Marked tissue eosinophilia within organizing chronic subdural hematoma membranes.

Chronic subdural hematoma (CSDH) with membranes is a common problem in neurosurgery. Despite its frequency, the pathogenesis of this lesion is poorly understood. We conducted a systematic pathologic review, with clinical correlation, of all CSDH presenting to our institution over a two-year period which had undergone pathological examination. Surprisingly, we found marked tissue eosinophilia in most CSDH membranes in the process of organization. Eosinophils were not evident in fresh hemorrhage or in older membranes which were completely organized. These results demonstrated that eosinophils are frequently present in large numbers in the early phases of chronic subdural membrane (CSM) formation but not associated with chronic, organized membranes. No specific clinical cause for tissue eosinophilia was identified in the cases reviewed. Though the significance of these observations remains unclear, recognition of eosinophilia as a pathological feature of organizing CSM is important in order to avoid confusion with other diagnoses.