Crossed signs in brainstem infarction
A brainstem infarction on one side typically gives crossed signs: cranial nerve signs on the damaged side, with limb weakness and loss of sensation usually on the opposite side.
Remember
In a brainstem infarction, the cranial nerve signs appear on the same side as the damage, while the limb weakness and loss of sensation usually appear on the opposite side.
The major sensory and motor tracts (pathways that carry sensation and movement between the brain and the body) all pass through the brainstem. A brainstem stroke tends to interrupt both.
Limb signs usually land on the side opposite the damage. The corticospinal tract (the main motor pathway to the limbs) crosses the midline in the lower medulla (part of the brainstem), so damage above that crossing weakens the opposite limbs. Pain and temperature fibres from the body cross in the spinal cord, so damage in the brainstem causes loss of pain and temperature in the body on the opposite side.
The cranial nerve nuclei are clusters of nerve cells in the brainstem that serve the face and head. They do not cross the midline, so damage to them causes signs on the same side as the damage.
A brainstem infarction on one side therefore typically produces crossed signs. Damage on the left side of the brainstem can cause reduced sensation of the left face with weakness of the right arm and leg. Another crossed picture is Millard-Gubler syndrome: in the lower pons (part of the brainstem), damage to the facial nerve fibres and the corticospinal tract causes facial weakness on the same side as the damage, with hemiplegia (paralysis of one side of the body) on the opposite side.
Key facts
- Cranial nerve signs
- appear on the same side as the damage, because the cranial nerve nuclei do not cross the midline.
- Limb signs
- are usually on the side opposite the damage, because the pathways cross the midline.
- Crossed pattern
- signs in the face on one side and in the limbs on the other point to the brainstem.
- Corticospinal tract
- the main motor pathway to the limbs crosses the midline in the lower medulla, so damage above that crossing weakens the opposite limbs.
- Pain and temperature
- the fibres from the body cross in the spinal cord, so damage in the brainstem causes loss of both on the opposite side of the body, while the pathways that carry facial sensation serve the same side as their nuclei.
- Shared blood supply
- the cerebellum shares its blood supply with the brainstem, so some strokes in the cerebellum can cause brainstem signs.
Exam traps
- Weakness on the same side as the damage with pain loss on the opposite side is the typical pattern of spinal cord hemisection (damage to one side of the cord), not brainstem damage, because the corticospinal tract has already crossed before entering the cord.
- Loss of limb movement and sensation on the same side as the damage is not typical of a brainstem infarction, because the main motor and sensory pathways have crossed or cross close to the level of the brainstem; same-side limb loss would suggest damage below where the pathways cross, such as in the spinal cord.
- Facial weakness and limb weakness on the same side as each other is not the typical pattern of a brainstem infarction, in which the face is affected on the damaged side and the limbs on the opposite side.
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