Consciousness is a subjective experience that encompasses an individual’s personal perception of “what it is like” to be themselves.
Disorders of consciousness (DoC) are medical conditions that affect the level of awareness and responsiveness of a person. They can be caused by various factors, such as brain injury, stroke, infection, or lack of oxygen. DoC can be classified into different categories based on the degree of impairment of consciousness. In this article, we will discuss the main types of DoC, their diagnosis, prognosis, and treatment options.
Types of DoC
According to the American Congress of Rehabilitation Medicine (ACRM), there are three main types of DoC: coma, vegetative state (VS), and minimally conscious state (MCS).
- Coma: Coma is a state of complete unresponsiveness to any stimuli, including pain, sound, or light. A person in a coma does not open their eyes, speak, or move voluntarily. Coma usually lasts for a few days or weeks, after which the person may either die, recover consciousness, or progress to a VS or MCS.
- Vegetative state: VS is a state of wakefulness without awareness. A person in a VS can open their eyes and have sleep-wake cycles, but they do not show any signs of awareness of themselves or their environment. They do not respond to commands, communicate, or have any purposeful behavior. VS can be either persistent (lasting more than a month) or permanent (lasting more than a year for traumatic causes or more than six months for non-traumatic causes).
- Minimally conscious state: MCS is a state of partial or fluctuating awareness. A person in a MCS can show some signs of awareness of themselves or their environment, such as following simple commands, answering yes or no questions, making eye contact, or reacting to emotional stimuli. However, these responses are inconsistent and may vary from day to day or hour to hour. MCS can be further divided into MCS plus (showing higher-level behaviors such as intelligible speech or functional object use) and MCS minus (showing lower-level behaviors such as visual pursuit or localization to pain).
Diagnosis of DoC
The diagnosis of DoC is based on clinical observation and assessment of the patient’s behavior and responses to various stimuli. The most widely used tool for this purpose is the Coma Recovery Scale-Revised (CRS-R), which evaluates six domains: auditory, visual, motor, oromotor/verbal, communication, and arousal. The CRS-R can help differentiate between coma, VS, and MCS, as well as detect signs of recovery or deterioration.
However, clinical assessment can be challenging and prone to error due to factors such as medication effects, fatigue, sensory impairment, or environmental distractions.
- Electroencephalography (EEG): EEG measures the electrical activity of the brain through electrodes attached to the scalp. EEG can reveal the level of arousal and the presence of sleep-wake cycles in patients with DoC. It can also detect covert responses to auditory or visual stimuli that may indicate some level of awareness.
- Magnetic resonance imaging (MRI): MRI uses a strong magnetic field and radio waves to create detailed images of the brain. MRI can reveal structural damage or abnormalities in the brain that may cause DoC. It can also measure brain metabolism and blood flow, which reflect brain function and activity.
- Functional magnetic resonance imaging (fMRI): fMRI is a type of MRI that measures changes in blood oxygen levels in the brain that occur when different regions are activated by a task or stimulus. fMRI can be used to assess the neural correlates of consciousness and identify regions that are involved in processing sensory information, language, memory, or emotions. fMRI can also be used to communicate with some patients with DoC by asking them to imagine performing certain tasks (such as playing tennis or walking around their house) that activate different brain areas.
- Positron emission tomography (PET): PET uses radioactive tracers that are injected into the bloodstream and accumulate in different organs according to their metabolic activity. PET can measure glucose metabolism and oxygen consumption in the brain,
In this context, neuroimaging methods, such as functional MRI and [18F]FDG-PET, have emerged as valuable tools for enhancing diagnosis and decision-making in patients with DoC. This article explores the significance of assessing brain dynamics in understanding consciousness and its implications for the development of new treatments.
Behavioral Assessment and Diagnosis of Disorders of Consciousness
The initial step in evaluating patients with DoC involves behavioral assessments, including responses to sensory stimuli, pain, and simple commands. Wakefulness (level of arousal) and awareness (content of conscious experience) are two fundamental components used to characterize consciousness.
Coma, characterized by the absence of wakefulness and awareness, can be followed by the unresponsive wakefulness syndrome (UWS), where patients regain eye-opening but lack awareness of self and the environment. Some patients progress from UWS to the minimally conscious state (MCS), exhibiting a broader range of non-reflexive behaviors, although functional communication remains impaired.
Brain Dynamics and Loss of Consciousness
Recent research has focused on studying the brain’s dynamical activity as a means to assess different states of consciousness. Studies have shown that loss of consciousness leads to reduced spontaneous neural activity and decreased functional connectivity between brain regions. Functional patterns of shorter duration and more random transitions between them have also been observed during reduced consciousness, indicating altered patterns of fluctuating functional connectivity.
Propagating Perturbations as a Window into Brain Dynamics
Observing how external perturbations propagate through the brain provides insights into brain dynamics and integrity. For example, even during deep sleep or under anesthesia, natural audio-visual stimuli can still be processed in sensory cortices but fail to integrate at higher-level cortical regions.
Artificial perturbations, such as transcranial magnetic stimulation, trigger comparable responses across conditions. However, the propagation of signals declines rapidly during deep sleep, anesthesia, or in patients with DoC. These observations have been used to classify the level of consciousness in patients and during anesthesia. Causal interactions, indicating the directionality of evoked responses, have also been shown to be sensitive to different states of consciousness and offer explanatory power in understanding neural mechanisms.
Assessing Brain Dynamics in Patients with Disorders of Consciousness
This study investigates the capacity of endogenous and exogenous events to propagate throughout the brains of patients with DoC using resting-state functional MRI. The researchers utilized model-free and model-based analyses to examine spontaneous endogenous events within the blood oxygenation level dependent (BOLD) signals and estimate pair-wise effective connectivity between brain regions.
In healthy brains, intrinsic relaxation times of BOLD signals exhibit a heterogeneous map of spatial distributions that become more homogeneous in patients, particularly in the UWS group. The model-based approach allows for the estimation of directional causal relations, simulating the asymmetrical propagation of exogenous perturbations.
The results reveal altered propagation properties in subnetworks within the patients’ brains, with the posterior regions failing to convey information, while broadcasting of information is reduced in subcortical, temporal, parietal, and frontal regions. These findings align with decreased cerebral glucose metabolism observed with [18F]FDG-PET, indicating that patients with prolonged DoC lack the capacity to propagate and integrate events necessary for conscious perception.
Conclusion
Understanding consciousness and assessing brain dynamics in patients with DoC is crucial for improving diagnosis and developing effective treatments. Behavioral assessments remain essential for diagnosis, but neuroimaging methods, such as functional MRI and [18F]FDG-PET, provide valuable complementary information.
By studying brain dynamics and the propagation of perturbations, researchers have identified altered functional connectivity and decreased capacity for information propagation in patients with DoC.
reference link : https://onlinelibrary.wiley.com/doi/10.1002/hbm.26386
