Primary CNS vasculitis: a rare, vasculitis of undefined etiology, mainly involving small arterial vessels in the soft meninges and brain parenchyma without other systemic inflammatory diseases. The main clinical manifestations are headache, mental confusion, and cognitive dysfunction, which lack specificity. At present, the clinical diagnosis of PACNS mainly relies on the comprehensive analysis of patients’ clinical manifestations, imaging examinations and pathological change characteristics, but it is more difficult to make a clear diagnosis. In this paper, we briefly review the source and epidemiology, etiology and pathogenesis, clinical manifestations, imaging examinations, laboratory tests, and diagnostic and therapeutic features of primary central nervous system vasculitis. 1. Source and epidemiology PACNS was first proposed in 1959 by Crav iot et al. and was also known as “granulomatous vasculitis”. In 1983, the term “isolated central nervous system vasculitis” was adopted, but some patients had varying degrees of extracranial vascular inflammation, and IAC was too limited, so Lie thought that PACNS was more appropriate, reflecting the anatomic site and clinical features of the lesion, but not limited by tissue characteristics. By 1986, only 46 cases of PACNS had been reported in the English literature, and a 10-year epidemiological survey by Watts in 2000 assessed the incidence of PACNS in the United Kingdom at 19.8 per 1,000,000, which was higher than the 10 per 1,000,000 estimated by Scott in 1982, but this does not necessarily mean that the incidence has increased. This is not necessarily an increase in incidence, but perhaps the result of an increase in the level of diagnosis. Currently, the consideration of vasculitis is increasing in patients with neurological lesions of uncertain etiology, thus making its diagnosis more important. 2. Etiology, pathogenesis and pathophysiological changes Except for a very few cases related to direct infection by microorganisms, most PACNS may result from autoimmune abnormalities induced by microbiogenic or non-microbiogenic immune complexes. Common causative agents include syphilis, bacteria, fungi, and viruses. The pathogenesis of PACNS is still unclear, but Kelley suggests that it may be a direct result of vascular damage caused by infection, or it may be a result of immune complex deposition, autoantibodies, and cell-mediated immune responses. Predisposing factors may be hypersensitivity reactions, possible genetic defects in certain individuals, and exposure to specific antigens of the immune system leading to a high risk of developing vasculitis. The pathophysiological changes of PACNS are: 1) permeable vascular inflammation involving soft and parenchymal vessels; 2) small to medium-sized vessels can be involved; 3) various types of pathology can be present: vascular granulomatous inflammation, lymphocytic infiltrative inflammation, and acute necrotizing inflammation. As the initial mechanism of stimulating inflammatory cells remains unclear, but eventually all can cause occlusion and thrombosis of the involved vessels, leading to ischemia and infarction in the affected vascular region. 3. Clinical manifestations PACNS can occur at any age, with prominent manifestations at the age of 4 0 ~ 6 0 years. It has been reported in the literature that 3-5% of cerebrovascular patients aged less than 50 years are caused by this disease. The incidence of PACNS is higher in men than in women (66% in men and 34% in women), with the brain being the most commonly affected site (95%), followed by the pons or midbrain (32%), cerebellum (18%), and spinal cord (16%). nerve root lesions. Focal symptoms include transient or persistent hemiparesis, sensory loss, motor ataxia, epilepsy, and cranial nerve damage; diffuse symptoms include headache, cognitive decline, confusion, fluctuating levels of consciousness, lethargy, and depression. It can manifest either as an acute onset course such as stroke and headache or as a chronic meningitis syndrome-like course. Currently, PACNS is clinically classified according to the type of pathology, and there are three main types: 1. GACNS (Granulomatous angiitis of the CNS): this type is pathologically characterized by typical granulomatous angiitis; 2. BACNS (Benign angiopathy of the CNS): it is a subtype with a predictable and more benign prognosis based on angiography without the need for intensive treatment; 3. Atypical PACNS: It refers to a subtype that does not meet the diagnostic criteria of GACNS or BACNS, but is angiographically or histopathologically confirmed as PACNS. These three different types of PACNS have their own characteristics. Among them, CNS granulomatous vasculitis (GACNS) accounts for about 20% of PACNS, is more common in males, and can occur at any age. The prodromal period is long and the onset is rarely acute. The most common symptoms are headache and altered mental status. Transient ischemic attacks, stroke, epilepsy, ataxia, visual changes, and aphasia are also characteristic of its clinical presentation. In contrast, benign angiopathy of CNS (BACNS) accounts for about 20% of PACNS and is more common in women. It often starts acutely, with headache or neurological manifestations as the main symptoms. Angiography is seen to be 