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Lesion localisation — the principle behind every neurology question
Every neurology question answers 2 questions: where is the lesion (cortex, capsule, brainstem, spinal cord, root, plexus, nerve, neuromuscular junction or muscle) and what is it (by tempo of onset) — and the combination of findings reveals the level.
Why it matters in the exam
Most clinical neurology questions hide a localisation question: weakness with brisk or absent reflexes, a sensory deficit with a level, "crossed" signs of a cranial nerve on one side and the body on the other, or fatigable weakness without sensory loss. Telling upper from lower motor neuron, and spotting what "does not fit" a level (for example sensory loss in muscle disease), lets you discard distractors quickly. Tempo also decides: seconds to minutes — vascular; hours to days — inflammatory or infectious; months — degenerative or neoplastic. The common distractor is choosing a diagnosis by one symptom without checking the whole combination, for example calling a stroke a bilateral leg weakness with a sensory level, which fits the spinal cord.
Key points
- Every question: where is the lesion and what is it.
- Upper neuron: brisk reflexes and spasticity; lower neuron: wasting, fasciculations and reduced reflexes.
- Crossed signs point to the brainstem; a sensory level to the spinal cord.
- Weakness without sensory loss: neuromuscular junction, muscle or anterior horn.
- Tempo of onset points to the cause: vascular, inflammatory or degenerative.
Summary table
| Level | Pointing findings | What does not fit |
|---|---|---|
| Cortex | Aphasia, neglect, hemianopia, face and arm weaker than leg, seizures | Cranial nerve signs on one side with the body on the other |
| Internal capsule and subcortical | Pure weakness of face, arm and leg equally, without cortical signs | Aphasia or neglect |
| Brainstem | Crossed signs: a cranial nerve on the same side and weakness or sensory loss of the body on the opposite side; vertigo, double vision | A sensory level on the trunk |
| Spinal cord | Sensory level, bilateral weakness, sphincter disturbance, upper motor neuron signs below the level | Facial involvement |
| Root, plexus and nerve | Radicular pain and deficit by dermatome (root); deficit in several nerves of a limb (plexus); deficit in the map of one nerve (nerve) | Brisk reflexes |
| Neuromuscular junction and muscle | Junction: fatigable weakness, ocular and bulbar, no sensory loss; muscle: symmetric proximal weakness, raised CK | Sensory loss |
A free exam question
מטופל הסובל מכאבי גב כרוניים, ביצע בדיקת EMG שהדגימה פיברילציות בשרירי hamstring gastrocnemius ולא הופק רפלקס H. ממצאים אלה מצביעים על פגיעה שורשית בשורש: (the question as examined, in Hebrew)
- א. L4
- ב. L5
- ג. S1
- ד. S2
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Sources
- DeJong's The Neurologic Examination, 8th edition (2019) — Localization in clinical neurology
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.