ORIGINAL ARTICLE - NEUROSURGICAL ANATOMY Recognition of peroneal intraneural ganglia in an historical cohort with Bnegative^ MRIs Thomas J. Wilson 1 & Marie-Nöelle Hébert-Blouin 2 & Naveen S. Murthy 3 & Kimberly K. Amrami 3 & Robert J. Spinner 1 Received: 5 January 2017 /Accepted: 20 February 2017 /Published online: 3 March 2017 # Springer-Verlag Wien 2017 Abstract Background The objective of this study was to review an historical cohort of patients with peroneal neuropathy and magnetic resonance imaging (MRI) read as negative for mass or cyst to determine if occult peroneal intraneural ganglion cysts can be identified on subsequent imaging review and to use this as an estimation of how under-recognized this patho- logic entity is. Method The patient cohort utilized in this study was a previ- ously published control cohort of 11 patients with peroneal neuropathy and MRI read as negative for mass or cyst. Clinical history, neurologic examination, and MRI studies of the knee were reviewed for each of the included patients. The primary outcome of interest was the presence of peroneal intraneural ganglion cyst on MRI. Results Overall, 7 of 11 (64%) patients in this historical Bnormal^ cohort had evidence of a peroneal intraneural gan- glion cyst on subsequent review of imaging. Deep peroneal- predominant weakness, knee pain, and tibialis anterior- predominant denervation/atrophy were seen more commonly in patients in whom an intraneural cyst was identified. Conclusions This retrospective cohort study provides evi- dence that peroneal intraneural ganglion cysts are an historically under-recognized cause of peroneal neuropathy, with 64% of this historical Bnegative^ cohort having evidence of a cyst on subsequent imaging review. Larger studies are needed to determine the treatment ramifications of identifying small cysts and to determine the clinical features suggestive of an intraneural ganglion cyst. Keywords Peroneal neuropathy . Intraneural ganglion cyst . Superior tibiofibular joint . Articular branch . Common peroneal nerve Introduction Peroneal neuropathy is the most common entrapment neurop- athy in the lower extremity [4]. This is typically idiopathic in origin but can be due to mass lesions, including intraneural ganglion cysts. Intraneural ganglion cysts are increasingly be- ing recognized as an important cause of peroneal neuropathy. In one study, 18% of cases were attributed to intraneural gan- glia [10]. We have recently described the nearly invisible intraneural ganglion cyst [11]. We have had multiple patients referred for evaluation with Bnegative^ imaging, who on sub- sequent review have had subtle evidence of an intraneural cyst. Despite increasing recognition, we believe that intraneural ganglion cysts are under-recognized. We have previously explored the clinical and electrodiagnostic correlates of peroneal intraneural ganglion cysts [12]. In that study, we utilized a cohort of patients with ultrasound or magnetic resonance imaging (MRI) evidence of an intraneural cyst and compared them to a cohort of patients with negative imaging. Patients were included in the negative cohort if they had an ultrasound or MRI that was initially read as negative, but the studies were never over-read by the study authors. In the context of what we have learned since the publication of that manuscript, Thomas J. Wilson and Marie-Nöelle Hébert-Blouin made an equal contribution to this manuscript * Robert J. Spinner Spinner.Robert@mayo.edu 1 Department of Neurosurgery, Mayo Clinic, 200 1st St. SW, Rochester, MN 55905, USA 2 Department of Neurosurgery, McGill University Health Centre, Montreal, Quebec, Canada 3 Department of Radiology, Mayo Clinic, Rochester, MN, USA Acta Neurochir (2017) 159:925–930 DOI 10.1007/s00701-017-3130-3