Idiopathic Peroneal Tenosynovitis Caused by a Previously Undiagnosed Auto-immune Condition.

In this case report, we present a unique case of idiopathic peroneal tenosynovitis in an otherwise healthy patient, presenting with a three-month history of pain over the lateral aspect of the right foot. Imaging revealed that fluid distention and synovial thickening distend the common peroneal tendon sheath and peroneus longus and brevis tendon sheaths. The patient was managed operatively with excision of the peroneus longus tendon, a side-to-side tenodesis, and Bröstrom-Gould lateral ankle ligament repair. Histologic examination was suggestive of a chronic inflammatory process possibly due to underlying autoimmune etiology. At three-month follow-up, the patient reported complete resolution of pain and is resuming normal activities without difficulty.