FP.09 Analysis of muscle MRI of a large cohort of chronic motor neuropathy/neuronopathy patients reveals characteristic features useful for diagnosis

Abstract

Chronic motor neuropathy/neuronopathy (CMNs) encompass a group of rare diseases characterized by muscle weakness mainly affecting distal muscles of the limbs. Muscle magnetic resonance imaging (MRI) can be a useful tool for the diagnostic workup but has not been explored systematically in these patients. This is a retrospective study collecting clinical, genetic and imaging data. Muscle MRI included T1-weighted and Short Tau Inversion Recovery (STIR) sequences. Fat replacement was quantified using the Mercuri score. Identification of patterns was performed using hierarchical clustering. We included 76 patients with a clinical and electrophysiological diagnosis of CMN. A pathogenic gene variant was identified in 29 subjects, a VUS was identified in 15 and 32 subjects remained without molecular diagnosis. We collected 25 whole-body and 51 lower limb (LL) MRIs. All patients but one had pathological findings. We identified features common to all patients regardless of the genetic diagnosis. Muscle fat replacement was predominant in the LL distal muscles (75/76 cases), but also affected the thigh in 59, the pelvis in 18 and the trunk/upper limb muscles in seven. The posterior compartment of the leg, including soleus and gastrocnemius, was the most affected (71/76) followed by the peronei (63/76). Asymmetric involvement was found in 19 patients. A distal to proximal gradient of fat replacement was observed in 39 patients (52%). Texture analysis showed a reticular pattern or "muscle islets" in 63 patients (83%). Hyperintensities on STIR were observed in 49/59 and were predominantly located in the distal LL muscles. A distal to proximal gradient in STIR was observed in 25 subjects. Besides features common to all individuals, we identified a pattern of muscle fat replacement characteristic of BICD2 and HSPB1 patients. We did not identify a correlation between fat content and age, disease duration or motor functionality. Muscle MRI of CMN patients reveals common features that could be helpful for the diagnosis of patients. We have observed a pattern of MRI involvement characteristic of patients with mutations in specific genes. There was no correlation between fat content and functional status or time of disease progression.