Consciousness Disorders

Abstract

Consciousness disorders are pathological states in which wakefulness, awareness, or both are impaired by acquired brain injury. This article distinguishes the major disorders along two dissociable dimensions — arousal (wakefulness) and awareness (the contents of experience) — and the states they define: coma, in which neither is present; the vegetative state, now often called unresponsive wakefulness syndrome, in which arousal returns without awareness; the minimally conscious state, in which fluctuating awareness reappears; and locked-in syndrome, in which awareness is intact but motor output is lost. It reviews the Glasgow Coma Scale, the problem of misdiagnosis, the discovery of covert awareness through functional neuroimaging, and the theories and treatments reshaping the field. Three interactive demonstrations explore the arousal-awareness map, the Glasgow Coma Scale, and a complexity measure of consciousness.

Keywords: consciousness disorders, coma, vegetative state, minimally conscious state, locked-in syndrome

Consciousness has two components that ordinary experience fuses but disease can pull apart. The first is arousal or wakefulness — the level of alertness that ranges from coma through drowsiness to full waking, gated by the brainstem and thalamus. The second is awareness — the contents of experience, the perceptions, thoughts, and intentions that the awake brain generates, supported by widespread cortical networks. In the healthy person the two rise and fall together: to be awake is to be aware. The disorders of consciousness are precisely the conditions in which this coupling breaks, so that a patient may be wakeful yet without any demonstrable awareness, or fully aware yet unable to move (Laureys et al., 2004; Bernat, 2006).

These are among the most consequential diagnoses in medicine, because the judgment of whether awareness is present in an unresponsive person governs decisions about treatment, rehabilitation, and the continuation of life-sustaining care. They are also among the hardest, because awareness can only be inferred from behavior, and a brain-injured patient may retain awareness while lacking any reliable way to express it. Much of the modern field is a sustained effort to see past that limit — to detect awareness that behavior conceals, using functional neuroimaging and electrophysiology rather than the bedside examination alone (Owen et al., 2006; Monti et al., 2010).

The National Library of Medicine files consciousness disorders in its Medical Subject Headings under both the neurological manifestations of disease and the neurocognitive disorders, defining them as “organic mental disorders in which there is impairment of the ability to maintain awareness of self and environment and to respond to environmental stimuli.” They are, in other words, a class of disorders — acquired pathologies of the nervous system — and this article treats them as such, describing the injuries that produce them and the clinical states they define.

Types of Consciousness Disorders

In the Medical Subject Headings, consciousness disorders sit beneath the broader headings of neurobehavioral manifestations and neurocognitive disorders, and the classification enumerates a single narrower descriptor directly under it (Table 1). MeSH is an indexing vocabulary built for retrieving the biomedical literature, not a clinical taxonomy, so this formal tree is coarse: it collapses the graded clinical states described in this article — coma, the vegetative state, the minimally conscious state, locked-in syndrome — under the general heading of unconsciousness, which the sections below unpack into the finer distinctions that guide diagnosis and prognosis. The one MeSH child is listed as the classification files it; it is not yet a separate article on this site and so is not linked.

The clinically useful distinctions are drawn not by depth alone but by which of the two dimensions is impaired and for how long. Coma is a state of unarousable unresponsiveness; the vegetative state is arousal without awareness; the minimally conscious state is the reappearance of fluctuating awareness; and locked-in syndrome is not a disorder of consciousness at all but a disorder of output that mimics one. The sections that follow organize the field along these clinical lines rather than the single indexing category above.

Coma and the Anatomy of Arousal

Coma is the deepest disorder of consciousness: a state of unarousable unresponsiveness in which the eyes remain closed, sleep-wake cycles are absent, and no stimulus can rouse the patient to purposeful response. It reflects failure of the arousal system — the ascending reticular activating system of the upper brainstem and its thalamic and basal-forebrain targets — either through direct damage to those structures or through diffuse injury to the cortex they activate (Bernat, 2006). Coma is by definition transient: within days to weeks the surviving brain either recovers, progresses to brain death, or transitions into one of the eyes-open states below, in which arousal has returned even when awareness has not.

The instrument that made coma measurable is the Glasgow Coma Scale, introduced by Graham Teasdale and Bryan Jennett in 1974 to replace vague terms like “stuporous” with a reproducible score (Teasdale & Jennett, 1974). It grades three responses — eye opening (1–4), verbal response (1–5), and motor response (1–6) — and sums them into a total from 3 (no response on any subscale) to 15 (fully responsive). A total of 8 or below conventionally defines coma and severe injury; 9–12 is moderate, and 13–15 is mild. The scale's simplicity and inter-rater reliability made it the global standard for grading acute brain injury, and it remains the first quantitative description most patients receive. The first demonstration lets the reader assign the three subscores and read off the total and its severity band.

