PDOC and pain
Last updated: April 14, 2025
In 2021, in a post on ‘Pain and PDOC’, I concluded, “In summary, the evidence suggests that the experience of pain is closely associated both neuro-anatomically and functionally with the same systems needed to generate and sustain consciousness and awareness of self. Consequently, it seems unlikely that damage sufficient to result in a prolonged disorder of consciousness could leave intact and functioning the mechanisms for generating the experience of pain.” Recently, in 2025, with Andrew Hanrahan, I wrote a paper, “Do some people with a prolonged disorder of consciousness experience pain? A clinically focused narrative review and synthesis.” concluding “someone with a prolonged disorder of consciousness is likely to be experiencing pain when they show typical pain behaviours. The experience is likely to be unpleasant, carry no additional meaning, and not recalled once the nociceptive stimuli end. The nature and quality of this experience will likely differ from that previously experienced when conscious.” In court in 2024, a barrister suggested my change of mind reflected a lack of expertise. I explained that, after further research, I had corrected my earlier misinterpretation of the evidence. The Court of Appeal accepted my position.This post considers what changed my mind and the implications for anyone involved with people with a prolonged disorder of consciousness. (PDOC)
References supporting most of the statements in this post can be found in our paper, “Do some people with a prolonged disorder of consciousness experience pain? A clinically-focused narrative review and synthesis.”
Table of Contents
Introduction: PDOC and pain
Pain is “An aversive sensory and emotional experience typically caused by, or resembling that caused by, actual or potential tissue injury.” It includes both a sensory and an emotional component; both are included in the word feeling, but sensation and emotion make a distinction. Furthermore, its cause can be actual or potential harm. Actual harm is detected by nociceptive (‘pain’) receptors that trigger nerve impulses, whereas potentiality arises from cognitive processes without any actual harm happening.
An unconscious person still has intact nociceptive receptors and their associated primary neural pathways. Therefore, in someone with a prolonged disorder of consciousness, actual harm generates nociceptive input that travels through the peripheral nerves and spinal cord to the brain.
The question is whether an unconscious brain generates:
- an aversive sensation (i.e. an unpleasant feeling) when a nociceptive stimulus happens?
- A memory of the sensation?
- an emotion associated with a nociceptive stimulus?
- an expectation or prediction of harm leading to an aversive sensory and emotional feeling?
- A sensory or emotional experience resembling that of pain without an actual or potential harm being present.
A person may be unconscious secondary to insufficient cerebral cognitive function, inadequate arousal, or both. The typical cause is mainly loss of generalised cognitive function due to widespread thalamocortical damage and dysfunction. Arousal is often reduced but present, as evidenced by the eye-opening sleep-wake cycle. Complete lack of arousal leads to permanent loss of eye-opening (i.e. coma), and coma rarely extends beyond six weeks.
Therefore, a severe reduction in cognitive power is the usual correlate of prolonged disorders of consciousness. As cognition is needed to recognise potential harm and generate a feeling resembling pain, these causes of pain are unlikely to be active.
Thus, this post only considers whether people with a prolonged disorder of consciousness (PDOC) react to a nociceptive stimulus by experiencing an unpleasant sensation (i.e. pain) and an emotional change, usually fear.
Origins of pain
Life is fragile; all living things can be harmed or killed. Plants can only adapt through natural selection of forms that avoid or resist harm; adaptation protects a species but not a plant. Animals can react individually, preventing or reducing damage so the individual survives and, eventually, the species survives. This strategy requires a means of detecting harm.
Animals developed nociceptors very early in evolution. Hydra, the most primitive organism with a nervous system, has heat nociceptors, which are also found in mammals. A harm detector is worthless unless the animal responds quickly, and animals with more complex nervous systems evolved ways for nociceptive signals to gain immediate attention: the stimulus generates a demanding sensation, which persists until the stimulus is reduced.
Nervous systems facilitate learning, which is quicker and more enduring when associated with strong emotions. Nociceptive stimuli likely generate emotions to increase learning from an event.
Thus, evolution probably generated neural networks that responded to nociceptive stimuli by developing a demanding sensation that could not be ignored and an emotion that increased learning about the context and consequences of the stimulus. Humans refer to the sensation as pain and the feeling as fear. The nociceptive sensation is added to the usual somatic sensation associated with the damage, allowing the animal to localise the harm.
