PDOC clinical features
Last updated: February 22, 2025
I have been assessing patients with disordered consciousness since working in neurosurgery in 1976. Initially, most patients were people who soon recovered consciousness. In 1986, I became responsible for the Oxford neurological rehabilitation service at the Rivermead Rehabilitation Centre and, in 1988, community patients at a young disabled unit, Ritchie Russell House. In both services, I saw many people with a prolonged disorder of consciousness (PDOC). In 1995, I started working as an expert advising the High Court when the withdrawal of Clinically Assisted Nutrition and Hydration was being considered; before 2019, all cases required a decision from the High Court. (See my historical review.) Other services also asked me to assess and advise on many patients clinically. Since the change in legal guidance in 2018, I have reviewed many more patients before withdrawing food and fluids. I estimate that I have seen over 1,500 patients since 1986.
I have recently reflected on what knowledge and skills an expert needs. This post considers the main clinical matters I have learned. When I started advising the court in 1995, the vegetative state was considered a definite binary state; a patient was or was not in a vegetative state. The apparent importance of determining this led to increasing attention to clinical features, finding considerable variation, culminating in the misleading claim that 40% of patients were misdiagnosed. My experience since 2018 has revealed so many unexplained PDOC clinical features that I am convinced that we must research clinical features and that this may help in our fundamental understanding of consciousness.
Table of Contents
Introduction
Although people have lived with profound disability due to brain damage for centuries, few survived for long, and anyone left completely unconscious died rapidly from starvation and dehydration. The encephalitis lethargica epidemic after the First World War left many people bordering on unconscious, being akinetic, but able to survive. After the Second World War, healthcare improvements gradually led to more people surviving brain damage with more severe losses, including being unconscious. The numbers and the severity of the losses continue to increase.
Initially, people considered patients to be in a stable and unchanging state of unconsciousness, even though they used the Glasgow Coma Scale to measure degrees of unconsciousness in the acute phase. More recently, some have recognised the variability within and between people categorised as having a prolonged disorder of consciousness. Others have interpreted the variability as misdiagnosis.
Clinical observations of people living in the Twilight Zone may influence the analysis and understanding of consciousness. [Twilight Zone: “a situation or conceptual area that is characterized by being undefined, intermediate, or mysterious” Oxford English Dictionary.] This post will explore some interesting, puzzling, or concerning issues generated by the PDOC clinical features I have seen.
PDOC: the population of interest.
This post concerns people who fall within the definition of a prolonged disorder of consciousness. Two documents provide criteria: the UK National Clinical Guideline (2020) and the original paper defining the minimally conscious state (2002). The criteria are closely aligned, and I will use Table 1.5 of the guideline.
To be considered no longer in a prolonged disorder of consciousness, the person should show repeatedly, consistently (i.e. not occasionally), and persisting over a few weeks:
- Functional use of objects such as a brush, comb, pen, or spoon
- Correct discriminatory choices, for example, between objects, photographs, or people
- Awareness of themselves, for example, accurately stating who they are and how they differ from or relate to others
- Awareness of their environment, for example, accurately reporting where they are, the weather, and the time of day (approximately)
I will not consider people born in this state or entering it before age five. However, I will consider people who enter this state from any cause, including progressive disorders.
Brain function.
As the case below illustrates, many people equate being unconscious with the complete absence of brain function.
The family of a person who had been in a prolonged disorder of consciousness for nine months and who grimaced and groaned when his arms were being stretched were concerned about the use of morphine. When I explained we needed to treat pain, they said, “But we were told he was unaware and had no brain function, so how can he feel pain?” I pointed out that he breathed, maintained his blood pressure, had a sleep-wake cycle, and spontaneously made purposeless movements and that all these functions came from brain activity. They had equated unconsciousness with total loss of brain function, perhaps because the clinician’s phrase (“no brain function”) suggested this.
This assumption is common. In the original UK legal case on withdrawal of treatment, Lord Keith of Kinkel wrote, “There are techniques available which make it possible to ascertain the state of the cerebral cortex, and in Anthony Bland’s case these indicate that, as mentioned above, it has degenerated into a mass of watery fluid.” [See Airedale Hospital Trustees v Bland [1992] UKHL 5 (04 February 1993)]
All people with a prolonged disorder of consciousness will still have living brain tissue, including in the cerebral cortex. This is easily seen in brain imaging, there is electrical activity on the electroencephalogram, and the patient may show sensory evoked potentials. Further, most patients have sleep-wake cycles with eye-opening and closure, and almost all will have spontaneous orofacial and eye movements.
