Taken together, these anatomic findings indicate that PVS is a state in which the cortex is either diffusely injured or effectively disconnected and isolated from the thalamus, or the thalamic nuclei are destroyed. In either the traumatic or anoxic types of PVS, atrophy of the cerebral white matter leads to secondary ventricular enlargement and thinning of the corpus callosum.
These observations notwithstanding, there is little doubt that the neuroanatomic and neurophysiologic basis of the vegetative state will prove to be complex or at least separable into categories defined by the locus of brain damage. In particular, a striking observation has been made by Owen and colleagues in a 23-year-old woman who had been vegetative for 5 months after a head injury. They observed localized cortical activity in the middle and superior temporal gyri in response to the presentation of spoken sentences that was comparable to the brain activity in normal individuals. Di and colleagues have similarly demonstrated in a limited proportion of vegetative patients that there can be brain activation to the patient’s own name and not to other names. These data suggest that some forms of mental processing can go on during a vegetative state but it is not clear if this situation is representative nor does it provide information about self-awareness, a requisite for consciousness. Further compelling demonstration of cognitive processing in vegetative and minimally conscious patients has been provided by Monti and colleagues by functional MRI. Five of their 54 patients, all with traumatic brain injury but none after anoxic ischemic damage, could willfully modulate focal brain activity by imagining playing tennis (frontal lobe activation) or mentally navigating a familiar place such as their home (temporal lobe activation). In one patient, this activity was used as a means of communication.
At a minimum, these demonstrations emphasize the care that must be taken in establishing diagnoses of PVS and minimally conscious states. Whether these findings with functional imaging simply reflect preserved islands of function in severe brain injury that cannot be examined clinically or whether they require an entire rethinking of the neurologic examination that determines the state of consciousness cannot yet be stated (see editorial by Ropper, 2010).
An additional observation of some consequence is the finding of purported axonal growth over time in a patient with traumatic brain injury who had been in a minimally conscious state (see below) for 19 years and then began to speak and comprehend, while remaining virtually quadriplegic. Voss and colleagues, using sophisticated MRI diffusion tensor imaging, have shown axonal sprouting in the posterior parietal and midline cerebellar regions. They compared the results of tensor imaging to a patient who had been in a minimally conscious state for 6 years without improvement and to 20 normal individuals. Their findings are subject to several interpretations, but axonal growth in the parietal lobes offers a potential explanation for the few instances in which recovery from severe injury does occur. When combined with the findings of Laureys and colleagues, a case can be made for the posterior parietal regions as necessary for integrated consciousness and also raise the possibility that certain islands of limited awareness may be dissociated from global brain function.
It is difficult to predict which comatose patients will later fall permanently into the vegetative or minimally conscious categories. Plum and Posner reported that of 45 patients with signs of the vegetative state at 1 week after onset, 13 had awakened and 5 of these had satisfactory outcomes. After being vegetative for close to 2 weeks, only 1 recovered to a level of moderate disability; after 2 weeks, the prognosis was uniformly poor. Larger studies by Higashi and colleagues have given similar results. As a rough guide to prognosis specifically in head injury, Braakman and colleagues found that among a large group of comatose patients, 59 percent regained consciousness within 6 h, but of those in a vegetative state at 3 months, none became independent. At no time before 3 or 6 months was it possible to distinguish patients who would remain in a vegetative state from those who would die. Further comments regarding recovery are made in the next section on the minimally conscious state.