Is there change in intelligence quotient in chronically ill schizophrenia patients? A longitudinal study in twins discordant for schizophrenia.
BACKGROUND
Intellectual deficits are commonly found in schizophrenia patients. These intellectual deficits have been found to be heritable. However, whether the intellectual deficits change over time and, if so, whether the change is related with an increased genetic risk for the disease are not known.


METHODS
We investigated change of intelligence quotient (IQ) in a twin sample of chronically ill schizophrenia patients, the discordant co-twins and healthy controls during a follow-up period of 5 years. A total of 52 twins completed two IQ assessments: nine patients [three monozygotic (MZ) and six dizygotic (DZ)], 10 unaffected co-twins (three MZ and seven DZ) and 33 healthy control twins (21 MZ and 12 DZ).


RESULTS
A significant interaction effect over time was found between IQ measurement and illness (F=4.22, df=1, p<0.05), indicating that change in IQ over time is significantly different between the groups. A stable course in IQ over time was found in the patients with schizophrenia (mean IQ from 109.78 at baseline to 108.44 at follow-up) relative to both the healthy control twins who showed a small increase (from 114.61 at baseline to 119.18 at follow-up) (t=2.06, p<0.05) and the unaffected co-twins (from 111.60 to 117.60, t=-2.32, p<0.05). IQ change in the unaffected co-twins of schizophrenia patients was comparable with that in healthy control twins (t=-0.49, p=0.63).


CONCLUSIONS
Patients with schizophrenia in the chronic phase of the disease, but not the discordant co-twins, show a lack of increase in IQ, which is probably due to environmental (non-genetic) factors related to the disease.
Rudolf Magnus Institute of Neuroscience, Department of Psychiatry, University Medical Center Utrecht, The Netherlands. A.Hedman@umcutrecht.nl
A M
AM
Hedman
N E M
NE
van Haren
G C M
GC
van Baal
R G H
RG
Brans
R
R
Hijman
R S
RS
Kahn
H E
HE
Hulshoff Pol
D000328:Adult; D002908:Chronic Disease; D015331:Cohort Studies; D004200:Diseases in Twins / Q000175:diagnosis* / Q000235:genetics*; D005260:Female; D005500:Follow-Up Studies; D059647:Gene-Environment Interaction; D006801:Humans; D007360:Intelligence / Q000235:genetics*; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D011594:Psychometrics; D012016:Reference Values; D012559:Schizophrenia / Q000175:diagnosis* / Q000235:genetics*; D012565:Schizophrenic Psychology; D014429:Twins, Dizygotic / Q000235:genetics / Q000523:psychology; D014430:Twins, Monozygotic / Q000235:genetics / Q000523:psychology; D014888:Wechsler Scales / Q000706:statistics & numerical data; D055815:Young Adult