Premorbid performance IQ de®cit inschizophrenia

Amminger GP, SchloÈgelhofer M, Lehner T, Looser Ott S, Friedrich MH,Aschauer HN. Premorbid performance IQ de®cit in schizophrenia.Acta Psychiatr Scand 2000: 102: 414±422. # Munksgaard 2000.

Objective: Performance IQ (PIQ) is often lower than verbal IQ (VIQ) inschizophrenic patients. Whether PIQ<VIQ precedes psychotic symp-toms in schizophrenia remains uncertain.Method: We investigated premorbid IQ scores in 63 subjects assessed at achild and adolescent psychiatric unit (mean age 13.1 years, SD 3.2), whoat follow-up in adulthood (mean age 30.9 years, SD 3.9) received alifetime RDC diagnosis of schizophrenia-related psychosis, affectivedisorder, or no psychiatric disorder.Results: Premorbid PIQ<VIQ signi®cantly differentiated the groupswith schizophrenia-related psychosis and no psychiatric disorder.Subjects with schizophrenia-related psychosis had a signi®cantly lowermean value for premorbid PIQ, but not VIQ, compared to subjects whodeveloped affective disorder or subjects without psychiatric disorder.Conclusion: Our results emphasize premorbid intellectual de®cits inschizophrenia. Those de®cits might largely be in consequence of animpairment on the PIQ scale.

G. P. Amminger1,M. SchloÈgelhofer1, T. Lehner2,S. Looser Ott3, M. H. Friedrich1,H. N. Aschauer4

1University Hospital for Child and Adolescent

Neuropsychiatry, Vienna, Austria, 2Laboratory of

Statistical Genetics, The Rockefeller University, New

York City, 3Department of Medical Genetics and the

Division of Developmental Behavioral Studies, New

York State Psychiatric Institute, New York City, USA

and 4Department of General Psychiatry, University

Hospital for Psychiatry, Vienna, Austria

Key words: schizophrenia; longitudinal studies; risk

factors; intelligence; Wechsler scales

G. Paul Amminger MD, University Hospital for Child and

Adolescent Neuropsychiatry, Waehringer Guertel

18±20, A-1090 Vienna, Austria

Accepted for publication June 19, 2000

Introduction

A signi®cant number of adult patients with schizo-phrenia exhibit cognitive de®cits in childhood. Someearly controlled studies on intelligence in childrenwho later developed schizophrenia (1±3) found anaverage de®cit of about 10 points in full scale IQ(FSIQ). This result was con®rmed by more recentfollow-back studies (4, 5). In accordance with these®ndings are results from birth-cohort studies (6, 7)on intellectual de®cits in pre-schizophrenic subjects.Unfortunately, all those studies either do not pro-vide information on verbal and performance IQscores in their samples or the assessments are basedon group intelligence tests, and it is unclear howthese measures relate to commonly used intelligencetests.

It is also well established that individuals withschizophrenia perform below normal comparisons(e.g. 8, 9) and patients with affective disorders (10)on a variety of standardized measures of intelli-gence. In several studies, in which the Wechslerintelligence scales were used, performance IQ (PIQ)was consistently lower than verbal IQ (VIQ) amongschizophrenic patients (for reviews: 8, 11). Further-more, it appears that this IQ pattern (PIQ<VIQ) is

rather typical for schizophrenia (12). Despiteevidence that lower PIQ than VIQ can already bedetected in young schizophrenic patients at an earlystage of their illness (13), the question as to whetherthis cognitive pattern can be observed prior topsychotic breakdown is uncertain. The aim of thisinvestigation was to determine if subjects withschizophrenia are characterized by premorbid de®citpredominantly on the performance scale as com-pared to the verbal scale.

Based on knowledge from earlier studies inschizophrenia (8, 12, 13), we tested the followinghypotheses:

(1) Subjects premorbid for schizophrenia-relatedpsychosis were expected to have lower pre-morbid PIQs in comparison with their VIQs(PIQ<VIQ) than those premorbid for affectivedisorder and those without psychiatric disorderin adulthood.

(2) Premorbid PIQ and FSIQ were expected to belower in the group with schizophrenia-relatedpsychosis compared to the group with affectivedisorder and the group without psychiatric dis-order in adulthood; we did not expect premor-bid VIQ to differ among the outcome groups.

