AbstractIntroduction/aimsInclusion body myositis (IBM) is a myopathic condition but in some patients has been associated with an axonal length‐dependent polyneuropathy. In this study, we quantified the cross‐sectional area of the sciatic and tibial nerves in patients with IBM comparing with Charcot–Marie–Tooth disease type 1A (CMT1A) and healthy controls using magnetic resonance neurography (MRN).MethodsMRN of the sciatic and tibial nerves was performed at 3T using MPRAGE and Dixon acquisitions. Nerve cross‐sectional area (CSA) was measured at the mid‐thigh and upper third calf regions by an observer blinded to the diagnosis. Correlations were performed between these measurements and clinical data.ResultsA total of 20 patients with IBM, 20 CMT1A and 29 healthy controls (age‐ and sex‐matched) were studied. Sciatic nerve CSA was significantly enlarged in patients with IBM and CMT1A compared to controls (sciatic nerve mean CSA 62.3 ± 22.9 mm2(IBM) vs. 35.5 ± 9.9 mm2(controls),p< 0.001; and 96.9 ± 35.5 mm2(CMT1A) vs. 35.5 ± 9.9 mm2(controls);p< 0.001). Tibial nerve CSA was also enlarged in IBM and CMT1 patients compared to controls.DiscussionMRN reveals significant hypertrophy of the sciatic and tibial nerves in patients with IBM and CMT1A compared to controls. Further studies are needed to correlate with neurophysiological measures and assess whether this finding is useful diagnostically.