Pain in PDOC

PDOC stands for Prolonged Disorder of Consciousness; the term covers two previously defined states: the vegetative state, and the minimally conscious state. This post considers the question, “Does a person in a prolonged disorder of consciousness experience pain?” This question covers both pain caused by care or treatment, and also pain arising secondary to consequences of nerve damage, such as from prolonged immobility or spasticity. I am frequently asked this question by the families of and clinicians caring for patients in a prolonged disorder of consciousness. In this post I will develop my own thoughts, and arguments. I will stress from the outset that no-one can know definitively, not least because no-one can know definitively what any other person experiences even when conscious. That said, I think we can come to some reasonably secure conclusions.

I started writing this after reading a legal case where the question of how much pain is experienced was a major concern. (here). The case concerned a child aged two, but the considerations are no different. This post will work through an argument step by step. The first step is to consider consciousness – what is it? The Oxford English Dictionary describes consciousness as “the state of being aware of and responsive to one’s surroundings“. This immediately raises two questions:

  • does lack of consciousness imply lack of responsiveness?
  • are consciousness and awareness synonymous?

Consciousness and responsiveness

Being unconscious is not the same as being unresponsive.

The importance of this distinction was brought home to me by the mother of patient who was, in our view, in a prolonged disorder of consciousness. When discussing our clinical findings and giving her the conclusion, that her son was completely unaware, she said “But he can’t be because he moves when I touch him and he opens his eyes when I talk.” We agreed that both her observations were accurate. I suddenly realised that we had been talking at cross purposes. She thought that being unaware meant being completely unresponsive. We though that being unaware meant (or was demonstrated by) a lack of behaviours that required the person to extract abstract meanings from stimuli and/or to required the person formulate abstract goals and show actions which carried meaning.

An unconscious person, and a person who is not aware, nevertheless responds to stimuli. Indeed, the Glasgow Coma Scale measures the depth of coma by measuring how responsive someone is. There are at least six levels of coma (scores 3/15 to 8./15), all being considered as being unconscious. Moreover unconscious people demonstrate may reflex responses such as constriction of pupils to bright light, startle to sudden loud noise, and tendon jerk reflexes.

Therefore it is important to understand that responses to stimuli do not necessarily imply conscious awareness. As will be discussed later, it is the complexity of the response that is important.

Reflex and automatic responses.

It is too easy to say about a response “Thats just a reflex response.” While most people understand the knee-jerk tendon reflex, and the reflex pupillary constriction to bright light, they quite rightly become suspicious if more complex responses are described as reflex. When I accidentally walk into someone, I say ‘Sorry’ and, if asked, I might describe that as a reflex. It is often said without conscious thought; I have apologised to more than one parking meter in London!

There are simple reflex responses which have one or two synapses. In plain English, the signal goes from an input to an output either directly or through one intermediate nerve cell. Tendon jerks are the simplest, withdrawal of a limb in response to pain is another. These can readily be termed reflexes, as they occur without conscious thought or awareness. Nevertheless, even these reflexes can be suppressed or enhanced consciously to a certain extent.

There are many automatic responses which are not normally within direct conscious control, but do involve more nerve cells and pathways than the simple reflex. Example include grimacing (drawing back of the lips) and increased pulse rate in response to pain, sudden startle movements of much on the body in response to a sudden loud noise, looking at an object entering the visual field fast, such as a ball coming towards one.

Thee are many automatic responses, of varying degrees of complexity, up to and including using standard phrases such as “OK thank you” in response to “How are you”. Most are less complex than that, and many can be consciously suppressed. Indeed, the presence of some automatic responses may indicate reduced conscious control over behaviour; the disinhibited behaviour seen in people who have taken too much alcohol would be an example.

Other responses are yet more complex, and usually depend upon a more complex process of analysing the stimulus or stimuli, and planning a response.

Therefore it is important to recognise that some quite complex responses to stimuli are automatic, in that they do not require or depend upon any complex processing of information. They are behaviours that are learned but so built-in that they occur without any conscious thought; consciousness may control them but it does not cause them.

Consciousness

Whole books are written about consciousness from many different perspectives. This brief few paragraphs will consider it from a neurological and practical perspective, sufficient to justify an answer to the question – does someone with a prolonged disorder of consciousness experience pain.

Consciousness is probably a record of what we have done, and the perception that we consciously control our behaviour, and make decisions is probably an illusion. This is widely felt to be the case. Two accessible papers will explain the evidence and how consciousness arises in detail for anyone interested. (here and here) In the meantime, you can consider consciousness as a delayed film, showing ‘you’ what ‘you’ experienced or decided a few milli-seconds ago.

