Acute Stress Disorder is the anxiety and behavioral disturbances that may develop within the first month after exposure to an extreme trauma. Usually, the symptoms begin during or shortly following the trauma. Such extreme traumatic events include rape or other severe physical assault, near-death experiences in accidents, witnessing a murder, or combat. The events of September 11, 2001 are a recent example that meets the criteria for an extreme traumatic event.

Common presenting signs and symptoms of Acute Stress Disorder include: generalized anxiety and hyperarousal, avoidance of situations or stimuli that elicit memories of the trauma, and persistent, intrusive recollections of the event via flashbacks, dreams, or recurrent thoughts or visual images. The symptom of dissociation, which is a perceived detachment of the mind from the emotional state or even the body, is a critical feature for the diagnosis. Dissociation also is characterized by a sense of the world as a dreamlike or unreal place, it may be accompanied by poor memory of the specific events, which in most severe form is known as dissociative amnesia.

Acute Stress Disorder is a diagnosis made by a mental health professional. There are specific diagnostic criteria which must be met before this diagnosis can be made.

A.   The person has been exposed to a traumatic event in which both of the following were present:
1. the person experienced, witnessed, or was confronted with an event or events that involved actual or threatened death or serious injury, or a threat to the physical integrity of self or others
2. the person's response involved intense fear, helplessness, or horror

B.   Either while experiencing or after experiencing the distressing event, the individual has three (or more) of the following dissociative symptoms:
1. a subjective sense of numbing, detachment, or absence of emotional responsiveness
2. a reduction in awareness of his or her surroundings (e.g., "being in a daze")
3. derealization
4. depersonalization
5. dissociative amnesia (i.e., inability to recall an important aspect of the trauma)

C.   The traumatic event is persistently re-experienced in at least one of the following ways: recurrent images, thoughts, dreams, illusions, flashback episodes, or a sense of reliving the experience; or distress on exposure to reminders of the traumatic event.

D.   Marked avoidance of stimuli that arouse recollections of the trauma (e.g., thoughts, feelings, people, conversations, activities, places).

E.   Marked symptoms of anxiety or increased arousal (e.g., difficulty sleeping, irritability, poor concentration, hyper vigilance, exaggerated startle response, motor restlessness).

F.   The disturbance causes clinically significant distress or impairment in social, occupational, or other important areas of functioning or impairs the individual's ability to pursue some necessary task, such as obtaining necessary assistance or mobilizing personal resources by telling family members about the traumatic experience.

G.   The disturbance lasts for a minimum of 2 days and a maximum of 4 weeks and occurs within 4 weeks of the traumatic event.

H.   The disturbance is not due to the direct physiological effects of a substance (e.g., a drug of abuse, a medication) or a general medical condition, is not better accounted for by Brief Psychotic Disorder, and is not merely an exacerbation of a preexisting Axis I or Axis II disorder.

Resources
Acute and Post Traumatic Stress Disorders. Mental Health Report of the Surgeon General, Chapter 4 - Anxiety Disorders. Available at: http://www.surgeongeneral.gov/library/mentalhealth/chapter4/sec2.html 
Long PW. Acute Stress Disorder, American Description. Internet Mental Health. Available at: http://www.mentalhealth.com/dis1/p21-an08.html 
Anxiety Disorders, 308.3 Acute Stress Disorder. Diagnostic and Statistical Manual of Mental Disorders - Fourth Edition (DSM-IV) Washington D. C.: American Psychiatric Association, 1994
Psychologynet. Anxiety Related Disorders - Diagnostic Criteria. http://www.psychologynet.org/stress.html 
American Psychological Association. Warning Signs of Trauma Related Stress. Available at: http://www.apa.org/practice/ptsd.html