As animals developed increasingly complex nervous systems, the drive to reduce the risk and consequences of harm supported the development of more complex processing of nociceptive stimuli, which allowed for better individual survival. Unsurprisingly, neural networks responding to nociceptive stimuli are present throughout the human brain, from the brainstem to extensive networks in both cerebral cortices.
In summary, neural networks activated by nociceptive stimuli permeate the nervous system because quick, effective responses to potential or actual harm significantly increase the probability of an individual surviving and passing on their genes.
Pain in humans
The International Association for the Study of Pain (IASP) recognises three types of human pain:
- Nociceptive; arising from stimulation of nociceptors by tissue injury
- Neuropathic; arising from damaged neural tissue
- Nociplastic; arising without evidence of tissue damage or nociceptor stimulation
However, this classification suffers from the usual challenges associated with categorising biological phenomena: pain is a continuous variable with many factors causing or exacerbating it, and few patients fit exclusively into one type. Nonetheless, it helps identify the likely major contributing factors.
Nociplastic pain has a sizeable psychologically driven component. People with a prolonged disorder of consciousness have minimal cognitive function and are, therefore, unlikely to have nociplastic pain. Neuropathic pain arises from long-standing tissue damage. It may occur in people with a prolonged disorder of consciousness, but as it is often unrelated to any external happening, one has less information about it.
This post mainly concerns nociceptive pain, although the content will probably apply to some neuropathic pain. The evidence and arguments will not apply to nociplastic pain (i.e. chronic pain).
In humans, networks are activated by nociceptive stimuli throughout the brain, including in the brainstem and midbrain, and many nuclei are involved. The hindbrain also has autonomic networks, including an emotional crying network. Nerve tracts link the cerebral hemisphere networks to lower networks, and there is likely a two-way interaction so that the cortex can influence lower centres.
The nociceptive stimulus is only effective by altering behaviour to avoid or minimise tissue damage. Specific behaviours, such as moving away from or removing the source, relate to the localisation and cause of the pain and depend on cognitive and cortical function. Less specific local reactions, such as arm withdrawal when the hand is damaged, are mediated through local reflexes. Furthermore, people in pain may benefit from signalling their pain to others; it might alert others to a danger or elicit support from others.
Some other behaviours arise that depend on cortical function, such as telling others about the pain. Many general behaviours do not rely on the cortex, including groaning, crying out, becoming agitated, crying (producing tears), increasing breathing and heart rates, grimacing etc. These are pain behaviours seen in people who are unconscious or have such severe cognitive impairment that they cannot use language or generate more specific behaviours.
These pain behaviours are all associated with pain in conscious people, significantly increasing when the pain becomes more severe. They are rarely associated with other causes. When observed, a nociceptive stimulus is the likely cause.
Prolonged disorders of consciousness (PDOC)
Consciousness is associated with cognitive function, and almost everyone with a prolonged disorder of consciousness has extensive bilateral hemispheric cortical and white matter damage, including the thalamus. The direct consequence is severe cognitive impairment, usually with minimal or no evidence of higher levels of information processing.
Consciousness depends on arousal. When someone is asleep, they are unconscious, but when they are aroused, consciousness returns. People with extensive brain damage sufficient to render them unconscious frequently have reduced arousal in the early stages. After acute injury, the arousal mechanism recovers sufficiently to reestablish a sleep-wake cycle. Nevertheless, the duration and extent of arousal are typically reduced.
A painful stimulus increases arousal, which is used to measure consciousness, as in the Glasgow Coma Scale. The complexity of the automatic reaction to pain is used to measure the depth of the coma. For instance, an extensor limb response is less complex than a flexion withdrawal reaction.
When someone is unconscious, many nursing and care procedures are needed to maintain health and potential function. Spasticity typically develops over a few weeks, so the requirement to undertake passive movement increases. Other care procedures include oral and tracheal suction, turning in bed and hoisting, inserting urethral catheters, caring for all tubes (e.g., gastrostomy and intravenous lines), and stretching joints to maintain skin integrity.
Many care procedures may cause pain in conscious people and likely generate nociceptive stimuli in unconscious people. Observations on a cohort of people with a prolonged disorder of consciousness demonstrated that most care procedures were associated with pain behaviours.
Pain, behaviour, and experience.
These pain behaviours do not require cortical cognitive function. Autonomic and motor behaviours occur together and end once the stimulus ends. The hindbrain is more resistant to damage, as evidenced by maintaining essential bodily functions controlled by the hindbrain, such as breathing and arousal. Therefore, neural networks will likely generate non-specific pain behaviours in the hindbrain.