Moreover, well over 50% will show other behaviours, which will be discussed later, such as repetitive stereotypical limb movement, dystonic posturing, localised response to pain, and vocalisation. They will also react differently to stressful stimuli such as pain, cold, and loud noise compared to warmth, soft lighting, and relaxing music.
Lesson:
Although many people think someone with a prolonged disorder of consciousness has no brain function, and some may believe there is no brain tissue, people with a prolonged disorder of consciousness still have significant brain tissue and function. They have an active, living brain that can respond to stimuli, change over time, initiate movements, and adapt to the environment to a limited extent.
Two aspects of consciousness.
Consciousness has many parts, with two main types. The first is intransitive consciousness (Bennett and Hacker). It has no object and refers to arousal or the level of consciousness.
A healthy person is unconscious when sleeping and regains consciousness when aroused naturally by the diurnal cycle or stimulation, such as an alarm. Disturbance of the arousal system may contribute to a person’s prolonged disorder by reducing the duration or level of arousal.
A few people never regain a natural diurnal rhythm, remaining with their eyes permanently closed. This state, which arises from severe damage to the ascending reticular activating system, is called a coma. However, it is still a prolonged disorder of consciousness, albeit one mainly manifesting with absent arousal. The reduction of cortical activity may contribute to the loss of arousal.
Arousal is typically manifested by eye-opening and is usually accompanied by being more reactive to external stimuli. However, one must always consider whether a lack of stimulation leads to a lack of arousal and unconsciousness. Two examples illustrate this.
One patient was assessed by many experts, including me, as being in a prolonged disorder of consciousness at the lower end. We discounted a carer’s reports of more complex reactions until we saw videos of the patient showing pleasure and interacting when outside and once in a car, apparently singing along with music.
Another was a person in a single room in a care home, rarely taken out. Formal structured assessment showed minimal responses, and daily nursing records confirmed this. She regularly attended a one-hour music session with others, but no report was returned to the staff. A video revealed her mouthing words of songs, smiling, interacting, and enjoying herself. At all other times, even with her family, she was unreactive,
The second is transitive consciousness, which refers to the content of consciousness. Cognitive functions in the cerebral cortices account for the second aspect of consciousness, which has two facets: a dispositional, unvarying state in the background, and an occurrent state, the active focus of attention. Most discussion concerns occurrent consciousness, which is what is in your mind at the time. The tests for emergence from a prolonged disorder of consciousness are influenced by arousal but mainly concern occurrent consciousness.
Lessons:
Thus, when assessing a patient, one needs to consider their arousal—how easily they are aroused, how long it lasts without external stimulation, and how it can be facilitated—and their behaviour when aroused—how much it is based on meaning and purpose.
Several consequences arise from the interaction between consciousness, arousal, and stimulation:
- Sedating drugs must be minimised;
- People should be observed in different social and physical contexts;
- Observations must consider the extent and duration of arousal.
Misleading PDOC clinical features.
I will now discuss behaviours that may occur but do not indicate consciousness, which may concern families and clinicians unfamiliar with the condition. Data about the frequency of these misleading PDOC clinical feeatures are unavailable. Any estimates given are based on my impression, a fallible source.
Minor spontaneous, unpredictable, purposeless movements are frequently seen. They include tremulous finger flexion-extension, facial twitches, blinking, eye movements, tongue and jaw movements, head-turning, swallowing, and leg movements. Their central significance is that they are frequently misinterpreted as related to a stimulus, such as a command or an attempt at communication.
More noticeable are myoclonic jerks in the face or limbs, focal epileptic muscle activity, and dystonic, athetoid, or choreiform limb or trunk movements. They have no implications for consciousness. Sometimes, a specific pharmacological treatment might be indicated, including withdrawal of major tranquillisers if they are being used, as they may precipitate dystonia.
Localising behaviours are also common. These include looking at a new, noticeable object moved into a visual field, looking at or turning the head towards a noise, pulling a limb away from pain, and looking or moving away from something. They are a slightly higher (more complex) level of response but do not imply awareness.
Larger-scale limb movements can occur. Some are repetitive, with a stereotypical pattern (all look similar), in which case they usually arise from basal ganglia dysfunction. Spasms, spontaneous or induced, may be seen generally in people with an evident high tone. Purposeless large-scale choreoathetoid movements are relatively rare. One may see movements such as scratching, putting a hand to the face, or moving the leg, arising without any apparent stimulus or purpose.
