Schizophrenia Research 114 (2009) 98–104

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Cognitive impairment in dual diagnosis inpatients with schizophrenia andalcohol use disorder

Victoria Manning a,⁎, Shai Betteridge b, Shamil Wanigaratne a, David Best c,John Strang a, Michael Gossop a

a National Addiction Centre, Maudsley Hospital/Institute of Psychiatry, 4 Windsor Walk, London SE5 8BB, UKb City and Hackney NHS Community Services, Top Floor E Block, St Leonard's Hospital, Nuttall Street, London, N1 5LZ, UKc Criminal Justice Research Centre, Room 822A,University of the West of Scotland, Hamilton Campus, Almada Street, Hamilton, Lanarkshire ML3 0JB, UK

a r t i c l e i n f o

⁎ Corresponding author. National Addiction CentrInstitute of Psychiatry, 4 Windsor Walk, Denmark HillTel.: +44 207 848 0814; fax: +44 207 701 8454.

E-mail address: v.manning@iop.kcl.ac.uk (V. Mann

0920-9964/$ – see front matter. Crown Copyright © 2doi:10.1016/j.schres.2009.05.020

a b s t r a c t

Article history:Received 2 July 2008Received in revised form 29 April 2009Accepted 24 May 2009Available online 21 June 2009

Cognitive impairment has been found independently among individuals with schizophreniaand individuals with alcohol use disorders. Less is known about the nature and severity ofcognitive impairment in patients with a dual diagnosis, though the co-occurrence of thesedisorders may further exacerbate cognitive impairment. The study investigates the possibleadditive effect of alcohol use disorder and schizophrenia on cognitive impairment amongpatients diagnosed with schizophrenia. Participants were inpatients with schizophrenia(n=30), inpatients with a dual diagnosis of schizophrenia and alcohol use disorder (n=30),and matched controls (n=30): all completed a comprehensive neuropsychological battery.Both patient groups were significantly impaired, relative to controls, across the battery. Dualdiagnosis patients were significantly more impaired than schizophrenia patients on delayedverbal memory, and executive functioning, primarily set-shifting, working memory, andplanning, and had higher psychiatric morbidity scores. The findings provide support for anadditive effect of the two disorders on cognitive impairment. These cognitive deficits mayaffect capacity to engage in treatment, increase risk of relapse, and adversely affect treatmentoutcomes. An understanding of the cognitive profile of people with dual diagnosis may help totailor treatment delivery to meet their specific needs, enhance cognitive strengths,accommodate deficits and improve treatment outcomes.

Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.

Keywords:Cognitive functioningImpairmentDeficitDual diagnosisAlcoholSchizophrenia

1. Introduction

Cognitive impairment is a central and enduring char-acteristic of schizophrenia (Hoff and Kremen, 2003; Bilderet al., 2000) and is common in individuals dependent onalcohol (Bates et al., 2002; Grant, 1987). Relatively little isknown of the possible ‘additive effect’ of heavy drinkingon cognitive impairment in patients with a dual diagnosis,(i.e. if the co-occurrence of the disorders exacerbates cog-nitive deficits associated with schizophrenia alone). Alcohol

e, Maudsley Hospital, London SE5 8BB, UK

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is the most frequently abused substance among individualswith schizophrenia (Cantor-Graae et al., 2001; Fowler et al.,1998; Miles et al., 2003). Between a quarter and one-third ofschizophrenia patients report current (past year) alcohol usedisorder (Menezes et al., 1996; Cantwell et al., 1999; Weaveret al., 2001), with higher rates among institutionalisedpatients (Isherwood and Brooke et al., 2001; Wheatley,1998).

Previous studies have reported mixed findings on theadditive effect of schizophrenia and alcohol use disorder oncognitive impairment. On a face recognition and set-shiftingtask, Nixon et al. (1996) failed to find differences betweenpatients with a dual diagnosis compared to those with onlyschizophrenia or alcohol dependence. In contrast, schizophre-nia patients with a history of alcoholism were found to show

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99V. Manning et al. / Schizophrenia Research 114 (2009) 98–104

greater (subtle but consistently identifiable) cognitive impair-ment on the Cognitive–Perceptual factor of the NeurologicEvaluation Scale (Allen et al., 2000). Using a comprehensivebattery of neuropsychological tests, greater cognitive impair-ment was found among dual diagnosis patients compared topatients with only schizophrenia or alcoholism (Allen et al.,1999). Other US studies have found evidence of an additiveeffect among older patients with dual diagnosis (Bowie et al.,2005; Mohammed et al., 2006). Our earlier work on a UKsample (Manning et al., 2007)was suggestive of greater globalcognitive impairment among dual diagnosis outpatients,though no specific deficits were observed, possibly due tothe insensitivity of the screening tool used (Mini Mental StateExamination).

