If there is a focal lesion related to the corticospinal or spinothalamic tracts, what type of assessment findings would you expect?

If there is a focal lesion related to the corticospinal or spinothalamic tracts, what type of assessment findings would you expect?

• A. Bilateral sensory loss
• B. Ipsilateral deficits
• C. Unilateral deficits on the contralateral side
• D. No deficits due to redundancy

Answer: (C)

The key idea is how these tracts cross over. The corticospinal tract, which controls voluntary movement, crosses at the level of the medullary pyramids, so a focal lesion that affects it above that crossing will produce motor weakness on the opposite side of the body. The spinothalamic tract, which carries pain and temperature, crosses in the spinal cord shortly after entry, so a lesion on one side will also disrupt contralateral pain and temperature sensations below the level of the lesion. Because a focal lesion interrupting either tract disrupts fibers that have already crossed, the resulting assessment findings are unilateral and on the opposite side to the lesion.

The key idea is how these tracts cross over. The corticospinal tract, which controls voluntary movement, crosses at the level of the medullary pyramids, so a focal lesion that affects it above that crossing will produce motor weakness on the opposite side of the body. The spinothalamic tract, which carries pain and temperature, crosses in the spinal cord shortly after entry, so a lesion on one side will also disrupt contralateral pain and temperature sensations below the level of the lesion. Because a focal lesion interrupting either tract disrupts fibers that have already crossed, the resulting assessment findings are unilateral and on the opposite side to the lesion.