Arterial-related craniospinal physiological compliance assessed by phase-contrast MRI and lumbar infusion test

Compartmental compliance is classically defined as the ratio between volume change and resulting pressure change. Nevertheless, the concept of craniospinal compliance is inherently complex, as it involves different physiological mechanisms that vary according to temporal and volumetric scales. During lumbar infusion tests, the craniospinal system is stressed artificially, whereas during a single cardiac cycle, it operates under dynamic physiological conditions. In this study, we propose a novel approach for estimating arterial-related craniospinal compliance under physiological conditions by combining data from phase-contrast MRI and lumbar infusion test. This physiological compliance is then compared with craniospinal compliance-related parameter derived from the infusion test. We retrospectively included 108 patients (73 ± 8 years; 77 men) suspected with normal pressure hydrocephalus. Each participant underwent MRI examination, followed by a lumbar infusion test, with a mean interval of 46 ± 43 days between procedures. Cerebral arterial flows were extracted from phase-contrast MRI to compute total cerebral arterial volume change over one cardiac cycle. Craniospinal pressure change over one cardiac cycle was derived from the amplitude of the average intracranial pressure pulse extracted from baseline recordings during the lumbar infusion test. Arterial-related craniospinal physiological compliance was calculated as the ratio of volume change to pressure change over a single cardiac cycle. In parallel, craniospinal infusion-derived compliance was calculated by dividing the infused volume (from infusion start to pressure plateau) by the corresponding average pressure change. Arterial-related compliances were significantly higher than infusion-based compliance (P < 0.001); however, moderate but significant positive correlations were observed between them (R = 0.46; P < 0.001). These findings indicate that the two craniospinal compliance estimates, although derived under different physiological and temporal conditions, are complementary. Both describe the behaviour of a single craniospinal system but capture distinct mechanisms of adaptation: the rapid, pulsatile adjustments to arterial inflow during the cardiac cycle and the slower compensatory responses to sustained volume loading during the infusion test. Our findings support the view that craniospinal compliance is not a fixed property but rather a dynamic parameter that depends on the physiological context and the specific question addressed. Recognizing this complexity may enhance diagnostic precision and inform better-targeted treatment strategies in patients with suspected normal pressure hydrocephalus.