Contradictory findings from the limited number of studiesin this area may arise from differences in sampling (e.g. in-patient versus outpatient, chronic versus acute schizophrenia,alcohol use versus alcohol dependence), methodology (cog-nitive domain examined, sensitivity of neuropsychologicaltest and performance parameter selected) and other limi-tations such as small samples reducing statistical power todetect group differences and failing to match on confoundingfactors such as premorbid functioning.

The present paper reports a further investigation of thisissue using a comprehensive neuropsychological batteryadministered to inpatients with a diagnosis of schizophrenia,half of whom had alcohol use disorders. The purpose of thestudy was to delineate the profile of cognitive deficits in dualdiagnosis patients and to determine the possible additiveeffect of alcoholism and schizophrenia on memory andexecutive functioning impairment, using matched controls.

2. Method

2.1. Sample

The overall sample comprised 60 inpatients drawn fromfour acute psychiatric wards in London and 30 controls.Inclusion criteria were: aged 18–65 years, a diagnosis ofschizophrenia, schizo-affective disorder or other psychoticepisode other than substance induced psychosis (ICD-10codes F20-29) provided by a treating clinician. Thirty patientsalso had alcohol use disorders (dual diagnosis). Exclusioncriteria included history of serious head injury (with loss ofconsciousness N5 min), primary drug use disorder, history ofsubstance misuse within the past five years (for the schizo-phrenia group), dyslexia or other learning disabilities, lack offluency in the English language and neurological disorders.Inclusion criteria for the 30 controls were that they matchedthe age, gender, ethnicity and years of education of the inpa-tients, no history of alcohol use disorder, drug use disorder ormental illness and fluent in English.

Eligible patients were approached in the second weekafter admissionwhen the purpose of the study was explained.A total of 174 patients were initially approached; of these 23did not meet inclusion criteria and 72 refused to participate,79 consented to participate but 19 failed to turn up forassessment or subsequently withdrew consent. The researchassessment lasted approximately 3 hour: breaks of up to10 minutes were permitted for cigarettes and refreshments.

2.2. Procedure

All patients met the Mini-International NeuropsychiatricInterview (Sheehan et al., 1998) criteria for current (withinthe past year) psychotic disorder. Dual diagnosis patients alsomet the MINI criteria for alcohol use disorder/dependence.In addition patients completed the AUDIT-5 item (Piccinelliet al., 1997) to assess problematic alcohol use; the MaudsleyAddiction Profile grid to assess drug use (Marsden et al.,1998); the Brief Symptom Rating Scale (Ventura et al., 1993),the Hospital Anxiety and Depression Scale to assess mood(Zigmond and Snaith, 1983); and the WHO Disability Scale(WHO, 2001) to assess everyday functioning.

All participants completed a computerised and a penciland paper neuropsychological battery assessing premorbidIQ (National Adult Reading Test; Nelson, 1991); generalintellectual functioning (Wechsler Abbreviated Scale ofIntelligence, Wechsler, 1999); verbal memory and learning,immediate, delayed and recognition memory (Verbal PairedAssociates subtest of the Wechsler Memory Scale, WechslerMemory Scale-III, Wechsler, 1997); verbal working memory(Letter Number sequencing subtest of the WMS-III); verbalinitiation and inhibition (Hayling Sentence completion Test,Burgess and Shallice, 1997); semantic and phonemic fluency(COWAT; Benton, 1968); visuospatial memory and learning(Visual Paired Associates), visual recognition memory(Pattern recognition memory), set-shifting (Intra/Extradimensional Shift) and planning (Stockings of Cambridge)on the CANTAB (Cantab,1999). Ethical approval for the studywas granted by the South London and Maudsley NHS Trust.

