A study by the Multi-Society Task Force on PVS concluded that the outcome from a vegetative state is better in traumatic as compared to nontraumatic cases. J.H. Adams and coworkers have proposed that this reflects differences in the state of thalamic neurons in the two situations. They suggested that after acute hypoxia, neurons subjected to ischemic necrosis are liable to be permanently lost; by contrast, in trauma, the loss of thalamic neurons is more frequently secondary to transsynaptic degeneration following diffuse axonal injury, allowing a greater potential for recovery. Many of these ideas are speculative.
Laboratory features The vegetative state is associated with a grossly abnormal EEG and is characterized by one of a number of abnormal patterns. There may be predominantly low-amplitude delta-frequency background activity, burst suppression, widespread alpha and theta activity, an alpha coma pattern, and sleep spindles, all of which have been described in this syndrome, as summarized by Hansotia (see Chap. 2). One important feature is a lack of the normal change in the background EEG activity during and immediately after stimulating the patient.
In all these clinical states, the profound and widespread dysfunction of the cerebrum is also reflected by extreme reductions in cerebral blood flow and metabolism, measured with positron emission tomography (PET) and other techniques. On the basis of PET studies in a patient with carbon monoxide poisoning, Laureys and colleagues observed that the main difference between the vegetative state and the later state in which the patient has recovered was the degree of hypometabolism in the parietal lobe association area. Anatomic changes in this same cortical region have been implicated in the transition from minimally conscious to a more awake state. The finding in PET studies that noxious somatosensory stimulation fails to activate the association cortices is consistent with the concept that large regions of cortex are isolated from thalamic input or that the critical parietal interpretive areas are isolated from the rest of the cortex.
Of practical value is the observation that the CT and MRI may show progressive and profound cerebral atrophy in cases of vegetative state. In the absence of this atrophy after several months or more, it may be unwise to offer a pessimistic prognosis.
The vegetative state blends into a less severe but still profound dementia that has been termed “minimally conscious state,” wherein the patient is capable of some rudimentary behavior such as following a simple command, gesturing, or producing single words or brief phrases, always in an inconsistent way from one examination to another (see Giacino et al). There is preservation of the ability to carry out basic motor behaviors that demonstrate a degree of awareness. The minimally conscious state is found as a transitional or permanent condition and is sometimes difficult to separate from akinetic mutism discussed further on. Any notion of such a patient’s self-awareness is conjectural, but there may be an impressive array of behaviors and activation of associative regions of the cerebral cortex that suggest that processing of external information is taking place beyond a rudimentary level (see discussion by Bernat). The causes and pathologic changes underlying the minimally conscious state are identical to those of the vegetative state, including the frequent finding of thalamic and multiple cerebral lesion. Viewed from on perspective, the distinction between them is one of degree.
It is useful to maintain a critical view of reports of remarkable recuperation after months or years of prolonged coma or the vegetative state. When the details of such cases become known, it is sometimes evident that recovery might reasonably have been expected. There are, however, numerous reported instances of partial recovery in patients—particularly children and young adults—who display vegetative features for several weeks or, as Andrews and Childs and Mercer describe, even several months after injury. Such observations cast doubt on unqualified claims of success with certain therapies, such as specialized sensory stimulation. Nevertheless, the rare occurrence of late recovery in adults must be acknowledged (see Andrews; Higashi et al; and Rosenberg et al, 1977) and a relation of awakening to the recovery of connections to the parietal lobes has already been mentioned.