A full blood count is now required every 3 months during treatment with dimethyl fumarate, and a baseline reference MRI should be performed before starting treatment.
If lymphocyte counts fall below 0.5 x109/L for more than 6 months, interruption of treatment should be considered. If treatment is stopped, patients should be monitored until lymphocyte levels return to normal. If treatment is continued, patients should be reminded to report symptoms of PML and the need for additional MRI should be considered. Treatment should be stopped immediately if PML is suspected.
PML is a rare but serious opportunistic infection caused by the John-Cunningham virus. Recommendations to reduce the risk of PML associated with dimethyl fumarate were initially introduced in April 2015 after a single fatal case was reported in a patient who had been receiving the drug for 4.5 years.