3. Results

Groups comprised equal numbers of men and women,with a mean age of 36.4 (±8.5) years, and, on average,11.5 years education, with no significant differences on anyof these social demographics across the three groups (seeTable 1). Groups comprised approximately 50% Caucasian(white British) participants with Black Afro-Caribbean/Afri-can participants the second most common ethnicity. Therewere significant differences between the patient groups andcontrols inmarital and employment status (withmore controlsbeing married and employed), but no differences between thetwo clinical groups.

No statistically significant differences were found in ageof onset of alcohol use, or age of regular drinking when dualdiagnosis patients were compared with schizophreniapatients and controls. Dual diagnosis patients consumedsignificantly larger quantities of alcohol per drinking occa-sion, consumed alcohol on significantlymore days in the pastmonth and obtained a significantly higher AUDIT score. Theiraverage age of onset of problem drinking was 28.3 (±9.1).All dual diagnosis patients scored more than 5 on the AUDIT,and met the MINI criteria for alcohol use disorder. In addi-tion, 22 (73%) met the criteria for alcohol dependence and 7patients (23%) reported having had a medically assisteddetoxification in their lifetime. About half (53%) of the dualdiagnosis patients reported illicit drug use in the past year,the majority (75%) used cannabis, with little use of otherdrugs.

Table 1Demographic and drinking data.

% / Mean (SD) Control Schizophrenia Dual diagnosis χ2 /F value, p

Age 35. 9 (7.5) 37.6 (7.9) 36.6 (10.2) 0.06, 0.94Gender (male) 15 (50%) 15 (50%) 15 (50%) –

Ethnicity (white) 15 (33%) 13 (29%) 17 (38%) 1.8, 0.77Marital status (single) 15 (50%) 26 (88%) 25 (83%) 20.6, b0.01Employment status (working) 22 (73%) 1 (3%) 0 (0%) 58.6, b0.001Education years 12.4 (2.2) 11.8 (1.8) 11.3 (1.5) 2.4, 0.09NART 101 (9.4) 99.5 (9.3) 98.1 (8.3) 1.2, 0.31Age of onset of schizophrenia – 25.0 (5.4) 23.0 (8.2) 1.0, 0.28Age first treatment episode – 27.0 (5.6) 25.6 (7.9) 0.76, 0.45Age of regular drinking 20.1 (4.3) 18.6 (1.3) 19.5 (5.8) 0.21, 0.81Age of onset of problem drinking – – 28.3 (9.1) –

Mean days used alcohol in past month 8.2 (6.8) 0.7 (1.4) 22.6 (8.5) 79.7, b0.001Mean alcohol amount per occasion 3.7 (1.9) 3.2 (2.0) 15.6 (10.8) 23.3, b0.001Mean score on the AUDIT 3.5 (2.0) 1.3 (1.9) 13.0 (3.8) 154.5, b0.001

100 V. Manning et al. / Schizophrenia Research 114 (2009) 98–104

3.1. Neuropsychological performance

3.1.1. Premorbid and current intellectual functioningThere were no significant differences between the groups

in level of premorbid functioning (F=1.2, p=0.31) with theNART score in the average range for all three groups.Significant differences were found in current intellectualfunctioning (IQ) with a higher mean score in the controlgroup (F=8.0, pb0.001). All three groups had a mean scorewithin the ‘average’ range; schizophrenia group 93.4(±10.4); dual diagnosis group 91.1 (±12.2) and controlgroup 102.7(±12.9). Table 2 shows differences in meanperformance on each subtest of the battery across the threegroups.

Schizophrenia and dual diagnosis patients demonstratedpoorer performance than controls on all measures. Therewere significant differences across the groups on almost alltests and on all performance parameters with the exception ofverbal recognition (which approached significance (F=2.6,p=0.08)). Tukey's post hoc analysis indicated that schizo-phrenia patients exhibited a significantly poorer performancethan controls on all tests except verbal recognition (VPA).

