Case: A 42-year-old male presents with a nine-month history of burning pain and numbness in his right calf which extends to the lateral aspects of the plantar surface of his foot. He has a full range of movement and normal power within his right leg and foot. There is no muscle atrophy and reflexes are normal. He has no history of trauma. What is the diagnosis?
Findings: The images demonstrate a multilobulated ganglion arising from the proximal tibiofibular joint (images D, F-G, red arrows); which extends along the articular branches of the tibial nerve to the popliteal fossa (images D-E, H, blue arrows). Extrusion of the cyst within the tibial nerve sheath from the level of the knee joint proximally over a length of four centimeters (images B-C, H, green arrows); with displacement of nerve fibers, representing the signet ring sign (image C). Perineural edema extends proximally within the tibial nerve to the sciatic bifurcation (image A).
Diagnosis: Intraneural ganglion of the tibial nerve.
Differential: Cystic peripheral nerve sheath tumor, cystic nerve degeneration from prior trauma, extraneural ganglion, adventitial cyst of popliteal artery, unusual dissecting “Baker’s cyst.”
Discussion: Intraneural ganglion cysts involving the peripheral nerves are an uncommon but recognized cause of neuropathy that can present with pain, paresthesia or muscle weakness. At the knee, the peroneal nerve is the most commonly involved, with tibial nerve involvement being extremely uncommon. The peroneal nerve may undergo primary intrinsic internal cystic degeneration after trauma especially after being impacted by a car bumper. Cystic degeneration may occur within a matter of weeks.
This case demonstrates the previously-described MRI “signet ring sign,” associated with intraneural ganglia, which represents the intraneural cyst positioned eccentrically within the epineurium of the nerve, with displacement of nerve fascicles. The communication with the proximal tibiofibular joint is well appreciated on MRI, representing the “tail sign” (red and pink arrows).
A ganglion is actually a “pseudocyst” and not a true epithelial-lined cyst. It arises from degeneration of tendon sheath or capsule with a slow leak or ooze that is marginated by fibrous tissue.
Treatment of symptomatic intraneural ganglion involves percutaneous decompression or surgical excision to reduce nerve compression and secondary muscle denervation.
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Editor’s Note: This case was submitted by Dr. Tiffany So, one of our scholarship winners for our December 2017 Melbourne course. Congratulations to Dr. So!