Estimating cognitive deterioration in schizophrenia
G. P. Amminger, J. Edwards, P. D. McGorry

Two recent studies failed to establish a relationship between the duration of untreated psychosis (DUP) and cognitive deterioration in first-episode patients (Barnes et al, 2000; Norman et al, 2001). Both studies used the premorbid IQ (estimated using the National Adult Reading Test (NART)) minus the current full-scale IQ (measured using the Wechsler Adult Intelligence Scale (WAIS)) to measure cognitive deterioration. The validity of this approach to assessing cognitive deficit is open to question.

We examined DUP and cognitive deterioration in 42 individuals (mean age 22.3 years; s.d.=4.1) with first-episode schizophrenia (Amminger et al, 2002). The revised version of the NART and WAIS (WAIS—R) were administered at clinical stabilisation and we have since taken the opportunity to apply the NART IQ minus WAIS—R full-scale IQ approach. Current IQ was higher than the estimated premorbid IQ in 38.1% of cases, suggesting an IQ increase.

The NART has been validated in older samples. We were therefore interested in the relationship between age at admission and IQ measures. NART IQ, but not WAIS—R full-scale IQ, was positively correlated with age at admission in our sample, (r=0.331, P=0.032). The WAIS—R ‘ vocabulary’ sub-test, suggested to be a better estimate of premorbid IQ than the NART (Russell et al, 2000), had also no relationship with age. It is possible that the NART underestimates premorbid IQ in young people with schizophrenia.

Age-standardised WAIS sub-tests are another method to estimate cognitive deterioration (Bilder et al, 1992). Performance on ‘information’ and ‘ vocabulary’ sub-tests are relatively stable, whereas the ‘ digit symbol’ sub-test is sensitive to brain insult. Bilder et al's (1992) deterioration index (DI), Math is based on the principle that a larger discrepancy between an individual's best and poorest performance on cognitive functions suggests cognitive loss. We found longer DUP, male gender, higher NART IQ and younger age at depression to be independent significant predictors of the DI (Amminger et al, 2002).

A cross-sectional test score (e.g. low-average full-scale IQ) cannot indicate deterioration on its own. In the absence of longitudinal data, indices reflecting decline from premorbid levels of functioning are required. Limitations of proxy methods need to be considered and studies which aim to validate measures of cognitive deterioration should be pursued.


Author's reply

Amminger et al raise some interesting issues. I certainly agree that the estimation of premorbid IQ, particularly in patients with schizophrenia, is challenging and that further validation studies on methods for making such estimates should be pursued.

More specifically with reference to our earlier paper on the relationship of DUP to cognitive functioning (Norman et al, 2001), Amminger et al argue for the likely superiority of Bilder et al's (1992) index as a measure of cognitive deterioration in contrast to estimates based on NART-estimated premorbid IQ minus current WAIS full-scale IQ. In this respect they note that 38.1% of patients in a recent study by their group showed higher current IQ than NART-estimated premorbid IQ. This would, of course, suggest an increase in IQ after illness onset — an unlikely occurrence. I have examined this issue in our data-set and found such a pattern in 17.8% of our sample, with the average discrepancy among these individuals being 8.4 points. I can also confirm that in our sample, as in Amminger et al's sample, NART scores were correlated with age at admission (r=0.24, P<0.05), but WAIS—R full-scale scores were not.

The substantive question, of course, is whether DUP is related to cognitive deterioration. Amminger et al report that they have found DUP related to deterioration based on Bilder's index. We had reported some results using Bilder's index in our earlier paper. I will take this opportunity to report further that when we examined correlations between our two indices of DUP and Bilder's deterioration index they were non-significant (r=0.06 and r=0.04). We are currently pursuing the issue of whether DUP may be related to recovery of cognitive functioning during the first year of treatment.

The discrepancy between our earlier findings and those of Dr Amminger and colleagues does not appear to be explained on the basis of use of the NART rather than Bilder index. Other variables related to sample composition may be relevant. Also of potential importance is the method of measuring DUP, which, as has been suggested elsewhere (Norman & Malla, 2001), also needs to be more carefully considered and standardised. In this, as in many areas of psychiatric research, cumulative progress is dependent on careful and comparable measurement across studies. I endorse Amminger et al's comments in this respect.