From disorder categories to symptom networks
Picture 1: DSM VI symptom network from http://www.psychosystems.org/
The Diagnostical Statistical Manual of Mental Disorders (currently in its 4th edition – DSM-IV) (http://allpsych.com/disorders/dsm.html) is a guide for psychologists to infer what type of mental disorder a person has. Knowing what type of disorder a person has, can help in determining the therapy that is most appropriate. In addition it is also important in terms of health care insurance, since (at least in the Netherlands) one can only get insured for therapy when one is diagnosed.
This categorical perspective however could be quite problematic in terms of finding out the mechanisms underlying a disorder. Therapy might be ineffective depending on the specific set of complaints/symptoms the person has. A disorder comprises of a number of symptoms. There could be a variety symptoms describing a disorder. Two people with a different set of complaints, thus can be diagnosed with the same disorder, or people with the same set of symptoms can be diagnosed with different disorders depending on the weight the patient puts on certain symptoms, the extent to which the patient is aware or sensitive of certain symptoms, and how the psychologist interprets these symptoms. Thus there is a subjective side of the patient describing his/her complaints and a certain subjective side for the psychologist or doctor who assigns the patient to a certain category.
Picture 2: example of how disorders can be connected
Depression is defined based on 8 symptom boxes (when 1 out of 2 core symptoms is present, and at least 4 other symptoms then a person is diagnosed with depression), while insomnia is defined when 1 out of 3 sleep symptoms is present and the person experiences severe daytime dysfunction for at least a month attributable to these sleep complaints.
The classification of people can obscure insight into mechanisms of the disorder when one doesn’t take into account the symptoms that make up the person having the disorder. In addition so called “bridge symptoms” (symptoms that are part of more than one disorder e.g. in the above picture concentration can be a bridge symptom between depression and insomnia) might explain the high comorbidity that exists between some disorders, like e.g. depression and anxiety, depression and insomnia…
Denny Borsboom has done a lot of interesting work in this regard, by constructing a network graph of the DSM-IV (see Picture 1). In a recent paper in PLoS One (2011) he elaborated on his network model and explained how the number of symptoms shared between two disorders predict the prevalence of comorbidity in the population. The idea is that instead of seeing symptoms as indicators of a certain disorder (the disorder which is considered a latent variable and underlying cause of the symptoms), one can see the symptoms as causally linked to one another (e.g. sleep deprivation -> fatigue -> irritability -> anxiety), i.e. one symptom can cause another symptom, which can cause other symptoms again on their own and eventually give rise to what we see on a more abstract level in the so-called disorder or combination of disorders (comorbidity).
I believe this is a very interesting way to look into disorders or better… symptom networks and could help us gain insight into how “disorders” arise and develop.
Denny Borsboom, Angélique O. J. Cramer, Verena D. Schmittmann, Sacha Epskamp, & Lourens J. Waldorp (2011). The small world of psychopathology PLoS ONE, 6 (11) : 10.1371/journal.pone.0027407.g001