Twelfth Cranial Nerve Lesions

Anatomy

Course

• Originates from the hypoglossal nucleus in the medial medulla oblongata.
• Exits the pre-olivary sulcus (between the olive and the pyramid).
• Leaves the skull via the hypoglossal foramen.

Innervation

The hypoglossal nerve innervates:

Muscle Group	Muscles
Intrinsic	All intrinsic muscles of the tongue
Extrinsic	All extrinsic tongue muscles except palatoglossus (CN X)

Pathology

Hypoglossal nerve lesions are usually associated with broader caudal cranial nerve pathology.

Causes

1. Central (CNS) Lesions
   • Vascular lesions of the medulla
   • Neurological disorders:
     • Multiple sclerosis
     • Motor neurone disease
     • Guillain-Barré syndrome
     • Poliomyelitis
2. Peripheral Lesions (posterior fossa / upper neck)
   • Trauma at the skull base
   • Mass lesions in the posterior cranial fossa:
     • Tumours
     • Abscesses
     • Aneurysms

Clinical Assessment

1. Inspection

• Look for wasting or fasciculations of the tongue → suggests LMN lesion.
• Bilateral LMN lesions can result in dysarthria.
• Fasciculations may be unilateral or bilateral.

2. Movement

• Ask patient to protrude the tongue:
  • Tongue deviates towards the weaker side in unilateral LMN lesion.
• Unilateral UMN lesions often cause no deviation, due to bilateral innervation.
• Bilateral UMN lesions → small, immobile tongue.

3. Bulbar Syndromes

• Pseudobulbar palsy: Bilateral UMN lesions (CN IX, X, XII)
• Bulbar palsy: Bilateral LMN lesions (CN IX, X, XII)

Investigations

Blood Tests

• FBC
• CRP
• ESR
• U&Es / Glucose

Imaging

• CT / CTA: Screens for mass lesions and aneurysms
• MRI: Most sensitive for detecting CNS, peripheral nerve, and head/neck lesions

Management

• Management is directed at the underlying cause where possible.

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Appendix 1

Anatomy of the Twelfth cranial nerve

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Appendix 2

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References

Publications

• Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 8e 2021
• Fuller G. Neurological Examination Made Easy. 6e 2019
• O’Brien M. Aids to the Examination of the Peripheral Nervous System. 6e 2023

FOAMed

• Coni R. Neuro 101: Cranial Nerves. LITFL
• Nickson C. Bulbar and pseudobulbar palsy. LITFL
• Nickson C. Pseudobulbar and bulbar palsies. LITFL
• Nickson C. The Brainstem Rules of Four. LITFL
• Ercleve T. The rule of 4 of the brainstem. LITFL
• Nickson C. Cranial nerve lesions DDx. LITFL