The nearly invisible intraneural cyst: a new and emerging part of the spectrum.

OBJECTIVE The authors have observed that a subset of patients referred for evaluation of peroneal neuropathy with "negative" findings on MRI of the knee have subtle evidence of a peroneal intraneural ganglion cyst on subsequent closer inspection. The objective of this study was to introduce the nearly invisible peroneal intraneural ganglion cyst and provide illustrative cases. The authors further wanted to identify clues to the presence of a nearly invisible cyst. METHODS Illustrative cases demonstrating nearly invisible peroneal intraneural ganglion cysts were retrospectively reviewed and are presented. Case history and physical examination, imaging, and intraoperative findings were reviewed for each case. The outcomes of interest were the size and configuration of peroneal intraneural ganglion cysts over time, relative to various interventions that were performed, and in relation to physical examination and electrodiagnostic findings. RESULTS The authors present a series of cases that highlight the dynamic nature of peroneal intraneural ganglion cysts and introduce the nearly invisible cyst as a new and emerging part of the spectrum. The cases demonstrate changes in size and morphology over time of both the intraneural and extraneural compartments of these cysts. Despite "negative" MR imaging findings, nearly invisible cysts can be identified in a subset of patients. CONCLUSIONS The authors demonstrate here that peroneal intraneural ganglion cysts ride a roller coaster of change in both size and morphology over time, and they describe the nearly invisible cyst as one end of the spectrum. They identified clues to the presence of a nearly invisible cyst, including deep peroneal predominant symptoms, fluctuating symptoms, denervation changes in the tibialis anterior muscle, and abnormalities of the superior tibiofibular joint, and they correlate the subtle imaging findings to the internal fascicular topography of the common peroneal nerve. The description of the nearly invisible cyst may allow for increased recognition of this pathological entity that occurs with a spectrum of findings.