Erdheim-Chester disease mimicking multiple sclerosis or a new association?

Erdheim-Chester disease (ECD) is a rare form of non-Langerhans cell histiocytosis that presents potential impairment of the central nervous system (CNS). Frequent CNS impairment makes ECD a disease worth considering in the differential diagnosis of multiple sclerosis (MS). We report the case of a patient initially diagnosed with relapsing-remitting MS with an atypical course who developed ECD during the disease progression.