Reduced exercise capacity in early-stage amyotrophic lateral sclerosis: Role of skeletal muscle

Our objective was to correlate skeletal muscle mass (SM) with cardiopulmonary exercise testing (CPET) descriptors of exercise capacity in patients with amyotrophic lateral sclerosis (ALS) and compare ALS CPET data with those of patients with mitochondrial myopathy (MM) and normal subjects (N). Twenty-four early-stage ALS patients (63±11 years) underwent bioelectrical impedance analysis of body composition, resting spirometry, and ramp CPET. Six MM and six N were used as controls (56 ± 7 and 63 ± 4 years, respectively). Results showed that ALS SM index was similar to that of N (9.0±2.1 kg/m2 vs. 10.4±1.9 kg/m2, respectively; p = n.s.), whereas peak VO2/kg SM was significantly lower (41.5 ± 11.6 ml/kg/min vs. 57.8 ± 7.5 ml/kg/min, respectively; p < 0.01). However, the heart rate/VO2 slope did not differ between ALS and N, being significantly higher in MM than in both ALS and N (6.1 ± 1.4 beats/ml/kg/min vs. 4.2 ± 1.1 beats/ml/kg/min vs. 3.8 ± 2.0 beats/ml/kg/min, respectively; both p < 0.01), excluding a marked skeletal muscle metabolic impairment in ALS. Neither cardiovascular nor ventilatory dysfunction was detected in ALS. Early-stage ALS patients show a SM similar to N, but with a reduced peak VO2/kg SM. Such a reduced peripheral O2 utilization is consistent with deconditioning as the main cause of impaired exercise capacity in this population.