Anatomical variations of the phrenic nerve: an actualized review

Abstract

Abstract Introduction: The phrenic nerve normally arises from ventral rami of C3, C4 and C5. It emerges laterally to the superior portion oflateral border of scalenus anterior muscle and presents a descendent course between subclavian artery and vein. It crosses anterior to internal thoracic artery and descends through the mediastinum, until the diaphragm muscle, to supply it with motor and sensitive fibers. Matherials and Methods: A bibliographic review was conducted, based on anatomy, neuroanatomy and surgical anatomy textbooks, published in Brazil and abroad, as well as a review of scientific articles, published over the last 20 years, available on research databases PubMed, Scielo, LILACS and MEDLINE, from keywords phrenic nerve, variation and anomaly. Results: Variations of the phrenic nerve are frequent, but they are not often discussed. Thus, we aimed to conduct an actualized review over the subject. Regarding the variations in the origin of the phrenic nerve, textbooks vaguely inform that it is mainly formed by C4, but the recent cadaveric studies pointed the segments C4 and C5 as the most common origin. About the variations in its course, the most described is its passage anterior to the subclavian vein, before reaching the thorax. However, the presence of accessory phrenic nerve represents the greatestvariation, mostly arising from nerve to subclavian. There are few reports in literature about the complications associated to these variations, but some are suggested, as the possibility of causing its damage during the puncture of the subclavian vein, when the nerve descends anterior to it, which may lead to a hemidiaphragmatic paresis. When variations are present, even simple procedures may cause injuries. Conclusion: Therefore it is fundamental to know the normal anatomy and the possible variations of the phrenic nerve, in order to perform safe procedures in its topography, as well as to enable a timely recognition of complications.