We still may not know what causes consciousness in humans, but scientists are at least learning how to detect its presence. A new application of a common clinical test, the positron emission tomography (PET) scan, seems to be able to differentiate between minimally conscious brains and those in a vegetative state. The work could help doctors figure out which brain trauma patients are the most likely to recover—and even shed light on the nature of consciousness.
“This is really cool what these guys did here,” says neuroscientist Nicholas Schiff at Cornell University, who was not involved in the study. “We’re going to make great use of it.”
PET scans work by introducing a small amount of radionuclides into the body. These radioactive compounds act as a tracer and naturally emit subatomic particles called positrons over time, and the gamma rays indirectly produced by this process can be detected by imaging equipment. The most common PET scan uses fluorodeoxyglucose (FDG) as the tracer in order to show how glucose concentrations change in tissue over time—a proxy for metabolic activity. Compared with other imaging techniques, PET scans are relatively cheap and easy to perform, and are routinely used to survey for cancer, heart problems, and other diseases.
In the new study, researchers used FDG-PET scans to analyze the resting cerebral metabolic rate—the amount of energy being used by the tissue—of 131 patients with a so-called disorder of consciousness and 28 healthy controls. Disorders of consciousness can refer to a wide range of problems, ranging from a full-blown coma to a minimally conscious state in which patients may experience brief periods where they can communicate and follow instructions. Between these two extremes, patients may be said to be in a vegetative state or exhibit unresponsive wakefulness, characterized by open eyes and basic reflexes, but no signs of awareness. Most disorders of consciousness result from head trauma, and where someone falls on the consciousness continuum is typically determined by the severity of the injury.
After the PET scans were complete, the researchers calculated how much glucose was being consumed by the most active hemisphere of the patients’ brains relative to the healthy controls. Patients in a state of unresponsive wakefulness had 38% as much metabolic activity as the controls, minimally conscious patients showed 58% as much activity, and people being roused back to consciousness (either from sleep or anesthesia), had 63% of the normal metabolic activity.
The decision to “pull the plug” and remove a loved one from life support is never simple, but the choice could easier if we understood which patients might wake up in time. Although it’s often hard for doctors and family to pinpoint important nuances that separate a lighter state of unconsciousness from a deeper one, the PET scan might help us fill in the gaps.
The PET scan data could correctly distinguish between minimal consciousness and severe unresponsive wakefulness with 89% accuracy, the team reports today in Current Biology. The test also seems to predict an individual’s ability to recover from disorders of consciousness: In a 1-year follow up,eight out of 11 patients whose PET scans showed 41% of normal activity or more (between unresponsive wakefulness and minimal consciousness) had regained consciousness, and the PET scan test predicted 88% of all outcomes correctly.
A simple test that can be used to distinguish between minimal consciousness and unresponsive wakefulness could be a huge boon to health care practitioners. Schiff says that if we can identify patients who are more likely to make a meaningful recovery, we can do a better job surrounding them with highly skilled workers who can give them the best possible care. “If this bears out, as it appears that it will, nobody should probably leave the hospital with a severe injury and a disorder of consciousness without having this measurement.”
In addition to its clinical value, the new study also sheds some light on the nature of consciousness. The predictive power of the PET scan comes from its ability to gauge the brain’s average metabolism. Exactly which regions in the brain are most active does not appear to play a major role in determining a person’s state of consciousness. The researchers suggest that this finding adds to a growing body of evidence that suggests consciousness doesn’t live in any one area of the brain, but is instead an emergent property of many parts working together. Or as Schiff puts it, “Disorders of consciousness are whole brain problems.”