A 42-year-old woman with acute leukaemia undergoing induction chemotherapy develops fever (38.9°C), cough, and dyspnoea on day 14 of neutropenia (ANC 200/μL). High-resolution CT chest shows multiple nodular infiltrates with a halo sign. Bronchoalveolar lavage culture grows Aspergillus fumigatus. She is started on voriconazole 6 mg/kg IV twice daily on day 1, then 4 mg/kg IV twice daily from day 2 onwards. After 48 hours of therapy, she develops blurred vision and mild confusion. What is the most appropriate management?