Table 2Group differences in mean raw scores [scale scores or standard score where availab

Neuropsychological domain Neuropsychological test Controlsa

(Mean/SD

General cognitive functioning MMSE 28.7 (1.3)General intellectual functioning FSIQ 102.7 (12Verbal memory (immediate) Verbal Paired Associates (VPA) I 20.2 (7.5)Delayed verbal memory VPA II 5.9 (2.0)a

Recognition (verbal) memory VPA recognition 23.6 (1.8)Visual recognition memory Pattern recognition memory 20.4 (13.2Visual memory Paired associate learning+ 4.8 (0.6)a

Phonemic fluency COWAT — FAS 46.5 (11.7Semantic fluency COWAT — animals 20.2 (5.2)Working memory Letter Number sequencing 9.7 (2.9)a

Set-shifting Intra/Extra dimensional Shift⁎⁎ 15.2 (8.9)Verbal inhibition Hayling Sentence completion 17.3 (2.9)Planning and organisation Stockings of Cambridge++ 7.3 (2.3)ab

Means on the same row sharing the same superscript letter differ significantly at p⁎ Kruskal Wallis Test of significant difference.⁎⁎ (total errors adjusted). NB: A higher score indicates poorer performance (total errors+ Stages completed.++ Mean problems solved in minimum moves.

Dual diagnosis patients exhibited significantly poorer perfor-mance than controls on all subtests.

3.1.2. Memory and executive functioningTo facilitate comparison of the schizophrenia and dual

diagnosis group relative to the control group across tests(using different performance parameters), raw scores foreach patient were transformed into standard (Z) scores usingthe mean performance of the control group to standardise thedata. The mean Z scores of the dual diagnosis and schizo-phrenia patients for memory and executive function subtests(respectively) are shown in Table 3.

Dual diagnosis patients obtained a significantly lower Z score(indicating greater impairment) than schizophrenia patientson delayed verbal memory (pb0.001). Dual diagnosis patientsscored lower on other measures of memory though none ofthese differences was statistically significant. A significantlyworse performance was found among dual diagnosis patientson working memory, set-shifting (cognitive flexibility), andplanning ability (all pb0.01).

A further analysiswasconducted to investigatewhetherdualdiagnosis patients were more impaired on overall memory and

le] on subtests.

)Schizophreniab

(Mean/SD)Dual diagnosisc

(Mean/SD)F /χ2, p value

ab 27.1 (2.6)a 26.1 (2.3)b 20.1, b0.001⁎.9)ab 93.4 (10.4)a 91.1 (12.2)b 8.0, b0.001ab [10.5] 9.9 (6.4)a [6.8] 7.4 (3.8)b [6.0] 36.9, b0.001b [10.1] 2.7 (2.3)ac [6.5] 1.4 (1.1)bc [4.9] 44.5, b0.001a 22.9 (1.2) 22.7 (1.9)a 2.6, 0.08)ab [−0.4] 18.3 (14.9)a [−1.6] 17.2 (12.3)b [−2.0] 7.5, b0.001b 4.0 (1.0)a 3.6 (1.0)b 11.8, b0.001)ab [11.7] 37.4 (12.6)a [9.6] 34.4 (8.1)a [8.7] 9.9, b0.001ab [10.3] 16.6 (4.2)a [8.0] 17.5 (3.0)b [8.8] 6.0. b0.01b [8.9] 7.4 (3.1)ac [6.7] 5.2 (2.7)bc [4.5] 17.4, b0.001ab 25.4 (18.6)ac 37.2 (17.4)bc 15.0, b0.001ab [9.3] 13.4 (3.7)a [6.3] 12.8 (2.7)b [5.7] 17.4, b0.001[−0.8] 5.8 (2.3)ac [−1.5] 4.0 (2.5)bc [−2.4] 13.7, b0.001

b0.05.

adjusted).

Table 4Correlations between drinking variables and cognitive impairment.

Bi-variate correlations Executive Functioning deficit Memory deficit

Alcohol frequency(days in past month)

−0.14, p=0.47 −0.05, p=0.80

Alcohol amount(average units per day)

−0.28, p=0.14 −0.38, pb0.05

Total consumption (units) −0.26, p=0.16 −0.05, p=0.80AUDIT score −0.31, p=0.10 −0.38, pb0.05MINI abuse symptoms −0.13, p=0.49 −0.17, p=0.38MINI dependence symptoms −0.48, pb0.01 −0.35, p=0.06Years of problem drinking a 0.30, p=0.52 0.02, p=0.97

a Partial correlation (controlling for the patients age).

Table 3Deficits in memory and executive functioning (mean Z score representingstandard deviations below healthy controls).

