National Survey of Health and Development, University College London
Child Psychiatric Unit, Institute of Psychiatry, London
National Survey of Health and Development, University College London
Department of Psychiatry and Behavioural Sciences, University College London
National Survey of Health and Development, University College London
Correspondence: Marcus Richards, MRC National Survey of Health and Development, Royal Free and University College Hospital Medical School, University College London, Department of Epidemiology and Public Health, 1-19 Torrington Place, London WCIE 6BT, UK. Tel: +44 (0)207 679 1737; Fax: +44 (0)207 813 0280; E-mail: m.richards{at}ucl.ac.uk
Declaration of interest Funding was provided by the Medical Research Council.
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ABSTRACT |
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Aims To determine risk of affective disorder in those classified with mild learning disability in the British 1946 birth cohort and to investigate whether this risk was accounted for by disadvantage in childhood and adulthood.
Method Learning disability was defined as the equivalent of an IQ
69 at age 15 years. The Present State Examination at age 36 years and the
Psychiatric Symptom Frequency Scale at age 43 years provided psychiatric
outcome measures.
Results Learning disability was associated with a fourfold increase in risk of affective disorder, not accounted for by social and material disadvantage or by medical disorder.
Conclusions Learning disability is strongly associated with risk of affective disorder, persisting well into midlife.
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INTRODUCTION |
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METHOD |
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Learning disability status
In line with ICD-10 criteria for mild learning disability (F70)
(World Health Organization,
1992), participants were classified as having mild learning
disability if they scored 2-3.33 s.d. units below the mean on the general
ability score of the Group Ability Test AH4
(Heim, 1955), equivalent to an
IQ score range of 50-69. The AH4 is a 130-item timed test with separate verbal
and non-verbal sections that are summed to yield the general ability score,
administered at age 15 years (Pigeon,
1968). The verbal items consist of analogies, comprehension and
numerical reasoning, whereas the non-verbal items consist of matching, spatial
analysis and non-verbal reasoning. An adequate measure to assess whether
participants met the ICD-10 criterion of impaired adaptive behaviour was not
available.
Psychiatric outcomes
Two principal psychiatric outcome measures were used. The first was a
shortened version of the Present State Examination (PSE;
Wing et al, 1974) at
age 36 years. This is a structured interview to elicit symptoms primarily of
depression and anxiety. PSE caseness was determined by the index of definition
(Wing & Sturt, 1978), using the cut-off for threshold disorder or above. The second was the
Psychiatric Symptom Frequency (PSF) scale
(Rodgers, 1994) an
interview-based scale for symptoms of anxiety and depression. A total symptom
score was obtained and a cut-score of 31 for classifying caseness in terms of
psychiatric disturbance gave an identical prevalence rate to that obtained
using the PSE (Paykel et al,
2001). This also allowed a measure of chronicity, where survey
members were classified as meeting case criteria for neither outcome (PSE and
PSF), either outcome or both outcomes. Neuroticism at age 26 years, measured
by the Maudsley Personality Inventory (MPI;
Eysenck, 1958), provided an
earlier indication of anxiety-proneness. Scores for all participants who
self-completed this instrument were included in the analysis. For descriptive
purposes, school teachers' behavioural ratings (disobedience, discipline
difficulties, restlessness in class, aggression and difficulty in making
friends) at age 15 also are reported.
Early background variables
The following variables were chosen to represent early social background
and early adversity:
For descriptive purposes, birthweight and age at reaching motor developmental mile-stones (age at first sitting up, standing and walking) also are reported.
Circumstances at age 36 years
The following variables were chosen to represent social, material and other
potential adverse circumstances at age 36 years:
Statistics
Logistic regression was used to determine the unadjusted association
between each early background variable (see above) and learning disability.
