Consciousness represents the core of our experience as human beings, and its alterations challenge our concepts of self and the meaning or value of life. For the physician, and especially the clinical neuroscientist, these alterations are among the greatest dilemmas, because the disorders that produce them often require swift diagnosis and management based primarily on the physical examination and most probable etiologies. The ancients, at least as far back as the Egyptians at the time of the Edwin Smith surgical papyrus, recognized that the brain was the seat of consciousness. However, the anatomical structures and physiological processes involved in awareness were not elucidated until the middle of the 20th century. The classic studies of Moruzzi and Magoun [1 established the importance of the midbrain reticular formation as the driving force of consciousness. McNealy and Plum y applied these concepts in their clinical observations, which were published in the 1960s, on patients with mass lesions. With some modifications by Ropper in recent years, their discussion of the anatomy of lesions altering consciousness, as amplified in the classic text by Plum and Posner,  remains the foundation for analysis of the patient with altered consciousness. The other major source of insight into mechanisms of consciousness comes from the study of patients with epilepsy, particularly by Penfield and Jasper [5 and by Gloor.  The various theories used to explain loss of awareness during complex partial seizures and absence seizures have helped elucidate the interaction of the cortex and the reticular system.
For the purposes of this book, consciousness describes that set of neural processes that allow an individual to perceive, comprehend, and act on the internal and external environments. It is usually envisioned in two parts: arousal and awareness. Arousal describes the degree to which the individual appears to be able to interact with these environments; the contrast between waking and sleeping is a common example of two different states of arousal. In contrast, awareness reflects the depth and content of the aroused state. Awareness is dependent on arousal, because one who cannot be aroused appears to lack awareness. Awareness does not imply any specificity for the modality of stimulation. This stimulation may be external (e.g., auditory) or internal (e.g., thirst). Attention depends on awareness and implies the ability to respond to particular types of stimuli (modality-specific). M
Many terms that describe gradations of consciousness populate the clinical literature. Stupor refers to a condition in which the patient is less alert than usual but can be stimulated into responding. Obtundation describes a patient who appears to be asleep much of the time when not being stimulated. This eyes-closed state is not electroencephalographic (EEG) sleep, however. Stuporous or obtunded patients respond to noxious stimuli by attempting to deflect or avoid the stimulus. The comatose patient lies with eyes closed and does not make an attempt to avoid noxious stimuli. Such a person may display various forms
of reflex posturing (defined later), but does not actively try to avoid the stimulus. After a period of coma, some patients may enter a vegetative state, in which the patient's eyes open and close, and the patient may appear to track objects about the room and may chew and swallow food placed in the mouth. However, the vegetative patient does not respond to auditory stimuli and does not appear to sense pain, hunger, or other stimuli. This is a state in which there is arousal but no awareness.
Delirium has been redefined in recent years by the psychiatric community through the Diagnostic and Statistical Manual of Mental Disorders. In this publication, delirium is defined as "a disturbance of consciousness that is accompanied by a change in cognition that cannot be better accounted for by a preexisting or evolving dementia. The disturbance develops over a short period of time, usually hours or days, and tends to fluctuate during the course of the day. There is evidence from the history, physical examination, or laboratory tests that the delirium is a direct physiological consequence of a general medical condition, substance intoxication or withdrawal, use of a medication, or toxin exposure, or a combination of these factors." [8
Many other terms have been applied to gradations of consciousness but lack consistent definitions and usage. For this reason, one should eschew their employment and concentrate on clearly describing what the patient does spontaneously and in response to auditory, visual, and somatic stimulation. This discipline results in clearer communication among the clinicians caring for the patient.
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