Schizophrenia Dual diagnosis T /U, p value

Memory subtestsImmediate verbalmemory

−1.37 (±0.8) −1.70 (±0.5) 1.8, 0.07

Delayed verbal memory −1.55 (±1.1) −2.21 (±2.2) 2.8, b0.01Verbal recognitionmemory

−0.37 (±0.6) −0.49 (±1.0) U=410.0, 0.87 a

Visual memory −1.27 (±1.5) −1.87 (±1.3) 1.6, 0.11Visual recognition −0.7 (±1.2) −1.0 (±0.9) 1.1, 0.30

Executive functioning subtestsWorking memory −0.76 (±1.0) −1.50 (±0.3) 2.9, b0.01Phonemic fluency −0.78 (±0.7) −1.03 (±0.7) 1.1, 0.27Semantic fluency −0.70 (±0.8) −0.53 (±0.6) 0.92, 0.36Verbal inhibition −1.32 (±1.3) −1.51 (±0.9) 0.67, 0.51Set-shifting −1.18 (±1.2) −2.50 (±1.9) 2.4, b0.01Planning −0.66 (±1.0) −1.41 (±1.1) 2.8, b0.01

a Mann–Whitney U test where scores not normally distributed.

101V. Manning et al. / Schizophrenia Research 114 (2009) 98–104

overall executive functioning when controlling for othervariables. Based on the Z scores a composite score of allmemorytests anda composite score of all executive functioning testswasconstructed. T-tests revealed significant differences in memory(mean=−4.7 schizophrenia and −6.4 dual diagnosis, t=2.3,pb0.05), and executive functioning (mean= −6.1 schizophre-nia and−9.1dual diagnosis, t=2.5,pb0.01). Logistic regressionwas conductedwithdiagnosis (schizophrenia ordualdiagnosis)as the dependent variable, and with the following variablesentered: composite executive functioning, composite memoryscore, premorbid IQ, age of onset of schizophrenia, and severityof psychiatric symptoms (HADS and BPRS).

Differences were found between the dual diagnosis andschizophrenia patients on executive functioning and psychia-tric symptoms. Dual diagnosis patients were significantlymore impaired on executive functioning (β=0.86, Wald=4.2, p=0.04) and had higher scores for psychiatric symptoms(β=1.04, Wald=4.4, p=0.04). The overall model was sig-nificant (χ2=11.0, pb0.01).

Drinking variables were not included as covariates in thelogistic regression analysis since many schizophrenia andcontrol patients did not use alcohol and there were noproblem drinkers. Correlation coefficients were calculatedbetween drinking variables and severity of cognitive impair-ment for the dual diagnosis group only (see Table 4). Statis-tically significant associations were found between compositememory score and quantity of alcohol consumed (r=−0.38,pb0.05), and the AUDIT measure of problematic alcohol use(r=−0.38, pb0.05). The negative association betweenmemory score and number of alcohol dependence symptomsapproached significance (r=−0.35, p=0.06). The compositemeasure of executive functioning was significantly associatedwith the number of alcohol dependence symptoms on theMINI (r=−0.48, pb0.01). No significant correlations werefound between either memory or executive functioning andfrequency of alcohol use or years of problem drinking.

4. Discussion

Both schizophrenia and dual diagnosis patients exhibitedmarked cognitive impairment across the neuropsychological

battery with poorer performance relative to matched controls.Dual diagnosis (schizophrenia andalcohol usedisorder) patientswere found to be more impaired than schizophrenia patientswithout substance use disorders on almost all measures andwere significantly more impaired on cognitive flexibility (set-shifting, working memory), planning and delayed verbalmemory. These findings support the suggestion that alcoholuse disorder further exacerbates cognitive impairment in pa-tients already compromised due to the schizophrenia. Also, thefailure of the MMSE to detect impairments that existed amongthese populations supports our earlier findings on outpatients.

Thefindings are consistentwith previous reports of greatercognitive impairment among alcohol-misusing patients withschizophrenia (Allen et al., 1999, 2000; Bowie et al., 2005;Mohamed et al., 2006). Although several studies havereported higher executive functioning in patients with a dualdiagnosis (Barnes et al., 2006; Carey et al., 2003; Herman,2004; Joyal et al., 2003; Potvin et al., 2005), an importantdifference between the present study and previous reportsconcerns the nature and severity of both psychiatric disorderand substance use among dual diagnosis patients. In the studybyCarey et al (2003) only 60%of patientswere diagnosedwithschizophrenia and only half of the ‘current’ users reportedsubstance use in the previous month, suggesting that theymight have been less problematic substance users than dualdiagnosis patients in the current study.

