A Systemic Review of Functional Near-Infrared Spectroscopy for Stroke: Current Application and Future Directions

I got nothing out of this, too many uses of the word 'could' means this is all speculative.

A Systemic Review of Functional Near-Infrared Spectroscopy for Stroke: Current Application and Future Directions

Muyue Yang^1,2, Zhen Yang³, Tifei Yuan⁴, Wuwei Feng⁵ and Pu Wang^1*

• ¹Department of Rehabilitation Medicine, Ruijin Hospital, Shanghai, China
• ²School of Medicine, Shanghai Jiao Tong University, Shanghai, China
• ³Core Facility of West China Hospital, Sichuan University, Chengdu, China
• ⁴Shanghai Mental Health Centre, Shanghai Jiao Tong University School of Medicine, Shanghai, China
• ⁵Department of Neurology, Medical University of South Carolina, Charleston, SC, United States

Background: Survivors of stroke often experience significant disability and impaired quality of life. The recovery of motor or cognitive function requires long periods. Neuroimaging could measure changes in the brain and monitor recovery process in order to offer timely treatment and assess the effects of therapy. A non-invasive neuroimaging technique near-infrared spectroscopy (NIRS) with its ambulatory, portable, low-cost nature without fixation of subjects has attracted extensive attention.

Methods: We conducted a comprehensive literature review in order to review the use of NIRS in stroke or post-stroke patients in July 2018. NCBI Pubmed database, EMBASE database, Cochrane Library and ScienceDirect database were searched.

Results: Overall, we reviewed 66 papers. NIRS has a wide range of application, including in monitoring upper limb, lower limb recovery, motor learning, cortical function recovery, cerebral hemodynamic changes, cerebral oxygenation, as well as in therapeutic method, clinical researches, and evaluation of the risk for stroke.

Conclusions: This study provides a preliminary evidence of the application of NIRS in stroke patients as a monitoring, therapeutic, and research tool. Further studies could give more emphasize on the combination of NIRS with other techniques and its utility in the prevention of stroke.

Introduction

Stroke, which refers to a medical condition in which insufficient brain blood supply results in cell death, is a major cause of death and disability worldwide (1, 2). Survivors are accompanied with the deterioration or loss of functions, for example, sensorimotor sequelae including motor weakness and impairment of voluntary motor control, spasticity, incoordination, apraxia, sensory loss/numbness, dysphagia, and dysarthria, and stroke could also lead to various cognitive and psychiatric deficits (3–5). These disfunctions are associated with cortical impairment due to insufficient blood supply and brain oxygenation. Therefore, monitoring the changes of brain circulation and oxygenation could timely reflect rehabilitation and recovery and the effect of therapy.

Neuroimaging has been shown to be an effective monitoring and therapeutic tool, evaluating the evolution of neural activity and stroke rehabilitation and recovery (6). Traditional methods such as functional magnetic resonance imaging (fMRI), positron emission tomography, electroencephalography (EEG) and magnetoencephalography (MEG)—have provided considerable initial insight into brain changes during recovery. However, several shortcomings including a confining monitoring environment, subject head fixation and high cost have limited applications in tasks which require constant movement or real-time monitor (7, 8).

Near-infrared spectroscopy (NIRS), introduced in 1977 by Jöbsis et al. as a monitoring tool of cerebral and myocardial oxygenation (9), has partially overcome these difficulties. NIRS is a non-invasive neuroimaging tool that has several potential advantages including real-time monitor, low price, simplicity, portability, relatively small equipment, and it's almost completely safe and non-invasive nature (8).

NIRS can be divided into continuous wave NIRS (CW NIRS), time domain NIRS (TD NIRS), and frequency domain NIRS (FD NIRS). CW NIRS emits continuous wave and measures the changes in the intensity of the light that passed through the tissue, whereas TD NIRS utilizes a short pulse of laser light and measures the arrival times of photons emerging from the tissue. FD NIRS records the intensity of the detected light as well as the phase shift. These signals can then be converted to the concentration of oxygenated (oxy-Hb) and deoxygenated hemoglobin (deoxy-Hb). One of the most common used algorithm is Modified Beer–Lambert Law (MBLL). CW NIRS could not measure absolute concentrations of oxy-Hb and deoxy-Hb, because this method assumes a homogenous tissue which is not true. This does not change the results of qualitative analysis, but may lead to error in quantitative outcome. TD NIRS recording the temporal broadening of the pulse as it penetrates the investigated area allows accurate quantification of the concentrations and has better spatial resolution (10). Neural activity increases oxygen demands, thus increasing cerebral blood flow due to neurovascular coupling. NIRS could capture the changes of oxy-Hb and deoxy-Hb to infer changes in brain activity (6, 11). Recent years have witnessed rapid development of the techniques of NIRS from the single-location measurements to two dimensions and then three dimensions. One of its most promising application is in brain-computer interface (BCI) which was firstly introduced by Coyle et al. (12). BCI use brain activity to control external devices bypassing the peripheral nervous system. NIRS presents as a valuable tool in its brain signal acquisition for its non-invasive nature and real-time monitoring. However, fNIRS-BCI system is still mainly used in researches due to slow information transfer rate and low accuracy (13).

Stroke includes two major types: ischemic, due to lack of cerebral blood flow, and hemorrhagic, due to bleeding. During ischemic stroke cerebral blood supply is disrupted by narrowing of vessels caused by atherosclerosis or embolism. In hemorrhagic stroke caused by hypertension or ruptured aneurysm blood flow is reduced due to direct blood loss or vessel compression. In either case, a significant decline in blood supply could be observed. The reduction in cerebral oxygenation results in injury of neurovascular unit, decreased neuronal activity and accumulation of anaerobic metabolites. NIRS could monitor oxygenation signal changes, thus reflecting this pathophysiological process (1).

NIRS is well-established as a safe and effective monitoring tool for stroke recovery, including upper limb, lower limb recovery, motor learning, cortical function recovery (8), cerebral hemodynamic changes, cerebral oxygenation (14), therapy (15, 16), clinical researches and evaluation of the risk for stroke (17). Brain-computer interfaces (BCIs), a newly emerging tool, use brain activity to control external devices, facilitating paralyzed patients to interact with the environment. NIRS combined with BCI offers great potential as a therapeutic tool (18, 19). In addition, NIRS has used in several clinical studies (20, 21), reflecting hemodynamic or oxygenation changes of brain, as well as in evaluation of the risks for postoperative stroke and muscle metabolism (22). NIRS has shown to be an effective and promising method, however, its clinical value still remains controversial (23). Therefore, to address these discrepancies, we conducted this systematic review to summarize the existing application of NIRS in stroke patients.