High-resolution MSK-US findings of a reversed palmaris longus muscle causing a painful mass with neurosensitive symptoms: a case report

The reversed palmaris longus muscle (RPLM) is an uncommon anatomical variant that can mimic a space-occupying lesion of the forearm.We present the case of a 13-year-old boy, a competitive martial arts, with a painful mass in the right forearm.A high-resolution musculoskeletal ultrasound (MSK-US) exhibited an ovoid muscle structure, isoechoic to adjacent muscles, with a typical fibrillar pattern, anomalously located in the distal third of the right forearm.The RPLM showed an inverted morphology, with a distal muscle belly and a proximal tendon.This anatomical variant is positioned adjacent to the median nerve (MN), resulting in local compression.Real-time dynamic MSK-US demonstrated thinning of the muscle belly during active contraction.Color and power Doppler US showed normal intrinsic vascularity and excluded neoplastic or inflammatory processes.A contralateral examination showed a normal palmaris longus tendon in the left forearm.An anterior right forearm RPLM was diagnosed by MSK-US.Management was conservative, focusing on patient education, physical therapy emphasizing stretching and strengthening, and a temporary reduction in martial arts practice intensity.This case report provides a detailed description of an RPLM diagnosed by MSK-US, highlighting its utility as a dynamic, accessible, and high-resolution tool.