Alcoholmay also have amore detrimental effect on cognitivefunctioning than many illicit drugs, and problematic or depen-dent use of alcohol may be associated with greater cognitiveimpairment. Those studies failing tofind evidence for an additiveeffect have examined predominantly co-morbid drug use ratherthan problematic alcohol use. Alcohol misusers comprised only34% of the sample in the study byHerman (2004) and 45% in thestudybyPotvin et al (2005). Theprocurementof illicit drugsmayalso require high level executive functions such as planning,problem-solving and social skills (Smelson et al., 2002; Salyersand Mueser, 2001). A recent meta-analysis of such studiesconcluded that schizophreniapatientsobtainedsimilarestimatesof overall cognitive capacities irrespectiveof additional substanceuse disorder with better performance among the dual diagnosisgroup on visual scanning and speed assessed by the Trail MakingTest (Potvin et al., 2008). However, in relation to workingmemory, exclusive use of alcohol was associated withinsignificantly lower scores. The authors also note that age andsubstance used may be linked to levels of functioning amongthose with co-morbid substance use disorder.

The findings of the current study have clinical implicationsfor screening, and for the treatment and rehabilitation of dual

102 V. Manning et al. / Schizophrenia Research 114 (2009) 98–104

diagnosis patients. Given the limited number of reliablecognitive screening tools and resource requirements of ad-ministering comprehensive batteries in busy clinical settings,the tests of planning/organisation, set-shifting and workingmemory may constitute candidate ‘screening’ measures.Coping with many real-life situations requires the sort ofcognitive processes assessed on tests of set-shifting (Burgesset al., 1998). Screening is important since verbal skills aregenerally preserved and deficits may therefore go undetected,or misinterpreted as the person being unmotivated. Aware-ness and sensitivity to cognitive deficits can enhance ther-apeutic rapport, identify those needing detailed assessmentand informways inwhich treatment delivery can be modifiedto capitalise on cognitive strengths.

During the weeks following inpatient admission, dualdiagnosis patients are likely to experience cognitive deficitsthat may limit the extent to which they can engage in andprofit from treatment. Psychological interventions requiringhigher-order cognitive functions should be delayed untiltime-dependent recovery has occurred. Particularly since thedeficits that differentiated dual diagnosis and schizophreniapatients were those showing little recovery after four weeksof abstinence among non-schizophrenia alcohol dependentpatients (Manning et al., 2008). Most psychological interven-tions place heavy demands on efficient cognitive functioning.For example, CBT requires the patient to attend to informa-tion, recall past experiences, hold onto thoughts and followconversation (working memory), integrate information, setand work towards new goals and self-monitor. Relapse Pre-vention requires cognitive flexibility — weighing up pros andcons of substance use and places heavy demands on theinitiation of ideas, problem-solving, inhibition and planning.It is therefore likely that with dual diagnosis patients suchinterventions may have limited effectiveness. If not graspedduring treatment, patients are unlikely to apply learnedpsychological constructs/principles to the real world to sup-port their long-term recovery. The 'revolving-door' pattern ofservice use associated with the dually diagnosed might bepartly due to the adverse effects of impaired cognitive func-tioning as a mediator of treatment engagement, effectivenessand outcome (Teichner et al., 2002; Carlsonn et al., 2006).

However, psychological interventions can be modified intheir delivery to accommodate attention, memory and exec-utive functioning deficits. For patients with limited atten-tional capacity, the length and pace of sessions should bereduced and their frequency increased. The amount ofmaterial covered in sessions and group sizes should beminimised to reduce distractions. To accommodate memorydeficits, regular use of summaries and reviews, repetition,rehearsal and simplified questions should be of benefit. Forexecutive function deficits, clinicians should encourage self-monitoring to support problems with impulsivity and adoptmore directive and highly structured therapeutic styles wherethere are problems with mental flexibility, poor initiation andproblems generating ideas. Finally, the use of goal ladders canhelp clients to organise and prioritise, when there areproblems in planning ability.

