Transcranial ultrasound in the critically ill patient: a narrative review

Abstract

Transcranial ultrasound is gaining widespread recognition as a useful bedside monitoring tool and non-invasive diagnostic device in the critically ill patient. The capabilities of transcranial ultrasound are themselves ever-increasing, and this, combined with improved physiological understanding, affords insights into pathophysiological processes often concealed from the bedside critical care clinician. Transcranial ultrasound remains unique in regard to its non-invasive, rapid, and critically composite blood flow velocity-centric (not pressure-centric) information. The mobility of transcranial ultrasound devices is of particular value to the largely immobile critically ill patient requiring multiple organ supportive therapies. In this review, we discuss some important origins of more modern composite techniques and highlight relevant major key concepts, whilst noting exciting frontier possibilities.

Keywords: Critical care; Doppler ultrasonography; Doppler ultrasound imaging; Intensive care; Point-of-care diagnostics; Transcranial.

Fig. 1

Cerebral vasculature demonstrated on TCCD in a patient who had previously undergone decompressive craniectomy for a traumatic brain injury. Image over-gained to demonstrate relative vessel positions

Fig. 2

Run-time chart showing intracranial pressure (ICP), mean flow velocity (FV), invasively measured cerebral perfusion pressure (CPP) and non-invasively measured cerebral perfusion pressure (nCPP). The ability of nCPP to track CPP is clearly demonstrated over time

Fig. 3

Simultaneously measured intracranial pressure (ICP), cerebral perfusion pressure (CPP), and the correlation coefficient Mx (CPP correlated with transcranial Doppler (TCD) measured mean flow velocity (FVm)) with each displayed against time. Above, ICP plateau-Lundberg A-waves- with corresponding periods of reduced CPP are shown. During these episodes, Mx becomes strongly positive, almost nearing 1. As such, during these spikes FVm is proportional to CPP, and autoregulation (where in health the correlation would be neutral or negative) can be seen non-functional

Fig. 4

Run-time charts showing intracranial pressure (ICP), arterial blood pressure (ABP), blood flow velocity (BFV), high-pressure arterial compartment compliance (C_a) and low-pressure venous and CSF compartment compliance (C_i) plotted against time on the X-axis. A rise in ICP is provoked by cerebral vasodilatation, causing a plateau wave of ICP. The rise in ICP is synchronised with a decrease in Ci and arterial dilatation denoted by an increase in Ca

Fig. 5

Transcranial colour-coded triplex and pulse-wave Doppler of the branching posterior cerebral artery with additional basal vein of Rosenthal waveform demonstrated within spectral Doppler trace

Fig. 6

Transcranial colour-coded Triplex of the contralateral transverse sinus with respiratory influence demonstrated

Fig. 7

A Brightness-mode structural image in the coronal plane via the transtemporal window. The contralateral temporal bone is visualised along with the cerebral hemispheres, and the lateral ventricles in the centre of the image. The echogenic skull base bony structures inferior

Fig. 8

Optic nerve sheath ultrasound. Two examples of optic nerve sheath images: A in a patient with intracranial haemorrhage secondary to an arterio-venous malformation, B in a healthy individual. The ONSDint (as per Hirzallah et al [66]), is denoted by the outer border of the radiolucent parallel lines shown and annotated. ONSDexternal corresponds to the wider, outer borders, of the outer dark parallel lines (not annotated). 3 mm depth is not shown, but is important for performing correct diameter measurement