Many patients vocalise, varying from occasional grunts or groans through more prolonged noises to apparent words unrelated to the context. Many patients swallow their saliva, and some may swallow food or fluids placed in their mouths.
People close to attaining consciousness may occasionally show more complex, higher-level behaviours. For example, one or two skilled therapists might occasionally engage a person in an activity or temporarily achieve a few correct answers to questions. Single, isolated snatches of purposeful movement or speech may occur very infrequently.
Many moderately complex behaviours are best considered automatic, generated by existing motor patterns. The main questions are: does this behaviour require the person to have extracted some meaning from the associated stimulus, or is it directed towards an abstract goal unrelated to contextual information? If so, one should establish whether the behaviour or other similarly complex behaviours occur frequently and consistently over several days. If so, the person may be regaining consciousness.
Lessons.
Families and many staff members interpret almost any movement as willed and evidence of awareness. However, a wide range of movements, some quite complex, are seen in people who are undoubtedly unconscious; they may also be quite frequent. Some are spontaneous, with no evident relationship to external stimuli; others are associated with a stimulus.
The main messages are that:
- A broad range of behaviours may be seen
- None of the usual movements seen require any complex cortical analysis or planning.
- Infrequent, more complex behaviours occur but are compatible with the person remaining in a prolonged disorder of consciousness.
- Most behaviours are best termed spontaneous and automatic, as reflex is too simplistic for many of the movements seen.
Behaviours, feelings & experience.
I did not mention one group of behaviours that may be seen. These are behaviours that typically indicate a person’s emotional state and feelings. Examples include looking puzzled or frightened; behaviours associated with pain, such as groaning, grimacing, or producing tears; similar behaviours typically associated with distress without any coincident, detectable pain-inducing stimulus; and behaviours such as smiling, singing, humming, etc, normally seen in people experiencing happiness or pleasure.
In many patients, behaviours indicative of pain may be seen, usually associated with a pain-inducing procedure. Episodes of apparent emotional distress without pain behaviours and stimuli are rare, but do occur.
One patient I saw was unresponsive to almost all stimuli, with a minimal response to sudden loud noise or bright light shining in his eyes. After entering his room and talking to the person with me, I turned to introduce myself and gave my usual introduction. Immediately, he screwed his face up, groaned and produced a high volume of tears. He was inconsolable for 1-2 minutes, slowly settling. There were no responses to anything else I said or did. Before I left, I said a formal farewell and the same reaction followed.
My interpretation was that my tone of voice or style triggered an episode of distress.
When pain or distress behaviours are observed, two questions arise: Are these behaviours evidence of consciousness, and if not, does the person experience the associated feeling?
Activities typically associated with pain, such as stretching tight muscles, suctioning the mouth and trachea, and many routine care procedures, are often accompanied by groaning, grimacing, increasing other movements, and tearing. The association is frequent and strong enough to accept a causal relationship. Nevertheless, one must ask if the unconscious person experiences something.
In 1991, Michael McQuillen wrote, “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.” In a review of relevant evidence to be published soon, I conclude that the person with a prolonged disorder of consciousness who shows behavioural responses indicative of pain when given a pain-inducing stimulus likely has an aversive feeling and experience.
I also conclude this unpleasant feeling will be forgotten once over, not be anticipated, and not felt in a specific location. It is generated by the brainstem and mid-brain networks and felt by the human being or person.
Feelings of pleasure are also possible, but since there are no pleasure receptors and much pleasure originates from cognitive processes, pleasure is likely infrequent.
Lessons.
The idea that people in a prolonged disorder of consciousness might feel or experience pain is a recent development despite being suggested over 30 years ago. The main lessons are:
- Pain behaviours indicate that some networks are processing nociceptive stimuli
- The person probably experiences an unpleasant feeling when showing pain behaviours
- The experience is unlikely to be remembered or anticipated
- The cause of pain behaviours must be investigated
- Pain-inducing procedures should be avoided or minimised, and the pain must be treated if possible.
Unresolved challenges.
A few PDOC clinical features remain difficult to explain, and one’s interpretation and use of the observation are challenging. A few patients show infrequent but unequivocal episodes of behaviours that require full consciousness but only briefly for a few seconds, never more than a minute.
