Multiple sclerosis (MS) remains an enigmatic disease. Not only is the cause unknown, but the last decade of work has led to uncertainty concerning some of the previously, most strongly held convictions about the disease. Recently, attention has shifted from understanding demyelination to understanding how axons are injured.

It has been attractive to hypothesise that axonal damage occurs with inflammation in white matter lesions. Histopathological and imaging provide clear evidence for axonal transaction in lesions.^1,2 However, rates of progression are independent of relapses³ and even treatments that prevent new inflammatory lesions⁴ do not slow progression of disability—a consequence of the progressive axonal degeneration.

This conundrum contributes to interest in study of primary progressive MS (PPMS), which is characterised by progression of disease from onset. Progression is related to axonal loss—just as in relapsing-remitting (RR) and secondary progressive MS (SPMS)—but imaging and histopathological studies both show less abundant white matter …