What are the effects and management of isolated 12th cranial nerve (CN) palsy?

Isolated 12th Cranial Nerve Palsy: Causes, Effects, and Management

MRI of the entire course of the hypoglossal nerve is the most appropriate initial imaging study for isolated 12th cranial nerve palsy, as neoplasms are the most common cause, particularly those involving the hypoglossal canal. 1

Clinical Presentation

Hypoglossal nerve (CN XII) palsy typically presents with:

• Dysarthria
• Deviation of the tongue toward the affected side upon protrusion
• Hemiatrophy of the tongue with fasciculations 2
• Difficulty with swallowing and speech 3

Etiology

The causes of isolated hypoglossal nerve palsy include:

Most Common Causes

• Neoplasms (most common overall):
  • Tumors involving the hypoglossal canal 1
  • Malignant tumors along the course of CN XII in the carotid space 1
  • Malignant tumors in the sublingual space 1
  • Schwannomas of the jugular foramen 2

Vascular Causes

• Internal carotid artery dissection 1
• Vascular lesions affecting the nerve's course 1

Other Important Causes

• Brainstem infarct (usually with additional neurologic deficits) 1
• Demyelinating disease 1
• Trauma (including iatrogenic from endotracheal intubation) 4, 3
• Inflammatory processes 1
• Infectious processes 1
• Idiopathic (rare) 5

Anatomical Considerations

The hypoglossal nerve has several segments where pathology can occur:

1. Nuclear origin: Dorsal medulla
2. Intracranial segment: Premedullary cistern dorsal to vertebral artery
3. Skull base segment: Through hypoglossal canal
4. Extracranial segment:
   • Carotid space
   • Anterior course inferior to hyoid
   • Terminal branches to tongue muscles

Diagnostic Approach

Imaging Studies

1. MRI with contrast (first-line):

   • Evaluates the entire course of the hypoglossal nerve 1
   • Directly images the brainstem, intracranial, skull base, and extracranial segments 1
   • Provides excellent soft tissue contrast 1
   • Protocols should include thin-cut high-resolution techniques 1
   • Pre- and post-contrast imaging is recommended 1
   • MRI has 100% sensitivity for detecting hypoglossal canal invasion by tumors 1

2. CT Neck with contrast (complementary):

   • Characterizes osseous integrity of hypoglossal canal and skull base 1
   • Include thin-cut high-resolution bone windows 1
   • CT has 87.5% sensitivity and 66.7% specificity for hypoglossal canal invasion 1

3. CTA Head and Neck:

   • Indicated when internal carotid artery dissection is suspected 1
   • Particularly useful in emergent settings 1

Additional Diagnostic Tests

• Electromyography of the tongue may help confirm denervation 6
• Denervation changes in the tongue can be detected on MRI, varying with the duration of palsy 1

Management

Management depends on the underlying cause:

1. Neoplastic causes:

   • Surgical resection when feasible
   • Radiation therapy (including gamma knife radiosurgery for schwannomas) 2
   • Referral to oncology for malignant tumors

2. Vascular causes:

   • Antiplatelet or anticoagulation therapy for dissection
   • Management of stroke risk factors

3. Iatrogenic/Traumatic causes:

   • Most cases resolve spontaneously within weeks to months 3
   • Speech therapy for persistent cases
   • Swallowing evaluation and therapy as needed

4. Inflammatory/Demyelinating causes:

   • Corticosteroids
   • Immunomodulatory therapy based on specific diagnosis

5. Idiopathic cases:

   • Supportive care
   • Some cases may not resolve 5

Pitfalls and Caveats

1. Inadequate imaging: Failure to image the entire course of the nerve may miss pathology 1

2. Overlooking associated symptoms: Nuclear lesions in the brainstem usually present with additional neurologic deficits 1

3. Misdiagnosis: Isolated hypoglossal nerve palsy can be mistaken for other conditions, leading to delayed diagnosis 5

4. Iatrogenic causes: Hypoglossal nerve injury can occur during difficult airway management or prolonged intubation but is often overlooked 4, 3

5. Combined syndromes: Hypoglossal nerve palsy can be part of multiple cranial nerve syndromes, particularly with lesions in the brainstem or jugular foramen 1

For optimal patient outcomes, thorough diagnostic evaluation with appropriate imaging and prompt management of the underlying cause are essential to prevent complications related to speech and swallowing dysfunction.