A new study suggests that it may not be uncommon for patients who are in a minimally conscious state to be misdiagnosed as being in a persistent vegetative state. The study underscores the necessity of using standardized and objective diagnostic criteria in diagnosing coma. However, it also leaves some important questions unanswered.
As background it is essential to understand a bit about consciousness and coma, for not all comas are created equally. In order to be conscious a person requires at least one hemisphere of the brain be mostly functioning and they require a functioning brainstem. The cortical hemispheres contain the gray matter – that part of the brain that thinks. So it makes sense that a certain minimal amount of gray matter is necessary to generate consciousness. As gray matter is damaged or inhibited from functioning one’s level of consciousness decreases until it descends beyond that fuzzy boundary into unconsciousness. When such unconsciousness is persistent we call that coma.
But interestingly the cortex by itself cannot generate consciousness. It requires constant prodding by a diffuse region in the brainstem (that primitive part at the base of the brain that connects the brain and the spinal cord) called the brainstem activating system. This region sends a constant barrage of electrical signals through the thalamus (the relay center of the brain) and then onto the cortex. Without this constant stimulation the cortex will lapse into sleep and coma.