According to the Diagnostic and Statistical Manual of Mental Disorders (DSM IV), Delirium is evidenced by:

• Disturbance of consciousness with reduced ability to focus, sustain or shift attention
• Changed cognition or the development of perceptual disturbance
• Disturbance develops in a short period of time and fluctuates over the course of the day
• History, physical examination, and laboratory findings show that delirium can be a physiological consequence of general condition; caused by intoxication; caused by medication; and caused by more than one etiology

The Confusion Assessment Method (CAM) uses four criteria to assess delirium

acute onset and fluctuating course inattention disorganized thinking altered level of consciousness

Delirium is easy to miss when it does not present classically with agitation and hallucinations. Most elderly patients with delirium are lethargic and somnolent. Delirium can be quickly screened for by recognizing its key features: acute onset, fluctuating course, inattention, and either disorganized thinking or altered consciousness. Patients with impaired vision, severe illness, cognitive impairment or dehydration on hospital admission are more likely to develop delirium. Delirium is likelier to develop in hospitalized patients who have physical restraints, weight loss, more than three medications added in a 24-hour period, an indwelling bladder catheter, or an iatrogenic event.