The patient in this case has Achilles tendon rupture associated with fluoroquinolone administration. The positive Thompson test (failure of the ankle to plantar flex when the calf muscles are squeezed) indicates that there is discontinuity between the gastrocnemius and soleus muscles and the heel. Figure 1 illustrates an Achilles tendon rupture.
The differential diagnosis includes plantaris tendon rupture, deep vein thrombosis, rupture of a popliteal cyst, Achilles tendinopathy, and retrocalcaneal bursitis. Plantaris tendon rupture, deep vein thrombosis, and popliteal cyst rupture all cause tenderness of the calf but not of the tendon itself. None of these conditions causes significant weakness. Achilles tendinopathy is associated with swelling and tenderness of the Achilles tendon, but no gap is present in the tendon. In patients with retrocalcaneal bursitis, the tenderness is at the back of the heel, not in the midportion of the Achilles tendon.
Although a diagnosis of plantaris tendon rupture is often made when a patient presents with calf pain after an athletic activity, in most cases the pathology is a tear of the muscle fibers of the medial head of the gastrocnemius muscle. The typical presentation is a tennis player who had a sudden sharp, disabling pain in the calf, hence the term "tennis leg." Examination may show tenderness, swelling, bruising, and weakness of ankle plantar flexion, but there is no tenderness or deformity of the Achilles tendon itself.
Deep vein thrombosis in the lower leg is characterized by pain, tenderness, and swelling of the calf but not of the Achilles tendon. No significant weakness of ankle plantar flexion is noted.
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