Brainstem Lesions
The brainstem has three levels — midbrain, pons, medulla — each containing distinct cranial nerve nuclei and long tracts. Localising a brainstem lesion to the correct level is one of the highest-yield neurological skills.
✦ The Mnemonic
"Mid Three-Four, Pons Five-Eight, Medulla Nine-Twelve"
Midbrain: CN III–IV · Pons: CN V–VIII · Medulla: CN IX–XII
Clinical Breakdown
The crossed sign rule: ipsilateral cranial nerve deficit + contralateral hemiplegia/sensory loss = brainstem lesion. The cranial nerve is at the level of the lesion (ipsilateral because it hasn't crossed); the long tract signs are contralateral (because the corticospinal and spinothalamic tracts have already crossed or will cross above/below the lesion).
Wallenberg syndrome (lateral medullary, PICA territory): the spinothalamic tract is damaged (contralateral body pain/temperature loss), the trigeminal nucleus (ipsilateral face pain/temperature loss), the vestibular nuclei (vertigo, nausea, nystagmus), the nucleus ambiguus (ipsilateral dysphagia and hoarseness — CN IX, X), the sympathetic fibres (ipsilateral Horner's), and the inferior cerebellar peduncle (ipsilateral ataxia). The pain/temperature dissociation (opposite face and body) is pathognomonic.
Locked-in syndrome: infarction of the ventral pons (basilar artery). The corticospinal and corticobulbar tracts are destroyed — the patient cannot move any voluntary muscles except the eyes (vertical eye movements spared via midbrain tectum, which is unaffected). The reticular formation and sensory pathways are intact — the patient is fully conscious and aware. Communication via vertical eye movements and blinking.
⭐ Clinical Pearl
Basilar artery occlusion is a catastrophic stroke — it supplies the entire brainstem via the basilar perforators. Presents with progressive loss of consciousness, cranial nerve palsies, quadriplegia, and respiratory failure. CT angiography confirms the diagnosis. Mechanical thrombectomy can be performed up to 24 hours (and beyond in selected cases) for basilar occlusion — even delayed recanalization can be life-saving.