Age / Sex : 42 / M

Chief complaint: Right ankle dorsiflexion loss (onset: 3 months ago)

                 Rigth knee pain, extending to ankle and toes

What is your impression?

Two weeks later, you can see the final diagnosis with a brief discussion of this case.

(Please submit only one answer)

Courtesy: Seun Ah Lee, Korea university medical center Ansan hospital

Intraneural ganglion cyst of peroneal nerve

Discussion

Findings:

Multiple, lobulating lesions are located along the course of the common peroneal nerve and its articular branch, with bright high SI. A connection between this lesion and proximal tibiofibular joint are suspected.

Differential Diagnosis:

 Extraneural ganglion cyst

Diagnosis: Intraneural ganglion cyst of peroneal nerve

Discussion:

Intraneural ganglion cyst is rare mucinous cyst within the epineurium of the peripheral nerves.

It is uncommon occurrence of the peripheral nervers, the most common type is the peroneal intraneural ganglion cyst. Other reported sites of involvement are the radial, ulnar, medial, sciatic , tibial and posterior interosseous nerves.

Numerous explanation have been proposed for the pathogenesis of the intraneural cyst. But the unifying articular theory is that mucinous cysts that are thought to arise when joint fluid dissects into an articular branch of a nerve through a capsular tear.

The most common location for intraneural ganglion cyst is the peroneal nerve near the fibular neck. Three MRI signs have been described;

(1)   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.

(2)   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 abscent when there is limited proximal extension of the cyst.

(3)   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.

References:

(1)   The clock face guide to peroneal intraneural ganglia: critical "times" and sites for accurate diagnosis. Skeletal Radiol. 2008 Dec;37(12):1091-9.

(2)   Peripheral nerve interneural ganglion cysts: MR findings in three cases. J Comput Assist Tomogr. 1998 Jul-Aug;22(4):629-32.