Acta Psychiatr Scand 2000: 102: 414±422Printed in UK. All rights reserved

Copyright # Munksgaard 2000

ACTA PSYCHIATRICASCANDINAVICAISSN 0001-690X

414

Material and methods

Subjects

We studied premorbid IQ scores in subjects who inadulthood developed schizophrenia-related psycho-sis (n=13), affective disorder (n=19) or nopsychiatric disorder (n=31). The mean age of thetotal group (n=63, male:female=44:19) at IQassessment (time 1) was 13.1 years (SD 3.2); themean age at diagnostic assessment in adulthood(time 2) was 30.9 years (SD 3.9).

Sample selection

Figure 1 displays the recruitment of the studysample. The study is based on a computer list ofsubjects (n=729), born between 1960 and 1971,who had been hospitalized at the UniversityHospital for Psychiatry, Vienna and had receiveda clinical diagnosis of schizophrenia (295.0±6,295.8±9), schizoaffective disorder (295.7) or affec-tive disorder (296.0±9) according to ICD-9 criteriaassigned at discharge from the hospital between1978 and 1992 (14). Of those patients, 40 had beenseen at the University Hospital for Child andAdolescent Neuropsychiatry, Vienna, prior to their®rst hospital admission in adulthood and receivedIQ assessment during this initial presentation. Toidentify a comparison group without psychiatricdisorder in adulthood we randomly selected another50 subjects, also born between 1960 and 1971. Theywere seen under the same conditions during thesame years at our child and adolescent psychiatricunit, but have not been hospitalized at our adultpsychiatric unit. (In those subjects we had nofurther information regarding psychiatric outcomesin adulthood at this point.) This procedure gave us atotal study sample of 90 subjects. Next, we ruled outcases from this sample with early-onset schizo-phrenia-related psychosis or affective disorder.Operational criteria checklist for psychotic andaffective illness (OPCRIT, version 3.31) (15) wasapplied to all available clinical information by arater blind to adulthood outcomes. Five cases metthe Research Diagnostic Criteria (RDC) (16) forschizophrenia-related psychosis and eight cases foraffective disorder at time 1. Those, and threesubjects diagnosed with epilepsy, were excludedfrom the study. Seventy-four subjects remained inthe study for diagnostic follow-up. All subjects (ortheir parents) were contacted by mail and informedabout our research project. In a subsequent semi-structured telephone interview conducted with thesubjects or with a parent, follow-up informationconcerning psychiatric problems and treatment inadulthood was collected. Subjects who were hospi-talized in other psychiatric clinics were asked for

permission to contact these institutions to requesttheir records. The RDC (16) were applied to clinicalinformation. Thirteen subjects ful®lled the lifetimeRDC for schizophrenia-related psychosis (10schizophrenia; three mainly schizophrenic schizo-affective disorder) at follow-up. Nineteen subjectsreceived a lifetime RDC diagnosis of affectivedisorder (four bipolar I disorder; 10 of majordepressive disorder; two of schizoaffective disorder,mainly affective, three intermittent depression).Two subjects diagnosed with schizophrenia andone with bipolar disorder were deceased at follow-up. Three subjects met RDC for anxiety disorder(all panic disorder), and three for substance abuse(drug and/or alcohol abuse). Thirty-one subjects didnot meet RDC for psychiatric disorder. In ®vesubjects diagnostic follow-up information wasinsuf®cient. Those, and subjects diagnosed withanxiety disorder or substance abuse were excludedfrom further analyses. Finally, we tested ourhypotheses in 63 subjects.

Table 1 shows number, sex, IQ test, childhoodpresenting symptoms, ages at assessments and agewhen ®rst met RDC in our study sample.

Assessments in childhood or adolescence (time 1)

IQs (FSIQ, VIQ and PIQ) were determined fromthe Wechsler Intelligence Scale for Children (WISC)(17) or the Wechsler Adult Intelligence Scale(WAIS) (18) during the subjects' initial presentationto our child and adolescent psychiatric unit. Fulltests, including all 11 WISC/WAIS subtests, wereadministered by trained and experienced clinicalpsychologists between 1975 and 1984. Subjectsunder the age of 16 years received the WISC andolder subjects the WAIS (see Table 1 for details).

