Question 14.2

Right half of cervical cord.

Discussion

Important features to process here are:

• A completely intact level of consciousness with normal cranial nerves
• Right sided motor weakness over the whole body
• Left sided loss of pain sensation in the upper limb

Now, for some localisation, showing the working:

• A completely intact level of consciousness with normal cranial nerves virtually excludes a lesion above the level of the medulla
• Because the arm is involved, we can establish that the lesion is above the level of the thoracic cord. 
• Nociceptive tract decussation occurs at or slightly above the level of their nerve root entry, and so the fibres from the left arm cross over to the right side shortly after joining the spinal cord.
• Motor tracts decussate at the level of the pyramids, which means to generate a right-sided hemiparesis with a lesion below the medulla, the spinal cord lesion must also be right-sided.

In case it helps, here is a crude diagram of these decussations: