Short communicationOpioid abuse and cognitive performance
Introduction
Opioid abuse is a significant public health problem worldwide, with more than nine million people estimated to abuse heroin (United Nations Office on Drugs and Crime, 2004). The United States has an estimated 980,000 chronic users of heroin, representing a greater than 50% increase in number of users since the early 1990s (Office of National Drug Control Policy, 2001). Methadone maintenance is the most widely used treatment for opioid dependence in the United States, with an estimated 179,000 patients enrolled in methadone maintenance treatment programs in 1998 (American Methadone Treatment Association, 1998). Methadone is a potent, long-acting synthetic opioid drug, and as such produces significant sedation and respiratory depression in non-tolerant individuals. However, profound tolerance quickly develops to the sedating effects of methadone, allowing patients to engage successfully in activities of normal living during long-term maintenance treatment.
Results of a small number of early studies in dependent opioid abusers enrolled in methadone maintenance treatment programs suggest only minimal impairment of psychomotor and cognitive/neuropsychological performance (Appel, 1982, Appel and Gordon, 1976, Gordon, 1970, Lombardo et al., 1976, Moskowitz and Robinson, 1985, Robinson and Moskowitz, 1985, Rothenberg et al., 1977). However, as Zacny (1995) commented in a review of effects of opioids on psychomotor and cognitive functioning, most early studies in this population have methodological problems such as use of a small sample size, absence of appropriate controls, failure to control for concurrent use of other drugs, and use of a very limited range of measures. A few recent studies that addressed some of these methodological problems provide evidence for performance impairment in dependent opioid abusers enrolled in methadone maintenance programs (Darke et al., 2000, Mintzer and Stitzer, 2002, Specka et al., 2000). However, these studies examined performance of MMP relative to non-drug abusing controls only, making it difficult to differentiate the effects of a history of long-term opioid (or polydrug) abuse from the effects of methadone maintenance itself. Although most of the early studies in MMP included a control group of former drug abusers, these studies have methodological problems as mentioned above (cf. Zacny, 1995). Furthermore, given that methadone doses have generally increased and that more sophisticated cognitive testing methods have been developed recently, a renewed look at the possible performance impairing effects of methadone itself is warranted.
Davis et al. (2002) examined neuropsychological performance in former opioid abusers enrolled in drug-free treatment programs, in addition to MMP and non-drug abusing controls (pain management patients). The incidence of impaired performance in the former opioid abusers fell between that in the MMP (highest incidence of impairment) and that in the non-drug abusing controls (lowest incidence of impairment). The incidence of impairment was significantly different in the MMP versus control groups, but no other paired comparisons were significant. Although the incidence of impairment in the former opioid abuser group was not significantly different from that in the MMP group, the pattern of results suggests that methadone maintenance is associated with additional impairment over and above that associated with long-term abuse, and that recovery of functioning may occur during abstinence. Recently, we developed a collaboration with drug-free treatment programs in our area enabling us to gain access to this highly relevant comparison population of former opioid abusers. Thus, the purpose of the present study was to attempt to differentiate the effects of a history of long-term abuse from the effects of methadone maintenance in our previous study (Mintzer and Stitzer, 2002) by comparing performance of a newly recruited group of currently abstinent former opioid abusers retrospectively to the two groups (MMP and matched non-drug abusing controls) reported on in our previous study.
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Participants
Twenty currently abstinent former opioid abusers were recruited from drug-free treatment programs and recovery houses in Baltimore, MD. This group of abstinent opioid abusers was compared retrospectively to two groups reported on previously in our laboratory (Mintzer and Stitzer, 2002): 18 opioid-dependent methadone maintenance patients recruited from outpatient methadone maintenance programs on the campus of the Johns Hopkins Bayview Medical Center and 21 matched controls without histories of
Participant characteristics
Participant demographics and recent drug use histories (% used in last 6 months, based on self-report) for the three groups are shown in Table 1.
The groups did not differ significantly with respect to gender, race, mean age, years of education, current reading level or estimated IQ (p > .05 in chi-square or three-way between subjects analysis of variance (ANOVA), as appropriate). However, there was a significant difference in employment status among groups such that a smaller percentage of
Discussion
Results of our previously reported comparison between MMP and matched controls (Mintzer and Stitzer, 2002; cf. Table 2) revealed significantly impaired performance for MMP relative to controls on the DSST (psychomotor/cognitive speed), trail-making A and B tasks, (psychomotor speed), the two-back task (short-term/working memory), and the gambling task (decision-making). In the present study, performance of a newly recruited group of currently abstinent former opioid abusers was compared
Acknowledgements
This project was supported by National Institute on Drug Abuse Research Grant DA-O5273. The authors thank Eva Costlow, Jennifer Kord, and Jeanene Pope for technical assistance, John Yingling for computer programming assistance and technical support, and Paul Nuzzo for assistance with data analysis.
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Present address: Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, Department of Health and Human Services, Baltimore, MD 21224, USA.