A 32-year-old man underwent a lumbar microdiskectomy and an incidental dural tear occurred. A hemilaminectomy was performed to obtain adequate visualization of the defect, and primary repair of the tear was performed. One month postoperatively he returns to the office complaining of severe headaches and occasional nausea which is worse with standing. He denies fever or chills. On physical exam his wound is well healed with no cellulitis or erythema. WBC and ESR are within normal limits. What is the most likely diagnosis?