Epidemiology of Anxiety Disorders Introduction
Anxiety disorders have a high impact on daily life (illness intrusiveness) and cause a great deal of suffering for the individual patient (Antony et al., 1998). They also have a substantial impact economically and incur a great deal of expenditure by society as a whole. Greenberg et al. (1999) report a total of $42.3 billion per year as direct and indirect expenses in the USA and there are no obvious reasons to assume that the picture for European countries would be very different (Costa, 1998; Martin, 1998).
In the last decades some large epidemiological studies have provided much information about the occurrence of psychiatric disorders in general and anxiety disorders in particular. The Epidemiologic Catchment Area study (NIMH) and the
National Comorbidity Survey (NCS) in the USA and the Munich Follow-up study in Europe are examples of landmark studies in this field (Regier et al., 1990b; Kessler et al., 1994; Wittchen et al., 1992). The WHO Study on Psychological Problems in General Health Care can be considered an intermediate stage between epidemiological and clinical study, and provides information on prevalence rates of mental disorders in primary care in 14 different countries world-wide (Sartorius et al.,1996).
Many other clinical studies of specific target populations have yielded much information.
Clinical studies often reveal different prevalence rates and comorbidity figures from population-based surveys and this is partly due to selection bias and severity of symptoms, and diagnostic criteria and instruments used. Therefore, Angst et al. (1997) advocate using sub-threshold syndromes to enhance further the validity of diagnostic systems. They state that if sub-threshold syndromes (especially concerning depression and anxiety) were included in diagnostic systems, the coverage of treated cases would be improved by nearly a third.
All excellent existing studies notwithstanding, there are still many reasons to be careful in making clear statements about the prevalence of mental disorders. We are facing different studies conducted in different countries in various settings: findings cannot be easily compared or generalised. Epidemiological studies have often used different diagnostic instruments, different sampling procedures, different case definitions, different time frames for the diagnoses (e.g. lifetime, six-month prevalence or current diagnoses) and different severity ratings for diagnostic decisions (Wittchen et al., 1992). The above reservations show the need for caution when interpreting the results.
Knowledge about prevalence rates of mental disorder does not automatically imply what needs to be done. There is a discrepancy between the real occurrence of disorders and the need for treatment or the possibility of finding the most adequate treatment for a diagnosed condition. Some filters can be taken into consideration.
The first filter is recognition and correct diagnosis by the general practitioner. It is estimated that about 50% of cases do not pass this filter. The second filter is the most adequate treatment of the diagnosed disease, again, which only half of the patients pass. When these two filters are taken together, only roughly about 25% of disordered subjects finally receive adequate treatment. In addition, for some disorders patients do not really need treatment, e.g. most specific phobias can be adequately dealt with by means of avoidance. This means that although specific phobias are much more prevalent than, for example, obsessive-compulsive disorder (11% and 2 - 3% respectively), the obsessive-compulsive disorder should have preponderance.
Anxiety disorders are by far the most common psychiatric disorders (25%), followed by major depression (17%) (Kessler et al., 1994). Lifetime prevalence rates for all anxiety disorders lumped together as found in the NCS are 19.2% for men, 30.5% for women (Kessler et al., 1994). There is a strong correlation between socio-economic status and anxiety disorders. The one-year prevalence as based on ECA data is 12.6% for all types of anxiety disorders, compared with 14.6% lifetime (Regier et al., 1998).
A final important introductory caveat is the issue of comorbidity. Comorbidity between disorders quite dramatically complicates the interpretation of many studies.
Even apart from the considerable comorbidity figures between the anxiety disorders themselves, comorbidity rates between anxiety disorders and depressive disorders are very high (especially panic disorder with agoraphobia, social phobia and obsessivecompulsive disorder), ranging from 30% for co-existing in time to 60% lifetime.
Comorbidity rates between, for example, generalised anxiety disorder (GAD) or post-traumatic stress disorder (PTSD) and other psychiatric disorders are even higher, about 80% for GAD and 90% for PTSD lifetime figures.
In general practice the comorbidity of anxiety disorders and depressive disorders is common, with the happy consequence that the chance of recognition and the likelihood of receiving treatment are increased (Sartorius et al., 1996). However, to complicate this further for the epidemiologist, there is also a substantial comorbidity between several medical conditions (e.g. cardial, pulmonary, cerebrovascular, gastrointestinal, diabetes and dermatological diseases) and anxiety disorders (especially panic disorder, GAD and agoraphobia) (Stoudemire, 1996).
In this chapter the epidemiological findings from some large population-based surveys and some smaller but relevant clinical studies will be reviewed, all DSM-IV anxiety disorders arranged by diagnostic category.