Disruption of brainstem monoaminergic fibre tracts in multiple sclerosis as a putative mechanism for cognitive fatigue: a fixel-based analysis

•We compared monoaminergic fibre tracts integrity in multiple sclerosis and controls.•Integrity was studied by diffusion-MRI and tractography, using fixel-based analysis.•Axonal damage within selective monoaminergic tracts was found in multiple sclerosis.•Dopaminergic and noradrenergic axonal damage was associated with cognitive fatigue.•Our results support the hypothesis that monoamines contribute to multiple sclerosis.