The Wechsler intelligence scales are oneof the mostwidely used psychometric instruments. They providemeasures of global intellectual ability (FSIQ) withsubscores for verbal (VIQ) and non-verbal (PIQ)intelligence. The verbal scale is comprised of thefollowing subtests: information, comprehensionsimilarities, arithmetic, vocabulary and digit span.The performance scale is comprised of the followingsubtests: picture completion, picture arrangement,block design, object assembly and digit symbol(coding). Wechsler's verbal and performance scalesdichotomy was constructed before factor analytictechniques became standard in research. The con-struct validity of the scales was later empiricallycon®rmed. In a study using data from the WISC-Rstandardization sample Kaufman (19) identi®edthree factors, verbal comprehension (information,comprehension, similarities and vocabulary), per-ceptual organization (picture completion, picturearrangement, object assembly and block design)

Premorbid performance IQ de®cit in schizophrenia

415

and freedom of distractibility (coding, arithmetic anddigit span). The perceptual organization and verbalcomprehension factors bear a clear resemblance toWechsler's PIQ and VIQ scores. Furthermore,performance and verbal scale appear to represent`¯uid' and `crystallized' cognitive functioning. Fluidintelligence represents the reasoning ability, depen-dent ultimately on the neural ef®ciency of the brain,while crystallized intelligence, in contrast, is highlydependent on learning experiences (20). It is generallyassumed that the verbal scale subtests vocabularyand comprehension measure crystallized cognitivefunction, and performance scale subtests block

design and object assembly measure ¯uid ability(19, 20).

Clinical symptomatology at time 1 was re-ratedfor this study according to criteria developed byRutter et al. (21): (1) psychotic symptoms; (2)emotional disorder with or without disturbance ofconduct; or (3) conduct disorder alone.

Diagnostic assessments at follow-up (time 2)

Lifetime RDC diagnosis (16) for psychotic andaffective illness were generated using the OPCRIT

Table 1. Number, sex, IQ test, childhood presenting symptoms, ages at assessments and age when ®rst met RDC of subjects with schizophrenia-related psychosis, affective

disorder and no psychiatric disorder

Schizophrenia-related psychosis

(n=13)

Affective disorder

(n=19)

No RDC disorder

(n=31) x2 df

Male:female ratio 11:2 11:8 22:9 2.653 2

WISC:WAIS ratio 5:8 10:9 18:13 1.412 2

Childhood-presenting symptoms 10.563* 4

Psychotic symptoms, % 23.1 10.5 0

Emotional problems, % 30.8 63.2 74.2

Conduct problems, % 46.2 26.3 25.8

Mean (SD) Mean (SD) Mean (SD) F df

Age, at IQ measurement 14.5 (3.5) 13.7 (2.7) 12.2 (3.1) 3.111* 2, 60

Age, at follow-up 31.5 (3.9) 31.5 (3.2) 30.3 (4.2) 0.719 2, 60

Age, ®rst met RDC 18.9 (1.9) 19.0 (2.1) Ð 0.013 2, 29

* P<0.05.

Comparison group Target group/patient control group

72 subjects

presented to child/adolescent psychiatry

90 subjects

presented to child/adolescent psychiatryand received IQ-assessment

63 subjects

STUDY SAMPLE(13 schizophrenia-related psychosis,

19 affective disorder, 31 no RDC disorder)

11 subjects

excluded on bases of follow-up diagnosis(3 anxiety disorder, 3 substance abuse,

5 insufficient information)

16 subjects

excluded on basis of childhood diagnosis(5 schizophrenia-related psychosis,

8 affective disorder, 3 epilepsy)

Fig. 1. Recruitment procedure.

Amminger et al.

416

system (15) which was applied to all available casenotes and hospital records. In addition, we evalu-ated whether RDC for other psychiatric disorderswere ful®lled, and the age when subjects ®rst metdiagnostic criteria. The resulting RDC categorieswere: (i) schizophrenia-related psychoses, includingschizophrenia, mainly schizophrenic schizoaffectivedisorder, and unspeci®ed psychosis; (ii) affectivedisorders, including psychotic and non-psychoticaffective disorders (mainly affective schizoaffectivedisorder, major depression, bipolar I disorder,bipolar II disorder, hypomania and intermittentdepression); (iii) anxiety disorders (generalizedanxiety disorder, panic disorder, phobic disorder);(iv) substance abuse (drug and/or alcohol abuse);and (v) no psychiatric disorder.

Statistical analysis

For these analyses, we examined the 63 subjects whohad both a complete WISC or WAIS test at time 1 inchildhood or adolescence and follow-ups yieldingadulthood clinical diagnosis of schizophrenia-relat-ed psychosis, affective disorder, or no RDC disorderat time 2. Following Cullari (12) we computedPIQ<VIQ on the basis of a point difference ascategorical assessment of de¯ated PIQ relative toVIQ. Additionally, PIQ±VIQ difference score (PIQminus VIQ), used previously by Goldberg et al. (13),was used as dimensional assessment of an indivi-dual's PIQ and VIQ performance. WISC and WAISshare the same concepts for PIQ, VIQ, and FSIQ.The study is investigating those concepts and notconcerned with the actual IQ scores. We, therefore,separately transformed WISC and WAIS derived IQscores to standard scores (Z-scores; 22) and pooledthem for further analyses. Proportions of PIQ<VIQvs. PIQoVIQ by clinical outcome groups we exam-ined by x2 test, applying exact tests when cell sizeswere small.

We applied factorial analyses of variance (generallinear model, general factorial procedure) (23), usingthe PIQ±VIQ difference score and IQ Z-scores(FSIQ, VIQ and PIQ) as dependent variables.Where appropriate, Tukey's HSD (honestly signi®-cant difference) test adjusting for multiple compar-isons was used to examine group differences. Analpha level of 0.05 was applied for all statistical tests,and all tests were two-tailed. Data analyses werecarried out with the SPSS 7.5 for Windows (23).

Results

(i) Adulthood clinical outcome and premorbid PIQ<VIQ

The proportion of subjects with PIQ<VIQ in thegroup with schizophrenia-related psychosis was

69.2% (nine of 13 subjects), 36.8% (seven of 19subjects) in the group with affective disorder, and16.1% (®ve of 31 subjects) in the group with noRDC disorder. x2 2r2 tables con®rmed a sig-ni®cant group difference between schizophrenia-related psychosis vs. no RDC disorder (x2=11.905,P=0.001) and a difference at a trend level forschizophrenia-related psychosis vs. affective disor-der (x2=3.239, P=0.072).

(ii) Adulthood clinical outcome and premorbid PIQ±VIQ

difference score

Mean values for the PIQ±VIQ difference score werex7.1 (SD 15.9, range x33±29), 4.7 (SD 16.3, rangex33±44) and 12.1 (SD 12.4, range x9±35) inschizophrenia-related psychosis, affective disorderand no RDC disorder, respectively. One-wayANOVA indicated signi®cant group difference(F=8.207, df=2, 60, P=0.001), and post hoccomparisons between outcome groups revealed asigni®cantly lower mean value for the PIQ±VIQdifference score, indicating poor PIQ relative toVIQ, in the group with schizophrenia-relatedpsychosis compared to the group with no RDCdisorder (P<0.001), and at a trend level comparedto the affective disorder group (P=0.068). Thedistribution for the PIQ±VIQ difference scores byoutcome groups are plotted in Fig. 2.

(iii) Adulthood clinical outcome and premorbid PIQ, VIQ andFSIQ

After transformation to Z-scores and pooling of thegroups tested with the WISC and the WAIS, one-way ANOVAs indicated that both PIQ (F=10.634,df=2, 60, P<0.001) and FSIQ (F=6.155, df=2, 60,

Fig. 2. Distribution of premorbid PIQ±VIQ differencescores in schizophrenia-related psychosis, affective disorderand no RDC disorder.

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417

P=0.004) signi®cantly differentiated the outcomegroups. Tukey HSD pairwise multiple comparisontests supported our hypothesis that subjects withschizophrenia-related psychosis had a signi®cantlylower premorbid PIQ and FSIQ than subjects withaffective disorder (P=0.002, P= 0.007 for PIQ andFSIQ, respectively) and subjects with no RDCdisorder (P<0.001, P=0.006 for PIQ and FSIQ,respectively). One-way ANOVA for clinical out-come groups revealed no signi®cant group differenceregarding VIQ (F=0.504, df=2, 60, P=0.607).

Table 2 shows means (standard deviations) andrange for premorbid PIQ, VIQ and FSIQ in subjectsassessed with the WISC and the WAIS by adult-hood clinical outcome.

