A systematic review of axon count measurement and reporting for nerve transfers in the upper extremity

Axon counts in the literature vary by source and method, and no single source summarizes all brachial plexus nerves. This review aims to compile these counts in one article and assess comparability across studies.

A systematic review was conducted in November 2023, according to the PRISMA guidelines. Four databases were assessed using different variations of the terms "nerve axon count* AND "upper extremity". 539 articles were found and screened, and after full text assessment of the pre-selected articles, 54 articles were finally retained. For these articles, the axon counts, SD, and ranges were collected for all upper extremities' nerves. If more than one article counted the axons of a specific nerve and if the difference between the counts was less than 100%, an average count was calculated.

In the 54 articles a total of 56 nerves were assessed. Axon counts of 18 nerves were done only in a single study. In the comparison between the averages in between studies, in 20 nerves the difference excided 100% between the highest and lowest value. For 16 nerves a metanalysis could been performed and presented in the article.

Our study revealed enormous differences in axon counts across various studies, reaching up to a factor of 87 between the averages from one study to another. Additionally, the quality of the methods of these studies varies, particularly since axon count is frequently reported as a secondary or tertiary outcome. This insight cautions surgeons against using axon counts from studies with differing methodologies to plan nerve transfers.