Epilepsia partialis continua revealing idelalisib-associated PML-IRIS: clinical and pathological features.

Idelalisib, a selective phosphatidylinositol 3-kinase delta (PI3Kδ) inhibitor, is a newly approved second-line drug for patients with chronic lymphocytic leukemia. Recent clinical trials have suggested a possible association between idelalisib treatment and development of progressive multifocal leukoencephalopathy (PML) due to John Cunningham virus (JCV) reactivation. Nevertheless, clinical course and radiological and pathological features of idelalisib-induced PML still need to be clarified. We provide here the first clinicopathological description of idelalisib-associated PML in a patient who developed epilepsia partialis continua (EPC) as the first manifestation of the disease. Since EPC could present without electroencephalogram alterations, it is crucial to recognize the clinical features of this epileptic condition. EPC is characterized by the presence of repetitive, irregular, clonic jerking, often associated with hemiparesis and involvement of distal rather than proximal muscle groups. Moreover, we highlight the importance of brain biopsy in selected cases when there is a high clinical suspicion of PML, despite negative JCV testing in the cerebrospinal fluid. The pathological finding of prominent inflammatory infiltrate observed here was consistent with a diagnosis of immune reconstitution inflammatory syndrome (IRIS). IRIS is often associated with PML as a paradoxical worsening of clinical symptoms due to an overreacting immune response, in the context of previous immunosuppression. The unprecedented pathologic observation of IRIS in idelalisib-associated PML provides further insights into the pathogenesis of this rare neurological side effect.