Cerebrovascular reactivity and dynamic autoregulation in ischaemic subcortical white matter disease

Background: It has been suggested that impaired cerebral autoregulation and vasodilatory capacity may play in role in the pathogenesis of the leukoaraiosis seen in small vessel disease. Adequate perfusion of the deep white matter of the brain depends on the relationships between blood pressure (BP), cerebral vasoreactivity and autoregulation. Methods: 24 h ambulatory BP measurement, quantitative volumetric MRI analysis of white matter lesion (WML) volume and transcranial Doppler ultrasound assessments of CO2 reactivity in response to hypercapnia and dynamic cerebral autoregulatory index (ARI) were undertaken in 64 patients with cerebral small vessel disease. Results: Subjects had mean 24 h BP 133/76 mm Hg (SD 13/9), median WML volume 7169 (IQR 20497) mm(3), mean CO2 reactivity 83.6 (SD 37.4)% and mean ARI 5.6 (SD 1.4) (range 0-9). In multivariate models, after adjusting for age, gender, vascular risk profile and WML volume, ARI correlated with 24 h mean BP levels (R-2 = 0.127, t = 2.440, p = 0.019) and CO2 reactivity correlated with duration of hypertension (R-2 = 0.085, t = -22.244, p = 0.029). In individuals with hypertension for more than 10 years, ARI also correlated with nocturnal BP dipping (r = 0.806, p = 0.002). ARI and CO2 reactivity were unaffected by WML volumes, and ARI and CO2 reactivity were unrelated. Conclusion: Cerebral autoregulation and CO2 reactivity are two distinct processes which are not related to WML volume but are related to BP levels and duration of hypertension, respectively. Greater nocturnal dipping was associated with higher ARI values, suggesting preservation of autoregulation in patients with increased vulnerability to reduced cerebral perfusion.