Per psycom.net, in 2013, the American Psychiatric Association adjusted the Diagnostic and Statistical Manual (DSM-V) to reflect the growing consensus that schizophrenia exists on a spectrum of psychotic disorders. Delusional disorder is on this spectrum. This makes sense to me, as I have had many of the symptoms of schizophrenia, however – quite fortunately – not all of them.

Down the rabbit hole

According to the Merriam-Webster online dictionary, a spectrum is a continuous sequence or range. This brings to mind a quote from The Matrix Trilogy: “You take the blue pill, the story ends, you wake up in your bed and believe whatever you want to believe. You take the red pill, you stay in wonderland, and I show you how deep the rabbit hole goes.” In fact, while I was experiencing psychosis, I felt as if I had taken “the red pill”.

Going by this spectrum model, how do we know that delusional disorder isn’t simply a mild version of paranoid schizophrenia? A study originally published in 2010 by Andreas Maneros, et al of the Martin Luther University of Halle-Wittenberg in Germany endeavored to answer this question. And, the results of the study, “Delusional Disorders—Are They Simply Paranoid Schizophrenia?” (Schizophr Bull. 2012 May; 38(3): 561–568) had reassuring results:

Significant differences between DD and PS were found: DD patients are, in comparison to patients with PS, significantly older at onset. Less of their first-degree relatives have mental disorders. They less frequently come from a broken home situation. First-rank symptoms, relevant negative symptoms, and primary hallucinations did not occur in patients with DD. Patients with DD were less frequently hospitalized, and the duration of their hospitalization was shorter. Their outcome is much better regarding employment, early retirement due to the disorder, and psychopharmacological medication. They more often had stable heterosexual partnerships and were autarkic. They had lower scores in the Disability Assessment Scale and in Positive and Negative Syndrome Scale. The diagnosis of DD is very stable over time.

The researchers went on to conclude: The findings of this study support the assumption that DDs are a separate entity and only exceptionally can be a prodrome of schizophrenia.

Well, that is a relief. I certainly have no interest in developing negative symptoms, and this study’s results give me hope that my delusional disorder diagnosis isn’t simply a prequel to paranoid schizophrenia. Yay.

Digging deeper

Consistent with the above study are the conclusions of Differences between delusional disorder and schizophrenia: A mini narrative review (World J Psychiatry. 2022 May 19; 12(5): 683–692) in which the authors endeavor to ask three questions: (1) Do epidemiological data differentiate DD from schizophrenia? (2) Do clinical features or psychiatric comorbidities differ in DD and schizophrenia? And (3) Are there data that show differences between DD and schizophrenia with respect to treatment response to either pharmacological or non-pharmacological treatment?

We found an overlap between the diagnosis of DD and schizophrenia, with boundaries often very blurred. As characterized in DSM-5, the middle age onset of DD distinguishes it from the earlier onset in schizophrenia. The literature gives a prototypical picture of schizophrenia as one of hallucinations, cognitive, and negative symptoms in addition to delusions, with function deteriorating over time. Relatively good function is maintained in DD. While this disorder is also characterized by symptoms other than delusions (mainly affective symptoms), delusions predominate. Treatment response to antipsychotic medication appears to be similar in the two conditions, although DD patients, as a group, are older, and would be expected, as one study has shown, to require comparatively lower doses to achieve symptom reduction... Specific symptoms, when targeted by cognitive behavioral therapies, respond in both DD and schizophrenia, although efficacy trials in DD are, to date, limited.

The content of delusions seems more understandable in DD than it often is in schizophrenia but the major theme is one of persecution in both conditions. In general, the prevalence rate for delusional disorder is significantly lower than that for schizophrenia.

Based on all of this information, I feel it’s safe to conclude that delusional disorder isn’t simply “schizophrenia light” – they are distinct diagnoses with distinct characteristics, however overlapping those characteristics (or symptoms) may be.