What are the differential diagnoses for a tongue that deviates to the left on protrusion?

Differential Diagnosis of Tongue Deviation to the Left

When the tongue deviates to the left on protrusion, this indicates left-sided hypoglossal nerve (CN XII) dysfunction, most commonly caused by neoplasms involving the hypoglossal canal or extracranial course of the nerve, requiring urgent MRI of the head, orbit, face, and neck with and without contrast. 1

Understanding the Anatomical Localization

The tongue deviates toward the side of weakness in lower motor neuron (LMN) hypoglossal nerve palsy. 1, 2 This occurs because:

• The hypoglossal nerve provides somatic motor innervation to all intrinsic and extrinsic tongue muscles (except palatoglossus) 1, 2
• When the left CN XII is damaged, the left-sided tongue muscles are weakened 3
• The unopposed action of the intact right-sided muscles pushes the tongue toward the weakened left side during protrusion 3, 4

Primary Differential Diagnoses by Anatomical Segment

The hypoglossal nerve can be divided into five segments, each with distinct pathologies: 5, 6

Skull Base Segment (Most Common)

• Neoplasms involving the hypoglossal canal are the most common cause of isolated hypoglossal nerve palsy 1
• Schwannomas, meningiomas, metastases, or chordomas affecting the skull base 1, 5
• Bony expansion of the hypoglossal canal may be visible on CT 6

Extracranial Segment (Carotid Space)

• Malignant tumors in the carotid space are the most common cause of extracranial CN XII palsy 1
• Internal carotid artery dissection can cause isolated acute CN XII palsy 1
• Tumors in the sublingual space affecting terminal nerve branches 1

Brainstem (Medullary) Segment

• Brainstem infarction affecting the hypoglossal nucleus 1, 7
• Demyelinating disease (multiple sclerosis) 1
• Nuclear lesions are usually accompanied by additional neurologic deficits indicating brainstem involvement 1, 2

Other Etiologies

• Inflammatory and infectious processes (meningitis, abscess) 1
• Trauma to the nerve along its course 1
• Vascular lesions or compression 1

Critical Diagnostic Approach

Imaging Strategy

MRI of the head, orbit, face, and neck with and without contrast is the preferred initial imaging modality. 1 This approach:

• Directly visualizes the entire course of CN XII from brainstem through skull base to extracranial segments 1, 8, 6
• Provides excellent soft tissue contrast to identify neuritis, nerve sheath tumors, and characterize the carotid space 1
• Can detect denervation signal changes in the tongue musculature that vary with duration of palsy 1

CT neck with IV contrast is an alternative or complementary examination 1:

• Superior for displaying bony anatomy of the hypoglossal canal 6
• Can detect bony expansion suggesting skull base pathology 6
• May be complementary to MRI in certain clinical scenarios 1

Clinical Examination Findings to Document

Look for these specific features that help localize the lesion: 1, 2, 3

• Tongue atrophy and fasciculations on the left side (indicates LMN lesion) 2, 3
• Dysarthria, particularly difficulty with lingual consonants 1, 2
• Associated cranial nerve palsies (CN IX, X, XI) suggesting jugular foramen or brainstem involvement 1, 8
• Fatty infiltration visible as decreased bulk on the affected side (chronic denervation) 2, 3

Common Pitfalls to Avoid

Do not confuse LMN with upper motor neuron (UMN) lesions: 2, 7

• LMN lesions: tongue deviates toward the side of the lesion with atrophy and fasciculations 2
• UMN lesions: tongue deviates away from the lesion side without atrophy or fasciculations 2, 7

Do not dismiss isolated hypoglossal nerve palsy as benign: 3

• It may be the initial or solitary sign of an intracranial or extracranial space-occupying lesion 3
• Urgent imaging is warranted even without other neurologic deficits 1

Do not confuse tongue deviation with facial asymmetry from facial nerve palsy: 2

• Facial nerve palsy affects facial muscles but not tongue protrusion 2

Do not overlook vascular causes: 1

• Consider CTA or MRA if acute onset suggests carotid dissection 1, 8
• Dissection can present with isolated CN XII palsy or multiple lower cranial nerve involvement 1

Do not assume all tongue deviation is from peripheral nerve pathology: 7

• Paramedian pontine infarction can cause contralateral tongue deviation if cortico-hypoglossal fibers are affected before decussation 7
• However, this would be an UMN pattern without atrophy or fasciculations 2, 7