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Increased amplitude of persistent inward currents (PICs) is observed in pre‐symptomatic genetically modified SOD1 mice models of amyotrophic lateral sclerosis (ALS). However, at the symptomatic stage this reverses and there is a large reduction in PIC amplitude. It remains unclear whether these changes in PICs can be observed in humans, with cross‐sectional studies in humans reporting contradictory findings. In people with ALS, we estimated the PIC contribution to self‐sustained firing of motoneurons, using the paired‐motor unit analysis to calculate the Δfrequency (ΔF), to compare the weaker and stronger muscles during the course of disease. We hypothesised that, with disease progression, ΔFs would relatively increase in the stronger muscles; and decline in the weaker muscles. Forty‐three individuals with ALS were assessed in two occasions on average 17 weeks apart. Tibialis anterior high‐density electromyograms were recorded during dorsiflexion (40% of maximal capacity) ramped contractions, followed by clinical tests. ∆F increased from 3.14 (2.57, 3.71) peaks per second (pps) to 3.55 (2.94, 4.17) pps on the stronger muscles (0.41 (0.041, 0.781) pps, standardised difference (d) = 0.287 (0.023, 0.552), P = 0.030). ∆F reduced from 3.38 (95% CI 2.92, 3.84) pps to 2.88 (2.40, 3.36) pps on the weaker muscles (−0.50 (−0.80, −0.21) pps, d = 0.353 (0.138, 0.567), P = 0.001). The ALSFRS‐R score reduced 3.9 (2.3, 5.5) points. These data indicate that the contribution of PICs to motoneuron self‐sustained firing increases over time in early stages of the disease when there is little weakness before decreasing as the disease progresses and muscle weakness exacerbates, in alignment with the findings from studies using SOD1 mice. Key points: Research on mouse model of amyotrophic lateral sclerosis (ALS) suggests that the amplitude of persistent inward currents (PICs) is increased in early stages before decreasing as the disease progresses.Cross‐sectional studies in humans have reported contradictory findings with both higher and lower PIC contributions to motoneuron self‐sustained firing.In this longitudinal (∼17 weeks) study we tracked changes in PIC contribution to motoneuron self‐sustained firing, using the ΔF calculation (i.e. onset‐offset hysteresis of motor unit pairs), in tibialis anterior muscles with normal strength and with clinical signs of weakness in people with ALS.ΔFs decreased over time in muscles with clinical signs of weakness. The PIC contribution to motoneuron self‐sustained firing increases before the onset of muscle weakness, and subsequently decreases when muscle weakness progresses. [ABSTRACT FROM AUTHOR]

Amyotrophic lateral sclerosis (ALS) is a fatal disorder characterized by degeneration of the upper and lower motor neuron. Among the at least 25 known genes associated with familial (hereditary) and sporadic ALS, mutations in fused‐in‐sarcoma (FUS) and superoxide dismutase 1 (SOD1) have been extensively investigated in the past years, with emphasis on altered excitability of affected neurons. Recently, we reported on hypoexcitability and increased cell death in a FUS/SOD1‐ALS–induced pluripotent stem cell–derived motor neuron model, which was partly reversible by a treatment with the potassium channel blocker 4‐aminopyridine (4‐AP). Based on this study, we aimed to examine this US Food and Drug Administration–approved drug as a potential individualized treatment for patients with ALS. We therefore retrospectively investigated 4 FUS/SOD1‐ALS patients who were prescribed 4‐AP. Two patients expressed an improved quality of life due to regain of facial muscle motor function and decreased disease progression rate, respectively. Together with recent pathophysiologic findings, this case series supports the need for clinical trials to examine the efficacy of this potential treatment in distinct ALS subgroups and disease stages. [ABSTRACT FROM AUTHOR]