Wakefulness Without Awareness

When a comatose patient begins to open their eyes and cycle between sleep and wakefulness but shows no sign of awareness of self or surroundings, the condition is the vegetative state. Arousal has returned — the brainstem has recovered — but the widespread cortical activity that awareness requires has not, so the patient is awake but, as far as any behavioral test can show, not aware (Laureys et al., 2004). Fred Plum and Bryan Jennett coined the term in 1972, and the Multi-Society Task Force later fixed the criteria and the prognostic terminology: a persistent vegetative state is one present at a month, and a permanent one is a probabilistic judgment that recovery is exceedingly unlikely, made at three months after non-traumatic and twelve after traumatic injury (Multi-Society Task Force on PVS, 1994).

Because “vegetative” is both pejorative and theory-laden — it presumes the absence of any inner life, which the field increasingly doubts — a European task force led by Steven Laureys proposed the neutral, purely descriptive term unresponsive wakefulness syndrome (UWS), naming what is observed (wakefulness without responsiveness) rather than asserting what is absent (Laureys et al., 2010). The two terms are used interchangeably today, with UWS increasingly preferred. The vegetative state or UWS is the clearest natural example of the dissociation this article turns on: arousal fully present, awareness absent. The second demonstration places it, and the other states, on the two-dimensional map of arousal and awareness.

Above the vegetative state on the awareness dimension lies the minimally conscious state (MCS), defined by Joseph Giacino and colleagues in 2002 as a condition of severely altered consciousness in which minimal but definite and reproducible behavioral evidence of awareness is present (Giacino et al., 2002). The patient may follow a simple command inconsistently, track a moving object with their eyes, reach for an object, or produce an intelligible word — behaviors that are absent in the vegetative state. The distinction matters because MCS carries a substantially better prognosis, and the boundary is often the difference between continuing and withdrawing rehabilitation. Later refinements split MCS into a lower tier (MCS−, showing only non-reflexive movement such as visual pursuit) and a higher one (MCS+, showing command-following or language), and identified emergence from MCS as the recovery of functional communication or object use (Bruno et al., 2011).

Distinct from all of these is locked-in syndrome, which is not a disorder of consciousness but its mirror image: a lesion of the ventral pons severs the motor pathways while sparing the arousal and awareness systems entirely, leaving a fully conscious patient unable to move or speak, typically able to communicate only through preserved vertical eye movements and blinking. Fred Plum, who also named this condition, stressed the catastrophe of mistaking it for a vegetative state — a fully aware person judged unaware — which is exactly the error the neuroimaging methods below were developed to prevent (Bernat, 2006).

Detecting Awareness the Bedside Misses

The diagnosis of these states rests on inferring awareness from behavior, and that inference is unreliable. When patients carrying a vegetative-state diagnosis are examined with the standardized Coma Recovery Scale-Revised (CRS-R) — a structured protocol developed by Giacino that probes auditory, visual, motor, and communicative function — roughly four in ten prove to be minimally conscious, their preserved signs having been missed by unstructured examination (Schnakers et al., 2009). Misdiagnosis is not a rare mishap but the base rate, and it is why guidelines now insist on repeated, standardized assessment rather than a single bedside judgment (Giacino et al., 2018; Kondziella et al., 2020).

The deeper problem is that some patients are aware but cannot behave at all, and for them a still better examination will never suffice. The landmark demonstration came from Adrian Owen and colleagues in 2006: a patient meeting every behavioral criterion for the vegetative state was asked, in the fMRI scanner, to imagine playing tennis and to imagine walking through her home, and her supplementary motor and parahippocampal areas activated indistinguishably from healthy volunteers — willful, task-appropriate brain activity in a patient who could not lift a finger (Owen et al., 2006). A larger study extended the method into a communication channel, using the two imagery tasks as “yes” and “no” so that a behaviorally unresponsive patient could answer autobiographical questions correctly through brain activity alone (Monti et al., 2010). This cognitive-motor dissociation — covert awareness detectable by neuroimaging in a patient who fails every behavioral test — is one of the most important findings in the field, and it reframes the vegetative diagnosis as a statement about behavior, not necessarily about consciousness.

