A 28-year-old man presents to the emergency department with acute onset proptosis, chemosis, ophthalmoplegia, and fever (38.5°C) for 2 days. He reports a history of sinusitis 1 week ago. On examination, visual acuity is 6/9 in both eyes, intraocular pressure is elevated, and fundoscopy shows disc edema. CT orbit with contrast shows inflammatory changes in the orbital fat with no discrete abscess. Blood cultures have been drawn. What is the most appropriate next step in management?