One patient, after four years in a verified vegetative state, interrupted a conversation by a group of farmers to answer, correctly a question one of them asked. [“Those white ducks. You know the ones I mean. What are they called?” Answer “Aylesbury duck”] For a few seconds, he followed the conversation, understood the question, knew the answer and said it. He returned to his previous vegetative state.
Another patient would sometimes reply to a carer who said, on entering the room, “Good morning Joan, how are you today?” Sometimes the reply was, “Very well, thank you. And how are you?” More usually, there was no response or an unintelligible sound. Occasionally, another phrase was uttered, comprehensible but unrelated to the greeting.
Other patients seem to hover on the border, with behaviours throughout the day. Sometimes, they react understandably to the environment, and at different times, the behaviours have no relation to anything happening. Each episode lasts a few minutes and may only occur every other day, 4-5 times a day.
Variability – unstable networks?
The central feature of a patient’s state and behaviours when in a prolonged disorder of consciousness is the variable and changing nature of PDOC clinical features over time in individual patients and, notably, between patients.
This variability has been overlooked. Only one variable phenomenon has been widely recognised: paroxysmal sympathetic hyperactivity, also known as autonomic storming, autonomic instability, paroxysmal sympathetic dysautonomia, diencephalic seizures, paroxysmal autonomic instability with dystonia syndrome, and other names. It manifests as spontaneous episodes of increased pulse and respiratory rate, increased blood pressure, pupillary dilatation, sweating, and abnormal movements. It is associated with more severe brain damage.
The pathophysiology of the autonomic instability is unknown. Factors precipitating instability include procedures that might cause pain and more benign stimuli, but many episodes appear spontaneously without apparent cause.
It does illustrate that variability occurs even in people with minimal or no consciousness. The brain also continues many other functions, such as breathing, controlling or secreting pituitary hormones, and maintaining cardiovascular parameters.
Prolonged disorders of consciousness arise from the disruption of cortical networks that analyse and process stimuli complexly. However, this does not mean that all other networks are entirely disrupted. The relative preservation of networks controlling respiration, blood pressure, and hormonal levels and the more complex behaviours and movements seen, such as dystonic posturing, show that some networks are still functioning.
Virtually all brain functions are based on networks. Disruption of the very large-scale cortical networks leads to loss of consciousness. However, many networks are less extensive, especially those serving less complex activities. Some networks likely have some residual functional capacity, limited in power or duration.
Therefore, one plausible explanation of the behavioural variability seen in many patients with a prolonged disorder of consciousness is that partially damaged networks fluctuate, sometimes producing more complex behaviours. The more extreme complex behaviours, such as the person who was minimally reactive for four years before answering a question posed in a conversation he could hear and never reacted afterwards, can be seen as statistical outliers; for a brief moment, a network functions at a high level.
Lessons.
Until about 2016, variability in the clinical behaviours of people with a prolonged disorder of consciousness was scarcely acknowledged. Instead, the result of variability was interpreted as a clinical misdiagnosis. With the increasing number of patients being assessed carefully across the much broader spectrum of disordered consciousness, variability has become too familiar to ignore, but no explanation has been proposed.
The lessons I have learned are:
- We have ignored clinical variability up to now
- We have no good evidence on its:
- Nature (frequency, extent etc)
- Causes
- Significance
- My hypothesis that the observed variability arises from instability in damaged neural networks needs development and testing.
- We should pay more attention to routine clinical observations and less to structured, planned, and controlled assessments.
Conclusions.
Writing this page has forced me to consider what I have learned about the clinical characteristics of people with a prolonged disorder of consciousness. My reflection has convinced me that our desire to classify and categorise patients in diagnoses, states, etc, a desire reinforced by management’s desire to classify and control clinical work, leads us to overlook vital observations. There is no logical basis for categorising someone as conscious or unconscious, just as there is no rational basis for classifying a purple colour as either red or blue.
Close observational studies of patients with a prolonged disorder of consciousness, including patients who have recently emerged over a few weeks or months, may give much data to elucidate the nature of consciousness. There is much speculation by philosophers and neuroscientists about the nature of consciousness in healthy people and its emergence in evolution; some data from people who have lost it and, sometimes, recover it slowly might reveal interesting phenomena.
We can already say consciousness is not a binary state; it is a continuous variable. We should not ask if this person is in a prolonged disorder of consciousness, but we should always ask how conscious this person is. Moreover, we must refine that to ask how aroused this person is, for how long, and what level of focused consciousness he has when aroused.
