Non-invasive Monitoring of Intracranial Pressure Using Transcranial Doppler Ultrasonography: Is It Possible?

Abstract

Although intracranial pressure (ICP) is essential to guide management of patients suffering from acute brain diseases, this signal is often neglected outside the neurocritical care environment. This is mainly attributed to the intrinsic risks of the available invasive techniques, which have prevented ICP monitoring in many conditions affecting the intracranial homeostasis, from mild traumatic brain injury to liver encephalopathy. In such scenario, methods for non-invasive monitoring of ICP (nICP) could improve clinical management of these conditions. A review of the literature was performed on PUBMED using the search keywords 'Transcranial Doppler non-invasive intracranial pressure.' Transcranial Doppler (TCD) is a technique primarily aimed at assessing the cerebrovascular dynamics through the cerebral blood flow velocity (FV). Its applicability for nICP assessment emerged from observation that some TCD-derived parameters change during increase of ICP, such as the shape of FV pulse waveform or pulsatility index. Methods were grouped as: based on TCD pulsatility index; aimed at non-invasive estimation of cerebral perfusion pressure and model-based methods. Published studies present with different accuracies, with prediction abilities (AUCs) for detection of ICP ≥20 mmHg ranging from 0.62 to 0.92. This discrepancy could result from inconsistent assessment measures and application in different conditions, from traumatic brain injury to hydrocephalus and stroke. Most of the reports stress a potential advantage of TCD as it provides the possibility to monitor changes of ICP in time. Overall accuracy for TCD-based methods ranges around ±12 mmHg, with a great potential of tracing dynamical changes of ICP in time, particularly those of vasogenic nature.

Keywords: Intracranial pressure; Non-invasive intracranial pressure monitoring; Transcranial Doppler Ultrasonography.

Fig. 1

Flow diagram representative of the methodological approach applied for the selection of articles

Fig. 2

PI behavior during drop in CPP observed in a traumatic brain-injured patient (source: Brain Physics Laboratory TBI Database, University of Cambridge). Dashed lines represent periods when PI increased due to increase in ICP, independently of changes in ABP. CPP cereberal perfusion pressure, PI pulsatility index, ICP intracranial pressure, ABP arterial blood pressure, TBI traumatic brain injury

Fig. 3

Systolic and diastolic flow velocities behavior during a drop of cerebral perfusion pressure during a plateau wave increase in ICP observed in a traumatic brain-injured patient (source: Brain Physics Laboratory TBI Database, University of Cambridge). FVd component in this case indicates inadequate cerebral perfusion. CPP cerebral perfusion pressure, FVs systolic flow velocity, FVd diastolic flow velocity, ICP intracranial pressure, TBI traumatic brain injury

Fig. 4

Representation of the CrCP interaction with ICP and WT in a situation of intracranial hypertension observed in a traumatic brain-injured patient (source: Brain Physics Laboratory TBI Database, University of Cambridge). During the increase of ICP, the CrCP also increases and WT decreases as an effect of preserved autoregulation. ABP arterial blood pressure, CrCP critical closing pressure, ICP intracranial pressure, WT wall tension, TBI traumatic brain injury

Fig. 5

Schematic representation of the black-box model (Schmidt et al. [20]), for nICP estimation. A known transfer function (represented by a linear model) between ABP and FV, alongside modification (TCD) characteristics are used as means to dynamically define the rules for a transformation of ABP into nICP (unknown transfer function—a linear model between ABP and ICP). ABP arterial blood pressure, FV cerebral blood flow velocity, TCD transcranial Doppler, nICP non-invasive intracranial pressure

Fig. 6

Example of vasogenic waves during CSF infusion test (Cardim et al. [44]). Shadowed areas in (a) and (b) represent ICP waves of vasogenic origin. It is possible to observe that at least for trends in time, there were good correspondence between ICP and nICP methods; nICP_BB non-invasive ICP method based on mathematical black-box model [6]; nICP_FVd non-invasive ICP method based on FVd [10]; nICP_CrCP non-invasive ICP method based on CrCP [18]; nICP_PI non-invasive ICP method based on PI