Symptoms of activity-induced weakness in peripheral nervous system disorders

Activity-induced weakness was reported in multifocal motor neuropathy (MMN) and chronic inflammatory demyelinating polyneuropathy (CIDP). This was attributed to activity-dependent conduction block (CB) arising in demyelinated axons. It is not known if activity-induced weakness is common, nor if it is specific for MMN and CIDP. We, therefore, carried out an investigation by questionnaire in 64 MMN patients, 52 CIDP patients, 48 progressive spinal muscular atrophy (PSMA) patients, and 30 normal subjects. Subjects were asked if they experienced an increase in weakness when performing 10 common tasks. The percentage of tasks causing activity-induced weakness was higher in the patient groups than in the normal subjects (p < 0.001). The risk of activity-induced weakness exceeding that in normal subjects was sixfold higher for each patient group when adjusted for sex, age, and a fatigue score. With further adjustment for scores of weakness and axon loss, no significant differences were found between the patient groups. In conclusion, activity-induced weakness is frequently reported in MMN and CIDP. It is, however, not specific for these neuropathies as PSMA patients reported it to the same extent.