USMLE® Step 1 Question of the Day: Compressed nerve
Published on Oct 25, 2023. Updated on Oct 24, 2023.
A 35-year-old man comes to the office for evaluation of right foot pain. The patient describes the pain as “burning” and located on the sole of the foot and the toes. The pain radiates to the calf and is exacerbated by prolonged standing and running. The patient has not suffered any trauma to the right foot. The patient currently runs 10 to 20 miles daily for an upcoming marathon. Past medical history is noncontributory. Vitals are within normal limits. Physical examination shows pain with passive dorsiflexion and eversion of the right ankle. Sensation to light touch and pressure are reduced over the plantar surface of the right foot. Tapping posterior to the right medial malleolus produces a tingling, electric shock sensation. Deep tendon reflexes are within normal limits. Compression of which of the following nerves is the most likely etiology of this patient’s symptoms?
B. Common peroneal
E. Lateral femoral cutaneous nerve
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The correct answer to today's USMLE® Step 1 Question is...
C. TibialBefore we get to the Main Explanation, let's look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
Incorrect: The femoral nerve can be compressed during prolonged maintenance in the dorsal lithotomy position or from pelvic fractures or hip dislocations. Patients present with sensory loss over the anterior thigh and medial leg, weakness of knee flexion and hip extension, and decreased patellar reflexes. A positive Tinel sign posterior to the medial malleolus is suggestive of tarsal tunnel syndrome.
B. Common peronealIncorrect: Compression of the common peroneal nerve occurs at the proximal fibula due to prolonged hyperflexion of the knee or from leg casting. Sensory symptoms occur at the posterolateral leg and dorsolateral foot and are often associated with foot drop.
Incorrect: The Ilioinguinal nerve provides innervation to the upper medial thigh and perineum. Injury to this nerve can occur during pelvic surgery, and presents with chronic suprapubic or groin pain. The ilioinguinal nerve does not innervate the sole of the foot.
E. Lateral femoral cutaneous nerve
Incorrect: Injury or impingement of the lateral femoral cutaneous nerve results in decreased anterior and lateral thigh sensation. Women who wear tight clothes are especially predisposed to impingement of this nerve. A positive Tinel sign, posterior to the medial malleolus, is suggestive of tibial nerve compression.
This patient presents with a sharp, burning pain and reduced sensation over the sole of the feet and a positive Tinel sign (shock-like sensation on tapping the tibial nerve behind the medial malleolus), consistent with tarsal tunnel syndrome, secondary to the compression of tibial nerve.
The tibial nerve is a branch of the sciatic nerve and traverses through the popliteal fossa and posterior compartment of the leg. It then enters the foot through the tarsal tunnel inferior and posterior to the medial malleolus where it innervates the plantar surface of the foot. Injury or compression of the tibial nerve at the tarsal tunnel results in sensory loss over the sole of the foot. Plantarflexion and inversion of the foot remains intact, as fibers innervating these muscles branch proximally to the tarsal tunnel. Common risk factors for tarsal tunnel syndrome include benign tumors/cysts, bone spurs, overuse injury, inflammation of the tendon sheath or swelling from a broken or sprained ankle.
In contrast, injury to the nerve at the popliteal fossa (e.g., deep penetrating trauma or knee surgery) results in loss of plantar flexion, loss of flexion of toes and weakened inversion. Consequently, patients often present with feet held in the calcaneovalgus position (everted at rest with loss of inversion and plantar flexion).
The tibial nerve may be injured at the popliteal fossa or at the tarsal tunnel. Patients typically have weakness of plantar flexion, flexion of toes and weakened inversion (in proximal injury), and sensory loss over the sole (in distal injury), respectively.
Leg. Morton D.A., & Foreman K, & Albertine K.H.(Eds.), (2019). The Big Picture: Gross Anatomy, 2e. McGraw-Hill.
Craig, A. (2013). Entrapment neuropathies of the lower extremity. PM&R, 5(5), S31-S40.
Immerman, I., Price, A. E., Alfonso, I., & Grossman, J. A. (2014). Lower extremity nerve trauma. Bulletin of the NYU Hospital for Joint Diseases, 72(1), 43.
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