Muscle MRI Contributes to the Differential Diagnosis Between Distal Myopathies and Distal Hereditary Motor Neuropathies

ABSTRACT Background Distinguishing between distal myopathies (DMs) and distal hereditary motor neuropathies (dHMNs) can be challenging because clinical, EMG and biopsy findings sometimes overlap. This study aims to identify distinctive muscle MRI features that can guide the diagnosis. Methods We collected clinical, genetic and muscle MRI data from patients with a confirmed diagnosis of DM and dHMN. We analyzed potential MRI characteristics to distinguish these conditions and to guide molecular diagnosis, such as the texture and pattern of infiltration. Results Seventy‐eight (71.5%) patients diagnosed with DMs and thirty‐one (28.4%) with dHMNs were included. A length‐dependent pattern of muscle involvement, a distal to proximal gradient of fat replacement along the length of the muscles and severe and widespread involvement of foot muscles were more common in patients with dHMNs. Muscle hypertrophy and asymmetry were more frequently observed in the DMs. A reticular pattern of fat infiltration was exclusive to patients with dHMNs, while the moth‐eaten pattern predominated in DMs. Muscle islands were more commonly identified in dHMNs (54.8%) but were also observed in 32% of patients with DMs. Conclusions Analysis of MRI features can help distinguish between DMs and dHMNs. A reticular pattern is an early feature of dHMNs while muscle islands are identified in advanced stages and in some forms of DMs, though not specific to neurogenic conditions. We recommend including foot muscles in the MRI protocol as they show extensive involvement in most dHMNs, while in DMs their involvement correlates with greater fatty infiltration of lower leg muscles.