3.2 Trajectory of illness from anxiety to depression and comorbidity with other conditions

Kupfer and colleagues (2012) have separately pointed out that 'major depressive disorder was assumed to precede generalized anxiety disorder until a 32-year prospective follow-up study challenged this notion', citing the work of Moffitt et al. published in the Archives of General Psychiatry in 2007, and that 'the reverse pattern seems to be frequently present', 'the combination of generalized anxiety disorder and major depression might represent an additional burden' and 'social anxiety disorder (social phobia) is now also regarded as an important and consistent risk factor for the development of severe depression' (Kupfer et al., 2012).

Creswell et al. (2014, p.674) highlight the significance that 'anxiety disorders are among the most common psychiatric conditions in young people […] often co-occur with other anxiety disorders, depression and behavioural disorders [and] are associated with increased rates of anxiety and depression in early adulthood…'.

Silk and colleagues (2012) have emphasised that 'anxiety disorders commonly precede the onset of depression in adolescence' and that 'epidemiological studies reveal that up to ¾ of depressed youth have a history of at least one anxiety disorder'. They cite a community study of adolescents that was published by Orvaschel et al. (1995) which found that '42% of youth with a first diagnosis of an anxiety disorder developed a second diagnosis of MDD [major depressive disorder] by one-year follow-up' and, further, that 'one of the best predictors of the presence of both depression and anxiety in youth is a family history of depression'. The authors put forward the hypothesis that two neurobehavioral vulnerability factors − ‘social evaluative threat’ and altered ‘reward processing’ − could be involved in the pathway from anxiety to depression in youth, and support this with evidence from behavioural and neuroimaging studies. They propose that these vulnerabilities 'are likely to be present in many, but not all, anxious youth, and if present, are likely to be exacerbated by pre-pubertal developmental processes in ways that create a potential spiral toward depressive disorder' (Silk et al. 2012).

Asselmann and Beesdo-Baum (2015) point to prospective epidemiological studies as being 'indispensable to inform on the course of anxiety disorders, as they are less susceptible towards biases'. According to the authors such studies have 'consistently found that anxiety disorders in childhood or adolescence strongly predicted the presence of the same condition (homotypic continuity) […] as well as other mental disorders (heterotypic continuity)' in later life, and particularly a higher 'risk of developing secondary mental disorders, especially depression'.

Anxiety and depressive symptoms are also known to be linked with conditions such as Alzheimer’s and other neurodegenerative disorders, cardiovascular disease, stroke or cancer, and can be comorbid with other psychiatric conditions, including bipolar affective disorder and schizophrenia.

The studies cited above emphasise the possible trajectory of illness from anxiety to depression, identify the potential for comorbidity between the two conditions, and acknowledge that symptoms commonly associated with anxiety and depressive-related illness co-occur in a number of other medical and psychiatric conditions. This reinforces the notion that anxiety and depression are heterogeneous conditions, and opens up the possibility for the existence of subtypes with disparate biological, psychological, social or environmental causes that warrant further investigation.