CLINICAL CASE | Spinal Cord Hemisection
A 21-year-old man suffered a gunshot injury. He was walking home from work with friends when he was hit by a stray bullet. He was unconscious when the ambulance arrived. When he regained consciousness in the emergency room, he reported that he was unable to move his right foot and that his right leg felt numb.
Neurological and radiological examination revealed that the bullet entered at about the mid-thoracic level. There was complete loss of right lower limb motor function and loss of tactile sensation (as well as other mechanosensations, including position sense and vibration sense), also on the right, at T10 and below. Pain sensation was examined with pin prick, and testing revealed an absence of pain on the left leg only, extending rostrally to T11. Figure 10–1A shows the distribution of sensory loss.
Answer the following questions based on your reading of this chapter, as well as review of Chapters 4 and 5 on mechanosensations and pain.
1. Why are pain and tactile sensations impaired on opposite sides of the body?
2. Why does the patient have preserved pain but not touch sensation over the T11 dermatome?
3. Explain why the leg on the side with the tactile sensation impairment is paralyzed, but not paralyzed on the side without pain? Key neurological signs and corresponding damaged structures Alternate distributions for pain and tactile sensory loss
The lesion damages two distinct populations of ascending somatic sensory fibers. Damaged ascending pain fibers ascend to the lesion site after crossing the midline caudal to the lesion. Damaged ascending tactile fibers ascend to the lesion site without decussating; they cross the midline rostral to the lesion, in the medulla. By following the pain and tactile circuits back to their origins in the periphery, the different sides of sensory loss are revealed. Preserved pain sense one to two segments caudal to tactile loss
Pain is perceived down to a more caudal level than tactile stimuli. Pain fibers cross the midline over multiple spinal levels (ie, they cross as they ascend). The first pain fibers to become damaged have entered the spinal cord one to two segments below the lesion (Figure 10–1A; see also Figure 5–1). Pain fibers entering at the level of the injury will bypass the lesion. It should be noted that the spinal injury can damage entering primary afferents themselves, and the nearby dorsal horn, and would be expected to produce a small amount of sensory loss just at the dermatomal level on the side of the lesion. However, because of dermatomal overlap (see Figure 4–5), this will be minimized and may even go unnoticed. Ipsilateral motor and tactile loss
The corticospinal tract decussates in the medulla (see Figure 10–4A). For this reason, the lesion interrupts the descending pathway controlling muscles on the same side (Figure 10–1A).