Brainstem and cranial nerve anatomy for lesion localisation
Where each cranial nerve nucleus sits in the midbrain, pons and medulla, the "rule of 4" for medial and lateral structures, and how crossed signs, named brainstem syndromes and nuclear versus fascicular lesions localise a lesion.
Tier 2Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- A brainstem lesion produces crossed signs: an ipsilateral cranial nerve deficit with contralateral long-tract signs.
- The cranial nerve number tells the level: 3 and 4 in the midbrain, 5 to 8 in the pons, 9 to 12 in the medulla.
- Medial structures (corticospinal tract, medial lemniscus, MLF, motor nuclei 3, 4, 6, 12) fall in paramedian artery territory. Lateral structures (spinothalamic, spinal trigeminal, sympathetic, spinocerebellar) fall in circumferential artery territory.
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Sources
- Principles of Neural Science (6th ed.)McGraw Hill (Kandel, Koester, Mack, Siegelbaum, 2021), 2021
- Localization in Clinical Neurology (8th ed.)Wolters Kluwer (Brazis, Masdeu and Biller), 2022
- Gates P. The rule of 4 of the brainstem: a simplified method for understanding brainstem anatomy and brainstem vascular syndromes for the non-neurologist. Intern Med J 2005;35:263-6AU/NZInternal Medicine Journal, 2005
- Last updated
- 14 November 2026
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