Discussion

In the present study, as expected, relatively poor pre-morbid PIQ in comparison with VIQ was observedsigni®cantly more often in individuals with schizo-phrenia-related psychosis compared to individualswith no RDC disorder, and at trend level seen moreoften in the target group than in affective disorder.Furthermore, the group with schizophrenia-relatedpsychosis, compared to the other outcome groups,obtained signi®cantly lower premorbid PIQ andFSIQ, while VIQ did not statistically differ amongthe groups. These ®ndings indicate that premorbidintellectual impairment in schizophrenia is largely aresult of an impairment on the performance scaleand correspond with several studies on intellectualfunctioning in schizophrenia (e.g. 8, 12, 13, 24±26).It is notable, however, that relatively low premorbidPIQ in comparison with VIQ was observable priorto the onset of schizophrenia. The premorbid PIQde®cit found in our target group is consistent with aprospective investigation by Ott et al. (27), whoreported premorbid PIQ, but not VIQ, at trend levellower in adolescents who developed schizophrenia-related psychosis in adulthood, compared withsubjects who developed major affective disorder or

no Axis I disorder. The appearance of PIQ de®citduring the premorbid phase is supported by birth-cohort studies (6, 7) and a population-based long-itudinal study (28) on school performance in Finnishchildren who developed schizophrenia, which allreported intellectual de®cits well before the onset ofpsychotic symptoms.

Premorbid PIQ±VIQ difference scores in subjectswith schizophrenia-related psychosis in our studywere remarkably similar to the distribution of PIQ±VIQ difference scores reported by Goldberg et al.(13, p. 263) in adolescent in-patients with psychoticdisorders. The high proportion of 69.2% of pre-schizophrenic subjects in our study characterized byPIQ<VIQ is consistent with Cullari (12) whoreported 68% with larger VIQ than PIQ in schizo-phrenic patients compared to 54% and 46% inpatients with affective and `other' disorder, respec-tively. The fairly consistent evidence that VIQ ishigher than PIQ among schizophrenic patients (8)contrast with Kljajic's report of identical frequenciesof PIQ<VIQ across ®ve diagnostic categories,including `schizophrenic' and `organic' (29).However, his experimental group comprised indivi-duals whose VIQs were 15 or more units higher thanPIQs, and controls whose VIQs were within 5 IQunits of their PIQs regardless of direction. Thismethodological procedure followed in the Kljajicstudy might account for the inconsistent ®ndings.

The results for the affective group in the presentstudy are supported by literature which indicatesthat individuals with schizophrenia perform belowindividuals with affective disorder on neuropsycho-logical tasks (10), and ®ndings that intellectualfunctioning in non-psychotic affective disordersdoes not differ signi®cantly from normal compar-isons (e.g. 30). Regarding premorbid psychiatricsymptomatology, emotional disorder was moreoften associated with adulthood outcomes ofaffective disorder and no RDC disorder, whereasin schizophrenia-related psychosis conduct disorderwas the most frequent presentation (see Table 1).This latter ®nding is in accordance with previous

Table 2. Premorbid IQ scores classi®ed by adulthood clinical outcome based on RDC diagnoses

Performance IQ Verbal IQ Full Scale IQ

WISCa WAISb WISC WAIS WISC WAIS

Adulthood clinical outcome Mean (SD) Range Mean (SD) Range Mean (SD) Range Mean (SD) Range Mean (SD) Range Mean (SD) Range

Schizophrenia-related 87.8 (10.1) 80±102 96.6 (16.6) 61±113 98.4 (15.7) 85±124 101.5 (15.1) 77±118 90.8 (10.8) 80±102 99.4 (14.6) 75±116

psychosis (n=13)

Affective disorder 105.2 (14.3) 90±134 114.2 (16.1) 90±136 102.5 (12.9) 85±123 107.3 (13.1) 91±130 106.0 (10.3) 86±119 112.0 (14.9) 91±134

(n=19)

No RDC disorder 112.8 (7.5) 99±127 113.2 (17.5) 77±140 99.8 (9.6) 87±120 102.3 (17.0) 75±125 106.7 (7.0) 98±123 108.5 (16.9) 78±135

(n=31)

a Numbers of subjects assessed with the WISC in schizophrenia-related psychosis, affective disorder, and no disorder are 8, 9 and 13, respectively.b Numbers of subjects assessed with the WAIS in schizophrenia-related psychosis, affective disorder, and no disorder are 5, 10 and 18, respectively.

