A narrative review presents a structured, physiology-based diagnostic strategy for patients with coma of unknown origin. Coma is defined as a state of unarousable unresponsiveness characterised by the complete absence of both arousal and awareness. It represents one of the most urgent and diagnostically challenging conditions in acute care, particularly when there is no obvious cause such as major trauma, cardiac arrest, or large territorial stroke. Prompt and systematic diagnosis is essential because many causes of coma are reversible, and delays in recognition can lead to preventable morbidity and mortality.
The review is grounded in current understanding of the neurobiology of consciousness. Consciousness depends on two interacting components: arousal, mediated by the ascending reticular activating system (ARAS) in the brainstem, and awareness, supported by frontoparietal cortical networks and their connections with the thalamus. Coma can result either from structural lesions that damage these systems directly, such as pontine haemorrhage or bilateral thalamic infarction, or from diffuse functional suppression caused by metabolic disturbances, toxins, sepsis, or endocrine disorders. The authors emphasise the concept of the “mesocircuit”, a network involving the thalamus, striatum, globus pallidus, and frontal cortex. Dysfunction within this circuit is increasingly recognised as a common final pathway for many forms of coma and may become a target for future therapies.
The proposed diagnostic algorithm begins with immediate stabilisation according to airway, breathing, and circulation principles. The airway should be secured when protective reflexes are absent. Ventilation should be assessed and normocapnia maintained, while haemodynamic instability and hypoglycaemia must be corrected without delay. Simultaneously, clinicians should obtain a focused history from family members, witnesses, and medical records, looking for clues such as intoxication, infection, seizures, or systemic organ failure.
The neurological examination remains the cornerstone of diagnosis. The authors recommend a structured assessment of consciousness level, pupillary and corneal reflexes, eye movements, motor responses, and respiratory patterns. While the Glasgow Coma Scale remains widely used, the Full Outline of UnResponsiveness (FOUR) score is preferred in critically ill and intubated patients because it incorporates brainstem reflexes and respiratory drive. Coma mimics such as locked-in syndrome, catatonia, and psychogenic unresponsiveness should be considered. Because consciousness can fluctuate and sedative drugs may obscure awareness, repeated examinations are essential, initially every 15–30 minutes and later every 2–4 hours once the patient is stable.
The review identifies several immediately reversible causes of coma, including hypoglycaemia, severe hyperglycaemia, Wernicke’s encephalopathy, electrolyte abnormalities, respiratory failure, opioid or benzodiazepine overdose, non-convulsive status epilepticus, bacterial meningitis, adrenal crisis, and myxoedema coma. Recognition and targeted treatment of these conditions are crucial to prevent irreversible brain injury.
Electroencephalography (EEG) is recommended early in all unexplained coma. Its principal role is detection of non-convulsive seizures and non-convulsive status epilepticus, conditions that are common in critically ill patients and often clinically silent. Continuous EEG may be required for prolonged monitoring, guided by seizure-risk tools such as the 2HELPS2B score. Beyond seizure detection, EEG provides insight into thalamocortical network integrity. The ABCD EEG classification ranges from severe delta-dominant suppression (pattern A) to normal alpha and beta rhythms (pattern D), reflecting progressive restoration of network connectivity and improving chances of recovery.
Neuroimaging is another central diagnostic tool. Non-contrast computed tomography (CT) is the first-line investigation for suspected structural causes because it rapidly identifies haemorrhage, hydrocephalus, mass lesions, and large infarcts. However, a normal CT does not exclude serious pathology such as early ischaemic stroke, posterior reversible encephalopathy syndrome, venous sinus thrombosis, encephalitis, or diffuse axonal injury. Magnetic resonance imaging (MRI), particularly diffusion-weighted imaging, is more sensitive for posterior fossa lesions, subtle cortical abnormalities, and diffuse injuries. Advanced diffusion imaging may also aid neuroprognostication.
Laboratory testing follows a stepwise approach. Immediate investigations include capillary glucose, arterial blood gas, electrolytes, renal and liver function tests, full blood count, coagulation profile, and blood cultures. Plasma ammonia is highlighted as an important second-line test because hyperammonaemia may indicate liver failure or occult urea cycle disorders. Depending on clinical suspicion, endocrine studies (thyroid function and cortisol) and broad toxicology screening should be added.
Lumbar puncture and cerebrospinal fluid (CSF) analysis are essential when central nervous system infection or inflammatory disease is suspected. Routine CSF analysis includes opening pressure, cell count, glucose, protein, lactate, Gram stain, and culture. Multiplex polymerase chain reaction panels can rapidly identify common pathogens, while paired serum and CSF antibody testing is recommended when autoimmune encephalitis is considered. The absence of CSF pleocytosis does not exclude viral or autoimmune disease, and repeat lumbar puncture may be necessary if suspicion remains high.
There are also emerging technologies that can transform coma care. Functional MRI and EEG can detect cognitive motor dissociation, revealing covert consciousness in behaviourally unresponsive patients. Portable MRI, automated pupillometry, near-infrared spectroscopy, blood biomarkers, wearable sensors, and artificial intelligence may allow continuous monitoring and more precise prognostication. The authors envision a shift from broad clinical phenotypes towards biologically defined “endotypes”, enabling personalised, mechanism-based treatment strategies.
The review proposes a practical and iterative diagnostic framework for coma of unknown origin. By integrating clinical examination, EEG, neuroimaging, laboratory testing, and CSF analysis within repeated reassessment loops, clinicians can rapidly identify reversible causes, localise brain dysfunction, and improve outcomes. The authors emphasise that understanding coma as a disorder of disrupted brain networks provides a foundation for future precision neurocritical care.
Source: Intensive Care Medicine
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