Local or diffuse thickening of intracranial or spinal dura mater; often with adhesion to underlying leptomeninges. Attributed to infections (syphilis, tuberculosis, fungi); autoimmune or vasculitic diseases (Wegener granulomatosis, sarcoidosis, rheumatoid arthritis, Behçet disease, Sjögren syndrome, giant cell arteritis); malignancy (dural carcinomatosis, skull metastases, histiocytosis, lymphoma). Idiopathic hypertrophic pachymeningitis (IHP) considered autoimmune.