Amminger et al.

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studies which reported high rates of behaviorproblems in children who developed schizophreniain adulthood (e.g. 5, 6, 31).

Premorbid PIQ de®cit and neurodevelopmental schizophrenia

The premorbid PIQ impairment in schizophrenia-related psychosis is consistent with the concept ofneurodevelopmental schizophrenia, which assumesthat a lesion occurring during early brain develop-ment may underlie childhood and adulthoodabnormalities (32, 33), and also with evidence thatintellectual de®cits in schizophrenia rather re¯ect astatic encephalopathy than a dementing disorder(34±36). Of speci®c interest with regard to our®ndings is a recent prospective study in children withserologically documented in utero exposure torubella, which found signi®cantly diminishedscores on WISC-R picture completion and blockdesign in childhood (both performance scale sub-tests) in subjects who developed psychosis inadulthood (37). However, there is a suggestionthat schizophrenia can be associated with excellentschool performance, indicating that only a subgroupof individuals with schizophrenia have premorbidintellectual de®cits (38). Deviation from the norm ineither direction might warrant further study as a riskfactor in schizophrenia.

Developmental implications

The development of schizophrenia is often precededby impairment of social competence and poorpremorbid adjustment in childhood followed byinterpersonal dif®culties and increasing psycho-pathology through adolescence and early adulthood(39, 40). Impairment in social competence might beassociated with de®cits in social cognition that maypredate psychotic symptomatology in schizophrenia(41). However, premorbid social de®cits in schizo-phrenia are only understood fragmentarily as to thenature of their underlying dysfunction (27).Cornblatt et al. (42) argued, based on ®ndings inclinically unaffected adults at genetically high-risk ofschizophrenia, that social information processingmight be disrupted by a chronic attention de®citleading to social indifference. On the other hand,perceptive abilities might also play an important rolein the development of social de®cits and relatedinterpersonal dif®culties (43). By the nature of theWechsler scales subtests it could be assumed thatboth attention de®cit and cognitive-perceptivedysfunction may contribute to diminished PIQrather than VIQ in individuals premorbid forschizophrenia.

What magnitude of PIQ±VIQ discrepancy isclinically relevant remains controversial. Kolb and

Whishaw (44), for example, proposed that adiscrepancy of 10 points informs much about achild's abilities and has practical (educational)signi®cance. In contrast, Kaufman (45) suggestedlimiting diagnostic interpretation to a wider dis-crepancy of 22 points or more, which he foundprevalent in less than 10% of the WISC-R standar-dization sample.

PIQ±VIQ discrepancy in normal children

Based on Kaufman's suggestion mentioned above,Mof®t and Silva (46) examined clinical signi®canceand developmental course of large PIQ±VIQdiscrepancy scores beyond the 90% percentile in alarge unselected birth cohort in a longitudinalprospective study, known as the Dunedin Healthand Developmental Study. In this investigationbrain dysfunction as de®ned by nervous systeminsult history, early neurological abnormality, orperinatal problems were not more prevalent amongchildren with PIQ±VIQ discrepancy beyond the 90%percentile (22±24 points or more) at the ages of 7, 9and 11 years than in children without this dis-crepancy. Additionally, the rates for motor skillsproblems and behavior problems in childhood werenot different between the groups, but signi®cantlymore problems with reading were observed in thediscrepancy group at age 11. Stability of substantialPIQ±VIQ discrepancy over two or more ages ofassessment was found to be moderate in 23% of thediscrepancy cases. Another 25% of the discrepancycases had PIQ±VIQ scores of 16±23 points andmaintained a notable amount of discrepancy, with-out meeting the 90% criterion. Stable discrepancycases were not signi®cantly different from non-stable cases for neurological or behavior problems.The authors concluded that substantial PIQ±VIQdiscrepancy is not the same sign in the generalpopulation as it is in subjects with neurologicaldisorders.

Regarding the direction of PIQ±VIQ discrepancyit is of interest that PIQ>VIQ was by far the mostfrequent pattern, seen in 85% of subjects withsubstantial PIQ±VIQ discrepancy at age 11.PIQ>VIQ, and not the reverse pattern, wasspeci®cally associated with poor academic achieve-ment at all ages throughout childhood.

