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  1. Niall Cartlidge
  1. Dr Niall E F Cartlidge, Department of Neurology, Royal Victoria Infirmary, Queen Victoria Road, Newcastle upon Tyne NE1 4LP, UK nefcartlidge{at}

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Neurologists are increasingly asked to see patients with acute neurological problems, many of whom have disorders of consciousness and awareness. The intensive care unit is a common source of referral of such patients where common questions include:

  • Why isn't the patient waking up?

  • Is the patient going to recover?

  • Is the patient brain dead?

 The neurologist in training needs to have a clear understanding of the basis of normal consciousness, to be aware of the range of disorders of consciousness, and particularly to be aware of the pitfalls in diagnosis.


Consciousness is a state characterised by awareness of self and environment and an ability to respond to environmental factors. Normal consciousness can be regarded as having two separate but closely interrelated components. The first of these is the arousal component of wakefulness. It is this that keeps the patient awake and which relates to the physical manifestations of awakening from sleep—for example, eyes being open, motor activity. The second component is the content of consciousness or the awareness of self and environment. This consists of the sum of psychological functions of sensations, emotions, and thoughts.

A detailed description of the pathophysiology of consciousness is beyond the scope of this article. In simple terms, normal consciousness requires an interaction between the reticular activating system of the brain stem and the cerebral cortex. The reticular activating system is responsible for arousal or alertness and the cerebral cortex is responsible for the content of consciousness or the awareness of self and environment.

It should be recognised that an individual's awareness of self can only be recognised by an observer on the basis of the responses made to …

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