Do some people with a prolonged disorder of consciousness experience pain? A clinically focused narrative review and synthesis

Objective To investigate the hypothesis that people with a prolonged disorder of consciousness experience nociceptive pain. Method A non-systematic literature review into the nature and neurophysiological basis of consciousness and pain likely function when someone has severe thalamocortical dysfunction; the behavioural manifestations of pain in people who cannot communicate; and how they relate to the experience. Findings Consciousness depends on thalamocortical integrity and is judged clinically by establishing the person's behaviour depends on extracting or using meaning. The experience of pain is also deduced from a person's behaviour, including increased purposeless motor movements, facial expressions, non-verbal vocal expressions and physiological (autonomic) changes such as tachycardia and tear production. Extensive brainstem and midbrain networks are activated by pain, including autonomic networks. Given their early evolution and location, they likely resist damage. The networks appear intrinsically resilient, functioning when damaged unless the damage is severe. Synthesis Someone with a prolonged disorder of consciousness usually has intransitive consciousness (arousal) that is not dependent on cortical cognitive processes and may have retained occurrent consciousness of mental states when aroused. Nociceptive stimuli elicit automatic but purposeless behaviours typically associated with pain. These behaviours are likely to be responses to this unpleasant mental state of occurrent consciousness that is limited to the time they show pain behaviours, with no memory of it. Conclusion The unconscious person with a prolonged disorder of consciousness exhibiting pain behaviours in response to nociceptive stimuli likely experiences pain without analysing its significance; they are unlikely to anticipate or remember it.