Case Report: Unilateral relapsing primary central nervous system vasculitis-expanding the phenotype.

Background: Unilateral relapsing primary central nervous system vasculitis (UR-PCNSV) is a scarcely reported subtype of PCNSV. It is characterised by frequent relapses with lesions confined to a single hemisphere. Herein, we expand the phenotype of UR-PCNSV, adding three cases to the existing 13 in the literature. Method: A retrospective review of clinic databases at two adult tertiary referral centres in New South Wales, Australia, was undertaken to identify cases of UR-PCNSV. Predefined inclusion criteria were (1) biopsy-proven PCNSV, (2) lesions confined to a single hemisphere, and (3) two or more relapses as evidenced by new enhancing lesions on MRI. Results: Three cases of biopsy-proven UR-PCNSV were identified. All demonstrated three or more relapses with new lesions confined to the same hemisphere. The mean age was 34.5 (± 8.6) years, and the median delay to diagnosis was 12 months (IQR 7.5-21). Headache was the first symptom in all patients, and they developed unilateral motor and sensory deficits. Cognitive impairment was a prominent feature in one and none developed seizures. CT and/or MR angiography showed normal results. MRI head showed both subcortical and cortical lesions with parenchymal and leptomeningeal enhancement. The protein level was normal in all patients, and one had a mildly raised white cell count (9 × 10 Conclusion: Compared with typical PCNSV, this rare unilateral, relapsing subtype has a younger age of onset, lower prevalence of angiographic abnormalities, and frequent relapses. Our patients had persisting lesion enhancement despite anti-CD20 mAb monotherapy and demonstrated hemiatrophy within the first year, indicating high inflammatory activity and a requirement for additional immunosuppression. This case series additionally highlights the overlapping clinical and radiological features of PCNSV and CNS demyelination, which may contribute to diagnostic delay.