Crystal-storing histiocytosis: An unusual relapsing inflammatory CNS disorder.

The differential diagnosis of acute leukoencephalopathy often focuses on central nervous system idiopathic inflammatory demyelinating diseases (IIDDs) such as multiple sclerosis (MS). However, a spectrum of conditions mimic IIDDs, therefore it is critical to consider whether symptoms, signs, imaging and/or response to therapies are compatible with the diagnosis. We describe a 32-year-old previously healthy woman presenting with a 2 year history of steroid-responsive relapsing episodes lasting 2-10 days characterized by transient visual blurring, right-hemiparesis, and spells of aphasia. MRI demonstrated multifocal, relapsing, predominantly white matter enhancing brain lesions, a longitudinally extensive cord lesion, and abnormal visual evoked potentials. Notably, some lesions persistently enhanced whereas others demonstrated progressive T2W hypointensity. Brain biopsy revealed an atypical plasma cell infiltrate and crystal-storing histiocytosis, which by mass spectrometry confirmed the presence of macrophages containing intracellular kappa-light chain restricted crystals. Bone marrow was negative. The patient did well for several years on pulse dexamethasone, however subsequent scans demonstrated increasing enhancement. Repeat biopsy demonstrated a clonal plasma cell proliferation. She was treated with melphalan, and has remained stable. Although this patient initially met McDonald criteria, atypical imaging prompted further workup, and advanced proteomic technology helped secured an accurate diagnosis. Crystal-storing histiocytosis should be considered in the differential diagnosis of inflammatory CNS disorders.