Functional near infrared spectroscopy as a probe of brain function in people with prolonged disorders of consciousness

Abstract

Near infrared spectroscopy (NIRS) is a non-invasive technique which measures changes in brain tissue oxygenation. NIRS has been used for continuous monitoring of brain oxygenation during medical procedures carrying high risk of iatrogenic brain ischemia and also has been adopted by cognitive neuroscience for studies on executive and cognitive functions. Until now, NIRS has not been used to detect residual cognitive functions in patients with prolonged disorders of consciousness (pDOC). In this study we aimed to evaluate the brain function of patients with pDOC by using a motor imagery task while recording NIRS. We also collected data from a group of age and gender matched healthy controls while they carried out both real and imagined motor movements to command. We studied 16 pDOC patients in total, split into two groups: five had a diagnosis of Vegetative state/Unresponsive Wakefulness State, and eleven had a diagnosis of Minimally Conscious State. In the control subjects we found a greater oxy-haemoglobin (oxyHb) response during real movement compared with imagined movement. For the between group comparison, we found a main effect of hemisphere, with greater depression of oxyHb signal in the right > left hemisphere compared with rest period for all three groups. A post-hoc analysis including only the two pDOC patient groups was also significant suggesting that this effect was not just being driven by the control subjects. This study demonstrates for the first time the feasibility of using NIRS for the assessment of brain function in pDOC patients using a motor imagery task.

Keywords: (Prolonged) disorders of consciousness; Brain function assessment in disorders of consciousness; Functional near infrared spectroscopy; M1, primary motor cortex; MCS, minimally conscious state; MI, motor imagery; MM, motor movement; SMA, supplementary motor area; SMART, Sensory Modality Assessment for Rehabilitation Technique; UWS, unresponsive wakefulness state; VS, vegetative state; fNIRS, functional near infrared spectroscopy; pDOC, prolonged disorders of consciousness.

Fig. 1

Waveguard cap and fibre arrangement. Caption: (A) Waveguard cap with an array of detectors and light sources and (B) fibre arrangement. The orange dots indicate sources position and the triangles indicate position of detectors. Each source - detector (triangle) pair forms a hypothetical channel (Ch). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Time course of changes in oxyHb (A) and deoxyHb (B) during the motor imagery task in each channel, single subject (subject number 4). Caption: (A) oxyHb and (B) deoxyHb on activation in a single subject to motor imagery task, dashed vertical lines represent start and stop of the task. Solid black lines present the pre and post-stimulus activation median values, red (A) and blue (B) lines present the 50th quantile (solid thick line) and the 10th and 90th quantile (dashed thin lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Time course of changes in oxyHb (A) and deoxyHb (B) during the motor imagery task in each channel, single subject (subject number 4). Caption: (A) oxyHb and (B) deoxyHb on activation in a single subject to motor imagery task, dashed vertical lines represent start and stop of the task. Solid black lines present the pre and post-stimulus activation median values, red (A) and blue (B) lines present the 50th quantile (solid thick line) and the 10th and 90th quantile (dashed thin lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Distribution of concentration changes in oxy and deoxy-haemoglobin. Caption: Distribution of concentration changes in oxy and deoxy-haemoglobin across four categories: vegetative state (VS), minimally conscious state (MCS), controls – motor movement (MM) and controls – motor imagery task (MI). Each dot denotes a subject and bold vertical lines indicate means. The horizontal axis denotes concentration changes expressed as a percentage of change in relation to the pre-stimuli haemoglobin concentration.

Fig. 4

Relative changes of oxy-haemoglobin concentration. Caption: relative changes of oxy-haemoglobin concentration in healthy subjects, minimally conscious state (MCS) and vegetative state (VS) patients over the right and left hemisphere on motor imagery (MI, all three groups) and motor movement (MM, controls only) task. The bars denote mean change and whiskers are for standard error of the mean.