The underlying neurophysiology and neuroanatomy is subject to debate, and there are many theories including one involving quantum theory! (here) For an overview of theories, see here and here.

Consciousness arises, in some way, from the entire brain. People with prolonged disorders of consciousness usually have damage that (a) includes both cerebral hemispheres, and also the thalamic nuclei and other basal ganglia; and (b) is diffuse and widespread. Therefore the damage will disrupt and often will prevent the function of any and all widespread cerebral neural networks.

That a prolonged disorder of consciousness is associated with widespread damage affecting all parts of both cerebral hemispheres is predicted by most of the theories about consciousness arising from the integrative function of neural networks (in the brain). Papers discussing this can be found here and here.

In summary, consciousness depends upon, and arises from large scale networks within the brain that traverse almost all areas, certainly from the basal ganglia to the whole of both cerebral hemispheres. Prolonged loss of consciousness arises from damage to the nerve cells and tracts that form these networks, and this damage must be widespread involving the basal ganglia and hemispheres.

Awareness and consciousness

The Oxford English Dictionary describes awareness as “knowledge or perception of a situation or fact”. In the context of this discussion, stating that a patient is or is not aware will be focused on perception. However it is not quite what is meant. Self-awareness is described as “conscious knowledge of one’s own character and feelings“, which is more focused on insight into one’s own state that it is focused on being aware in contrast to be unaware.

Yet the central question asked about someone who is, in common language, unconscious is “Are they aware of anything?” and this can be refined into three questions: “Is she (or he) aware of what is going on around her?“; and “Is she aware of her own bodily sensations and what is happening to her?“; and “Is she aware of her situation?”

These questions require the patient to recognise sensations as sensations that imply something more, as something requiring analysis, which ends up with the person developing an internal model of their situation and appreciating that they have done so. This is a very complex activity. It is the function of the many networks within the brain that, if not the same as those underlying consciousness, will be very closely related.

The question is perhaps best phrased as “Does the patient have ‘self-awareness’, the ability analyse sensory stimuli and interpret them as experiences with meaning?

Although this is slightly off the point (which is about pain), it leads on to a brief comment on the assessment of awareness. Only a human can decide if another human is aware. There is no unique, single observation, test, or behaviour that proves it. It is a judgement based on the observed or recorded pattern of behaviours.

The questions I ask myself are, “Do these behaviours require the person to abstract meaning from stimuli received? Do these behaviours depend upon the person having an abstract goal or to appreciate that the behaviour carries a meaning? Or are there other more probable explanations? “ As part of this, it is important to see repeated and different examples, to make coincidence and automatic behaviours an unlikely explanation.

My first conclusion is that consciousness and awareness are closely intertwined concepts, and depend upon identical or closely intertwined neural structures and mechanisms. My second conclusion is that a behavioural assessment allows a clinical judgement of the extent of self-awareness, better termed and awareness of him- or her-self as a person. The third conclusion is that this judgement depends upon evidence that the person extracts conceptual meaning from stimuli, through generating specific actions related to that meaning and/or and can form internal conceptual meaning that leads to that achieve a clear purpose.

Experience.

An interesting recent paper explores the nature of ‘subjectivity’, which is equivalent to a person’s ‘experience’. (here) The hypothesis is set out be the author in these words:

“In this paper, a different perspective is adopted: the modes of information processing by neuronal circuits are examined to explore to what extent a subjective and personal perspective might arise as an emergent property of the complex neural networks in the brain. This analysis reveals an intrinsic relational nature of neuronal elaboration; in particular, such relational nature appears to be inherently self-centred; furthermore, all activities in the brain (anything which is sensed/experienced) are analyzed and perceived in terms of their vital, emotional and operative relevance for oneself. This suggests that a subjective dimension (self-centred relational analysis) and a personal perspective (emotional, affective, operative relevance for the self) intrinsically characterize cerebral activity, and are not “added” a posteriori by some subjective observer function (consciousness) to an initially detached, objective representation of reality.

The core ideas are shown in figure 5 of the paper. In essence, the brain breaks down the analysis of any stimulus to determine what the stimulus represents, where in space, relative to the person, the stimulus arises, and the emotional associations attached to the stimulus which influence how strongly the stimulus is memorised. The output of this analysis is a personally-relevant representation which is then further processed by other areas within the cerebral cortex.

