Neuralgic amyotrophy (NA) is a peripheral nervous system disorder involving multifocal distribution. Although nerve ultrasonography has shown potential for detecting NA lesions, no established detection method exists for distal forearm NA. A 59-year-old man presented with weakness of the muscles innervated by the left posterior interosseous nerve (PIN), median nerve (MN), anterior interosseous nerve (AIN), and ulnar nerve (UN), following severe left shoulder pain. This case suggests that nerve ultrasonography can help accurately diagnose distal forearm NA.

Introduction/Aims: Cross-sectional area (CSA) reference values using ultrasonography vary widely for lower extremity peripheral nerves. In addition, there is a lack of data on the muscular branches of the tibial nerve and the anatomical variations of the sural nerve. We aimed to evaluate the ultrasonographic reference values for lower extremity peripheral nerves considering different anatomical variations and physical factors. Methods:The CSA of the lower extremity nerve was measured at 10 sites. In addition to establishing reference values, differences in the CSA owing to anatomical variations were verified. The relationship between CSA and physical factors, such as age, height, weight, body mass index, and ankle circumference, was also examined.Results: A total of 100 healthy Japanese volunteers were recruited. The mean CSA of the sural nerve significantly differed depending on its formation pattern (1.4-1.8 mm 2 ). The mean decreases in CSAs from the proximal to distal tibial and fibular nerves within the popliteal region significantly differed based on the fine branching pattern. The maximum value of the mean decreases in CSAs in the tibial and fibular nerves reached 7.2 and 2.5 mm 2 , respectively. With respect to physical factors, age and ankle circumferences were associated with CSA at several measurement sites.Discussion: Fine branching from the tibial and fibular nerves and sural nerve formation may affect CSA measurements. The establishment of accurate CSA reference values requires consideration of anatomical variations in the peripheral nerves of the lower extremity.

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