Early detection of peroneal neuropathy by ultrasound
A 12 year old boy presented with left ankle swelling and pain. He sustained left ankle sprain while playing at school. On examination, active ankle sprain dorsiflexion toe dorsiflexion and eversion were normal and there was no sensory impairment as well. Only left toe extension impairment was noted. The ultrasound was done to evaluate the soft tissue injury of the ankle. Initially, we focused on the ankle area for a sprain. When comparing the bilateral peroneus longus muscles, we found the loss of normal muscular echogenic appearance over the entire left side. Tracing this back to the origin of peroneus muscle, we found a hypoechoic cyst with a well demarcated margin neighboring the left peroneal nerve just posterior to the fibula head []. Lower limb nerve conduction and electromyography study for peroneal nerve were done. The left common peroneal nerve injury over the fibular head area was found (conduction velocity: 37 ms, normal value: More than 41 ms; peak amplitude: 1.7 mV, normal value: More than 2.0 mV in nerve conduction velocity (NCV) study; only increased polyphasic motor unit action potential was found in electromyography study). In order to more thoroughly evaluate the origin of the mass lesion and adjacent structure, a knee MRI was done. The MRI showed a 33 mm synovial cyst as originating from the proximal tibiofibular articulation with neither rupture nor regional infiltration []. A left PTFJ synovial cyst with extraneural compression over the common peroneal nerve was diagnosed.\nSurgery was performed and the synovial cyst was exposed in the PTFJ area. Compression of the common peroneal nerve by the synovial cyst at fibular head and neck was found during surgical exploration. The compressed nerve was thin by about 60% in diameter when compared to the noncompressed area. Total excision of the synovial cyst was performed and a synovial cyst with a fibrous wall was confirmed on histopathology. After 3 months of the surgery, the physical examination revealed normal muscle power of the peroneal muscle and normal gait pattern was obtained. He returned to sports postoperatively. One year later, the subsequent conduction velocity study of the common peroneal nerve revealed the parameters had returned to normal range (NCV: 46 ms; peak amplitude: 3.7 mV in NCV study; no polyphasic motor unit action potential was found in electromyography study). There was no recurrence of the cyst at 3 years of followup.