Posttraumatic Stress Disorder can occur after an individual experiences an extreme traumatic event or an event that involves actual or threatened death or serious injury to oneself or someone else.  Fear, helplessness, and/or horror are experienced as a result.  The individual continually re-lives the event via flashbacks, memories, nightmares, and/or intrusive thoughts, images, or perceptions.  Avoidance of associated stimuli, physiological reactivity, psychological distress, and numbing are also present.  Other symptoms include an inability to recall aspects of the trauma, avoidance of  associated stimuli, feelings of detachment, hypervigilance, insomnia, difficulties concentrating, exaggerated startle response, and impaired social and occupational functioning.

The above symptoms must persist for more than one month.  If the symptoms are of shorter duration, then a diagnosis of Acute Stress Disorder is made.

American Psychiatric Association: Diagnostic and Statistical Manual of Mental Disorders, Fourth  Edition, text Revision. Washington, DC American Psychiatric Association, 2000.