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Introduction Transverse sinus stenosis is the most sensitive diagnostic marker for idiopathic intracranial hypertension (IIH), present in over 90% of cases. The etiology of sinus stenosis can be extrinsic (due to brain compression) or intrinsic (due to luminal architectural irregularities). Etiology impacts treatment effect and durability, however the proportion of intrinsic versus extrinsic stenosis and precision of etiologic determination is not known.Material and Methods We identified a consecutive series of 115 adult patients with lumbar puncture opening pressure >20mm Hg and magnetic resonance venogram (MRV) showing some degree of stenosis. Collected data included clinical presentation, prior medical history, imaging findings and treatment. Six clinicians (3 endovascular neurosurgeons (NUS), 3 neuroradiologists (RAD)) retrospectively evaluated MRV images to determine the degree and nature of venous sinus stenosis (VSS). Stenosis was graded according to the Farb score (0-4 points on each side depending on percent stenosis). Etiology was determined to be either intrinsic or extrinsic. Agreement among graders was measured using free marginal kappa values.Results The patient cohort was majority female (87.8%) with an average age of 34.4, BMI of 40.4, and lumbar puncture opening pressure of 32.5 cm. Papilledema was present in 44.3% of patients. The overall proportion of intrinsic stenosis was 30.0%, with no significant difference between specialties (χ 2=0.03, p=0.86). However, evaluations significantly differed between graders within each specialty (NUS: 21.7-39.1%; RAD: 20.9-43.4%; χ2 NUS=8.85, p=.012; χ2 RAD=18.44, p<.001).When analyzing patient-matched grading, there was complete agreement (unanimity) regarding etiology in 46/115 (40%) of cases, which was more common among RAD (60%) than NUS (50%). Near-consensus (5/6 agreement) was achieved for 28% of cases, while there was relative disagreement (4/6) and discordance (3/6) for 17% and 15% of cases, respectively. NUS had a free marginal kappa of 0.34 (95%CI 0.22-0.46, 66.96% overall variation), signifying fair agreement on the Landis and Koch scale. Radiology had a free marginal kappa of 0.47 (95%CI 0.35-0.59, 73.33% overall variation), signifying moderate agreement. The total agreement across all 6 graders had a kappa of 0.45 (95%CI 0.36-0.54, 72.58% overall variation), also indicating moderate agreement. Contrasted MRV was available for 105/115 patients and inter-grader agreement did not differ between MRVs with contrast and those without (p = 0.6906). The average Farb score across all patients was 3.50. It was higher for extrinsic stenosis (3.51) than intrinsic stenosis (2.73), and this difference was statistically significant (p < 0.0001). The average lumbar puncture opening pressure was not significantly different for patients with intrinsic and extrinsic stenosis, respectively (32.5 vs. 31.6, p=0.6480).Conclusion In a consecutive series of patients with intracranial hypertension and abnormalities on MR venogram, the overall proportion with intrinsic sinus stenosis was 30%. There was fair agreement among endovascular neurosurgeons and better, moderate agreement among neuroradiologists regarding etiology, which did not differ based on contrast administration. The subjectivity of etiologic assessment makes it difficult identify patients who are at increased risk of treatment failure with stent-adjacent stenosis.Disclosures A. Anandarajah: None. S. Mohajeri: None. V. Nguyen: None.