LETTER TO THE EDITOR - EDITORIAL Do deep ulnar intraneural ganglion cysts demonstrate a shared but unrecognized propagation pattern? Pierre Laumonerie 1,2 & Nicholas M. Desy 3 & Kimberly K. Amrami 4 & Robert J. Spinner 2,5 Received: 30 August 2016 /Accepted: 14 September 2016 # Springer-Verlag Wien 2016 To the Editor: Over the past decade, advances have been made to elucidate the patterns of formation and propagation of intraneural ganglion cysts based on a unifying articular theory [4]. Unlike the tenden- cy for intraneural ganglion cysts to ascend (i.e., extend proximal- ly) within a parent nerve via an articular branch from a synovial joint, previous descriptions of cysts specifically involving the deep ulnar nerve [5, 6] suggested them to be outliers in that they descended (i.e., extended distally) into the palm. Inspired by two recent publications in Acta Neurochir [3, 5], we wondered if this observation on the directionality was accurate. A medial plantar intraneural ganglion cyst, which was prominent in the midfoot, was recently demonstrated to occur during the last phase of a triphasic mechanism: this intraneural cyst, which arose from the subtalar joint, ex- tended in the medial plantar nerve, crossed over in the tibial nerve and descended down the medial and lateral plantar nerves [3]. We reinterpreted the only case our group has treated of an intraneural cyst in the deep branch of the ulnar nerve at the wrist [5]: this patient who had undergone a previous ulnar nerve operation at Guyon’s canal at another institution was reevaluated by us and found to have a deep ulnar nerve palsy and an intraneural cyst; this cyst was reported to support the articular theory based on its origin from the triquetrohamate joint, and to extend distally intraneurally into the midpalm. The patient was treated by decompressing the deep ulnar intraneural cyst and resecting the articular branch. He made full clin- ical recovery and postoperative MRI showed no evidence of recurrence. Our central hypothesis related to the dy- namic nature of intraneural cysts: did unrecognized prox- imal extension along the deep ulnar branch occur with cross-over within the common epineurial sheath of the ulnar nerve trunk and descent down the deep and super- ficial branches of the ulnar nerve? – i.e., an observation that would further unify propagation patterns. To our surprise, we confirmed this hypothesis. Reanalysis of the history and physical examination find- ings revealed subtle involvement beyond the motor loss in the deep ulnar nerve branch: subjective numbness in the little finger and two-point sensation of 6 mm in the ulnar 1–1/2 digits (versus 4 mm in the radial sided digits). Reinterpretation of the MRIs showed an unrecognized cyst extending along the proximal deep ulnar nerve to the ulnar nerve at Guyon’s canal where there was evi- dence for a wedding ring sign at the level of the ulnar nerve cross-over. A cyst also was seen in the superficial branch of the ulnar nerve (Fig. 1). Reinterpretation of the operative photographs demonstrated an unrecognized cyst in the proximal portion of the deep ulnar nerve, proximal to the articular branch (Fig. 2). We previously demonstrated that deep ulnar intraneural ganglia have joint connections [2, 5, 6], even when the * Robert J. Spinner spinner.robert@mayo.edu 1 Department of Orthopaedic Surgery, Hôpital Pierre Paul Riquet, Toulouse, France 2 Department of Neurologic Surgery, Mayo Clinic, Rochester, MN, USA 3 Department of Orthopedic Surgery, Mayo Clinic, Rochester, MN, USA 4 Department of Radiology, Mayo Clinic, Rochester, MN, USA 5 Mayo Clinic, Gonda 8-214, Rochester, MN 55905, USA Acta Neurochir DOI 10.1007/s00701-016-2972-4