Measuring Consciousness and Restoring It

If awareness can hide from behavior, the field needs a measure of consciousness that does not depend on behavior at all — and the leading candidate grows out of theory. Integrated information theory (IIT), developed by Giulio Tononi, holds that consciousness corresponds to the capacity of a system to be at once integrated (its parts act as a unified whole) and differentiated (it can take on a vast repertoire of distinct states); either property alone is insufficient (Tononi et al., 2016). The rival global neuronal workspace theory, developed by Stanislas Dehaene and Jean-Pierre Changeux, instead locates consciousness in the broadcasting of information to a frontoparietal network that makes it globally available for report, memory, and control (Dehaene & Changeux, 2011; Mashour et al., 2020). The two theories disagree about mechanism and anatomy, and adjudicating between them is a central project of contemporary consciousness science (Bayne et al., 2016).

IIT yielded a practical tool. The perturbational complexity index (PCI), developed by a group including Tononi and Melanie Boly, perturbs the cortex with a pulse of transcranial magnetic stimulation and measures the algorithmic complexity of the EEG response — high when the brain's reaction is both widespread and differentiated, low when it is local or stereotyped (Casali et al., 2013). Empirically PCI separates conscious from unconscious states across wakefulness, sleep, anesthesia, and brain injury with an approximate cutoff near 0.31, and it detects consciousness in some behaviorally unresponsive patients — an objective index that requires no behavior and no cooperation. The third demonstration builds the intuition, letting the reader vary integration and differentiation and watch a complexity index cross the consciousness threshold.

Detection has begun to enable treatment. Nicholas Schiff and colleagues showed that deep-brain stimulation of the central thalamus — the hub of the arousal system — could produce measurable behavioral improvements in a patient who had been minimally conscious for six years, the first evidence that the residual circuitry of a chronically injured brain can be driven toward greater responsiveness (Schiff et al., 2007). Pharmacological routes exist too, most strikingly the paradoxical arousal that the sedative zolpidem produces in a minority of patients. Systematic reviews now frame prognosis and emerging therapy together, cautioning that recovery from disorders of consciousness is more common and more delayed than once believed, which raises the stakes of both accurate diagnosis and active treatment (Edlow et al., 2021).

Worked Example

The Glasgow Coma Scale is worth computing by hand because its structure explains both its usefulness and its limits. The total is the sum of three independently scored responses: eye opening E (1 = none, 2 = to pain, 3 = to speech, 4 = spontaneous), verbal response V (1 = none, 2 = incomprehensible sounds, 3 = inappropriate words, 4 = confused, 5 = oriented), and motor response M (1 = none, 2 = extension, 3 = abnormal flexion, 4 = withdrawal, 5 = localizes to pain, 6 = obeys commands):

GCS = E + V + M

Consider a patient who opens their eyes only when a painful stimulus is applied (E = 2), makes incomprehensible sounds but no words (V = 2), and withdraws a limb from the painful stimulus without localizing to it (M = 4). The total is GCS = 2 + 2 + 4 = 8. Because 8 lies at the conventional threshold — a score of 8 or below defines coma and severe brain injury — this patient is comatose, and the score of 8 typically triggers airway protection because the patient can no longer be relied on to guard it. Now suppose that a day later the same patient opens their eyes to speech (E = 3), speaks confusedly but is understandable (V = 4), and obeys a simple command (M = 6): the total rises to 3 + 4 + 6 = 13, a mild injury, and the three-point jump in the motor score — from withdrawal to command-following — is the single most prognostically important change, because obeying a command is behavioral evidence of awareness. The worked case also exposes the scale's weakness: an intubated patient cannot be scored verbally, and a locked-in patient obeys no motor command yet is fully conscious, so a low GCS grades the output, not the awareness the later methods in this article were built to detect.

Discussion

The story of the disorders of consciousness is the progressive separation of two things that intuition binds together. Coma research established that arousal has its own brainstem machinery; the vegetative state showed that arousal can return without awareness; locked-in syndrome showed that awareness can persist without any motor output; and cognitive-motor dissociation showed that awareness can persist without behavioral output of any kind, detectable only inside a scanner (Laureys, 2005; Owen et al., 2006). Each step widened the gap between what a patient can do and what a patient may experience, and each made the clinical task harder rather than easier, because every behavioral criterion the field relies on measures the former as a proxy for the latter.

This is why the field has invested so heavily in measures that bypass behavior. The high rate of misdiagnosis under bedside examination (Schnakers et al., 2009), the demonstrations of covert awareness (Monti et al., 2010), and the development of behavior-independent indices like PCI (Casali et al., 2013) are one continuous response to a single problem: consciousness is private, and the injured brain may have lost every ordinary means of making it public. The ethical weight is considerable, since decisions about the continuation of care have sometimes been made about patients later shown to retain awareness — which is why current guidelines treat the diagnosis as provisional and demand repeated, standardized, and where possible instrumented assessment (Giacino et al., 2018; Kondziella et al., 2020).

