Lesion localisation
Introduit| English | Sens |
|---|---|
| aphasia | aphasia |
| sensory level | sensory level |
| polyneuropathy | polyneuropathy |
| motor neuron disease | motor neuron disease |
Not what, but where
- A neurologist's first question is not "what is the disease?" but "where is the lesion?". The answer comes from the pattern of signs, not from any one sign.
- Each level of the nervous system has one feature that no other level has, and that feature names the level.
Cortex
- Cortical lesions cause loss of higher functions (aphasia 失语症, neglect, a field defect or seizures), with weakness or numbness of the face, arm and leg on the OPPOSITE side.
- The face, arm and leg are affected on the same side as each other.
Brainstem
- Brainstem lesions give CROSSED signs: a cranial nerve problem on the side of the lesion, with weakness or numbness of the body on the other side.
- That is because the cranial nerve nuclei sit in the brainstem, while the long tracts there have not yet crossed.
Weakness of the whole left side of the face with a weak right arm and leg. Where is the lesion?
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Crossed signs, a cranial nerve on one side and the body on the other, mean the brainstem, on the side of the cranial nerve.
Spinal cord
- Cord lesions give a sensory level 感觉平面 on the trunk, upper motor neuron signs below it, and lower motor neuron signs at it.
- Bladder and bowel control are often affected.
A sensory level on the trunk points to the spinal cord.
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Only the cord has segmental levels with normal sensation above and loss below.
Peripheral nerve and root
- Peripheral lesions give weakness and sensory loss in the territory of one nerve, one root or many nerves together, with lower motor neuron signs and lost reflexes.
- There are no upper motor neuron signs.
Absent ankle reflexes, numb feet on both sides and downgoing plantars. Which level?
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Lost reflexes, a stocking pattern and no UMN signs point to a polyneuropathy.
One unique feature per level
- Cortex owns higher functions; brainstem owns cranial nerves with crossed signs; cord owns a sensory level; periphery owns nerve and root territories.
- Work through the levels one at a time and say why at each step.
Match each pattern to its level.
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One unique feature per level.
Put the levels in order from top to bottom.
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Work down the levels and test each one's unique feature.
Worked example: four steps to a stroke
- (Fictional) Right lower-face weakness with a normal forehead, a weak right arm and leg, and aphasia. Aphasia → left cortex. Forehead spared → upper motor neuron facial weakness.
- Face and body weak on the SAME side (not crossed) → above the brainstem. Right-sided weakness → left hemisphere: left middle cerebral artery territory.
Lower-face weakness with a normal forehead means —
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The forehead receives input from both hemispheres, so it is spared when one side's upper motor neurons fail.
Worked example: burning feet
- (Fictional) Burning pain in both feet, pain sensation lost up to the knees on both sides, absent ankle reflexes, normal (downgoing) plantar responses.
- Symmetrical, longest nerves first, reflexes lost, no UMN signs: a peripheral polyneuropathy 多发性神经病. A cord lesion would give a level on the trunk and UMN signs below it.
Three common slips
- Localising from one sign. One sign has many possible addresses; the pattern has one.
- Forgetting that brainstem signs are crossed. It is the single most useful clue.
- Missing UMN and LMN signs together. Wasting and fasciculations with brisk reflexes and no sensory loss is motor neuron disease 运动神经元病.
Wasting and fasciculations with brisk reflexes and no sensory loss suggest —
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UMN and LMN signs together without sensory loss is the classic motor neuron disease picture.
Recap
- Cortex: higher function loss plus opposite-side signs. Brainstem: crossed signs.
- Cord: a sensory level, UMN signs below, LMN signs at the level.
- Periphery: nerve or root territory, LMN signs, no UMN signs.