Acute pachymeningitis mimicking subdural hematoma in a patient with polyarteritis nodosa

Pachymeningitis is a very rare neurologic manifestation of polyarteritis nodosa (PAN). This report describes a case of acute pachymeningitis that was, initially, misdiagnosed as subdural hematoma on the brain CT of a patient with PAN. A 45-year-old man, who had been diagnosed as having PAN 6 months previously, came back to the emergency room with complaints of sudden headache, nausea, vomiting, and diplopia for 3 days before his hospital admission. Initially, the noncontrast enhanced brain CT findings showed high densities in the bilateral tentorial and posterior parafalcial area, which suggested a small amount of subdural hematoma. However, the subsequent MRI findings revealed pachymeningitis of the bilateral tentoria and falx. He was treated with high-dose steroid and cyclophosphamide pulse therapy. Thereafter, his symptoms gradually resolved.