Measuring glymphatic function: Assessing the toolkit

Abstract

Glymphatic flow has been proposed to clear brain waste while we sleep. Cerebrospinal fluid moves from periarterial to perivenous spaces through the parenchyma, with subsequent cerebrospinal fluid drainage to dural lymphatics. Glymphatic disruption is associated with neurological conditions such as Alzheimer's disease and traumatic brain injury. Therefore, investigating its structure and function may improve understanding of pathophysiology. The recent controversy on whether glymphatic flow increases or decreases during sleep demonstrates that the glymphatic hypothesis remains contentious. However, discrepancies between different studies could be due to limitations of the specific techniques used and confounding factors. Here, we review the methods used to study glymphatic function and provide a toolkit from which researchers can choose. We conclude that tracer analysis has been useful, ex vivo techniques are unreliable, and in vivo imaging is still limited. Finally, we explore the potential for future methods and highlight the need for in vitro models, such as microfluidic devices, which may address technique limitations and enable progression of the field.

Keywords: aquaporin-4; cerebrospinal fluid; efflux; glymphatics; imaging; influx; methods; microfluidics; parenchyma; periarterial; perivenous; tracer.

Figure 1

Structure of the glymphatic system. (1) Cerebrospinal fluid (CSF), produced by the choroid plexus, is found in the ventricles and subarachnoid space; (2) CSF enters periarterial spaces; (3) CSF flows from the periarterial space to the brain parenchyma, termed “influx”; (4) Here, CSF mixes with interstitial fluid (ISF), and the CSF-ISF fluid is exported into the perivenous space, termed “efflux”; (5) The CSF-ISF fluid can drain into dural lymphatics via arachnoid cuff exit (ACE) points, which facilitate direct communication between the dura and parenchyma. Some fluid also drains directly into the blood via arachnoid granulations, which contain dense populations of immune cells for surveillance. The CSF-ISF fluid is eventually exported to extracranial lymphatics. Created with BioRender.com.

Figure 2

Schematic representation of the methods used to study glymphatics. Created with BioRender.com. DCE-MRI: Dynamic contrast-enhanced magnetic resonance imaging; DTI: diffusion tensor imaging; MREG: magnetic resonance encephalography; SPECT/CT: single-photon emission computed tomography/computed tomography.

Figure 3

A proposed framework for selecting an appropriate method to study the glymphatic system. The choice of method depends on the specific research question being asked. MRI: Magnetic resonance imaging; SPECT/CT: single-photon emission computed tomography/computed tomography.

Figure 4

The first step towards a glymphatics microfluidic model. Reprinted with permission from Soden et al. (2022). Created with BioRender.com.