ESOC 2026 | Guidance for diagnosing and classifying primary angiitis of the central nervous system (PACNS)

PACNS, primary central nervous system angiitis, is a rare cerebrovascular disease involving vessels of any size in the brain, and it is a primary disease. So it is a vasculitis directly involving brain vessels, brain leptomeningeal and spinal vessels, without any association with systemic involvement. This is the most important organ-specific vasculitis in the general vasculitis classification, and its pathogenesis is not very well known, but several autoimmune reactions towards cellular antigens in the vessel wall are being investigated. And this disease is a rare but over-suspected and sometimes over-diagnosed in the general population of stroke patients. We don’t have any specific finding in clinical presentation, instrumental tests, and any specific finding also in imaging and pathological findings for PACNS. Biopsy is the main cornerstone of the diagnosis in the literature, but large vessel vasculitis cannot be diagnosed by biopsy because the danger outweighs the benefit in these patients. And pathological findings, histopathological findings, are not really specific. They are specific of vascular damage involving all layers in the vessel wall, but they are not specific about the causative agent. Post-infectious vasculitis, systemic vasculitis with CNS involvement, have the same pathological appearance of PACNS. So even biopsy is not enough to make a proper diagnosis. So in this context, when non-invasive diagnosis using several neuroradiological techniques, advanced MRI imaging, but also new generation DSA has been mostly used, making less confident the use of biopsy in some patients. There is the need of a strict definition of background experience and neurovascular tips dedicated to this disease. The main component of this neurovascular team, of course, should be vascular neurologists dedicated to rare neurovascular disease and neuroradiologists dedicated as well to this disease for diagnostic purposes. But the team should also include neurosurgeons in order to define the proper execution and the indication of biopsy. Pathologists, neuropathologists as well. And also immunologists and or rheumatologists. But the main differential diagnosis of PACNS are CNS-based diseases. So the main core of the disease is the central nervous system. Within the central nervous system, the spinal cord involvement is rarer than the brain involvement. Only 38 cases have been reported in the literature. The diagnosis is only pathological, but in some of these patients, the diagnosis was performed using the contemporary involvement of the CNS of the brain, and not only biopsying the spine, the spinal cord. And several subtypes of the disease are known. The main categorization of the disease is according with the size of the involved vessel. So we have two general categories, two general subtypes of PACNS, large vessel PACNS and small vessel PACNS. Large vessel include large and medium-sized vessels in the brain, and small vessel include only vessel with a diameter less than 500 microns. So in a more rigorous view, less than 250 microns. It’s not well defined. But the modern DSA can reach up to this limit. So there are some cases in the recent literature where we have a positive biopsy. Positive biopsy is classical for small vessel PACNS. And also, we have a positive angiogram with involvement of the more distal medium vessel, so at the neighbor between medium and small vessels. But we don’t have any description of any PACNS with a contemporary involvement of large and small vessels. But secondary vasculitis, for example, post-infectious vasculitis, may affect both large and small vessels. This is one of the most interesting and practical clinical clues to make a differential diagnosis and to guide the diagnostic pathway.

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