Nov 14 2011

Bedside Test for Consciousness

The diagnosis of coma, specifically persistent vegetative state (PVS), is not as straightforward as it might seem. The current standard of care is to perform a thorough neurological exam in order to assess function. The limitation here, however, is that we are inferring brain functioning from what the patient can do. Some parts of the neurological exam are straightforward, like pupillary reflexes. Reflexes are a more direct way of interrogating the nervous system because the examiner is able to see the response, and therefore the integrity, of one specific pathway.  This is very useful in determining which parts of the nervous system are working and which are damaged.

Coma, however, is a condition of impaired consciousness – so we also need to determine, to the best of our ability, the exact level of consciousness of a patient. This is where it can get a little tricky. The parts of the exam that are most useful for determining level of consciousness are response to various types of stimuli and the ability to follow commands. If I say to a patient (without any non-verbal cues), “show me two fingers on your right hand,” and they then raise two fingers on their right hand, that tells me a lot about their function. They can hear, they can understand language sufficiently to understand the command, they are aware enough to make sense of the command and the context of the exam (they know I want them to do something), and they have the motor pathways necessary to move their fingers on the right hand.

The flip side of that, however, is that if the patient is unable to show me two fingers it could mean that they are deaf, aphasic (impaired language), delirious, or paralyzed in that hand. I cannot know for sure that their lack of response is due to impaired level of consciousness. We can compensate for this somewhat by giving various commands – using various body parts, language that is easiest to understand, and as loud as possible to be heard. But this is only a partial compensation. A patient who is locked-in, for example (conscious but paralyzed below the eyes), would give the same lack of response as someone who is in a PVS.

There is also the potential to overcall non-specific responses. Patients in a PVS still have sleep-wake cycles. They open their eyes and look around, and may grimace or move their limbs. They do not, however, give any specific response to their environment. It is not uncommon, however, for family members, or inexperienced health care providers, to misinterpret non-specific movements as if they were responses to the environment. By chance alone the random behaviors of the patient in a PVS may correlate with something happening in their environment, like someone speaking to them. So to be sure they are really responsive there needs to be a specific and repeatable response to stimuli.

The ability to reliably diagnoses PVS vs other conditions that may mimic PVS is important in order to give an accurate diagnosis and prognosis to family members and caretakers. There is also the possibility that if a person in an apparent PVS actually has some awareness, that it may be possible to communicate with them if we can get access to their consciousness despite their physical limitations. There is therefore the need for more sensitive and specific tests for consciousness, to supplement the neurological exam.

I have written here previously about research into this problem, mainly using two diagnostic technologies – EEG and functional MRI scanning. These are both functional tests that look at brain activity. The theory behind recent research is that perhaps a person in an apparent PVS can have a specific response to a command that we can detect with EEG or fMRI, even though they are unable to signal their awareness with physical movement. In the recent issue of The Lancet is the latest in this line of research.

Cruse et. al. used EEG monitoring on 16 patients who, by clinical exam, are in a PVS, and 12 healthy controls. They asked the subjects to imagine themselves clenching their right hand, and then separately wiggling their toes. In 9 of the 12 healthy control the researchers were able to detect specific EEG responses in their motor cortex to the two distinct commands. That is a bit disappointing in terms of the sensitivity of the test. In 75% of the healthy controls the EEG activity reflected the specific commands they were given, but in 25% the EEG could not detect this.

I find this disappointing because, at present, I feel the most useful application of such diagnostic tests to supplement the exam is to confirm that a patient is truly in a PVS, facilitating decisions about long term care. With a 25% false negative rate, however, a negative test will not sufficiently confirm PVS in order reassure families that they are, for example, making the right decision in limiting care.

In patients in the study in PVS by clinical exam, 3 could “repeatedly and reliably generate appropriate EEG responses to two distinct commands, despite being behaviourally entirely unresponsive.” This result is similar to prior research showing that some patients in apparent PVS show signs of awareness when EEG or fMRI scanning is used.

While we can know for certain what the false negative rate is from this study, because we can know for certain that the healthy controls were, in fact, conscious, we cannot know what the false positive rate is. In other words, we cannot yet be sure what these results mean. It implies that there is more going on in the brains of those three subjects than their exam would indicate, and that’s an interesting result in itself. But we cannot know if that specific brain activity is a true indicator of conscious awareness. There is a great deal of subconscious processing in the brain, and perhaps those processes are disinhibited in a patient with impaired consciousness.

In other words – this and other studies indicate specific brain activity in response to commands, but that does not necessarily mean that subjects have any meaningful awareness of that activity.

The results of this and similar studies still need further validation. It would be nice to go back to the same set of patients and see if the results are reproducible over time, by examiners blinded to the previous results. It would also be a good indicator that the results are meaningful if they correlate to some other marker or predictor of severity of damage. What do the brains of those PVS subjects who showed responsiveness look like? Are there other reasons to think their damage may not be as severe as those in a true PVS, or perhaps they have a different kind of damage.

