Bolus injection test for measurement of cerebrospinal fluid absorption: A study of pressure-time, pressure-volume and volume-time relationship

Abstract

The significance of the bolus injection test lies in supplying data for evaluation of the cerebrospinal fluid (CSF) bulk flow. It provides pressure-time (PT) curves by the process of direct recording, while the pressure-volume (PV) and volume-time (VT) plots are subsequently computed. While a single PT curve offers information about the character of decompression of the CSF system, a series of PT curves may be converted into a PV plot which permits ascertainment of the degree of distensibility of the system, the availability of the reserve space, and the patency of the drainage devices under stress. The reserve space has been defined as a space occupied alternatively either by blood in the venous bed or by CSF fluid excess in the CSF space. Obliteration of this compartment turns the CSF system into the inelastic one, in which the casual increase in fluid volume produces high-pressure increments. The VT curves show that the rate of absorption is a function of pressure, and hence, the higher the pressure rises the faster the excess of fluid is drained out of the system. This mechanism breaks down on occasions, and 2 types of absorption defects may be demonstrated by abnormal VT curves. In the first type of absorption defect, the CSF system fails to empty the reserve space and to restore its distensible property but the high-pressure drainage is not impaired; in the second type the rapid high-pressure absorption is at fault and the drainage is performed by slowly working pathways. The bi-compartmental character of semi-log curves indicates that different mechanisms may be involved in low- and high-pressure absorption. The shunting diversion seems to be justified in cases with a high-pressure absorption defect.