A susceptibility-weighted imaging qualitative score of the motor cortex may be a useful tool for distinguishing clinical phenotypes in amyotrophic lateral sclerosis

Objectives To distinguish amyotrophic lateral sclerosis (ALS) and its subtypes from ALS mimics and healthy controls based on the assessment of iron-related hypointensity of the primary motor cortex in susceptibility-weighted imaging (SWI). Methods We enrolled 64 patients who had undergone magnetic resonance imaging studies with clinical suspicions of ALS. The ALS group included 48 patients; the ALS-mimicking disorder group had 16 patients. The ALS group was divided into three subgroups according to the prevalence of upper motor neuron (UMN) or lower motor neuron (LMN) impairment, with 12 subjects in the UMN-predominant ALS group (UMN-ALS), 16 in the LMN-predominant ALS group (LMN-ALS), and 20 with no prevalent impairment (C-ALS). The Motor Cortex Susceptibility (MCS) score was defined according to the hypointensity of the primary motor cortex in the SWI sequence. Its diagnostic accuracy in differentiating groups was evaluated. Results The MCS was higher in the ALS group than in the healthy control and ALS-mimicking disorder groups ( p < 0.001). Among ALS subgroups, the MCS was significantly higher in the UMN-ALS group than in the healthy control ( p < 0.001), ALS-mimicking disorder ( p = 0.002), and LMN-ALS groups ( p = 0.002) and higher in the C-ALS group than in the healthy control group ( p = 0.019). An MCS value ≥ 2 showed specificity and a positive predictive value of 100% in the detection of both UMN-ALS and C-ALS patients. Conclusions The assessment of MCS in the SWI sequence could be a useful tool in supporting diagnosis in patients suspicious for ALS with prevalent signs of UMN impairment or with no prevalence signs of UMN or LMN impairment. Key Points • The hypointensity of the primary motor cortex in susceptibility-weighted imaging could support the diagnosis of ALS. • Our new qualitative score called MCS shows high specificity and positive predictive value in differentiating ALS patients with upper motor neuron impairment from patients with ALS-mimicking disorders and healthy controls.