Abstract Body: Background
Prior studies have demonstrated that the incidence of stroke and rate of infarct progression varies across the 24-hour cycle and that circadian rhythm is associated with the brain’s response to reperfusion and neuroprotection. However, the pathophysiology of this variation is poorly understood. In this study we assessed changes in collateral status with time of day as a possible mechanism behind this variation.

Methods
Using data from the 7 randomized EVT trials in the HERMES collaboration, patients with analyzable baseline CT angiograms were selected. Presenting collateral status on CTAs was rated using the Regional Collateral Score (rCS). The effect of time of onset (last known well) and time of scan on collateral status was assessed. Time of day was analyzed as: 1) binary day-night (23:00–06:59/07:00–22:59); 2) 4-hour intervals (23:00–02:59/03:00–06:59/07:00–10:59/11:00–14:59/15:00–18:59/19:00–22:59) and 3) a continuous variable as per Consortium International pour la Recherche Circadienne sur l’AVC recommendations.

Results
Among 1250 patients with LVOs, 1086 (86.9%) had daytime and 164 (13.1%) had night-time stroke onset. Baseline demographic and clinical features of patients that differed (Table 1) were night-time onset patients were younger (62.0 vs 67.1 years), less often had highly severe (NIHSS>16) deficits (43.6% vs 56.3%), had longer onset-to-arrival (186 vs 122 minutes), and longer onset-to-imaging (218 vs 159 minutes). In multivariable analysis, categorizing based on last known well time, there were no significant difference in rCS score by night vs day (7.5±2.0 vs 7.4±2.3 p=0.99) or by 4-hour intervals (7.8±1.9 vs 7.3±2.1 vs 7.4±2.2 vs 7.5±2.2 vs 7.5±2.3 vs 7.1±2.4 p=0.24) (Figure 1). Similarly, categorizing by scan time, there were no significant differences in rCS score by night vs day (7.3±2.3 vs 7.4±2.2 p=0.60) or by 4-hour intervals (7.2±2.4 vs 7.6±2.0 vs 7.3±2.2 vs 7.4±2.1 vs 7.5±2.3 vs 7.6±2.3 p=0.22) (Figure 1). The relationship between continuous time and rCS mapped by LOESS regression also did not indicate substantial rCS variation with time of onset (Figure 2).

Conclusion
Our study does not demonstrate a strong relationship between time of day and regional collateral score. This was at least partially driven by a ceiling effect as most patients had good collateral scores. Further studies to identify patient and imaging characteristics that may explain the relationship between circadian variation of stroke outcomes are needed.