What are the key landmarks to identify the hypoglossal nerve?

Key Landmarks to Identify the Hypoglossal Nerve

The hypoglossal nerve can be reliably identified using several anatomical landmarks, with the posterior belly of the digastric muscle serving as the most consistent landmark, as the nerve typically runs 5 mm deep to the junction of the tendon and belly of this muscle.

Anatomical Course of the Hypoglossal Nerve

The hypoglossal nerve (CN XII) has a complex course that can be divided into several segments:

1. Cisternal segment: Originates from the dorsal medulla as rootlets situated along the anterior margin of the lower two-thirds of the olive in the preolivary sulcus 1

2. Intracanalar segment: Traverses the hypoglossal canal surrounded by a venous plexus 1

3. Extracranial segment: Travels caudally within the carotid space and then courses anteriorly inferior to the hyoid 1

4. Terminal segment: Distributes branches to the intrinsic and extrinsic muscles of the tongue (except the palatoglossus muscle) and the infrahyoid strap muscles via the ansa cervicalis 1

Key Surgical Landmarks for Identifying the Hypoglossal Nerve

Primary Landmarks:

• Posterior belly of the digastric muscle: The hypoglossal nerve is typically located within 1 cm deep to the junction of the tendon and belly of this muscle, with an average depth of 5.48 ± 2.24 mm 2

• Digastric tendon: The hypoglossal nerve is located 3-7 mm (mean 5 mm) inferior to the digastric tendon 3

• Hypoglossal triangle: A triangular region bordered by:

  • Descending hypoglossus laterally
  • Transverse hypoglossus inferiorly
  • Inferior border of the stylohyoid muscle superiorly 3

Secondary Landmarks and Measurements:

• Relationship to carotid arteries:

  • Crosses the internal carotid artery 29.93 ± 5.99 mm superior to the carotid bifurcation 4
  • Crosses the external carotid artery 15.19 ± 6.68 mm superior to the carotid bifurcation 4

• Relationship to bony landmarks:

  • 12.24 ± 3.71 mm superior to the greater horn of hyoid bone 4
  • 17.16 ± 4.40 mm inferior to the angle of the mandible 4
  • 39.08 ± 5.69 mm from the tip of the mastoid process 4

• Relationship to occipital artery: The crossing point between the hypoglossal nerve and occipital artery is approximately 8 mm superior to the emergence of the occipital artery from the external carotid artery 3

Clinical Implications and Surgical Considerations

• In 73% of cases, the hypoglossal nerve loop is located inferior to the digastric tendon 4

• Atypical variations exist, such as the hypoglossal nerve coursing superficial to the internal jugular vein, which may increase risk of injury during neck surgery 5

• Most hypoglossal nerve palsies are due to neoplasms involving the hypoglossal canal, making knowledge of these landmarks critical for surgical planning 1, 6

• Careful identification of these landmarks is essential during procedures such as facial nerve anastomosis surgery, where rapid and accurate localization of the hypoglossal nerve is required 2

Imaging Considerations

• MRI with contrast is the preferred imaging modality for evaluating the entire course of the hypoglossal nerve, with 100% sensitivity for detecting hypoglossal canal invasion 6

• CT provides complementary information about the osseous integrity of the hypoglossal canal and skull base 6

• Thin-cut high-resolution heavily T2-weighted imaging combined with MRA provides detailed visualization of the hypoglossal nerve 1

By understanding these anatomical landmarks and relationships, surgeons can minimize the risk of iatrogenic injury to the hypoglossal nerve during procedures involving the neck region.