The association between learning disability and the principal psychiatric
outcomes (PSE at age 36 years and PSF at 43 years) also was investigated using
logistic regression. To examine the extent to which any association between
learning disability and adverse psychiatric outcome was accounted for by early
disadvantage, these associations were adjusted for the early background
variables. The extent to which any association was specific to midlife was
examined by adding MPI Neuroticism at 26 years to the models for ages 36 and
43 years (the influence of learning disability thus being assessed in relation
to early adulthood to these ages) and by adding the PSE at 36 years to the
model for PSF at 43 years (for similar reasons). Finally, to test whether any
association between learning disability and the PSE was accounted for by
current adverse circumstances, adjustment was made, in turn, for each variable
representing circumstances at age 36 years (see above).
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RESULTS |
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Early background variables
Table 1 shows that although
there was no gender difference in the likelihood of being classified as having
learning disability, those with learning disability were significantly more
likely than the comparison group to have had a manual social class background,
a mother with no educational qualifications, to be part of a large family and
to have experienced overcrowding at 15 years. There was no association at the
5% level between parental divorce or death and learning disability, or
hospitalisation for at least 3 weeks by age 5 years and learning disability
(no participant with learning disability had been hospitalised during
childhood or adolescence for diseases of the central or peripheral nervous
system).
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Those with learning disability were significantly more likely to have had low birth weight (<2.5 kg). However, there was no difference at the 5% level between those with learning disability and the comparison group in time to reach the motor developmental milestones of sitting, standing or walking.
Behavioural and psychiatric variables
There was clear evidence of mental disturbance in adolescence and middle
adult life in those with learning disability
(Table 2). Thus, they were
significantly more likely than the comparison group to be rated by teachers as
showing behavioural disturbances at 15 years, and were significantly more
likely to meet PSE and PSF case criteria at 36 and 43 years, respectively.
Indeed, joint risk of behavioural disturbance (defined as scoring at least one
of the five teacher ratings in Table
2) and PSE caseness was significantly greater in the group with
learning disability than in the comparison group (2=43.86,
P<0.001). Of the psychiatric measures, only MPI Neuroticism at 26
years (not shown) did not distinguish between the two groups (regression
coefficient=0.54 (95% CI -0.65 to 1.72), P=0.38).
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Table 3 shows that the associations between learning disability and the two principal psychiatric outcomes were not attenuated by adjustment for gender, early background variables and MPI Neuroticism. Indeed, coefficients for the PSE were strengthened by the inclusion of these variables. Nor were they attenuated by the addition of birth weight into the model (not shown). However, the association between learning disability and the PSF total score at age 43 years fell below the 5% level after adjusting for the PSE at age 36 years, suggesting no significant increase in cases of affective disorder after age 36 years.
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Because those with learning disability were more likely to have a father of manual social class, these analyses were repeated after weighting to allow for the stratified social class sampling used to select this cohort. The pattern of results was similar to that of the unweighted ones presented.
To test whether the association between learning disability and the PSE was accounted for by current adverse circumstances (i.e. at age 36 years), the variables representing these circumstances (see Method) were added in turn to the model, already adjusted for gender, the early background variables and MPI Neuroticism. Only financial hardship attenuated the association to any notable degree, although the coefficient remained significant at the <0.001 level, as it did after adjusting for each of these variables in turn.
Finally, in terms of chronicity, those with learning disability were
significantly more likely than the comparison group to meet the case criteria
for both psychiatric measures (2=28.45, P<0.001).
For meeting the criteria on neither measure, proportions were 67.5% and 90.4%,
for meeting the criteria on either measure they were 22.5% and 8.1% and for
meeting the criteria on both measures they were 10.0% and 1.5% in the group
with learning disability and the comparison group, respectively.
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DISCUSSION |
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These results also build on those of van Os et al (1997), who initially reported an association between low cognitive ability and affective disturbance in this cohort by showing that this association was not accounted for by adverse social or material circumstances in childhood or adulthood.
There are several potential limitations of this study. First, the number of learning disability cases was relatively small in this birth cohort, which limits the ability to perform stratified analyses, such as examination of gender differences. Second, two different instruments were used at different times to measure psychiatric disturbance. Both measure a similar spectrum of symptoms of milder disorder, and a PSF cut-score was employed to give similar case rates for the PSE index of definition (Paykel et al, 2001). However, there are significant methodological differences between the two instruments, and the validity of adjusting the PSF cut-score in this way depends on the assumption (for which there is some support) of constant prevalence over the 7 years separating the measures (Paykel et al, 2001). Third, the PSE and PSF were designed for the general population, and are not specifically validated for use in learning disability. Although standard diagnostic criteria may become less appropriate in the general population the more severe the learning disability (Royal College of Psychiatrists, 2001), we have sought to minimise this potential bias by excluding those with moderate, severe and profound learning disability.