Current Directions

The most active current effort is to move behavior-independent detection from the research scanner to the bedside and into routine prognosis. Task-based fMRI and EEG paradigms that ask an unresponsive patient to follow commands by modulating brain activity are being standardized and combined, and cognitive-motor dissociation detected early after injury is emerging as a predictor of later recovery, suggesting that covert command-following should inform prognosis rather than merely revealing it (Edlow et al., 2021). Alongside this, resting-state metrics — PCI and related measures of the complexity and integration of spontaneous brain activity — are being validated as diagnostic markers that require no cooperation from the patient at all (Casali et al., 2013; Tononi et al., 2016).

A second direction is the direct test of the competing theories of consciousness against each other and against the clinical data. Adversarial collaborations now pit integrated information theory against the global neuronal workspace using pre-registered predictions, in the hope of resolving which network properties actually track awareness (Bayne et al., 2016; Mashour et al., 2020). The therapeutic frontier is expanding in parallel, from thalamic deep-brain stimulation toward less invasive neuromodulation and pharmacological arousal, on the premise — increasingly supported — that some chronically unresponsive brains retain circuitry that can be driven back toward responsiveness (Schiff et al., 2007; Edlow et al., 2021).

Common Misconceptions

A vegetative patient is in a coma.

They are different states. A comatose patient has neither arousal nor awareness and does not open the eyes; a vegetative (unresponsive wakefulness) patient has recovered arousal — eyes open, sleep-wake cycles present — but shows no awareness (Laureys et al., 2004).

An unresponsive patient is definitely unaware.

Around 40% of patients judged vegetative are found to be minimally conscious on standardized assessment, and functional neuroimaging reveals covert, willful awareness in some who fail every behavioral test — cognitive-motor dissociation (Schnakers et al., 2009; Owen et al., 2006).

Locked-in syndrome is a disorder of consciousness.

It is not: the locked-in patient is fully conscious. The lesion severs motor output while sparing arousal and awareness, so the danger is the opposite one — mistaking a fully aware person for an unaware one (Bernat, 2006).

Glossary

Arousal.

The level of wakefulness or alertness, gated by the brainstem reticular activating system and its thalamic targets; the first of the two dimensions of consciousness.

Awareness.

The contents of conscious experience — perceptions, thoughts, and intentions — supported by widespread cortical networks; the second dimension of consciousness.

Cognitive-motor dissociation.

The condition of a patient who shows no behavioral awareness yet produces willful, command-following brain activity detectable by functional neuroimaging or EEG.

Coma Recovery Scale-Revised.

A standardized behavioral protocol probing auditory, visual, motor, and communicative function, used to distinguish the vegetative from the minimally conscious state.

Coma.

A state of unarousable unresponsiveness with the eyes closed and no sleep-wake cycles, reflecting failure of the arousal system; by definition transient.

Glasgow Coma Scale.

A clinical scale grading depth of coma as the sum of eye (1–4), verbal (1–5), and motor (1–6) responses, from 3 to 15; a total of 8 or below defines coma.

Global neuronal workspace.

The theory that a stimulus becomes conscious when it is broadcast to a frontoparietal network that makes it globally available for report, memory, and control.

Integrated information theory.

Tononi's theory that consciousness corresponds to a system's capacity to be at once integrated (unified) and differentiated (able to take on many distinct states).

Locked-in syndrome.

A state of preserved arousal and awareness with near-total paralysis from a ventral pontine lesion, communication typically limited to vertical eye movements; a disorder of output, not consciousness.

Minimally conscious state.

A condition of severely altered consciousness showing minimal but definite and reproducible behavioral evidence of awareness, such as inconsistent command-following or visual pursuit.

Persistent vegetative state.

A vegetative state present at one month after injury; distinguished from a permanent vegetative state, a probabilistic judgment that recovery is exceedingly unlikely.

Perturbational complexity index.

A behavior-independent measure that perturbs the cortex with transcranial magnetic stimulation and quantifies the algorithmic complexity of the EEG response, with a consciousness cutoff near 0.31.

Unresponsive wakefulness syndrome.

The neutral, purely descriptive term proposed for the vegetative state, naming the observed wakefulness without responsiveness rather than asserting the absence of any inner life.

Vegetative state.

A state of recovered arousal — eyes open, sleep-wake cycles present — with no behavioral evidence of awareness of self or environment.

