Son, Byung-chul

BACKGROUND Recently, the distribution of subparaneurial ganglion cysts involving the common peroneal and tibial nerves was described based on MRI analysis. Crossover in extreme cases was observed at the sciatic bifurcation through a mechanism comparable to a geyser, enabled by fenestrations in the epi- and paraneurium at the level of the bifurcation. The goal of this study was to investigate whether the geyser theory for crossover holds for other locations. OBSERVATIONS The authors’ prospectively maintained database of > 1100 cases of intraneural ganglion cysts in the world’s literature was queried for the existence of subparaneurial ganglion cysts at locations other than the knee. A previously unrecognized case involving the ulnar nerve originating from the elbow joint was found. Reinterpretation of the patient’s MRI studies showed circumferential subparaneurial distribution of a cyst over a long distance. Based on their previously reported geyser theory, the authors wondered if crossover had occurred at the level of the medial cord. A fenestration in the epi- and paraneurium at this level was confirmed on histological analysis of a normal cadaveric specimen. LESSONS This illustrative case shows that the geyser theory also holds in other locations. Surgical treatment of these subparaneurial cysts still follows the principles of the unified articular theory. https://thejns.org/doi/10.3171/CASE25671

OBJECTIVE The formation and propagation of intraneural ganglion cysts (INGCs) is being elucidated by the unified articular theory. Its principles include a connection for joint fluid to egress from a synovial joint via an articular branch to a parent nerve and cyst following the path of least resistance, dependent on pressures and pressure fluxes. The occurrence of so-called extreme peroneal or tibial INGCs in the popliteal fossa extending to the sciatic nerve has been reported. One rarely described variant with a circumferential cyst within the subparaneurial compartment outside the epineurium of nerve(s) has been previously illustrated, but its mechanism and morphology have not been clarified. In this study, the authors aimed to investigate this type of cyst to challenge the principles of the unified articular theory. METHODS Four novel cases of patients with peroneal INGCs and "extreme subparaneurial cyst(s)" of the sciatic nerve and its distal branches were investigated: 3 arose from the superior tibiofibular joint (STFJ) and 1 from the knee joint. Three other cases of recognized extreme subparaneurial cyst (2 peroneal and 1 tibial from the STFJ) and 1 case of a peroneal subparaneurial cyst in the literature were reinterpreted. Data on clinical presentation, MR images, and surgical results were analyzed. RESULTS In all 8 cases, subparaneurial extension was observed to different degrees along the sciatic, tibial, common peroneal, sural, and deep and superficial peroneal nerves, as was subparaneurial-to-subparaneurial communication at the sciatic nerve bifurcation (i.e., crossover). Sequential MRI performed in 7 patients showed variable dynamic changes, including extreme ascent and descent. Extraneural rupture of the subparaneurial cyst, with spread into the surrounding tissue, was present at the sciatic nerve bifurcation in 6 cases. CONCLUSIONS The authors provide pathoanatomical and pathophysiological evidence supporting that extreme subparaneurial cysts follow the principles of the articular theory. They propose a distribution pattern that explains the occurrence and evolution of extreme subparaneurial cysts along the sciatic nerve and its distal branches in patients with peroneal or tibial INGCs and subepineurial cysts. Crossover in the subparaneurial compartment allows potentially extensive circumferential distribution within connected nerves. Also, dynamic factors can lead to dramatic changes in cyst size and appearance from reabsorption or extraneural rupture. In Part 2 of this study, the authors provide evidence showing that a fenestration in the epineurium allows cysts to pass from the subepineurial-to-subparaneurial, subparaneurial-to-subparaneurial, and subparaneurial-to-subepimyseal compartments.

OBJECTIVE Extreme subparaneurial ganglion cysts are poorly understood. In Part 1, the authors demonstrated that patterns of distribution in the subparaneurial space are consistent with the principles of the articular theory for intraneural ganglion cysts (INGCs). How a cyst transfers from the subepineurial to the subparaneurial compartment remains to be determined. In this part, the authors provide evidence supporting that cyst transfer occurs at the sciatic nerve bifurcation based on MRI and histological analysis, and introduce the geyser theory as an explanation for the redistribution of cysts. METHODS The authors analyze MR images obtained in patients with extreme subparaneurial cysts as well as newly identified cases of nearly extreme subparaneurial cysts (i.e., cysts visible in the subparaneurial compartment but not to the extreme degree as defined in Part 1) for potential crossover sites. They also analyze histological sections from 10 cadavers around the sciatic nerve bifurcation for the presence of openings in the epineurium and paraneurium. RESULTS MRI studies performed in 8 patients in Part 1 and 8 newly identified examples with nearly extreme subparaneurial INGCs showed evidence to support communication between different compartments at the sciatic nerve bifurcation (subepineurial-to-subparaneurial and subparaneurial-to-neighboring soft tissues). Openings in the epineurium of the common peroneal and tibial nerves at the sciatic nerve bifurcation were consistently found in all 10 cadaver specimens, as were openings in the paraneurium. CONCLUSIONS The authors expand on the evidence to support cyst redistribution at the sciatic nerve bifurcation for INGCs in the knee region. They put forth a logical explanation for the development of a spectrum of extreme INGCs (ranging from a faint amount of cyst around the epineurium [wedding ring sign] to extreme subparaneurial INGCs [owl eyes]) based on principles seen in a geyser. The authors believe that radiological occurrence of crossover can be explained by the presence of fenestrations in both the epineurium and paraneurium at the sciatic nerve bifurcation, allowing cyst crossover in two steps: from the subepineurial to the subparaneurial space ("cross") and between a shared subparaneurial space of different nerves ("over"). Finally, the frequent occurrence of extraneural rupture ("cross-out") is due to cyst transfer into the soft tissue compartment via an opening in the outermost circumneurial layer.