Study limitations

Several limitations and possible shortcomings of ourstudy need to be mentioned. First, as the sample ishighly selected and comprised of subjects whoexperienced psychiatric problems in childhood oradolescence our ®ndings cannot be extended toschizophrenia in general. Also, PIQ<VIQ in the

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419

group with schizophrenia-related psychosis occurr-ed only in males. As there were only two females inthe target group, our results are limited to male pre-schizophrenic subjects. Notable in this context is arecent report by Purcell et al. (26), who found signi®-cantly more men than women with schizophrenia toexhibit PIQ<VIQ. The high proportion (84.6%) ofmale subjects in our group with schizophrenia-related psychosis, however, is consistent with theliterature on gender differences and premorbidfeatures in schizophrenia (47). Secondly, the sizeof the target group in our study is small. Thepossibility that our ®ndings were due to chancefactors is unlikely, however, because they were basedon directional hypotheses. Thirdly, considering theproportion of normal children with better verbalthan perceptional abilities (48, p. 741) and the factthat PIQ<VIQ can also typically be observed inchildren with Asperger's syndrome and autism (49),premorbid PIQ<VIQ is only a risk factor withoutspeci®city for schizophrenia. However, other well-established risk factors for schizophrenia, e.g. delaysin developmental milestones, abnormalities in socialfunctioning and behavior, cognitive de®cits, motordysfunction and attentional dysfunction, are alsonot speci®c to schizophrenia, although effects aremost dramatic in this group (50). Fourthly, howperformance and verbal scale tests relate to majorneuroanatomical and functional brain systems isunclear and our study does not provide informationregarding possible pathophysiological mechanismscausing poor PIQ. Although substantial PIQ<VIQis often interpreted as indicative of right hemisphericdysfunction and VIQ<PIQ to indicate left hemi-spheric dysfunction, this classi®cation is not sup-ported by neuropsychological studies (51). Becausethey are timed, performance scale tests might bemore susceptible than verbal scale tests to attentionde®cits and de®cits in motor co-ordination. Ourstudy does not control for attention de®cits andde®cits in motor co-ordination and therefore cannotestimate their in¯uence on PIQ in our sample.Fifthly, it could be argued that the fact that thegroup with schizophrenia-related psychosis scoresnearer the population mean of 100 regarding FSIQthan the two other groups accounts for groupcontrasts. Inspection of means for VIQ and PIQ,however, shows that FSIQ differences are mainly aresult of poor PIQ, whereas VIQ was almost equalamong the groups. Sixthly, due to the nature of ournon-psychiatric control group which has beenrecruited through a child psychiatric service the®ndings cannot be extended directly to unselectedpopulations.

A strength of the present investigation is the studysample which is derived from a larger total sample

using stringent selection criteria, excluding early-onset schizophrenia cases from the investigation.Additionally, except for one subject from theschizophrenia-related psychosis group and anotherfrom the affective disorder group, patients weredrug-free at the time of IQ assessment. Further-more, the long follow-up interval of almost 18 yearsand the mean age of 31 years attained at the end offollow-up makes it unlikely that individuals from thecontrol groups will develop schizophrenia in thefuture.

The ®ndings here emphasize that intellectualfunction in schizophrenia is, to a considerabledegree, independent of psychopathology and sug-gests a neuropsychological imbalance that may bestable over time and course of the disorder. Thisview is supported by results from family studies (52,53) showing subtle neuropsychological impairmentsin non-psychotic relatives of schizophrenic patientsthat are not associated with clinical symptoms ofschizophrenia. Given that IQ assessment clearlypredated the onset of schizophrenia-related psycho-sis (mean in years 4.2; SD=3.4; minimum=0.5,maximum=10.3), our study gives evidence thatlower PIQ relative to VIQ is part of the premorbidpicture of this illness. Prospective studies inpopulations at high risk for schizophrenia wouldbe necessary to investigate the predictive quality ofPIQ<VIQ for the development of schizophrenia.

Acknowledgements

The authors thank Dr L. Erlenmeyer-Kimling, Division ofDevelopmental Behavioral Studies of the Department of MedicalGenetics, New York State Psychiatric Institute, and Dr JeanAddington, Department of Psychiatry, University of Calgary, forhelpful discussion and reading of this manuscript. This researchwas supported in part by Oesterreichische Nationalbank grantNo. 7866 to Dr Amminger.

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