Key Researchers

Melanie Boly (contemporary). Professor at the University of Wisconsin-Madison; neuroimaging researcher on disorders of consciousness whose work bridges clinical assessment and integrated information theory. ORCID - Google Scholar - Faculty Page

Joseph T. Giacino (contemporary). Professor at Harvard Medical School and Spaulding Rehabilitation Hospital; co-defined the minimally conscious state and developed the Coma Recovery Scale-Revised. ORCID - Google Scholar - Faculty Page

Bryan Jennett (1926-2008). Late professor of neurosurgery at the University of Glasgow; co-created the Glasgow Coma Scale with Graham Teasdale and co-coined the persistent vegetative state with Fred Plum. Wikipedia - Wikidata

Steven Laureys (contemporary). Founder of the Coma Science Group at the University of Liege and researcher at Universite Laval; proposed the term unresponsive wakefulness syndrome and has led the neuroimaging study of residual awareness. ORCID - Wikipedia - Wikidata - Google Scholar

Adrian M. Owen (contemporary). Professor at Western University; demonstrated covert awareness in the vegetative state using fMRI mental-imagery tasks, establishing cognitive-motor dissociation. ORCID - Wikipedia - Wikidata - Google Scholar

Fred Plum (1924-2010). Late professor of neurology at Weill Cornell Medical College; co-author of the classic monograph on stupor and coma and coiner of both “persistent vegetative state” (with Bryan Jennett) and “locked-in syndrome.” Wikipedia - Wikidata

Nicholas D. Schiff (contemporary). Professor of neurology at Weill Cornell Medical College; pioneered central-thalamic deep-brain stimulation in disorders of consciousness and the mesocircuit hypothesis of recovery. Google Scholar - Faculty Page

Graham Teasdale (contemporary). Emeritus professor of neurosurgery at the University of Glasgow; co-creator with Bryan Jennett of the Glasgow Coma Scale, the global standard for grading impaired consciousness. Wikipedia - Wikidata - Faculty Page

Giulio Tononi (contemporary). Professor of psychiatry at the University of Wisconsin-Madison; originated integrated information theory and co-developed the perturbational complexity index as an objective measure of consciousness. ORCID - Wikipedia - Wikidata - Google Scholar

Frequently Asked Questions

What are disorders of consciousness?
They are states in which acquired brain injury impairs wakefulness, awareness, or both. Because consciousness has two dissociable components — arousal and awareness — the disorders are classified by which is affected: coma abolishes both, the vegetative state spares arousal but not awareness, and the minimally conscious state shows partial awareness (Laureys et al., 2004).

What is the difference between coma and the vegetative state?
A comatose patient is unarousable, with the eyes closed and no sleep-wake cycles. A vegetative patient has recovered arousal — the eyes open and sleep-wake cycles return — but shows no behavioral sign of awareness. The vegetative state is therefore wakefulness without awareness (Laureys et al., 2004; Multi-Society Task Force on PVS, 1994).

Why is the vegetative state now called unresponsive wakefulness syndrome?
Because “vegetative” is pejorative and presumes the absence of any inner life. A European task force led by Steven Laureys proposed unresponsive wakefulness syndrome as a neutral term that describes what is observed — wakefulness without responsiveness — rather than asserting what is absent (Laureys et al., 2010).

What is the Glasgow Coma Scale?
It is a clinical scale that grades depth of coma as the sum of three responses — eye opening (1–4), verbal response (1–5), and motor response (1–6) — giving a total from 3 to 15. A total of 8 or below defines coma and severe injury (Teasdale & Jennett, 1974).

How often are these states misdiagnosed?
Frequently. When patients diagnosed as vegetative are examined with the standardized Coma Recovery Scale-Revised, roughly 40% are found to be minimally conscious, their signs of awareness having been missed by unstructured bedside examination (Schnakers et al., 2009).

Can an unresponsive patient be aware?
Yes. In a landmark study, a patient meeting every behavioral criterion for the vegetative state generated willful, task-appropriate brain activity when asked to imagine playing tennis in an fMRI scanner — covert awareness that no behavioral test could detect (Owen et al., 2006; Monti et al., 2010).

Is locked-in syndrome a disorder of consciousness?
No. In locked-in syndrome a brainstem lesion severs motor output while sparing arousal and awareness, so the patient is fully conscious but unable to move or speak, communicating typically through eye movements. The clinical danger is mistaking it for a vegetative state (Bernat, 2006).

Can consciousness be measured objectively?
Increasingly. The perturbational complexity index perturbs the cortex with transcranial magnetic stimulation and measures the complexity of the EEG response, separating conscious from unconscious states without requiring any behavior, with an approximate cutoff near 0.31 (Casali et al., 2013; Tononi et al., 2016).

References