Cerebellar tonsillar hemorrhages in primary central nervous system vasculitis: a potential radiologic marker

Background Primary central nervous system vasculitis (PCNSV) is a rare inflammatory vasculopathy with diverse clinical and radiological manifestations that often delay diagnosis. Although abnormalities on magnetic resonance imaging (MRI), including susceptibility-weighted imaging (SWI), fluid-attenuated inversion recovery (FLAIR), and contrast-enhanced imaging, are increasingly recognized, a characteristic imaging pattern that may facilitate diagnosis has not yet been described. We aimed to evaluate cerebellar tonsillar hemorrhages (CTH) as a potential imaging marker of PCNSV. Methods We conducted a retrospective observational study at a tertiary neuroscience referral center in India. The discovery cohort included 51 patients with biopsy-proven PCNSV and 44 hypertensive microangiopathy controls who underwent MRI between 2018 and 2024. The imaging findings were independently reviewed for CTH and associated enhancement patterns. The reproducibility of this imaging pattern was subsequently assessed in an independent replication cohort of 55 patients with digital subtraction angiography (DSA)-proven PCNSV, evaluated between 2018 and 2026. Results CTH were identified in 35/51 (68.6%) biopsy-proven PCNSV patients compared with 9/44 (20.4%) hypertensive controls (p<0.001). In the biopsy-proven discovery cohort, CTH demonstrated a sensitivity of 68.6% (95% confidence interval [CI], 54.1–80.9%) and specificity of 79.5% (95% CI, 64.7–90.2%) for distinguishing PCNSV from hypertensive microangiopathy and remained associated with PCNSV after adjustment for age and sex (adjusted odds ratio [OR] 12.52, 95% CI 3.46–56.94). This association persisted in exploratory age-restricted sensitivity analyses. Associated punctate or linear contrast enhancement was present in 33/35 (94.3%) CTH-positive biopsy-proven patients and was not observed in hypertensive controls with available contrast-enhanced MRI (0/4). In the independent DSA-proven replication cohort, CTH were observed in 41/55 (74.5%) patients, of whom 23/39 (58.9%) demonstrated associated cerebellar tonsillar enhancement on contrast-enhanced MRI. Interobserver agreement for CTH detection was high in the combined discovery cohort (Cohen’s κ = 0.94) and the DSA-proven replication cohort (κ = 0.91). CTH frequently occurred in the absence of clinical ataxia or diffuse brainstem involvement. Conclusion CTH represent a frequent and reproducible imaging finding in both biopsy-proven and DSA-proven PCNSV. The presence of CTH, particularly when accompanied by punctate or linear enhancement, may improve the radiological suspicion of PCNSV and help distinguish inflammatory microangiopathy from hypertensive small-vessel disease.