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Back pain and foot drop
A 45-year-old man has had two weeks of low back pain radiating down the lateral leg to the top of the foot. He now catches his toes when he walks.
Step 1 of 4
Dorsiflexion and great-toe extension are weak. Inversion and hip abduction are also weak. The dorsum of the foot is numb. Patellar and ankle reflexes are normal.
Where is the lesion?
Correct answer: C
Both an L5 root lesion and a common fibular nerve lesion cause foot drop and dorsal foot numbness. Weak inversion and hip abduction point to the L5 root, because inversion (tibial nerve) and hip abduction (superior gluteal nerve) are spared in a fibular neck lesion.
Show answer
C
Both an L5 root lesion and a common fibular nerve lesion cause foot drop and dorsal foot numbness. Weak inversion and hip abduction point to the L5 root, because inversion (tibial nerve) and hip abduction (superior gluteal nerve) are spared in a fibular neck lesion.
Step 2 of 4
Which disc herniation most likely explains this?
Correct answer: B
A typical posterolateral herniation compresses the traversing root, numbered for the lower vertebra: L4–L5 → L5. A far-lateral L4–L5 herniation compresses the exiting L4 root; L5–S1 posterolateral compresses S1 (see intervertebral disc).
Show answer
B
A typical posterolateral herniation compresses the traversing root, numbered for the lower vertebra: L4–L5 → L5. A far-lateral L4–L5 herniation compresses the exiting L4 root; L5–S1 posterolateral compresses S1 (see intervertebral disc).
Step 3 of 4
Which new finding would make this an emergency?
Correct answer: C
Saddle anesthesia with urinary retention suggests cauda equina syndrome, which needs an emergency MRI and decompression. The other features are typical of an ordinary radiculopathy.
Show answer
C
Saddle anesthesia with urinary retention suggests cauda equina syndrome, which needs an emergency MRI and decompression. The other features are typical of an ordinary radiculopathy.
Step 4 of 4
Which bedside test is the most specific for lumbar disc herniation?
Correct answer: B
The straight leg raise is sensitive but not specific; the crossed straight leg raise (raising the asymptomatic leg reproduces pain in the symptomatic one) is highly specific.
Show answer
B
The straight leg raise is sensitive but not specific; the crossed straight leg raise (raising the asymptomatic leg reproduces pain in the symptomatic one) is highly specific.
Key points
- Foot drop + weak inversion and hip abduction → L5 root; normal inversion → common fibular nerve.
- Posterolateral L4–L5 → L5; L5–S1 → S1.
- Saddle anesthesia + retention → emergency MRI.
Study: Intervertebral disc, Common fibular nerve, Spinal nerves and roots, Cauda equina