In other words, nociceptive stimuli:
- Activate primary neural tracts, delivering stimuli to residual neural networks in the brain
- Hindbrain neural networks are likely still active, but cortical networks are not
- The behaviours associated with nociceptive stimuli are probably generated by the hindbrain neural networks.
The next question concerns the unpleasant feeling generated by nociceptive stimuli. This feeling is not itself localised; localisation is provided by the associated somatosensory stimuli when they reach the cortex. The powerful aversive feeling is an essential part of the protective process and is likely to be associated with other non-specific features associated with pain.
Therefore, the hindbrain neural networks likely generate the unpleasant feeling as an integral part of their output.
Next, in many patients, analgesics such as morphine reduce pain behaviours. This may just e due to a sedative effect, reducing arousal, but it may indicate a specific effect on the feeling.
This hypothesis cannot be proved because the person cannot tell us whether they experience an unpleasant feeling and, if so, what it is like. In 1991, Michael McQuillen asked, “Can people who are unconscious or who are in the persistent vegetative state feel pain?” and he answered, “Although by definition the unconscious patient cannot tell you that he perceives pain, available data suggest that he may; therefore, you cannot know that he doesn’t.”
The remaining philosophical question is, “Who is experiencing this aversive feeling?”
I start by emphasising that no separate mind is associated with the body. The mind is as integrated into the body as the kidneys are. Second, although the brain is essential in generating a mental state, the mental state is not located solely in the brain. The mental state belongs to the body, just as blood pressure belongs to the body, not the heart or any other component.
In a conscious person with an undamaged brain, the self-aware person is associated with high-level cognitive function, memory, and other features primarily arising from cortical function. These aspects of personhood are reduced or lost in someone with a prolonged disorder of consciousness. Nevertheless, the body is still alive, has some residual functioning brain, and is still human.
All one can say is that a human being feels an unpleasant experience. Given the loss of cognitive functions, including memory, it is unlikely that the unpleasant feeling:
- is remembered once the stimulus ends; this requires memory, which will be severely impaired or absent.
- is anticipated before it starts; this depends on memory and cognitive processing which are severely impaired or absent.
- generates fear; this depends on understanding it, another impaired cognitive process.
PDOC and pain: consequences
The primary consequence is that this analysis supports the current clinical approach– if it looks like pain, treat it as pain.
Unconscious patients receive many medical and care procedures, and a significant proportion are likely to cause pain. While no one deliberately undertakes procedures to cause pain, some procedures may be unnecessary. It is easy to carry out ‘routine care’ without critical consideration. One must consider the balance between adhering to a routine and the benefit achieved.
Many procedures are relatively brief and may generate only minor pain behaviours. Although we cannot know the severity of any unpleasant feeling, we can assume a relationship between the extent of pain behaviours and the severity of the pain. When a procedure is associated with more severe pain, one must consider using analgesic drugs prophylactically if possible. Again, the benefits and harms of the drug must be considered.
Next, current and future pain must be considered when discussing a person’s best interests. This applies to all decisions, such as whether to have an operation, but it is vital when considering continuing active medical treatments such as gastrostomy feeding and hydration.
Closely associated, one must recognise that this analysis may affect carers and relatives. Although pain behaviours have consistently been recognised and treated, most people (including me) comforted themselves by believing that an unconscious person could not experience anything. This now seems unlikely. Staff caring for people with a prolonged disorder of consciousness may already be distressed; this analysis may increase that.
Conclusion
The title of this post, “Prolonged Disorders of Consciousness and Pain,” raises two philosophical issues: Who is the unconscious person, and what is pain? It has suggested we consider the patient to be a human who continues to have sensory feelings, at least from painful stimuli, and the associated behaviours and physiological changes identify that pain. The first conclusion follows from a review of evolutionary biology and neural networks activated by pain, and the second from a brief consideration of the nature of personhood, which belongs to the whole body, not any part of it. The outcome of this analysis supports current practice to treat pain behaviours as if they represent pain but strengthens the need to do so. It may also influence decisions made in the person’s best interests.
To answer the initial five questions, the unconscious brain probably:
- Generates an unpleasant sensation when a nociceptive stimulus occurs.
- does not generate
- A memory of the sensation
- An accompanying emotional response.
- An expectation or prediction of nociceptive pain
- An experience resembling pain in the absence of nociceptive stimulation