Early detection of peroneal neuropathy by ultrasound.

A 12 year old boy presented with ankle sprain. The physical examination revealed mild weakness of ankle dorsiflexion. An ultrasound was done for the soft tissues of the ankle. In addition to relative atrophy of the peroneus longus muscle, a compressive common peroneal nerve (CPN) lesion with a synovial cyst at the level of the proximal tibiofibular joint (PTFJ) was accidently found. Since there were features of CPN compression by the synovial cyst, total excision of the cyst was performed. After the operation, muscle strength improved and the neurological deficit subsided. Therefore, the early diagnosis of PTFJ synovial cyst with nerve injury was crucial in order to achieve a better result. The obscure nature of clinical presentations can delay the diagnosis, which may potentially lead to a poor prognosis after treatment in such cases. This report highlights the fact that ankle sprain do need a thorough clinical work up in some cases.