Abstract
Background
Individuals with a schizophrenia spectrum disorder (SSD) are commonly reported to experience intellectual and neurocognitive impairments which may commence prior to the onset of other symptoms. Considerable cognitive heterogeneity is known to exist within the SSD population. Several lines of inquiry have attempted to characterize the variability in IQ trajectory by comparing the relationship between estimates of premorbid and current intellectual function. Three putative IQ trajectories have been consistently identified in the literature: a preserved IQ (PIQ) and compromised IQ (CIQ) trajectory respectively reflective of intact and impaired current intellectual function in the absence of a meaningful decline from premorbid estimates; and a deteriorated IQ (DIQ) trajectory defined by a meaningful disparity between estimated premorbid and current intellectual function. The aim of the present review was to comprehensively and systematically summarize our current understanding of the variability in putative IQ trajectories within the SSD population.
Methods
The literature search was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. The databases: PubMed, Scopus, and Web of Science were used to locate relevant literature from January 1980 to October 2018. The search strategy was created as part of a larger systemic review centered around three concepts: SSD, cognition and subgrouping. Only eligible records pertaining to the investigation of IQ trajectory subtypes in SSD were included in this report.
Results
The search strategy yielded a total of 2439 records, from which 14 studies were identified that classified participants into cognitive subgroups based on putative IQ trajectories. The classification criteria employed varied slightly across studies, however on average 31% of SSD participants investigated were classified as having PIQ, 42% as having DIQ and 20% as having CIQ. PIQ participants were consistently reported to outperform the DIQ and CIQ subgroups across the majority of cognitive domains, with meaningful differences between the DIQ and CIQ subgroups uncommon; although, subgroups were consistently reported not to differ in attention and executive function. The PIQ participants were consistently reported to exhibit control equivalent intellect, however subpar neurocognitive function. A select number of studies reported the PIQ participants as having better clinical and functional ratings compared to the remaining subgroups.
Discussion
The findings highlight the possibility that three distinct trajectories of intellectual decline exist within the SSD population. It appears that a proportion of individuals with SSD have relatively preserved intellectual functioning and exhibit neurocognitive abilities near-equivalent to healthy controls, as well as better clinical and functional presentation relative to the remaining SSD population. A second subgroup appear to experience compromised intellectual/neurocognitive function that appears to predate illness onset. Finally, a proportion of SSD individuals appear to experience a decline in intellectual function from premorbid estimates. More comprehensive examination of the clinical and functional relevance of this classification method is required.