Box 1. Disorders of consciousness

To be awake is to be in a state of engaging with the environment. To have agency is to interrogate this environment with some goal or purpose. This capacity to engage and interrogate seems to go awry in various disorders that affect cognition. And nowhere is this incapacitation more patent than in coma and related conditions.

Coma. The patient seems to be asleep and cannot be awoken. There is no spontaneous organized behaviour, not even pushing away of an irritant, and there is no evidence of any awareness of sensory cues — no response beyond reflexes mediated by the brainstem and the spinal cord. Most patients in coma do not recover meaningful neurological function, but many do progress to states that are clearly distinguishable from coma, as described below.

Persistent vegetative state (PVS). This is similar to coma in all respects except that, at times, the patient does not seem to be asleep. The eyes may be open, and spontaneous, non-purposeful, roving eye movements occur. PVS is probably the result of a return of some of the functions that would govern the sleep–wake cycle, albeit in the absence of a functioning cerebral cortex¹⁴. As a result, it is only the brainstem and perhaps a few islands of dysfunctional (or disconnected) cortex that regain function.

Minimally conscious state (MCS). In contrast to PVS, patients show occasional signs of arousal and organized behaviour. Nevertheless, for the most part, there is a profound deficit in consciousness. Indeed, functional brain-imaging studies¹⁵ indicate that parts of the cortex may be able to function even when the patient seems to be unconscious. This observation and the differential prognosis of MCS and PVS call for more careful classification of patients, which may currently be biased towards PVS¹⁶. M.N.S. & R.K.