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The present study compares prevalences of SAD and
S-SAD in Winnipeg residents of 1) wholly Icelandic descent and 2)
non-Icelandic descent. A finding of significantly different prevalences
of SAD and S-SAD in these 2 groups would presumably strengthen the
case for the importance of genetic factors in the etiology of SADs.
Methods
Means of Measurement
Prevalence rates of SAD and S-SAD were measured in the 2 study
populations using the SPAQ, an instrument for investigating changes
in mood and behaviour with the seasons (3). The SPAQ has several
components, including 6 scales that measure seasonal variations
in mood, appetite, weight, sleep, energy, and social activity. The
sum of the scores on these 6 scales yields the seasonality score
(SS), which can range from 0 to 24. Further, the questionnaire evaluates
the degree to which seasonal changes are experienced as a problem
and identifies the months of the year in which subjects feel best
and worst.
The SPAQ-based criteria for SAD, and other psychometric properties
of the SPAQ, have recently been reviewed (20,21). Further, the SPAQ-based
diagnoses were checked against clinical diagnoses, with which they
agree reasonably well (22–24). Unfortunately, not all SPAQ-based
studies use the same criteria for SAD and S-SAD. In the present
study, our criteria for SAD were an SS ³ 11, “feeling
worst” in a period including January or February, and experiencing
seasonal changes as a problem to at least a “moderate”
degree. Our criteria for S-SAD were either 1) SS ³ 11 and experiencing
seasonal changes as a problem “mildly” or “not at
all” or 2) SS = 9 or 10 and experiencing seasonal changes at
least mildly. Again, the period of feeling worst had to include
January or February.
Study Populations and Survey Methods
Previous studies have identified a population of
wholly Icelandic descent living in Manitoba (25,26). Like the population
of Iceland, from which it cannot be distinguished anthropometrically
(27), this population exhibits a remarkable degree of genetic homogeneity
(28).
In the present study, 250 adult Winnipeg residents of wholly Icelandic
descent were identified, and in January 1993, we sent them the SPAQ,
together with a covering letter. Of the 250, 210 responded (84%).
In the final statistical analysis, 6 sets of answers were excluded
because of missing values. Thus, the final Icelandic study population
comprised 204 individuals, ranging in age from 18 to 74 years.
To construct the non-Icelandic population, we purchased a random sample of 1000 residential addresses from the Manitoba Telephone System database. This database, which is updated frequently, contains the names, addresses, and telephone numbers of all telephone-service subscribers in Winnipeg. The SPAQ was sent, along with a covering letter, to all 1000 addresses during the first half of June 1993. One-half of the letters asked for a male respondent, and one-half asked for a female respondent. Of these mailings, 949 were received by addressees, and 51 were returned as undeliverable. Then, 3 weeks after the initial mailing, a second mailing went out to those who had not returned the questionnaire from the first mailing. The Winnipeg Area Study arranged and administered the purchase of the address list and the mailings. We ultimately received back 515 filled-out questionnaires (54.2%). In the final statistical analysis, 66 sets of answers were excluded due to missing values. The final non-Icelandic study population, therefore, comprised 449 individuals, ranging in age from 18 to 74 years.
Icelandic Canadians in Manitoba form a culturally coherent group,
which has traditionally exhibited an enthusiastic willingness to
participate in scientific studies with an Icelandic connection.
The high rate of response to our mailed questionnaire was not unexpected
for this group. In contrast, our non-Icelandic population was drawn
at random from among the adult residents of Winnipeg, who form a
more culturally diverse group than the Icelandic Canadians and have
no tradition-bound enthusiasm for projects connected with Iceland.
The Icelandic study population was significantly
younger than those in the non-Icelandic population (P = 0.005).
Women outnumbered men in both populations, but there was no statistically
significant difference in the distribution of men and women in the
2 populations. Table
1 provides an overview of the demographic features of both the
Icelandic and the non-Icelandic respondents.
Statistical Analysis
Age standardization was performed using the world
standard population, truncated to ages 15 to 74 years (29). We used
multiple linear regression to find the dependence of SS on age,
sex, and descent (non-Icelandic or Icelandic). Age- and sex-standardized
prevalence rates with 95% confidence intervals (CIs) were computed
using the program package CIA (30). Because these CIs are approximate,
they are not suited to the statistical comparison of prevalence
rates based upon small samples.
Odds ratios (ORs) between the 2 population groups
were calculated by logistic regression adjusted for age and sex
for 1) SAD versus S-SAD plus No SAD; 2) S-SAD versus No SAD; and
3) SAD plus S-SAD versus No SAD.
In addition, polytomous logistic regression was applied to fit a
model to the data, using the program package SPIDA (31). Polytomous
logistic regression is an extension of logistic regression, allowing
the dependent variable to take values in addition to 0 and 1; in
this case, it allows 3 values: 0 = No SAD, 1 = S-SAD, 2 = SAD. In
this way, odds ratios for each of SAD and S-SAD versus No SAD can
be computed simultaneously.
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Results
Seasonality Scores
The age-standardized SS proved markedly higher in
the non-Icelandic population than in the Icelandic population.
