A 63 year old woman with white matter lesions and pachymeningeal inflammation.

We describe the case of a 63-year-old woman with CNS Rosai-Dorfman disease, presenting with diffuse dural infiltration, mimicking idiopathic hypertrophic pachymeningitis, and right vertebral artery dissection. Her symptoms included a progressive 11-month history of vertigo, gait ataxia, and right thalamic stroke. A diagnosis of CNS Rosai-Dorfman disease was made following open dural biopsy, and later confirmed on autopsy studies. The autopsy demonstrated widespread dural infiltration by inflammatory cells, principally large histiocytes, many of which exhibited emperipolesis, a characteristic finding in Rosai-Dorfman disease. A second pathological finding on autopsy was the presence of multiple demyelinating plaques (with preservation of axons), located in the corpus callosum, periventricular white matter, and multiple brainstem segments. These were consistent with a diagnosis of multiple sclerosis. This case description serves to remind clinicians that CNS Rosai-Dorfman disease—although uncommon—may present as a focal, dural-based, hemispheric mass lesion, or as diffuse pachymeningeal inflammation. Our case was also unusual due to the co-existence of CNS Rosai-Dorfman disease, multiple sclerosis, and polycythemia vera (all rare diseases) in a single patient. Although the overlap of disorders may have been co-incidental, one could raise the question whether all three disorders were triggered by the same underlying dysimmune state.