Self-disorders, according to the Examination of Anomalous Self-Experience (EASE) which is a semi-structured interview manual published in Psychopathology in 2005 that measures self-disorders both qualitatively and quantitatively, all share in common "a disorder or deficiency in the sense of being a subject, a self-coinciding center of action, thought, and experience" (footnote removed). A link to the manual's full-text can be found here:
no/getfile.php/NLSH_bilde% 20og%20filarkiv/Pulsen/ Kunnskapsbygging/Tekstfiler/ EASE.pdf
Self-disorders include dissociative-like phenomena, disturbances in the stream of consciousness, transitive phenomena, and changes in basic world experience. Studies have shown self-disorders to selectively aggregate in the schizophrenia spectrum:
RESULTS:Basic self-disturbance scores [measured using the EASE] were significantly higher in patients with a schizophrenia spectrum diagnosis (n = 8) compared to patients with other psychotic diagnoses (n = 8).CONCLUSIONS:The findings are consistent with previous work indicating that the disturbance of the basic sense of self is more characteristic of schizophrenia spectrum psychosis than other psychoses. This may have implications for early diagnosis, clinical formulation and intervention.
This shows that people with schizotypal disorder have similar levels of self-disturbance to non-affective psychosis (mostly schizophrenia patients), showing significant differences from other mental illness (which includes patients with bipolar disorder, major depression, OCD, etc.):
Another study with the same result (emphasis added):
Converging evidence indicates that self-disorders (SDs) selectively aggregate in schizophrenia spectrum conditions. The aim of this study was to test the discriminatory power of SDs with respect to schizophrenia and nonschizophrenia spectrum psychosis at first treatment contact. SDs were assessed in 91 patients referred for first treatment through the Examination of Anomalous Self-experience (EASE) instrument. Diagnoses, symptoms severity, and function were assessed using the Structural Clinical Interview for the DSM-IV, Structured Clinical Interview for the Positive and Negative Syndrome Scale, Calgary Depression Scale for Schizophrenia, Young Mania Rating Scale, and Global Assessment of Functioning-Split Version. Most patients found it highly relevant to talk about SDs. EASE total score critically discriminated between schizophrenia, bipolar psychosis, and other psychoses. The EASE total score was the only clinical measure that showed a significant and robust association with the diagnosis of schizophrenia. Systematic exploration of anomalous self-experiences could improve differential diagnosis in first-treatment patients.
Though we don't have the full-text here, another article cites the above article, saying that it shows that when controlling for the "symptomatic PANSS dimensions" self-disorders still aggregated in the schizophrenia spectrum:
The difference in SD between schizophrenia and bipolar psychosis, observed by Haug et al,59 remained significant after controlling for the differences between the groups on the symptomatic PANSS dimensions.
This shows another look at the unity between schizophrenia and schizotypal disorder in terms of self-disorders, again showing their "selective aggregation" in the schizophrenia spectrum:
This study uses a pre-EASE scale of self-disorders and compares them in "residual schizophrenia and psychotic bipolar illness in remission":
Results: Diagnosis of schizophrenia was associated with elevated scores on the scales measuring perplexity (loss of immediate meaning), disorders of perception, disorders of self-awareness, and marginally so, disorders of cognition.
com/doi/10.1034/j.1600-0447. 2003.00105.x/abstract; jsessionid= D2717824A891EB491D3725FECEA165 6A.f04t02
This look at self-disorders is interesting and can possibly bring about greater insight into the schizophrenia spectrum as a whole, showing where and how it differs from other mental illnesses, like psychotic bipolar disorder and OCD, thus bolstering the Kraepelinian dichotomy. Here is a study that looks at a possible relationship between self-disorders and insight in schizophrenia, and actually gives a very good introduction to self-disorders. For example, it mentions that in all the foundational texts on schizophrenia, self-disorders were seen as important:
The notion of disordered self as the core disturbance of schizophrenia appears in all foundational texts on schizophrenia (eg, Kraepelin, Bleuler, Minkowski, Jaspers, and Schneider) but was only recently revived in contemporary psychiatry.
This is under the section "The Disordered Self in Schizophrenia," a very interesting read into the core of the schizophrenia spectrum. Here is the link to the full text:
This explores those self-disorders in that section (footnote removed):
This basic self-world structure is disturbed in schizophrenia spectrum disorders, ie, it is constantly challenged, unstable, and oscillating, resulting in alarming and alienating anomalous self-experiences (also termed “self-disorders”), typically occurring already in childhood or early adolescence. The patients feel ephemeral, lacking core identity, profoundly, yet often ineffably different from others (Anderssein) and alienated from the social world. There is a diminished sense of existing as a bodily subject, distortions of the first-person perspective with a failing sense of “mineness” of the field of awareness (eg, “it feels as if the thoughts aren’t really mine”), and a deficient sense of privacy of the inner world. There is a significant lack of attunement and immersion in the world, inadequate prereflective grasp of self-evident meanings (perplexity), and hyper-reflectivity (eg, “I only live in my head” and “I always observe myself”). Although patients often suffer from self-disorders, the latter are usually lived in an ego-syntonic way, as modes rather than as objects, of the patients’ experience, ie, often affecting more the “how” than the “what” of experience. What is important to emphasize at this point is that the self-disorders, reflecting the unstable basic self-world structure, destabilize the natural ontological attitude and may throw the patient into a new ontological-existential perspective, an often solipsistic framework, no longer ruled by the “natural” certitudes concerning space, time, causality, and noncontradiction. Unconstrained by these certitudes, the world may appear as only apparent or staged, ontologically mind-dependent, prone to noncausal relations, and the patient may experience a unique access to deeper layers of reality, which are inaccessible to others. Often, these experiences evoke a specific sense of grandiosity, leaving others to be seen as oblivious to the true nature of reality and only concerned with everyday trivialities.
Self-disorders seem like they can discriminate between the schizophrenia spectrum and other mental illnesses and seems to show traits present in the schizophrenia spectrum that are typically absent outside of it, even in other psychoses. Even the non-psychotic schizotypal disorder has an aggregation of self-disorders, which suggests that self-disorders are more of a "schizophrenia spectrum thing" than a "psychosis thing."