Conversely, the Psychiatric Assessment Schedule for Adults with a Developmental Disability (PAS-ADD), an instrument specifically designed to diagnose and measure degree of psychiatric illness in learning disability (Moss et al, 1993), would not have been suitable for use in the general population. However, because this instrument is based in part on the PSE, the latter, although less likely to be valid for learning disability than the PAS-ADD, should have some degree of face validity in this population. Similarly, the PSF was relatively brief and administered by lay interviewers, and should not have presented an undue challenge to those with mild learning disability.
Potential reasons for affective disorder in learning disability
In summarising potential reasons for the high risk of affective disorder in
learning disability, Scott
(1995) identified four basic
factors: medical, including general conditions such as a brain disorder;
environmental neglect or inconsistency; an effect of learning disability
regardless of cause; and reverse causality (i.e. psychiatric disorder leading
to impaired intellectual performance). Of these, Scott notes that the last is
the least likely, because low intelligence usually predates the psychiatric
disorder. Regarding medical conditions, those with learning disability in this
study were no more likely to have been hospitalised in early life than the
comparison group, and there were no participants with learning disability in
the analysis who had been hospitalised at this time for central or peripheral
nervous system conditions. As with Maughan et al
(1999), those with learning
disability had significantly lower birth weight than controls. This is
consistent with evidence that birth weight is associated directly with
cognitive development (Breslau,
1995; Richards et al,
2001) and with susceptibility to stress
(Nilsson et al,
2001). Again, however, risk of affective disorder in learning
disability was maintained even after adjusting for low birth weight.
Furthermore, there was no evidence of delayed motor milestones in this group,
a phenomenon linked to risk of schizophrenia
(Jones et al, 1994)
and affective disorder (van Os et
al, 1997) in this cohort. The lack of clear involvement of
neurological disease or a neurodevelopmental syndrome is consistent with the
group in this study representing mild learning disability.
The second factor identified by Scott (1995) involves environmental neglect or inconsistency. As already noted, however, we did not find that adverse circumstances in childhood accounted for the increased risk of affective disorder in learning disability. Indeed, as Scott notes, "Being reared in adverse conditions does not explain the increased incidence of those disorders which have not been shown to be strongly related to any specific environmental influences, for example most emotional disorders" (p. 631). Maughan et al found that early social adversity played a more significant role in the NCDS. One possible discrepancy is that these authors included behaviour problems as an explanatory variable in this respect, whereas we treated these as outcomes.
The third factor identified by Scott (1995) is psychiatric disorder as an effect of learning disability regardless of cause. Adverse circumstances in adulthood did not account for risk of affective disorder in learning disability in the present analysis, just as adverse circumstances in early life did not. However, there may be more subtle factors that were not taken into account, such as coping capacity and self-worth (Scott, 1995).
Implications for later life
What are the implications for risk of affective disorder in later life
among those with learning disability? In a clinic-based study, Cooper
(1997) found higher rates of
anxiety and depression in those with learning disability aged 65 years and
older than in a younger group with this disability. The present results
suggest high risk of affective disorder, which, although not increasing over
time, is at least persistent in mid-life. If high risk of affective disorder
is confirmed in older people with learning disability at the population level,
then the strategic planning in the health and social services for older people
with learning disability (Holland,
2000; Department of Health,
2001) must also take account of psychiatric vulnerability.
Continued assessment of affective state in individuals with learning
disability as they progress through mid-life into old age is therefore of
considerable importance.
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Clinical Implications and Limitations |
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LIMITATIONS
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REFERENCES |
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Received for publication April 4, 2001. Revision received June 27, 2001. Accepted for publication July 6, 2001.