A 12-year-old with Acquired Immunodeficiency Syndrome (AIDS) with a CD4+ count of 50 cells/μL presents with a headache and confusion. A lumbar puncture reveals an opening pressure of 30 cm H₂O and the cerebrospinal fluid is positive for cryptococcal antigen. In addition to antifungal therapy, what is the most critical initial management step for this patient’s increased intracranial pressure?