Peroneal Intraneural Ganglion Cyst

Intraneural ganglia are mucinous cysts that are thought to arise when joint fluid dissects into an articular branch of a nerve through a capsular tear. As a result, intraneural ganglia are typically tubular in shape. This is in contrast to the more common extraneural ganglion cysts, which tend to be globular. Both intra- and extra-neural ganglion cysts can be present in the same patient.

Peroneal intraneural ganglion cysts are almost always symptomatic, with patients presenting with pain and neuropathy. The much more common extraneural ganglion cysts are usually asymptomatic, but when symptomatic, patients usually present with pain rather than neuropathy

The most common location for intraneural ganglia is the peroneal nerve near the fibular neck.

The MRI findings of a peroneal intraneural ganglion are characteristic. But to appreciate the findings, an understanding of the course of the common peroneal nerve and its articular branch is necessary.

The sciatic nerve (pink, SN) descends along the posterolateral knee and terminates above the knee into the larger tibial nerve (yellow, TN) and the common peroneal nerve (green with dots, PN). The tibial nerves descends centrally along the popliteal fossa. The common peroneal nerve curves anterolaterally along the fibular head. An articular branch (green) crosses transversely ("transverse limb") across the anterior aspect of the fibular neck and then ascends ("tail") to supply the proximal tibiotalar joint. The other branches of the common peroneal nerve are not shown.

In the case of a peroneal intraneural ganglion, fluid is thought to dissect from the proximal tibiofibular joint (pink oval) via the articular branch into the common peroneal nerve. Occasionally, enough pressure exists for the fluid to dissect further into the sciatic nerve.

Three MRI signs have been described:

• The transverse limb sign: This refers to the appearance of the articular nerve branch as it transversely crosses over the anterior surface of the fibular neck. This is said to be pathognomonic.
• The signet ring sign: This refers to the eccentric displacement of fascicles by the cyst within the outer epineurium of the common peroneal nerve. Commonly seen, this sign may be absent when there is limited proximal extension of the cyst.
• The tail sign: This refers to a cystic connection between the articular branch of the common peroneal nerve and the joint. Due to low spatial resolution, differentiation of a neural from a non-neural connection to the joint is often not possible.

The diagram on the top right is an oblique view of the proximal calf, showing the nerves of interest. The T1-weighted images in the left column trace the sciatic nerve (SN) from the top down, as it divides into the tibial nerve (TN, yellow arrow) and common peroneal nerve (PN, green arrow). The bottom images show the transverse limb of the articular branch on T1- and T2-weighted sequences. An extraneural cyst can also be seen posterior to the tibia (red X).

The two T2-weighted images also show evidence of denervation (orange) in the muscles of the anterior compartment. The more cephalad T2-weighted image also shows the tail of the articular branch (green arrow), as well as nerve fascicles arranged eccentrically within the epineurium (signet ring, white arrow). The sagittal T2-weighted image shows both the transverse limb and the tail (green arrows).

Intra- and extra-neural ganglion cysts are treated differently, underscoring the need for accurate diagnosis. Intraneural ganglion cysts are usually treated by decompression (not resection) of the cyst followed by disconnection of the articular branch (to reduce the chance of recurrence). Extraneural ganglion cysts are treated by resection. Treatment for both types of cysts also involves resection of the proximal tibiofibular joint.

References

Spinner RJ, Luthra G, Desy NM, Anderson ML, Amrami KK. The clock face guide to peroneal intraneural ganglia: critical "times" and sites for accurate diagnosis. Skeletal Radiol. 2008 Dec;37(12):1091-9.