medwireNews: Researchers have failed to find an improvement in cognitive performance in patients with schizophrenia who use or have used cannabis, in findings that refute those of several other studies.
"Not only did our patients fail to show improved cognitive performance associated with (past or present) cannabis consumption, but longitudinal cannabis consumption was associated with poorer social cognition," the researchers note.
"Furthermore, current cannabis use was related to worse working memory performance when adding a low premorbid IQ and an earlier age at illness onset to the explanatory model."
Manuel Cuesta (Complejo Hospitalario de Navarra, Pamplona, Spain) and colleagues did confirm, however, that cannabis use had a significantly greater effect on the cognitive performance of siblings of patients with schizophrenia, relative to the general population.
While worsening processing speed, attention, declarative memory, and social cognition in controls could be explained by a combination of cannabis and tobacco use, in addition to earlier age at cannabis use onset or lower IQ, cannabis use in siblings of patients with schizophrenia negatively and independently influenced declarative memory. When associated with an earlier age of cannabis use initiation, it also had a negative effect on processing speed tasks.
"These results can be explained by the greater vulnerability to the cognitive effects of cannabis in siblings," say Cuesta et al in the European Archives of Psychiatry and Clinical Neuroscience.
"Studies have shown that the genetic risk of psychosis may be associated with sensitivity to cannabis; thus, this increased sensitivity could lead to greater cognitive effects in siblings."
The researchers studied the relationship between lifetime use of cannabis, assessed over a 10-year period, and cognitive performance in 42 patients with schizophrenia, 35 of their unaffected siblings, and 42 mentally healthy individuals.
Pearson correlations showed that cannabis use among patients was not significantly associated with composite scores for processing speed, attention, declarative memory, working memory, or executive function.
Longitudinal cannabis consumption significantly worsened performance on the Mayer-Salovey-Caruso emotional intelligence test (MSCEIT) and the association between current cannabis use and performance on the MSCEIT nearly reached significance.
"The differential pattern of associations between cannabis use and cognitive performance in patients compared with siblings and controls can be explained based on the negative effects of illness on cognition," Cuesta and team explain.
"Our patient sample showed cognitive function arrest in almost all of the domains assessed compared with healthy controls. Therefore, it is arguable that the effects of the illness per se were greater than the effects of cannabis consumption."
Indeed, the researchers found that cannabis use in the previous 10 years and lifetime cannabis use significantly increased positive symptoms, while longitudinal cannabis use over the past 10 years, current use, and lifetime cannabis use increased negative symptoms.
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