Functional Neuroimaging in Patients With Disorders of Consciousness: Caution Advised.

Abstract

P in patients with disorders of consciousness is a complex clinical problem, which can be impacted by multiple factors. The current approach to the assessment of patients with disorders of consciousness involves standardized behavioral examinations. Functional neuroimaging investigates evidence of neural responses in the absence of behavioral signs. In 2006, the earliest study reporting the use of neuroimaging in a patient with disordered consciousness used a functional magnetic resonance imaging (fMRI) mental imagery task to suggest the presence of residual cognitive capability in a patient diagnosed as being in a vegetative state, now referred to as unresponsive wakefulness state.1 Since then, fMRI2 and electroencephalography (EEG)3 have been used to provide evidence of preserved cognitive processes in patients in varying states of consciousness. These and other studies have helped to define cognitive motor dissociation (evidence of command following during a motor imagery task)4 and higher-order cortex motor dissociation (association cortex responses during language and music stimuli).5 Most recently, the optical neuroimaging technique functional near-infrared spectroscopy (fNIRS) has been used to investigate neural responses in patients with disorders of consciousness.6 fNIRS measures the hemodynamic response to neural activity using a method similar to pulse oximetry. It uses multiple sources and detectors (optodes) to noninvasively measure the absorption of near-infrared light through the skull and produce maps of regional cerebral oxygenation. Like fMRI, fNIRS measures the dynamics of oxygen delivery resulting from localized neuronal activation.7 Simultaneous EEG-fNIRS studies deliver the possibility of a direct bedside measure of neurovascular coupling,8 and multimodal (although not necessarily simultaneous) imaging protocols are now being proposed for use in patients with disorders of consciousness. Kazazian et al9 described a protocol, which will combine fMRI, EEG, and fNIRS studies across the first 10 days postinjury in 350 acutely brain-injured patients, with follow-up imaging at 12 months. The physiological signals measured by fMRI, EEG, and fNIRS as well as other factors relevant to the clinical use of these techniques are shown in Table 1. It is clear that functional neuroimaging technologies are contributing new perspectives to the assessment of disorders of consciousness beyond those accessible through clinical behavioral assessments.10 To date, studies have broadly been limited to clinical settings with the appropriate level of technical expertise and support infrastructure. However, continuing innovations in, and the availability of, low-cost wearable neuroimaging technologies (especially fNIRS) are likely to expand their use when studying patients in the acute and chronic phases of disordered consciousness. It is, therefore, timely to consider the array of context-specific challenges in data acquisition, analysis, and interpretation presented by the advent of this new horizon of functional neuroimaging, and to provide recommendations for addressing these challenges.