Intraneural ganglion cyst of the peroneal nerve

A 59-year-old male with no history of prior injuries presented with progressive pain in the anterolateral aspect of the left leg, sensory loss to the dorsum of the foot and weakness during ankle dorsiflexion and foot eversion. Physical examination revealed a mass on the proximal lateral aspect of the leg.

Magnetic resonance imaging (MR) of the left knee was performed, with axial (Figures 1a–1h) and sagittal (Figures 2a and 2b) proton density-weighted images, with fat suppression, showing a large hyperintense cystic lesion (blue arrows), with tubular, beaded configuration, located adjacent to the common peroneal nerve (yellow arrows), with no fat plane between the cyst and the nerve, reflecting intraneural location, oriented longitudinally along the course of the nerve, compatible with an intraneural ganglion cyst. This intraneural ganglion cyst originates from the proximal tibiofibular joint, with a tail that lies anteromedial to the proximal fibula, crossing the fibula from medial to lateral (transverse limb sign). There is a slight hyperintensity of the peroneal nerve, suggesting neuropathy. No significant denervation hyperintensity of the muscles of the anterior and peroneal compartments of the leg is seen.

Peroneal neuropathy is the most common compressive neuropathy of the lower extremity and the third most common focal neuropathy overall, following median and ulnar neuropathies [1]. Multiple causes have been identified, with external compression being the most common, although traumatic injuries, prolonged bed rest, metabolic disease and tight casting may also produce peroneal nerve neuropathy [2,3]. Intraneural ganglion cysts are a rare cause of peroneal neuropathy. They consist of non-neoplastic mucinous cysts lined by flat spindle-shaped cells found within the epineurium of a nerve [4].

Although the pathogenesis of intraneural ganglion cysts remains unclear, they are thought to originate from small tears in the joint capsule of adjoining joints, allowing fluid to propagate along the epineurium of the parent nerve, dissecting the nerve sheath and forming a cyst within the nerve [3,4]. The proximal tibiofibular joint is most commonly involved in cases of common peroneal nerve intraneural ganglion cyst [3].

The resulting displacement and compression of the nerve fascicles may result in symptoms of common peroneal nerve neuropathy, which include weakness or inability to perform ankle dorsiflexion, foot eversion and hallux extension, which may develop acutely or gradually [5,6]. There may also be sensory impairment to the anterolateral aspect of the leg and dorsum of the foot, with paraesthesia, numbness, and pain [5,6].

The imaging diagnosis of intraneural ganglion cysts of the peroneal nerve can be made using ultrasonography or MR imaging. MR imaging can be used to evaluate the exact anatomical location of T2 fat-suppressed hyperintense cystic lesions along the peroneal nerve or its branches. Intra or extra-neural location may be determined through the absence or presence of a fat plane between the cyst and the nerve. MR imaging or ultrasonography may also be used to assess any communication to the adjacent tibiofibular joint through a narrow pedicle connecting the intraneural cyst to the joint (tail sign). The tail may lie anteromedial to the proximal fibula, crossing the fibula from medial to lateral (transverse limb sign). MR imaging can also assess any denervation changes to the anterior and lateral compartments of the leg [4,7].

The cyst was surgically removed. If diagnosed and treated early, neuropathy is reversible. Adequate imaging characterisation is important to guide effective surgical treatment, as surgical intervention should target and remove the articular connection and not only the cyst itself. Isolated cyst decompression without disconnecting from the joint has high rates of recurrence [3,7,8].

Written informed patient consent for publication has been obtained.