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PML Risk in MS — RACP Adult Medicine MCQ

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HardNeurologyPML Risk in MSRACP Adult Medicine

A 45-year-old woman with relapsing-remitting MS on dimethyl fumarate develops progressive lymphopaenia with an absolute lymphocyte count of 0.3 × 10⁹/L persisting for 6 months. What is the major infectious risk?

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Correct answer: D — Progressive multifocal leukoencephalopathy (PML)

Persistent lymphopaenia (ALC <0.5 × 10⁹/L for >6 months) on dimethyl fumarate significantly increases the risk of PML caused by JC virus reactivation. Dimethyl fumarate should be ceased if ALC remains <0.5 × 10⁹/L for >6 months. Regular monitoring of lymphocyte counts (at least 6-monthly) is mandatory. MRI brain should be performed to assess for PML, and JCV antibody status should be checked.

Reference: eTG – 2025 – Neurology; TGA – 2024 – Dimethyl Fumarate Safety Update