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Introduction Since recognised by Cravioto and Feigin in the 1950s, primary angiiitis of the central nervous system (PACNS) remains a rare entity with unclear pathogenesis. Particularly, the typical disease pattern is symmetrically distributed supratentorial white matter lesions. In this case, we describe a primary angiitis atypically affecting the brainstem and cerebellum in the setting of smouldering myeloma.Case A 55 year-old gentleman, with treated smouldering myeloma, presented in May 2023 with sudden horizontal diplopia and subacute headache. Examination demonstrated a right internuclear ophthalmoplegia and this correlated with MRI pontine diffusion restriction. His diplopia improved and he remained clinically well for two years. Screening for systemic vasculitides, thrombophilia and coagulation testing were normal. Bone marrow studies demonstrated an unchanged myeloma cell population and stable paraprotein (32g). A lumbar puncture revealed 8 monocytes without raised protein. However, repeat MRIs demonstrated further pontine, cerebellar and lobar microhaemorrhages. Despite radiological progression, the patient declined biopsy as he remained asymptomatic. Weekly plasmapheresis was commenced to assess and treat potential paraprotein-related hyperviscosity. Despite this, a generalised tonic-clonic seizure eventuated in January 2025. On follow-up MRI 12 days post-seizure a cerebellar folia T2 hyperintensity was identified as a biopsy target. The biopsy was proceeded to and demonstrated a granulomatous vasculitis that was negative on beta-amyloid immunostaining. He has been commenced on corticosteroids with cyclophosphamide.Conclusion This case represents an unusual instance of PACNS with brainstem and cerebellar preponderance. Whilst no direct myeloma implication could be identified, an altered immunological self-tolerance needs to be considered.