• Miguel A Reina, Xavier Sala-Blanch, Enrique Monzó, Olga C Nin, Paul E Bigeleisen, and André P Boezaart.
• From the Centro de Estudios Universitarios (CEU) San Pablo University School of Medicine, Madrid, Spain (M.A.R.) the Department of Anesthesiology, Madrid-Montepríncipe University Hospital, Madrid, Spain (M.A.R., E.M.) the Human Anatomy and Embryology Unit, Faculty of Medicine, University of Barcelona, Barcelona, Spain (X.S.-B.) the Division of Acute and Perioperative Pain Medicine, Department of Anesthesiology (O.C.N., A.P.B.) the Department of Orthopedic Surgery (A.P.B.), University of Florida College of Medicine, Gainesville, Florida the Department of Anesthesiology, University of Maryland School of Medicine, Baltimore, Maryland (P.E.B.).
• Anesthesiology. 2019 Jun 1; 130 (6): 1007-1016.

BackgroundThere is confusion regarding the spread of intraneurally injected local anesthetic agents during regional anesthesia. The aim of this research was to deliberately inject a marker that does not leave the neural compartment into which it is injected, and then to study the longitudinal and circumferential spread and possible pathways of intraneural spread.MethodsAfter institutional review board approval, we intraneurally injected 20 and 5 ml of heparinized blood solution under ultrasound guidance into 12 sciatic nerves in the popliteal fossa and 10 median nerves, respectively, of eight fresh, unembalmed cadavers using standard 22-gauge "D" needles, mimicking the blocks in clinical conditions. Ultrasound evidence of nerve swelling confirmed intraneural injection. Samples of the nerves were then examined under light and scanning electron microscopy.ResultsExtrafascicular spread was observed in all the adipocyte-containing neural compartments of the 664 cross-section samples we examined, but intrafascicular spread was seen in only 6 cross-sections of two nerves. None of the epineurium, perineurium, or neural components were disrupted in any of the samples. Spread between the layers of the perineurium was a route of spread that included the perineurium surrounding the fascicles and the perineurium that formed incomplete septa in the fascicles. Similar to the endoneurium proper, subepineural compartments that did not contain any fat cells did not reveal any spread of heparinized blood solution cells. No "perineural" spaces were observed within the endoneurium. We also did not observe any true intrafascicular spread.ConclusionsAfter deliberate intraneural injection, longitudinal and circumferential extrafascicular spread occurred in all instances in the neural compartments that contained adipocytes, but not in the relatively solid endoneurium of the fascicles.

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