Coupled with this there are networks that can increase or decrease the processing of different streams of stimuli, which gives rise to selective attention through altering the content passed on to the cortex.

The paper then suggests that different ‘products’ then arise from the processed stimuli which are intrinsically personalised by the initial processing: knowledge, personal meaning, symbolic representation, and many other higher order phenomena.

The paper is an attempt to understand how subjective experience arises. The details are unlikely to be correct in every detail. The important point made by the paper is that the emergence of many of the aspects of consciousness that we take for granted – a sense of own’s own body, a feeling of being present personally in a place at a time, memory of the past and expectation of a future, imagination etc – all arise from and depend upon very complex processing of data in multiple networks distributed throughout most of the brain. Significant structural damage to these networks will disrupt and usually stop the processing, removing the possibility of subjective experience.

Pain

Long books are written about pain, from many perspectives. These few paragraphs will focus on aspects that are germane to the question. A brief introduction to some of the important ideas and facts can be found here.

Nocioceptive pain reflexes are present in all animals down to single-celled creatures such as amoebae. In animals with nervous systems there are pathways that take information from nocioceptors (organs that detect harmful stimuli) up to the central areas where responses are generated. Many of these responses are automatic; withdrawal from pain, increased heart rate, increased alertness and so on. Response to stimuli associated with tissue damage have been present from the first stages of cellular life. Unless one attributed experience of pain to all living animal cells, the occurrence of automatic, and at times complex responses to nocioceptive (painful) stimuli cannot be taken as proof of awareness or experience.

To reinforce this, all living creatures

Unsurprisingly there is much research into and even more debate about the underlying neuroanatomical, neurophysiological, and chemical basis of the experience of pain. Some important points arise.

The areas of pain that are active when pain occurs are widespread, which led to the concept of a pain matrix. However it is probable that the pain matrix is similar to or the same as the networks that are responsible for analysing all sensory input. (here)

It is also widely recognised, and indeed is common experience, that psychological factors such as emotional state, expectation, fear that pain may arise and son on have a major impact upon the experience of pain. For example, pain is part of depression. (here) No-one doubts that depression is a major cause of pain, yet it is not a result of nocioceptive stimuli arising from any specific part of the body consequent upon tissue damage.

The experience of pain also raises philosophical problems. This is illustrated by asking, “When you hit your thumb with a hammer, where do you feel the pain; in your thumb, or in your brain?” This is an example of a potential mereological fallacy: “ascribing to a part of a creature attributes which logically can be ascribed only to the creature as a whole.” (here) In other words you feel the pain; it is not ‘felt’ or experienced by any one part of you. It arises from you, your whole body. Without a damaged thumb there would be no pain, and without a brain you would not perceive the pain, and without your heart your brain would not function.

I will extract from clinical and experimental studies the main points relevant to the question:

  • there is no single specific cerebral location that ‘feels’ pain or that is always involved;
  • the experience of pain involves emotion, expectation and many other phenomena over and above tissue damage;
  • the areas of the brain usually activated by pain and involved in the experience of pain can be activated in the absence of a nocioceptive input;
  • the experience of pain is associated with widespread cerebral networks not dissimilar to the networks involved in being conscious and being aware.

In summary, the evidence suggests that the experience of pain is closely associated both neuro-anatomically and functionally with the same systems that are needed to generate and sustain consciousness and awareness of self. Consequently, it does not seem likely that damage sufficient to result in a prolonged disorder of consciousness will leave, intact and functioning, the mechanisms for generating the experience of pain.

CONCLUSION

My conclusion is that patients who are in a prolonged disorder of consciousness are very unlikely to experience pain, meaning that it is unlikely that the person has any reflective awareness of himself or herself sufficient to think, ‘this hurts’ or ‘I am suffering pain’. My reasoning is as follows. Lack of consciousness, and hence lack of awareness of yourself as a person with a past, a present, and a future, arises from a failure of networks in the brain that process and analyse incoming stimuli (information). These networks extend through most of the brain, including both cerebral hemispheres and the basal ganglia lying underneath and within the cerebral hemispheres. The experience of pain arises from a similar network, if not the same network. Moreover, experience of any kind is a part of consciousness and awareness of oneself. There is no separate system for pain, different from the system that supports consciousness. Therefore there is no mechanism by which some could possibly experience pain while still being unconscious. The behaviours and movements seen in unconscious people when given or having sensory input that would cause a conscious person to experience pain are automatic behaviours, not requiring and not indicative of consciousness.

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