Asymptomatic progressive multifocal leukoencephalopathy during natalizumab therapy with treatment.

We report a case of asymptomatic progressive multifocal leukoencephalopathy (PML) detected on regular MRI screening in a 40-year-old patient with subsequent benign course with 12 months follow-up. The patient had a history of aggressive inflammatory multiple sclerosis, prior mitoxantrone therapy, Stratify John Cunningham Virus test positivity (Quest Diagnostics, Madison, NJ, USA), and 5 years of natalizumab monotherapy. The initial MRI detection of PML was both atypical and subtle. Early diagnosis and intervention, and pre-emptive treatment for immune reconstitution inflammatory syndrome with high dose steroids, as well as empirical mirtazapine and mefloquine, were associated with a benign PML disease course and outcome.