Background
The historical concept of borderline conditions refers to the pathology on the border between neurosis and psychosis. In DSM-III the conditions were divided into specific but also somewhat overlapping diagnostic criteria for Borderline Personality Disorder (BPD) and Schizotypal Personality Disorder (SPD). This phenomenological overlap, which results in co-occurrence of the two diagnoses, remains a clinical challenge to this day.

Methods
To address this issue we examined the co-occurrence of SPD and BPD according to the established DSM-IV/-5 diagnostic criteria. A literature search was conducted including studies that employed a structured interview with defined BPD and SPD criteria.

Results
Studies from 20 samples were included (i.e. 15 patients, 3 community and 2 forensic samples). For patients diagnosed primarily with BPD, 1–27% also met the criteria for SPD and for patients diagnosed primarily with SPD, 5 – 33% showed co-occurrence with BPD. In the forensic samples, co-occurrence for primary BPD was 10% and 67 – 82% for primary SPD. In the community samples, co-occurrence for primary BPD was 29% and 50% for primary SPD. The pattern of co-occurrence across community samples was particularly heterogeneous.

Conclusion
The identified co-occurrences for BPD and SPD were considerably sample-dependent, and samples and measurements were generally too heterogeneous for a precise meta-analysis. Forensic and community samples generally showed higher co-occurrences, but these findings were characterized by potential methodological limitations.