Tendinopathy Pain Mimicking Neuropathy on Nerve Conduction Study Resolved with Ultrasound-Guided Steroid Injection Treatment: A Case Report.

We describe the case of a 41-year-old Asian man who was initially given a diagnosis of peroneal neuropathy but who later received a diagnosis of tenosynovitis of extensor digitorum longus (EDL). The patient initially presented with left lateral ankle numbness, pain, and decreased range of dorsiflexion after an 8-km walk. Peroneal neuropathy was first diagnosed on the basis of reduced compound muscle action potential (CMAP). Conversely, ankle ultrasound revealed normal peroneal nerve but considerable EDL tenosynovitis. Ultrasound-guided injection of triamcinolone and lidocaine into the tendon sheath was performed for pain relief. At 1-month follow-up, CMAP amplitude was restored, and ultrasound imaging revealed normal EDL structure without signs of tenosynovitis. This case serves as a reminder that clinicians must pay attention to multiple factors affecting CMAP, including tendinopathy and pain, to avoid misinterpretation.