Not All Strokes Are Strokes An Example of Diagnostic Confirmation Bias.

A 72-year-old woman presented with a complex partial seizure and right hemiparesis, after a four-week history of cognitive decline, apraxia and speech disturbance. She previously had chronic lymphocytic leukaemia (CLL) and had finished chemotherapy one year prior to presentation. She was receiving monthly intravenous immunoglobulins for bronchiectasis. Brain imaging showed hypodensity in the left temporo-parietal regions. Cerebrospinal fluid was positive for the JC virus, leading to a diagnosis of progressive multifocal leucoencephalopathy (PML). She remains alive, eight months following initial presentation. The case was valuable for reflective practice in avoiding diagnostic (confirmation) bias because the treating team pursued an incorrect diagnosis of stroke and secondary seizure after radiology findings appeared consistent with this. Additionally, PML has not previously been reported in individuals with CLL receiving immunoglobulin therapy, and may explain the relatively benign course in this individual patient. This offers a potential research question for disease modifying treatments in PML. LEARNING POINTS: This case highlights new insights into an uncommon but important condition: always consider progressive multifocal leucoencephalopathy when immunocompromised patients present with neurological symptoms.A full differential diagnosis should always be considered, even in the context of a more 'plausible' diagnosis.Avoid premature closure and confirmation bias as cognitive errors in diagnostic reasoning.