Panic Disorder (PD) and panic disorder with agoraphobia (PDA) are not homogenous syndromes. The variety of signs and symptoms that can characterize them include the worry of having additional attacks and behavioral alterations to avoid such attacks, such as avoiding driving on freeways or avoiding other situations typically associated with such attacks. The DSM-IV requires at least four of thirteen enumerated somatic or cognitive symptoms, which allows for considerable variance in symptomatic presentation (identical to the point about major depression made in a prior blog). The current nosologic system does not allow for etiologic-based or variability-based variants for either PD or major depressive disorder (MDD). Therefore there is considerable symptomatic variation. I should note before going farther that panic attacks are found in other conditions and a panic attack alone is insufficient to meet criteria for panic disorder (PD).
During the 1960s, 1970s, and early 1980s the prevailing view was that hyperventilation was the immediate cause of panic attacks. In this construction, an individual exhales more carbon dioxide than is metabolically produced, thus causing decreased carbon dioxide levels in the blood which is called respiratory alkalosis and which was hypothesized to cause the panic attack. Unfortunately for the proponents of this theory, studies showed that lowered carbon dioxide levels caused panic attacks in a minority of patients and thus their cause was left open to question.
A few years later in 1993, Briggs et al. began investigating panic symptom variation in an effort to derive panic subtypes. They examined data collected as part of the Cross National Collaborative Panic Study (CNCPS) and found support for a respiratory and non-respiratory types of panic. The respiratory subtype was characterized by difficulty breathing, choking or smothering feelings, chest pain or discomfort, fear of dying, and paresthesias (feelings of pins and needles), as well as endorsement of other panic symptoms. The non-respiratory type of PD was characterized by racing heart, tremors, sweating, and dizziness. I should note that these findings are based on patients seeking treatment and thus does not address the question of whether there is a difference between treatment seekers with PD and those who do not seek treatment. There could be other subtypes not identified by this research.
Now come R. Robertson-Nay and K.S.Kendler, who published the results of their recent research in Psychological Medicine 2011. They used two new analytical and statistical approaches to evaluate panic symptoms across six datasets which included both community samples as well as patients seeking treatment who met criteria for PD or PDA. Their findings supported those of Briggs et al., distinguishing respiratory and non-respiratory subtypes of panic symptoms. Although they found that the respiratory type endorsed a higher rate of symptoms, this does not necessarily mean that it is more severe, nor does it necessarily mean that the causes are different for the two forms. Psychosocial factors, co-morbidities, and longitudinal data were not examined, nor were panic attacks that were part of other disorders. Nonetheless, this is an important step forward and generates more specific questions which should be evaluated to more completely understand the cause and modulating factors affecting symptomatic presentation.