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

Abstract 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. Given the patient's clinical-radiological characteristics, two diagnostic possibilities were proposed: the coexistence of both diseases or a new presentation of ECD mimicking MS. We conducted a literature review, analyzing the various diagnostic and therapeutic possibilities.