A 21-year-old college lacrosse player is brought to the team physician for sudden onset lower back pain for the past 4 hours. He reports that he was doing deadlifts during training and felt a pop in the lower back. Vital signs are within normal limits. Physical examination reveals a postitive straight leg raise test on the right. Magnetic resonance imaging (MRI) of the lumbar spine reveals a paracentral herniation in the L3/4 intervertebral disk compressing the right spinal nerve root.
Based on these findings, additional testing will likely reveal which of the following findings?
The correct answer is:
D) Diminished sensation over the right medial ankle and 1st toe
This answer choice correctly describes an L4 dermatomal distribution. In the lower lumbar regions, the spinal nerve trajectories are angled such that paracentral intervertebral disk herniations affect the nerve root below. Since this patient is diagnosed with an L3/4 paracentral disk herniation, the most likely nerve root affected is the L4 nerve root. If the patient were diagnosed with a far lateral disk herniation, L3 would be the most likely nerve root affected.
Answer Choice A: Diminished sensation over the left medial ankle and 1st toe, is incorrect. In the lower lumbar regions, the spinal nerve trajectories are angled such that paracentral intervertebral disk herniations affect the nerve root below. While this describes an L4 dermatomal distribution, the patient is diagnosed with a compression of the right spinal nerve root. The nerve root is distal to where sensory fibers cross in the spinal cord, thus an ipsilateral spinal nerve injury will affect the ipsilateral limb.
Answer Choice B: Diminished sensation over the left medial thigh, is incorrect. This answer choice describes the L3 dermatomal distribution. A paracentral disk herniation will affect the nerve root below, while a far lateral disk herniation will affect the nerve root at the level of the injury, which would be L3 in this case. Additionally, the injury occurred on the right side and will not have diminished sensation of the contralateral leg.
Answer Choice C: Diminished sensation over the right lateral thigh, is incorrect. This answer choice describes the L5 dermatomal distribution. This dermatome is not likely affected by this injury.
Answer Choice E: Diminished sensation over the right medial thigh, is incorrect. While a sensory deficit will occur in the right lower extremity, this answer choice describes the L3 dermatomal distribution. Therefore, this is unlikely to be affected by a paracentral disk herniation since in the lower lumbar regions, the spinal nerve trajectories are angled such that paracentral intervertebral disk herniations affect the nerve root below.
Key Learning Point
In the lower lumbar regions, the spinal nerve trajectories are angled such that paracentral intervertebral disk herniations affect the nerve root below. Far lateral disk herniations will affect the nerve root at the level of the injury.
