Altered mental status (AMS) is a general term used to describe various disorders of mental functioning that can range from slight confusion to coma.   Given the vagueness of the term, it is imperative to understand its key components before considering a differential diagnosis. Fundamentally, mental status is a combination of the patient's level of consciousness (i.e., attentiveness) and cognition (i.e., mental processes or thoughts); patients may have disorders of one or both.  For example, patients with meningitis may have impaired consciousness (i.e., altered sensorium, decreased attentiveness) with intact cognition, whereas patients with dementia may have a normal level of consciousness with impaired cognition. However, more frequently, patients exhibit altered levels of consciousness plus cognition: for example, with delirium, a relatively common and sometimes fatal cause of AMS.
Studies conducted in emergency departments have observed that AMS is the primary reason for the visit in about 4% to 10% of all patients.   AMS is more common in certain types of patients. For example, up to 25% to 30% of older patients present to the emergency department with AMS.   Importantly, a diagnosis of AMS can be difficult to unravel in an older patient with dementia, given that dementia is one of the primary risk factors for delirium.  The prevalence of dementia is approximately 1% at the age of 60 years, and doubles every 5 years, reaching a plateau at around age 85 years between 30% and 50%. 
Normal state of consciousness consists of either the state of attentiveness in which most people function while not asleep, or one of the recognised stages of normal sleep from which the person can be easily aroused. Abnormal state of consciousness is more difficult to categorise, and many terms are used. Some of the more common terms include:
Hyper-alert: heightened arousal with increased sensitivity to immediate surroundings. Hyper-alert patients can be verbally and physically threatening, restless, and/or aggressive. 
Confused: disorientated; bewildered, and having difficulty following commands.
Delirious: disorientated; restless, hallucinating, sometimes delusional.
Somnolent: sleepy, responding to stimuli only with incoherent mumbles or disorganised movements.
Lethargic: reduced level of alertness with decreased interest in the surrounding environment. 
Obtunded: similar to lethargy; the patient has a lessened interest in the environment, has slowed responses to stimulation, and tends to sleep more than normal, with drowsiness in between sleep states.
Stuporous: profoundly reduced alertness and requiring continuous noxious stimuli for arousal. 
Comatose: state of deep, unarousable, sustained unconsciousness. 
Assistant Professor of Medicine
University of Massachusetts Medical School
GB declares that he has no competing interests.
Dr Gary Blanchard would like to gratefully acknowledge Dr David Dosa, a previous contributor to this topic. DD declares that he has no competing interests.
Assistant Clinical Professor of Medicine
Division of Neurology
Duke University Medical Center
TC has worked as a paid speaker for GlaxoSmithKline and Pfizer in 2008 and 2009.
Consultant in Old Age Psychiatry
Mental Health Unit
KA declares that he has no competing interests.
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