Sympathetic Acute Stress Disorder is caused by the release of excessive adrenaline and norepinephrine into the nervous system. These hormones may speed up a person's pulse and respiratory rate, dilate pupils, or temporarily mask pain. This type of ASD developed as an evolutionary advantage to help humans survive dangerous situations. The 'fight or flight' response may allow for temporarily-enhanced physical output, even in the face of severe injury. However, other physical illnesses become more difficult to diagnose, as ASD masks the pain and other vital signs that would otherwise be symptomatic.

The DSM-IV specifies that Acute Stress Disorder must be accompanied by the presence of dissociative symptoms, which largely differentiates it from PTSD.

Dissociative symptoms include a sense of numbing or detachment from emotional reactions, a sense of physical detachment - such as seeing oneself from another perspective, decreased awareness of one's surroundings, the perception that one's environment is unreal or dreamlike, and the inability to recall critical aspects of the traumatic event (dissociative amnesia).

In addition to these characteristics, ASD can be present in the following four distinct symptom clusters;

Intrusion symptom cluster

Recurring and distressing dreams, flashbacks, and/or memories related to the traumatic event.

Intense/prolonged psychological distress or somatic reactions to internal or external traumatic cues.

Negative mood cluster

A persistent inability to experience positive emotions such as happiness, loving feelings, or satisfaction.

Avoidance symptom cluster

The avoidance of distressing memories, thoughts, feelings (or external reminders of them) that are closely associated with the traumatic event.

Arousal symptom cluster

Sleep disturbances, hyper-vigilance, difficulties with concentration, easily startled, and irritability/anger/aggression.