A 46-year-old man comes to the clinic because of 2 weeks of lower back pain radiating down his right leg. The pain began after he lifted a heavy box while moving furniture. He has no fever, weight loss, bowel or bladder dysfunction, or history of cancer. Neurologic examination shows decreased sensation over the lateral leg and dorsum of the foot. Strength is mildly decreased with dorsiflexion of the right great toe. Patellar reflexes are 2+ bilaterally, and Achilles reflexes are 2+ bilaterally.
Which of the following spinal nerve roots is most likely affected?
The correct answer is:
C) L5
This patient has sensory loss over the lateral leg and dorsum of the foot with weakness of great toe dorsiflexion, which localizes to the L5 nerve root. L5 radiculopathy commonly results from posterolateral herniation of the L4-L5 intervertebral disc, which compresses the traversing L5 nerve root. The L5 dermatome includes the lateral leg and dorsum of the foot, and L5 motor involvement can impair dorsiflexion of the great toe through the extensor hallucis longus. Normal patellar and Achilles reflexes are also consistent with L5 radiculopathy because those reflexes primarily assess L4 and S1, respectively.
Answer choice A: L3, is incorrect. L3 radiculopathy typically causes pain or sensory loss over the anterior thigh and may cause weakness of hip flexion or knee extension. It would not be expected to cause sensory loss over the dorsum of the foot or weakness of great toe dorsiflexion.
Answer choice B: L4, is incorrect. L4 radiculopathy often causes sensory loss over the medial leg and may reduce the patellar reflex. Although L4 contributes to ankle dorsiflexion, isolated weakness of great toe dorsiflexion with sensory loss over the dorsum of the foot is more characteristic of L5 involvement.
Answer choice D: S1, is incorrect. S1 radiculopathy typically causes sensory loss over the lateral foot and posterior calf, weakness of plantarflexion, and a decreased Achilles reflex. This patient’s Achilles reflex is normal, and his weakness involves great toe dorsiflexion rather than plantarflexion.
Answer choice E: S2, is incorrect. S2 contributes to sensation of the posterior thigh and upper posterior leg and participates in sacral plexus function. An S2 lesion would not produce the classic pattern of dorsum-of-foot sensory loss and great toe dorsiflexion weakness seen in this patient.
Key Learning Point
L5 radiculopathy causes sensory loss over the lateral leg and dorsum of the foot and may cause weakness of great toe dorsiflexion, while the patellar reflex primarily tests L4 and the Achilles reflex primarily tests S1.
