A 43-year-old woman had herself admitted to the hospital after complaining of headaches. Two days later, she was unable to move
either her hands or feet and was, in effect, a quadriplegic. A neurologic examination revealed no loss of either cranial nerve reflexes and sensation from the body or head. Subsequent magnetic resonance imaging scans and other neurological diagnosis indicated that the damage to the central nervous system was quite limited. The most likely focus of the lesion was in the: __________
d) midline at the border of the medulla and spinal cord
Explanation:
The decussation of the pyramids occurs at the most caudal aspect of the medulla near its border with the spinal cord. Therefore, damage to the midline region would most likely affect corticospinal fibers that arise from both sides of the cerebral cortex and that normally pass to both the cervical and lumbar levels on both sides of the spinal cord. The net results here would result in an upper motor neuron paralysis of all four limbs. The position of the decussation of the pyramids is the only locus where a single lesion (among the choices presented) could affect pyramidal fibers bilaterally. Therefore, the other choices are incorrect because lesions at any of these other sites could not affect pyramidal fibers bilaterally
