Our understanding of schizoaffective illness can be organized in five different theories. One approach holds that schizoaffective illness is its own disease, separate from others, as appears to be the case superficially by its separate diagnostic criteria in DSM-IV. A second model holds that schizoaffective illness represents a middle clinical picture on a psychotic continuum that extends from bipolar illness to schizophrenia; in other words, this model rejects the Kraepelinian dichotomy of bipolar illness and schizophrenia. A third model argues that schizoaffective illness represents the comorbidity of affective illnesses and schizophrenia, thereby maintaining the Kraepelinian dichotomy and explaining overlap symptoms as chance co-occurrence. A fourth theory views schizoaffective illness as basically a variant of bipolar illness, and a fifth sees schizoaffective illness as a variant of schizophrenia.
Five models of schizoaffective illness
1. A separate illness
2. An intermediate form on the continuum of psychosis
3. Comorbidity of schizophrenia and affective illnesses
4. A more severe variant of bipolar illness
5. A less severe variant of schizophrenia
This is the aspect of diagnosis that receives the most attention from clinicians. From this perspective, the term “schizoaffective” simply applies to those individuals with continuous psychotic and mood symptoms. Unlike mood illnesses, psychotic symptoms are not brief. And unlike schizophrenia, mood symptoms are not absent. Clinically, many patients seem to fall into this overlap region. In fact, the original paper describing the occurrence of such patients with such overlap was published in 1933. Indeed, Kraepelin himself observed that a good number of patients had such overlap of manic-depressive and dementia praecox symptoms. Hence, the fact that such overlap occurs is almost universally accepted, even by Kraepelin, who originated the idea that mood and psychotic illnesses differ.
By itself, the presence of overlap does not invalidate the diagnoses of schizophrenia and mood illnesses. This is partly because symptoms are only one of four diagnostic validators. This is also partly because a difference in symptoms is not an all-or-nothing phenomenon. In other words, to say that schizophrenia and mood illnesses differ in symptoms is not to say that they never overlap. It only means that they usually don’t overlap. And indeed, some well-done symptom prevalence studies have shown that patients with mood and psychotic symptoms tend to differentiate into two big groups, one with mainly mood symptoms and one with mainly psychotic symptoms, although there is some overlap.
It is sometimes argued that the mere existence of schizoaffective illness is a counterexample to the Kraepelinian dichotomy of schizophrenia and mood illnesses. As should be clear from the above considerations, this is not the case. Some overlap is expected; and symptoms are only one aspect of diagnostic validation. To refute the Kraepelinian diagnostic schema, one would also need to look at genetic, course, and treatment response data.
Schizoaffective illness is not found mainly in families of persons with schizoaffective illness. Rather, various studies suggest a unique pattern. In some studies of families of persons with bipolar illness, there is an increased prevalence of schizoaffective illness, bipolar type. In some studies of families of persons with schizophrenia, there is an increased prevalence of schizoaffective illness, depressed type. And in a number of well-executed studies comparing both major groups, schizoaffective illness is more prevalent in families of persons with schizophrenia or bipolar illness than in control populations or than in families of persons with schizoaffective illness. (The most definitive genetic study is here).
These results are consistent with a number of possibilities. In some persons, schizoaffective illness, bipolar type appears to be a more severe variant of bipolar illness. In others, schizoaffective illness, depressed type appears to be a less severe variant of schizophrenia. In still others, since it seems to run in families of persons with both schizophrenia and bipolar illness, only two explanations seem possible: (1) Schizoaffective illness may indeed be the counterexample to the Kraepelinian dichotomy between bipolar illness and schizophrenia; no distinction between any psychotic illnesses can be made and they should all be seen as one continuum and (2) schizoaffective illness may simply represent the comorbidity of having, by chance, schizophrenia and bipolar illness (or unipolar depression) at the same time, just as one might have diabetes and asthma at the same time. So far then, the genetics of schizoaffective illness mainly argues against the concept of a separate illness, but the four other possibilities remain open.
The course of schizoaffective illness is better than schizophrenia but worse than affective illness. This would be consistent with being on the middle of a continuum between those two other conditions, or with the chance comorbidity of the two conditions.
There are few studies of treatment of schizoaffective illness, but it is generally thought that these patients require long-term treatment with antipsychotic agents, as in schizophrenia, and long-term treatment with either mood stabilizers (bipolar type) or antidepressants (unipolar depressed type) as in the corresponding affective illnesses. This treatment response pattern is consistent with all four models except the separate illness model.
Its appearance in DSM-III through 5 notwithstanding, there is no evidence that schizoaffective illness represents a separate illness distinct from schizophrenia and bipolar illness. Studies of symptomatology vary, but some important and well-done studies tend to find a difference in symptoms in psychotic and affective populations that more or less falls along the lines of Kraepelin’s dichotomy of schizophrenia and affective illnesses. While there are overlap areas, such overlap is empirically expected in a real-world population of persons (or animals or any other grouping). Therefore, studies of phenomenology can be interpreted as leaning against the single psychosis continuum model.
If schizoaffective illness represents a comorbidity of schizophrenia and bipolar illness, one would expect an epidemiological prevalence that is significantly lower than the other two. In other words, schizoaffective illness should be very infrequent, since comorbidity should not be overly frequent by chance. Clinical impressions to the contrary notwithstanding, epidemiological prevalence studies indeed demonstrate that schizoaffective illness appears to be very infrequently diagnosable in the general community, at a level of less than 0.5%, which is much lower than accepted prevalence rates for schizophrenia (1%) and bipolar illness (2-4%).
In sum, PL advises three considerations on schizoaffective illness:
1. Some persons experience mainly bipolar mood symptoms, with only some excess of psychosis. These persons are diagnosable with schizoaffective illness, bipolar type, seen as a severe variant of bipolar illness. By and large, they need aggressive mood stabilizer treatment and perhaps somewhat less aggressive antipsychotic treatment. They have a relatively good prognosis.
2. Some persons experience mainly psychotic symptoms, with only some excess of unipolar depressive symptoms. These persons are diagnosable with schizoaffective illness, depressed type, seen as a somewhat less severe variant of schizophrenia. By and large, they need aggressive antipsychotic treatment, and perhaps somewhat less aggressive antidepressant treatment. Their prognosis, though better than in schizophrenia, is usually only fair. This group is to be distinguished from schizophrenia with comorbid major depressive episodes; in the latter case, a patient may experience one or two or only a few depressive episodes that are brief, spaced apart, and often psychosocially triggered. In schizoaffective illness, depressed type, depressive symptoms are more frequent and more persistent, though still often less so than psychotic symptoms.
3. Some persons appear to be truly schizoaffective: they experience psychotic and affective symptoms in more or less equal amounts. This group represents the true comorbidity of schizophrenia and affective illnesses, has an intermediate outcome, and requires aggressive, persistent, long-term treatment with both antipsychotic agents and either mood stabilizers or antidepressants.
If clinicians try to differentiate apparently schizoaffective patients in this manner, they will encounter three groupings. By thinking about these patients according to the group that best describes them, they will be able to better target treatments.