In all age groups in the non-Icelandic population
and in all but the youngest and oldest age groups in the Icelandic
population, the SS was higher among women than among men.
Multiple linear regression analysis showed that the
mean SS decreased with increasing age by about 0.67 per 10 years
(P < 0.001) in the combined Icelandic and non-Icelandic
population. The regression coefficient for woman versus man was
1.09, P = 0.001; for non-Icelandic versus Icelandic descent,
it was 1.51, P < 0.001.
Seasonal Affective Disorders
The age- and sex-standardized prevalences of SAD
and of combined SAD plus S-SAD were markedly higher in the non-Icelandic
population than were in the Icelandic population (Table
2): 9.1% versus 4.8% for SAD (P < 0.01) and 25.0%
versus 17.7% for SAD plus S-SAD (P < 0.01). The age- and
sex-standardized prevalence of S-SAD was higher in the non-Icelandic
population than in the Icelandic population (15.9% vs 12.8%), but
this difference was not statistically significant (P >
0.10).
Sex- and age-adjusted ORs (non-Icelandic vs Icelandic) for SAD,
S-SAD, and SAD plus S-SAD were calculated first by logistic regression.
For SAD, the OR proved to be 3.3 (P < 0.008); for SAD
plus S-SAD, it was 2.1 (P < 0.002). The ORs for SAD and
S-SAD were then recalculated using polytomous logistic regression,
which yielded similar results (Table
3).
Table 3
also shows the dependence of SAD and S-SAD upon age and sex. For
SAD, the calculated OR for age per 10 years in the combined Icelandic
and non-Icelandic population was 0.71 (P = 0.004). In other
words, if 2 individuals of the same descent and gender are separated
in age by 10 years, the older individual is 29% less likely to suffer
from SAD than the younger. For S-SAD, the calculated OR for age
per 10 years was 0.80 (P = 0.005). In the combined population,
the OR for women versus men were 3.23 for SAD (P = 0.001)
and 1.46 for S-SAD (P = 0.10).
The variables weight, education, and marital status were also tested
but did not prove to have predictive significance for SAD or S-SAD.
Discussion
In the present study, we found that prevalences of
SAD and combined SAD plus S-SAD in Winnipeg residents of wholly
Icelandic descent differed significantly from the prevalences found
in Winnipeg residents of non-Icelandic descent. For those not of
Icelandic descent, the OR for SAD was more than 3 times as high,
and the OR for SAD plus S-SAD more than twice as high, as for those
of Icelandic descent. The prevalence of S-SAD differed noticeably
between the 2 groups, but this difference was not statistically
significant.
These results strongly support the suggestion of
earlier studies (9, 17–19) that genetic factors play an important
role in the etiology of SADs, for the above-mentioned differences
are evidently not explained by such environmental factors as ambient
light or climate, which appear to be identical for both populations.
Taken together with earlier findings, the present results hint
further at the etiologic importance of environmental factors unrelated
to latitude or ambient light. Magnússon and Axelsson (9)
found the prevalence of SAD in Manitobans of wholly Icelandic descent
who live in the Interlake District (50.5°N latitude) to be 1.3%.
The present study finds the prevalence of SAD in Winnipeg residents
(50°N) of wholly Icelandic descent to be 4.8%. According to
all available indications, the Icelandic population in Winnipeg
is not genetically distinguishable from the Interlake study population;
the 2 populations reside at an effectively identical latitude, being
separated by only 90 km. That the genetic similarity of these populations
has not been verified by the analysis of genetic material but relies
instead upon anthropomorphic data (27,28) is a limitation of the
present study. DNA analysis was not possible at the time when the
series of studies upon which the current findings are built was
initiated (9,11,17,25,26). However, it may soon become practical
to assess the genetic similarity of the study populations by this
means.
Prior to the present study, the direct variation with latitude in
the prevalence of SADs appeared to hold for genetically comparable
populations. The differences in the prevalences of SAD now found
in the genetically similar Interlake and Winnipeg populations do
not fit comfortably with this rule, but because the differences
are not statistically significant (P = 0.20), the point is
still inconclusive.
Further, it must be noted that the prevalence of SAD in the combined
Winnipeg and Interlake Icelandic populations (3.0%) is rather close
to the prevalence found in Iceland as a whole (3.6%), despite the
very large difference in latitude between the Manitoba locations
(50° to 50.5°N) and Iceland (63.4° to 66.5°N). For
combined SAD plus S-SAD, the comparable figures are 10.4% and 11.6%.
The descriptive element of the latitude hypothesis would lead us
to expect much larger differences, and only further investigation
can reveal why this expectation is not fullfilled.
The etiologic element of the latitude hypothesis, according to
which light deprivation is a principal causal factor in the development
of SADs, is still persuasive on the basis of the light-related,
seasonal onset, the remission of symptoms, and the efficacy of phototherapy.
Nonetheless, it is becoming clear that the expression of these disorders
is also heavily dependent upon genetic factors, and there are at
least some indications that environmental factors unrelated to latitude
or ambient light may also be important. This study does not indicate
what these additional factors might be; hence, this topic requires
further investigation. Discerning these factors would presumably
help us to supplement drug and phototherapy regimes in SAD treatment.
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