TECHNICAL NOTE - PERIPHERAL NERVES Intraoperative intravenous fluorescein as an adjunct during surgery for peroneal intraneural ganglion cysts Jonathan J. Stone 1 & Christopher S. Graffeo 1 & Godard C.W. de Ruiter 2 & Michael G. Rock 3 & Robert J. Spinner 1 Received: 8 January 2018 /Accepted: 16 January 2018 # Springer-Verlag GmbH Austria, part of Springer Nature 2018 Abstract The intraoperative use of intravenous fluorescein is presented in a case of peroneal intraneural ganglion cyst. When illuminated with the operative microscope and yellow filter, this fluorophore provided excellent visualization of the abnormal cystic peroneal nerve and its articular branch connection. The articular (synovial) theory for the pathogenesis of intraneural cysts is further supported by this pattern of fluorescence. Further, our report presents a novel use of fluorescein in peripheral nerve surgery. Keywords Peripheral nerve . Peroneal nerve . Intraneural ganglion cyst . Articular branch . Fluorescein Introduction Peroneal intraneural ganglion cyst is increasingly recognized as possible etiology for foot drop [21], which if managed with inappropriate surgical technique can cause further nerve inju- ry, or predispose to cyst recurrence [4, 11, 15]. The mucinous dilation of the peroneal nerve is hypothesized to occur second- ary to communication of the superior tibiofibular joint through an articular branch, which allows egress of joint (synovial) fluid and intraneural propagation into the parent nerve [20]. Clinical, radiologic, histologic, and cadaveric studies provide extensive evidence for the articular (synovial) theory for the development of intraneural ganglion cysts [18, 19]. Additional intraoperative investigation using modern neurosurgical tools such as fluorescent dye may demonstrate further insight into the pathogenesis of intraneural cysts, as well as provide a tool for improving surgical outcomes. The use of fluorescein in neurosurgery was first described in 1948 to aid in the biopsy of brain tumors [12]. Parallel long- term usages in Ophthalmology and Urology support its safety and ease of administration [24, 26]. The side effect profile is favorable: minor reactions such as nausea or urticaria are un- common (1.3%) and major adverse events such as respiratory distress (0.03%) or cardiac disturbance (0.02%) are rare [22]. The development of operating microscopes with a dedicated yellow 560-nm filter further prompted the use of fluorescein in Neurosurgery, and has been shown to improve gross total resection rates for neoplasms, and highlight key anatomy for vascular pathology [3, 9, 14]. Our report describes a novel application of fluorescein to peripheral nerve surgery and con- stitutes the first instance of its use to demonstrate the anatomic relationship between a peroneal cyst and its articular branch, further contributing to the growing literature emphasizing the importance of the articular branch in the pathogenesis and treatment of this disease. Case illustration An otherwise healthy 49-year-old man presented with pain migrating from his right knee into the dorsum of his foot with associated with sensory loss. Over 6 weeks, this progressed to a foot drop that worsened with walking and improved with rest or cessation of weight bearing. At presentation, 3 months after the onset of his symptoms, he had MRC grade 1 foot dorsiflexion and toe extension. He had normal eversion, in- version, and plantar flexion. He had diminished sensation, worse in the first dorsal web space but also present over the dorsum of the foot. There was a positive Tinel’ s sign at the fibular neck. MRI demonstrated a long, multilobulated tubular * Robert J. Spinner Spinner.Robert@Mayo.edu 1 Department of Neurologic Surgery, Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA 2 Department of Neurosurgery, Hague Medical Center, The Hague, The Netherlands 3 Department of Orthopedic Surgery, Mayo Clinic, Rochester, MN, USA Acta Neurochirurgica https://doi.org/10.1007/s00701-018-3477-0