The onset of both acute and chronic anxiety, as mentioned, is rare before age 18 years or after 35 to 40 years of age (average age of onset 25 years). The condition in some series is twice as frequent in women as in men and there is clearly a high familial incidence. In one study (Wheeler et al) there was a prevalence of 49 percent among the grown children of patients with anxiety neurosis, compared with 5 percent in the general population. Slater and Shields found that there was a concordance rate of 40 percent in identical twins, compared with 4 percent in dizygotic twins. Among the relatives of index cases, the mothers suffered from anxiety neurosis more often than did the fathers; in the latter, alcoholism was more frequent than in the population at large (Modlin). A clear pattern of inheritance has not been established, but it approximates that of autosomal dominance with incomplete penetrance. The psychodynamic theories that attempt to provide a unified explanation of these diverse anxiety states were reviewed by Nemiah and we do not dismiss them but cannot comment authoritatively.
The symptoms of an anxiety resemble those of fear in many ways, although nearly always the former are longer in duration and less distinct. The most important difference, however, is that the cause of fear is known to the patient, whereas that of anxiety is not. The most extreme but not inconceivable interpretation of anxiety is the James-Lange theory of emotion, mentioned earlier, which attributes the psychologic experience entirely to the accompanying physical symptoms.
On the physiologic and biochemical side, it has been observed that anger provokes an excessive secretion of norepinephrine, whereas fear is accompanied by increased secretion of epinephrine. Actually, fear activates the autonomic nervous system as a whole and the increase in epinephrine is more than counterbalanced by a parasympathetic discharge. Attention has been focused on overactivity of the locus ceruleus and upper brainstem nuclei as the possible anatomic substrates of anxiety (Judd et al). Other studies have implicated serotonergic centers. Evidently, the responsiveness of the autonomic nervous system in these patients remains heightened and a number of stimuli (cold, pain, muscular effort) may produce abnormal responses in pulse, respiration, oxygen consumption, and work performance. Another interesting abnormality (noted by Cohen et al) is that the blood lactic acid levels in response to exercise are higher than normal. The presence of these changes does not necessarily mean that they are causal; they are as likely secondary to other factors such as poor physical condition and apprehension associated with the syndrome. Nevertheless, some investigators have found that infusions of lactic acid can trigger panic attacks in persons with anxiety neurosis (Liebowitz et al). Subsequently, numerous other theories of causation have been proposed based on the reported provocation of panic attacks by a number of different substances—carbon dioxide, yohimbine, gamma-aminobutyric acid (GABA), isoproterenol, and others. None provides a comprehensive biologic explanation.
Studies correlating cerebral function and blood flow indicate that when panic is induced by an intravenous injection of sodium lactate, there is an immediate increase in blood flow to the cortex of both temporal lobes. In states of fear, the tips of the temporal lobes and the amygdaloid nuclei are known to become activated. In the relaxed period between panic attacks, the right limbic system and the parahippocampal gyrus are abnormally active in some studies. As with the aforementioned biochemical models, these seem to be more reflections of brain activity in response to the psychic experience than they are explanations. Nevertheless, parts of the limbic system are presumably involved in a germinal way in the production and perpetuation of anxiety and its related states.