Neuropsychological functioning in methadone maintenance patients versus abstinent heroin abusers

Several studies have reported on neuropsychological status as an important contributing variable in drug abuse rehabilitation outcomes. However, few studies have dealt with cognitive impairment in methadone maintenance patients (MMP), despite the fact that methadone is the most frequently used opioid substitution treatment in European countries. The objective of the present study is to contrast the neuropsychological performance of MMP with that of abstinent heroin abusers (AHA). Participants were matched with respect to age, education, pre-morbid IQ, employment status and lifetime drug abuse, and they underwent a set of tests aimed at assessing visuo-spatial attention, processing speed and executive functions. Although processing speed and attention deficits have previously been the focus of studies with MMP, executive functions have not received a similar degree of attention. The purpose of comparing matched MMP and AHA is two-fold: firstly, to test the differential effects of current opioid consumption and past opioid abuse on cognitive-executive performance and secondly, to assess the potential consequences of opioid-related neuropsychological deficits. Results showed a significantly slower performance by MMP on processing speed, visuo-spatial attention, and cognitive flexibility tests (Five Digit Test (FDT) parts 1 and 3; Oral Trails (OT) parts 1, 2; Interference 2-1), and less accuracy in working memory and analogical reasoning tests extracted from the Wechsler Adult Intelligence Scale (WAIS III). Effect sizes for significant comparisons ranged from 0.67 to 1. These results seem to suggest that methadone consumption by itself induces significant cognitive impairments that could compromise drug-treatment outcomes in MMP.