When a person has feelings of terror that strike suddenly and repeatedly with no warning (a panic attack) she or he may develop panic disorder. Panic disorder is different from a one-time or isolated attack because it is characterized by panic attacks that are repeated and unexpected. The panic attacks occur when the person is not in any real threat or danger.

People who suffer from repeated panic attacks soon learn that they can't predict when a panic attack will happen. They start to worry about when and where the next panic attack will happen. Because of this, they often start to avoid situations where they think they might have a panic attack, or where they think help might not be available if they need it. Avoiding situations like this is also unhealthy since it can develop into what are called specific panic-induced phobias (like a fear of driving) or agoraphobia, which is when a person is unable to go beyond known and safe surroundings because of extreme fear and anxiety. Avoiding places because of agoraphobia can lead to serious disability in panic disorder.

People who develop panic-induced phobias begin to avoid situations that they fear will trigger a panic attack, and their lives may become more and more limited as a result. Their work may suffer because they can't travel or get to work on time. Relationships may become rocky since panic attacks, or even the fear of them, take over the lives of the affected person and those close to them.

Panic disorder may progress to a more advanced stage where the person becomes afraid of being in any place or situation where they think they might not be able to escape, or won’t be able to get help if they have a panic attack. This condition is called agoraphobia. It affects about a third of all people with panic disorder. Sometimes people with agoraphobia can become "housebound" because of their anxiety. Some people with agoraphobia do not have panic attacks and cannot remember a time when they had panic attacks.

Typically, people with agoraphobia fear being in crowds, standing in line, entering shopping malls, and riding in cars or public transportation. Often, these people restrict themselves to a "zone of safety" that may be only their home or the immediate neighborhood. If they try to move past the edges of this zone, they become very anxious. Sometimes a person with agoraphobia is unable to leave home alone, but can travel if accompanied by a particular family member or friend. Even when they restrict themselves to "safe" situations, most people with agoraphobia continue to have panic attacks at least a few times a month.

People with agoraphobia can be seriously disabled by their condition. Some are unable to work, and they may need to rely on other family members, who must do the shopping and run all the household errands, as well as go with the person on rare trips outside the "safety zone." After time, the person with agoraphobia typically leads a life of almost total dependency and great discomfort.

Also, sleep may be disturbed because of panic attacks that occur at night. This may make the person wake up in the middle of the night in terror. The experience is so frightening that some people who have nocturnal (night-time) panic attacks become afraid to go to sleep. This only makes the condition worse because they will suffer from exhaustion. Also, even if there are no nocturnal panic attacks, sleep may be disrupted because of chronic, panic-related anxiety.

The sensations experienced during a panic attack are often mistaken for the symptoms of a heart attack or other life-threatening medical conditions. As a result, doctors often aren’t able to diagnose panic disorder until extensive and costly medical procedures fail to provide a correct diagnosis or relief.

But what causes panic disorder? The next section can help explain.