Recent findings present “good evidence that at least some patients in the vegetative state are conscious” (The Lancet, 10th November 2011).
Published Online November 10, 2011 DOI:10.1016/S0140- 6736(11)61591-2 http://www.thelancet.com/
The vegetative state (VS) is a disorder that can be defined as wakefulness without conscious awareness of self and environment.
Researchers typically accept a distinction between the contents and levels of consciousness.
Contents of consciousness are defined as subjective experience— eg, the taste of coffee, feeling of pain.
In the study of levels of consciousness, three distinct stages of degraded consciousness have been described: coma, the vegetative state (VS), and the minimally conscious state (MCS). Differentiation between the stages is based on behavioral criteria. Patients in the vegetative state differ from those in a coma because they can be aroused, yet both groups are considered fully unconscious. Patients in the minimally conscious state are believed to have fluctuating consciousness and are distinguished from the vegetative state when an outside observer (a doctor in most cases) thinks the patient has a minimum understanding of self or the environment (eg, a voluntary attempt to communicate). Other patients with severe brain injury, who are not in the minimally conscious state, are typically believed to be more conscious than minimally conscious patients.
Up to 43% of patients diagnosed as vegetative are reclassified as (at the least) minimally conscious when assessed by experienced teams (Schnakers C, Vanhaudenhuyse A, Giacino J, et al. Diagnostic accuracy of the vegetative and minimally conscious state: clinical consensus versus standardized neurobehavioral assessment. BMC Neurol 2009; 9: 35; Childs NL, Mercer WN, Childs HW. Accuracy of diagnosis of persistent vegetative state. Neurology 1993; 43: 1465–67; Andrews K, Murphy L, Munday R, Littlewood C. Misdiagnosis of the vegetative state: retrospective study in a rehabilitation unit. BMJ 1996; 313: 13–16).
However, a further subset of conscious patients could exist who are undetected even after extensive clinical investigation in specialized centers. Findings from functional neuroimaging studies have called into question several of the core principles that underpin diagnosis of the vegetative state; in particular, the extent to which clinicians can truly consider that a patient is unaware of themselves and their environment simply because they show no overt behavioral responses to external stimulation.
In The Lancet, Damian Cruse and colleagues’ study (Bedside detection of awareness in the vegetative state: a cohort study, published online November 10, 2011 DOI:10.1016/S0140-6736 (11) 61224-5) examines 16 patients in the vegetative state.
The investigators showed that three (19%) of the 16 patients could generate EEG responses to two commands involving motor imagery, although the patients were otherwise behaviorally unresponsive.
But how can this discovery be understood?
With Cruse and colleagues’ study these findings present good evidence that at least some patients in the vegetative state are conscious. However, the methods in all these experiments are indirect and investigate a factor other than consciousness alone.
So far, most researchers have interpreted these published results as suggesting that many patients in the vegetative state are wrongly diagnosed; however, these studies have an even stronger consequence. The real underlying issue is that the levels of consciousness have little to do with consciousness—ie, subjective experience. A more plausible interpretation is that vegetative and minimally conscious states distinguish between different levels of cognitive and communicative abilities, which is a different matter than subjective experience per se.
A new classification system seems to be necessary.