesions at the jugular foramen or brainstem may affect multiple lower cranial nerves (CN IX-XII) simultaneously, creating complex clinical pictures 1, 5.