NEFL and major depressive disorder: NfL provides a measure of axonal pathology and neurodegeneration across a range of neurological conditions.55 A small number of studies in adults with major depressive disorder but without head injury history/cognitive problems have shown group-level elevations in NfL, although the mechanistic relationship between elevated NfL and neuropsychiatric symptoms is uncertain.56-58 Hence, it may be that increased levels of NfL indicate the presence of brain pathology that leads to neuropsychiatric problems after repetitive sports RHI/TBI.