Cerebral Function in Coma, Vegetative State, Minimally Conscious State, Locked-in Syndrome, and Brain Death

Progress in intensive care has increased the number of patients who survive severe acute brain injury. The majority of these patients recover from their coma within the first days after the insult, others will take more time and go through different stages before fully or partially recovering awareness (e.g., minimally conscious state, vegetative state) or will permanently lose all brain functions (i.e., brain death). One of the most challenging problems facing intensivists is understanding the natural history of severely brain injured patients and their possibility of recovery. Clinical practice shows how delicate it is to infallibly recognize unambiguous signs of conscious perception of the environment and of the self in these patients. This complication is reflected in the frequent misdiagnosing of the locked-in syndrome, minimally conscious state and vegetative state [1, 2]. First, objective assessment of residual brain function in severely brain injured patients is difficult because motor responses may be small or inconsistent. This is even more so because consciousness is not an all-or-none phenomenon but part of a continuum [3]. Second, there is a theoretical limitation to the certainty of our clinical diagnosis, since we can only infer the presence or absence of conscious experience in another person [4]. In the present paper, we will first try to define consciousness as it can be assessed at the patient’s bedside. We then review the major clinical entities of altered states of consciousness encountered in the intensive care unit. Finally, we discuss the functional neuroanatomy of these conditions as assessed by positron emission tomography (PET) scanning.