There are also implications for the rehabilitation ofcognitive deficits. Cognitive functioning can decline in theabsence of intervention among patients with schizophrenia(Heaton et al., 2001; Eberhard et al., 2003). Cognitive reme-

diation therapy (CRT) has been used to ameliorate cognitivedeficits in patients with schizophrenia (Krabbendam andAleman, 2003; Wykes et al., 2002, 2003) and targets thecognitive functions that differentiated schizophrenia fromdual diagnosis patients in this study. A recent meta-analysisfound that cognitive remediation was associated with im-provements with a medium effect size for cognitive perfor-mance and psychosocial functioning. It is concluded thatwhen combined with psychiatric rehabilitation, CRT mayimprove functional outcomes (McGurk et al., 2007). Mostrecently, Silverstein et al. (2009) found greater attentivenessin group sessions and higher levels of skill acquisition amongschizophrenia patients who had undergone ‘attention shap-ing’ — a behavioural intervention aimed at improving atten-tiveness and learning of social skills. The authors suggest thatenhancing cognitive abilities within the environmentalcontext where difficulties are experienced could lead togains in attention and functional outcome. Unfortunately aswith most CRT studies, patients with co-morbid substanceuse disorders were excluded and thus the feasibility and ef-fectiveness of these interventions with the dually-diagnosedremains untested. The rehabilitation of memory and execu-tive functioning deficits is important because these functionsare needed not only to profit from treatment but to initiateand maintain behaviour change (abstinence). They are alsovital to support psychosocial adaptations after treatment(Marlatt and Gordon, 1985; Tiffany, 1990) and to achieveoptimal long-term outcomes. Poor mental flexibility mayleave dual diagnosis patients less capable of adoptingappropriate coping methods in the face of alcohol craving.

Interpretation of these findings should take account of thestudy limitations. It is not possible to infer direction of effect.It is possible that these deficits existed before patientsengaged in problem drinking, although the absence of adifference in premorbid IQ between the clinical and controlgroups does not support this. Longitudinal research such asthat undertaken byHarrison et al (2007), is needed to addressremaining questions on the nature and course of cognitiveimpairment and its temporal relationship with the onset ofschizophrenia and alcohol use disorder. Also, the observedcognitive deficits cannot be attributed to alcohol alone sincemany of the dual diagnosis patients were also illicit drug(mainly cannabis) users. It should also be acknowledged thatthe study provides evidence for the additive effect of alcoholuse disorder above and beyond the deficits typically seenamong patients with schizophrenia. More compelling evi-dence for the additive effect will emerge if these findings arereplicated in a second comparison population of alcohol de-pendent patients without schizophrenia. However findingappropriate matches in terms of the severity of alcohol prob-lems among both groups of inpatients is a highly importantyet improbable task.

Despite these limitations, the study supports the hypoth-esis that alcohol use disorders are associated with greatercognitive impairment among schizophrenia patients. Some ofthe patients with the greatest clinical need may profit theleast from treatment, leading to poorer outcome and a likelyreappearance in treatment. Cognitive impairment has a directand indirect impact on functional outcome (independent,interpersonal and vocational functioning) (Evans, 2003;Green et al., 2000; Meltzer et al., 1996). Dual diagnosis

103V. Manning et al. / Schizophrenia Research 114 (2009) 98–104

patients constitute one of the most challenging client groupsfaced bymental health and addiction services today and placea considerable burden on resources, (Kastrup, 1987; Drakeand Wallace, 1989; Wright et al., 2000). An understanding ofthe nature and severity of cognitive impairments in the du-ally diagnosed and their prognostic implications can helpinform diagnosis, treatment and rehabilitation planning.Future research should explore the role for cognitive rehabil-itation of memory and executive function deficits and aimto advance our understanding through the application ofintegrated neuropsychological and neuroimaging techniqueswith this population.

Role of funding sourceFunding for the wider study in which the current study was embedded

was from the Department of Health Drugs Misuse Research Initiative Grant.The funders had no further role in study design; in the collection, analysisand interpretation of data; in the writing of the report; and in the decision tosubmit the paper for publication.

ContributorsVM managed the literature searches, designed the study, collected and

analysed the data, undertook the statistical analyses and wrote the first draftof the manuscript with the help of MG. SB provided training in theadministration of neuropsychological tests and assisted the interpretation ofthe neuropsychological data. SW, DB, SB, JS and MG contributed to thesubsequent drafts of the paper. All authors approved the final version.

Conflict of interestThere was no conflict of interest including financial, personal or other

relationships with other people or organisations.

AcknowledgmentsThe authors would thank Ms Emily Titherington and Ms Isabella

Schrover for their help in data collection and scoring of neuropsychologicalassessments.

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