Irene occasionally experiences unpredictable episodes of intense dread accompanied by chest pains and a sensation of smothering. Since her symptoms have no apparent cause, they would probably be classified as indicative of:

Irene must be suffering from some phobia or a panic attack.

Explanation:

Panic attack are sudden instances when a person experiences extreme fear.

During panic attacks, a persons various physical reactions are triggered.

A panic attack has no apparent cause.

Common symptoms include: numbness, loss of control, increased heart rate, feeling of chest congestion, breathlessness etc.

Panic attacks can prove to be fatal.