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Earlier studies suggested more severe overall cognitive impairments in deficit versus non-deficit schizophrenia; however, the specific contribution of different cognitive domains to this overall cognitive impairment remains unclear. The purpose of this study was to compare the two subtypes in general cognitive functioning as well as in individual cognitive domains using the composite score approach. One hundred and forty-three patients fulfilling the criteria for the deficit syndrome were compared with 123 patients diagnosed with non-deficit schizophrenia. Neurocognitive functioning was assessed by a neuropsychological test battery measuring the domains of sustained vigilance/attention, working memory, short-term memory, verbal memory, cognitive flexibility, and ideation fluency. Using the raw neuropsychological measures, we calculated a global index of cognitive impairment and domain-specific composite z-scores. Association between these composite scores and the deficit syndrome was examined by logistic regression analysis. After adjusting for relevant covariates including sex, age, education, smoking, and antipsychotic dose, results indicated a significant increase in the likelihood of deficit syndrome as a function of global (OR = 5.40; 95% CI 3.02-9.65) as well as domain-specific impairments (OR > 2 for all individual domains except for short-term memory). Cognitive flexibility was an independent predictor (OR = 2.92; 95% CI 1.47-5.80), whereas other cognitive domains demonstrated no unique contribution to the general cognitive impairment. Patients with deficit schizophrenia suffer from a more severe degree of neurocognitive impairment, which is qualitatively similar to the dysfunction seen in non-deficit schizophrenia. However, our results indicate that cognitive flexibility is specifically impaired in deficit versus non-deficit patients and may therefore represent a core feature of this subtype.

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Genetic association studies have yielded extensive but frequently inconclusive data about genetic risk factors for schizophrenia. Clinical and genetic heterogeneity are possible factors explaining the inconsistent findings. The objective of this study was to test the association of commonly incriminated candidate genes with two clinically divergent subgroups, non-deficit (SZ-ND) and deficit-schizophrenia (SZ-D), and symptom severity, in order to test for replication of previously reported results. A homogeneous sample of 280 schizophrenia patients and 230 healthy controls of Hungarian, Caucasian descent were genotyped for polymorphisms in schizophrenia candidate genes NRG1, DTNBP1, RGS4, G72/G30, and PIP5K2A. Patients were divided into the diagnostic subgroups of SZ-ND and SZ-D using the Schedule for Deficit Syndrome (SDS), and assessed clinically by the Positive and Negative Symptom Scale (PANSS). SNP8NRG241930 in NRG1 and rs1011313 in DTNBP1 were associated with SZ-ND (P = 0.04 and 0.03, respectively). Polymorphisms in RGS4, G72/G30, and PIP5K2A were neither associated with SZ-ND nor with SZ-D. SNP8NRG241930 showed association with the PANSS cognitive and hostility/excitability factors, rs1011313 with the negative factor and SDS total score, and rs10917670 in RGS4 was associated with the depression factor. Although these results replicate earlier findings about the genetic background of SZ-ND and SZ-D only partially, our data seem to confirm previously reported association of NRG1 with schizophrenia without prominent negative symptoms. It was possible to detect associations of small-to-medium effect size between the investigated candidate genes and symptom severity. Such studies have the potential to unravel the possible connection between genetic and clinical heterogeneity in schizophrenia.