100% abnormal, MRI is 77% abnormal, and CSF is normal or mildly abnormal. If angiographic follow-up is performed after treatment, complete or near-complete recovery can be achieved. Hajj-Ali et al. reported 16 cases of BACNS, which showed a mean age of 40 years, a male to female ratio of 1:4.3, headache as the main symptom, and normal or mildly abnormal CSF. DSA showed symmetrical stenosis of multiple inferior vessels on both sides, often with post-stenotic dilatation. The clinical prognosis is good, but the etiology of the vascular lesions remains unclear and can be true vascular inflammation or reversible vascular occlusion or vasospasm. Finally, atypical PACNS, which occurs mainly in sites such as the spinal cord, can have a GACNS-like presentation clinically but no granulomatous features on CNS biopsy. May have abnormal CSF findings, but exclude patients with BACNS diagnosis. MRI is more sensitive than CT, and its most common manifestation is extensive cortical and white matter damage, which can be bilateral with hemorrhagic foci of different sizes; the foci can be irregularly streaked, banded, or diffusely vascularized in the brain parenchyma, and some of them have been followed up. New lesions may appear in some cases, while pre-existing lesions may shrink or disappear. Cerebral angiography is a major diagnostic tool for PACNS, and abnormal changes are seen in approximately 60% of patients, mainly in the form of multiple alternating stenoses and dilatations, but also in the form of bead-like changes in the arteries and aneurysm formation. Transcranial Doppler can detect proximal large vessel abnormalities. MRA can detect abnormal vascular changes, but it cannot show the smaller diameter of the involved intracranial vessels. Therefore, it cannot replace conventional cerebral angiography. In 1988, Calabrese and mallek proposed diagnostic criteria for the diagnosis of PCNSV: 1. symptoms of neurological deficit that cannot be explained by other diseases; 2. angiography showing characteristic manifestations of vasculitis or CNS biopsy showing vasculitis; 3. exclusion of secondary CNS vasculitis, such as infection, sickle cell anemia, Moyamoya disease, systemic vasculitis , vasospastic migraine and rare metabolic vascular lesions. 5. Laboratory tests General laboratory parameters lack sensitivity and specificity for identifying CNS vasculitis caused by autoimmune abnormalities. In recent years, the detection of anti-neutrophil antibodies has been found to play an important role in the diagnosis of small vessel vasculitis, and the significance of these autoantibody tests remains to be determined. Examination in the cerebrospinal fluid reveals that microorganisms may be present in the cerebrospinal fluid as well as abnormal increases in cells within the cerebrospinal fluid in patients with infectious vasculitis, tuberculous vasculitis may manifest as hypoglycemia and hypochlorhydria, and syphilitic vasculitis often presents with positive findings for syphilis-related antibodies or antigens. There are no specific changes in cerebrospinal fluid examination in autoimmune vasculitis, the most common change is mild elevation of cerebrospinal fluid protein with mild lymphocytic reaction or the presence of neutrophilic leukocytes and positive oligoclonal zone band, but positive oligoclonal zone band can be seen in cerebrospinal fluid of clinical brain inflammatory lesions, this point lacks specificity for diagnosis. laboratory examination of 62 cases of children with PACNS by Benseler et al. showed that The percentage of elevated hematocrit was 51%, elevated C-reactive protein was about 74%, elevated lgG was about 35%, and abnormal CSF was about 39%, which may also show elevated white blood cell count and protein content. The diagnosis of PACNS is very difficult and depends mainly on the clinical manifestations, imaging findings and pathological changes. Although imaging is also important, the gold standard for diagnosis is still pathological examination. Currently, the diagnostic criteria for PACNS are: 1. Clinical symptoms are mainly headache and multifocal neurological deficits, which persist for at least 6 months or the first symptoms are very severe. 2. Angiography may reveal multiple arterial segmental stenoses. 3.Exclude systemic inflammatory or infectious diseases. 4.Soft meningeal or brain parenchyma biopsy confirms vascular inflammatory changes without evidence of microbial infection, atherosclerosis and tumor. PACNS caused by infection must be treated with appropriate anti-microbial drugs after clear diagnosis. Combination corticosteroids and immunosuppressive agents are preferred for the treatment of autoimmune vasculitis, but there is no uniform standard for the dose and duration of treatment. Currently, most investigators believe that in patients with acute focal neurological damage, if the cerebrospinal fluid examination is normal and the diagnosis of vasculitis relies on angiography, high-dose corticosteroids combined with calcium antagonists can be used for a short period of time (3-6 weeks). ~After ~12 months, intermittent intravenous administration for 2-6 months is required to consolidate the effect. The prognosis of this disease is poor, and if not treated aggressively, the disease can progress and even die.