The Science Behind Comas

The brain is the most complex organ in the body. How cool is it that your personality, emotions, feelings, the habits you learn, and memories you store are all because of this mass of tissue containing billions of neurons? The brain does so much for us. However, it is not unstoppable. When the brain goes through a traumatic injury, it can "shut down."  But how exactly does that happen? 

So, imagine a long day of school, then going to sports practice, and then going to a part-time job. Sounds like a long and tiring day, right? After spending lots of time on your feet and exerting a lot of energy your body and mind will want more rest. This might lead you to go to bed earlier or sleep in the next day so that your body can recharge. Sleeping may look similar to a coma, however it is not. A coma is a defined as a state of unarousable unconsciousness in which the alert system of the brain fails to produce the correct stimuli that would jumpstart the motor pathways and systems that allow this person to interact and respond to their surroundings.  

A comatose patient
(NPR)

So why does this happen? Responding to the different stimuli in our environment is a very natural thing but when our body goes through an experience that triggers certain disorders, a coma can result. These disorders that result in comas can include structural brain lesions, metabolic and nutritional disorders, external toxins, infections in our central nervous system, septic illness, seizures, hypothermia and hyperthermia, or a serious trauma. All of these can impede the function of the Ascending Reticular Activating System (ARAS). The ARAS functions as our alert system, and allows to to respond and react to external and internal changes. It is a complex system of pathways that maintains our state of wakefulness and are involved in arousal. When these pathways sustain intense damage, it directly correlates to unconsciousness of sleep.

A simplified representation of the ARAS Pathway via bm-science

A study was done by various researchers with high-level technology to better understand the pathways of ARAS and how it is intertwined with the state of a coma. Simply stated, the ARAS pathway is a huge set of connected nuclei in our brain that does the job of regulating our wakefulness and sleep-wake transitional periods. The scientists knew that disruption of the pathway could result in a coma, but what specific issue causes it? They came to the conclusion that a loss of consciousness was due to a disconnection from the brainstem arousal nuclei present in the thalamus and the basal forebrain, located in the cortex. The thalamus, also known as the 'relay center', stops sending signals to the rest of the brain to keep up with its normal bodily processes. Therefore, an injury serious enough produces this disruption in the pathway and in turn the normal circadian-sleep cycle which produces PVS in many cases. Below is an image that shows the difference in the ARAS pathway systems between a comatose patient and a wakeful one. It is clear that the ARAS pathway is not stimulated at all in the comatose patient. The comatose patient exhibited a decrease in neural connectivity in the upper ARAS pathway, the prefrontal cortices, and the areas marked with blue arrows. 

Difference in ARAS pathway reach in patient with cerebral infractions compared to a patient that sustained no injuries. 

(Impaired Consciousness Due to Injury of Ascending Reticular Activating System Study)

So we know what happens when the ARAS pathway shuts down and we know what it looks like compared to a wakeful, non-comatose patient. This has helped scientists determine that being asleep and being in a persistent vegetative state are very different. 

This article took information from various studies that supported the hypothesis that metabolism and brain waves observed during persistent vegetative state (PVS) differ greatly from the ones observed during sleep. What was their evidence to support this hypothesis? 

They used the science of electroencephalography (EEG) to monitor brain rhythms during different states of unconsciousness. The scientists determined that being in a coma is more similar if you were comparing it to deep sleep, which is when our body is less easily woken up. In this state, our bodies are working slower and our metabolic rate has decreased. The contrast between the EEG rhythms in comas and in sleep is that when we sleep, our body is also going through periods of REM (rapid eye movement) sleep. This type of sleep takes of 20-25% of our time resting. During REM we dream more vividly and our brain is stimulated to a higher level. It is essential in maintaining and creating memories. Scientists have still not definitively concluded as to whether comatose patients experience REM cycles. However scientists have concluded that the vegetative state does not dream or function the same way we do when we sleep. 

Comparison of Brain Waves in Three Different States of Unresponsiveness

(Sleepline)

Generally, the longer a patient is in a coma, the less likely they are to come out of it. The level of a coma is measured by the Glasgow coma scale. The recovery rate of a coma varies. Depending on the length of the unconscious time period or the depth of it, percentages can differ. A paper done by David Bates at the Department of Neurology in Newcastle determined that the likelihood of a good recovery in all comatose patients is only 10%. 

The Glasgow Coma Scale from The George Washington School of Medicine

 Overall, scientists are still finding new ways to look deeper into the millions of pathways and systems our brain has. It is an intricate system and we still have much to discover. As new, more advanced technologies are created, we can begin to understand more and more how the brain works when a traumatic injury is sustained and what we can do to better the percentage of recovery. .