Introduction

Natalizumab (Tysabri, Biogen-Idec Inc, Cambridge, Massachusetts, USA) is an effective drug for the treatment of multiple sclerosis (MS). Patients treated with natalizumab are at higher risk of developing progressive multifocal leukoencephalopathy (PML). MRI has become crucial in the drug surveillance in terms of an early (preferably presymptomatic) detection and monitoring of natalizumab-associated PML with special regard to the occurrence of an immune reconstitution inflammatory syndrome (IRIS).1

Natalizumab-associated PML is characterised by rather heterogeneous imaging findings particularly in the early disease course. Recently, a rare PML imaging pattern with punctiform enhancing lesions has been described in a patient with an asymptomatic course of natalizumab-associated PML.2 It remains unclear what this inflammation pattern in acute PML represents regarding the underlying histopathology and possible consequences of the disease progression or PML-IRIS development. In this case report, we describe a patient with acute natalizumab-associated PML presenting with punctate enhancing lesions suggestive of perivascular inflammation and discuss the importance of correctly interpreting this imaging finding in terms of differentiating productive JC virus infection from early presymptomatic PML-IRIS manifestation.