Delusional disorder is defined by the DSM-IV-TR (2000) as a month or more of nonbizarre delusions.
A delusion is a “false belief based on incorrect inference about external reality that is sustained despite what almost everyone else believes and despite what constitutes incontrovertible and obvious proof or evidence to the contrary.” Or, to put it in laymen’s terms, a delusion is a belief that a person holds onto very strongly despite the fact that no one else believes it or that there is overwhelming evidence to the contrary, or both.
Bizarre delusions are those that are almost, if not completely, impossible in reality, such as aliens replacing your brain with a machine that they use to download information, whereas nonbizarre delusions could be things that could actually happen, but are unlikely, such as being loved by a model you’ve never met or being slowly poisoned by your neighbor.
Individuals with delusional disorder do not necessarily suffer from schizophrenia, they may show no other impairment in functioning, they are not suffering from a long standing mood disorder (such as major depressive disorder or bipolar disorder), and they are not experiencing their nonbizarre delusions because of substance abuse or a medical condition.
The best way to differentiate between OCD and delusional disorder is that a person with OCD recognizes that their obsessions or compulsions are excessive, whereas someone with delusional disorder would not know that their beliefs are not realistic. However, there are individuals with OCD who are less able than others to recognize the excessiveness of some of their beliefs or behaviors, and therefore the diagnosis of both disorders at the same time is possible.
In this case, what would typically happen in therapy is that a person would be prescribed an atypical antipsychotic medication to decrease the strength of the obsessions, and then some CBT and ERP would be tried to deal with the compulsions.