Anatomical Relationship Between the Vagus and Hypoglossal Nerves at C4 Level
At the level of C4, the vagus nerve typically travels laterally to the common carotid artery within the carotid sheath, while the hypoglossal nerve has already coursed anteriorly and is located more anterolaterally in the neck. 1
Anatomical Course of Both Nerves
Vagus Nerve (CN X)
- The vagus nerve exits the skull through the jugular foramen alongside the glossopharyngeal nerve and has a close relationship with the hypoglossal nerve at the skull base 1
- After exiting the skull, the vagus nerve descends within the posterior aspect of the carotid sheath alongside the internal jugular vein and common carotid artery 1
- At the C4 level, the vagus nerve typically maintains its position within the carotid sheath, running lateral to the common carotid artery in approximately 94.3% of cases 2
- Less common variations include the vagus nerve running medial (2.9%), posterolateral (1.4%), or posterior (1.4%) to the common carotid artery 2
Hypoglossal Nerve (CN XII)
- The hypoglossal nerve exits the skull through the hypoglossal canal and initially has a close relationship with the vagus nerve near the skull base 3
- After exiting the skull, the hypoglossal nerve travels caudally within the carotid space and then courses anteriorly inferior to the hyoid bone 3
- By the level of C4, the hypoglossal nerve has typically already coursed anteriorly to innervate the tongue muscles 3, 4
- The average distance of the hypoglossal nerve is approximately 27.7 mm to the carotid bifurcation, 9.3 mm to the hyoid bone, and 54.7 mm to the inferior border of the cricoid cartilage 2
Clinical Significance
- Advanced imaging techniques, particularly MRI with thin-cut heavily T2-weighted sequences and contrast-enhanced MRA, provide detailed visualization of these nerves and their relationships 1
- Awareness of the anatomical relationship between these nerves is crucial during surgical procedures in the neck to prevent iatrogenic nerve injury 2, 4
- The vagus and hypoglossal nerves may have interconnections forming a neural plexus, which can complicate identification during surgery 4, 5
- A previously undescribed branch arising from the dorsal aspect of the hypoglossal nerve at the level of the atlas has been found to join branches from C1 and C2 nerves to innervate the atlanto-occipital joint 5
Imaging Considerations
- MRI is the preferred modality for direct visualization of these cranial nerves and their relationships 1
- Thin-cut heavily T2-weighted contrast-enhanced modified balanced SSFP sequences and contrast-enhanced MRA focused on the posterior skull provide detailed imaging of these nerves 1
- High-resolution 3-D T2-weighted imaging combined with MRA and 3-D T1-weighted contrast-enhanced sequences can effectively visualize these nerves and potential neurovascular compression 1
Anatomical Variations
- The vagus and hypoglossal nerves may have both false communications (connected only by connective tissue) and true communications (with actual nerve fiber exchange) 6
- True communications between these nerves, when present, are typically directed toward the vagus nerve 6
- At the skull base, the vagus and hypoglossal nerves are often in close contact but usually can be separated during careful dissection 6