Hope you don't mind, I saw this and answered it over in the schizophrenia/psychosis forum first! I'm just going to copy-paste because it's the same question:
BPD is often referred to as being very close in symptoms (with a different core) to bipolar, maybe that's what you're thinking of? Also, there are some symptoms of BPD that can be similar to the kind of thought distortion schizophrenia often presents with, but with BPD they're usually triggered by events, especially ones to do with personal relationships. It also seems pretty common for people with BPD to also be diagnosed with psychosis, but BPD alone is not a psychotic disorder.
Quote:
Impaired Perception and Reasoning
People with borderline disorder often say that they have difficulty with their memory, especially under stress. You may misperceive experiences, expecting the worst from others, even when nothing nothing negative is intended. You may have difficulty with your concentration,, and with organizing your thoughts and behaviours. You may not be able to think a complex problem through adequately and determine reasonable alternatives, and the consequences of these alternatives. Although these problems are not listed as such in the diagnostic criteria for borderline disorder, they have been shown to be present in people with the disorder through careful neuropsychological testing, and they constitute important features of the disorder.
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Borderline Personality Disorder Demystified: An Essential Guide for Understanding and Living with BPD, Robert O. Friedel, MD
He goes on to list several specific symptoms: brief episodes of paranoid thinking, dissociative symptoms, magical thinking, depersonalization, and unstable self-image or sense of self. At another point, he does clarify that "it now seems clear that borderline disorder is not related to schizophrenic disorders."
(As an aside, I really recommend this book, it's written by a professional with lots of experience and it's well researched! There are a lot of junky pop-science books out there on BPD that aren't written by professionals (How To Stop Walking On Eggshells for one!) or that are written by professionals who don't really know much about BPD. Friedel is great, and he talks about a lot of the uncertainty in the diagnosis and the ongoing research.)
Personally, as someone with BPD and psychosis, I feel like they intertwine a lot. The worse psychotic episodes I had were almost always triggered by my BPD, and the kind of paranoia I get over people and relationships and things like that feels basically psychotic to me, it's that overwhelming and "real". A lot of the cognitive problems I had I'm still not sure if they were due to psychosis or BPD (or possibly Bipolar since we're still not sure if that's a possibility). Even though the definition has changed, I can see why it was originally named "borderline" as in the border between neurosis and psychosis. But schizophrenia as a diagnosis is a lot more complicated than
just psychosis, and sometimes it doesn't even include what we typically consider psychosis, like catatonic schizophrenia. I wouldn't say BPD has much in common with schizophrenia.
Anyway, mental illness is basically categorized as symptom clusters and there's a
lot of overlap between many different mental illnesses. It's also complicated because many people, especially people with chronic and severe mental illness, have comorbid disorders!