One of my first patients had a panic disorder which did not respond at all to any of the usual treatments (cognitive behavioral therapy, and various medication trials) and it turned out that she had a severe B12 deficiency. Her panic cleared completely with the first B12 injection.
Other patients may have an underlying seizure disorder (e.g., temporal lobe syndrome), adrenal or thyroid dysfunctions, with PMS, postpartum, postmenopausal, and with significant inflammation or infection (which alters brain production of serotonin, dopamine and glutamate). I recall one woman who had chronic inflammation as a result of both a chronic sinus condition and work in a very moldy environment.
Patients with bipolar disorder have a 20% chance of having a panic disorder and panic disorder is associated with increased risk of suicide, major depression and other anxiety disorders.
Patients with panic disorder have a significantly higher rate of gastrointestinal complaints, such as irritable bowel syndrome.
In general most panic disorder patients tend to describe themselves as always having been fearful and shy as children, have discomfort with aggression, and low self esteem.