Secondary progression is not the only explanation.

Multiple sclerosis is an inflammatory demyelinating disorder of the central nervous system. Its presentation is variable and its course and prognosis are unpredictable. Approximately 85% of individuals present a relapsing-remitting form of the disease, but some patients may evolve into a progressive course, accumulating irreversible neurological disability, defining its secondary progressive phase. Despite all the advances that had been reached in terms of diagnosis, many decisions are still taken based only on pure clinical skills. We present the case of a patient that, after being diagnosed with a clinically isolated syndrome many years ago, seemed to be entering in a secondary progressive course, developing a clinical picture dominated by a progressive gait disturbance. Nevertheless, multiple sclerosis heterogeneity asks for some clinical expertise, in order to exclude all other possible causes for patients' complaints. Here we present an important red flag in the differential diagnosis of secondary progressive multiple sclerosis.