The transition zone of a nerve describes a region of a few millimeters where the myelin sheath changes from central to peripheral type. This zone is susceptible to mechanical irritation and is implicated in neurovascular compression syndromes such as trigeminal neuralgia (CN V), hemifacial spasm (CN VII), vestibular paroxysmia (CN VIII) and glossopharyngeal neuralgia (CN XI).
The location of the transition zone relative to the root entry zone for a cranial nerve can be useful in understanding whether a contacting vessel is likely to cause neuralgia or not. For CN V, VII and IX the transition zones are located very close to the root entry zone (within a few millimeters). CN VIII has a transition zone further along the nerve (around 10 mm) 1.
Transition zone locations
- CN V: 4 mm from root entry zone
- CN VII: 2 mm from root entry zone
- CN VIII: 10 mm from root entry zone
- CN IX: 1.5 mm from root entry zone