The only pattern to emerge so far is that patients with traumatic brain injury seem to have a greater chance of showing responsiveness than those with non-traumatic brain injury. This study showed that as well, with 20% of traumatic and 9% of non-traumatic subjects showing a response (but the small number of subjects in the study makes those numbers statistically unreliable). This does make sense, however, because non-traumatic injuries (like anoxic injury – lack of oxygen to the brain) tend to cause damage to the whole brain. Traumatic injury, however, can damage certain parts of the brain while leaving others relatively unharmed, creating the potential for paralysis in excess of loss of consciousness.

Before these new techniques become standard, however, I think that researchers need to indicate that they mean something practical. Specifically the results should make some prediction about prognosis or response to rehabilitation or a specific intervention. If they do not predict anything about outcome, that would call into question the significance of the results. Perhaps the technology is just detecting small but irrelevant differences in the residual brain function of those with serious damage.

While this technology is hopeful and should be pursued, it will likely be years before we understand what the true implications of this research are.


9 responses so far

9 thoughts on “Bedside Test for Consciousness”

  1. banyan says:

    Why can’t you just ask Bill Frist?

  2. Kawarthajon says:

    I don’t understand statistics enough to know what they mean when they say that the “classification accuracy” of the EEG for the patients who showed a response to the instructions was within the 68%-78% range. Could someone help me understand this?

    I wonder if it is a stretch to assume that because the coma patients had this reaction that they are conscious of the statements they heard? Are there any other possible explanations for this? Can people’s brains react in a passive way to environmental stimulus, but not relate to being conscious?

    I also wonder if there are any ways to back this study up, like using an fMRI to assess the same scenario in the same patients that showed signs of being conscious of the instructions?

    These kinds of stories make me think about what it must be like for those poor people, if they are really conscious of the world around them, but unable to respond in any perceivable way.

  3. daedalus2u says:

    I am at Neuroscience 2011 and saw a very interesting poster on the swallowing reflex. I don’t remember the details, but there was an inhibitory reflex that blocked swallowing without an actual bolus of food going down. You can swallow once, or a few times, but there is a limit. You can’t swallow with nothing in your mouth indefinately. I can only do it once.

    The explanation was that because food and air go down the same tube, there has to be very robust protection against food going the wrong way. Reptiles and fish don’t have the same reflexes because (presumably) with their slower metabolism, air isn’t as important.

    Asking a patient to swallow would test a whole different set of nerves.

  4. SARA says:

    As time goes on, we will presumably develop more sensitive and targeted methods of monitoring brain activity. I wonder what brain areas we would want to test in order to prove consciousness and even to rate the level of consciousness? What would those testing methods will be?
    Will we find a way to monitor chemical reactions? Will we drill down our ability to watch the electrical impulses at a minute level – ie – which neurons are firing and to wear?

    I guess I want to know what scientists, who are frustrated by the inability to accurately decipher what these current tests mean, wish for in a “better” machine to monitor the brain activity?

    Will we ever be able to decipher the level of self awareness, of intellectual loss, of emotional pain suffered by someone in that condition?

  5. neilgraham says:

    Presumably an eliminative materialist of the future would have no problem. She would simply throw the whole notion of consciousness into the waste basket at the bottom of the patient’s bed.

  6. Nitpicking says:

    @Daedalus2U, keep in mind that fish don’t breathe with lungs with the obvious exception of lungfish. It’s just not possible for swallowed food in the throat to block their breathing.

    And in reptiles, the trachea and esophagus don’t cross as they do in humans, so they also don’t choke on food.

  7. sonic says:

    Accuracy (in a binary -yes/no situation-) is a measure of correct results divided by the total results–
    In situations that aren’t binary (scalar quantities) the math can get more complex (more or less right answers)- but the concept is the same.

    Without access to the actual article it is impossible to know exactly what they are talking about. I don’t have that access.

    I went to the Coma Science Group for more info– there are lots of articles about the difficulties of diagnosis in coma patients.
    Here is one about diagnostic accuracy and the vegitative state–
    They claim misdiagnosis as high as 43%– (That’s similar to what the article is saying).


  8. Yes, but keep in mind, patients who were not truly in a PVS were in a minimally conscious state (MCS), which is just barely above PVS. They have not demonstrated this matters to prognosis.

    Also, I think it is a stretch at this point to say that the responsive patients have conscious awareness of what is going on (a state we would call locked in). It is much more likely in my opinion that their brains are capable of responding to stimuli without consciousness.

  9. sonic says:

    Dr. N–
    Brains do respond to stimuli without consciousness regularly- don’t they.

    It seems that true confirmation of consciousness would be difficult– One could interview people who ‘woke-up’ after coma– but it still would be difficult to know what they were reporting on.
    The way the test is run it can only find positives– and like you say it isn’t clear positive for what.
    This seems a very conservative test (only positives– false or otherwise). Negatives don’t count as demonstrated by the misses in the ‘known to be conscious’ group.
    Perhaps that is appropriate given the level of unknown in this case.

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