151

JAD

OOX4S

Mania and seasonality

in the southern

hemisphere

Summary

Admissions for mania over a 9-year period in New Zealand were analysed, including data from four separate regions spanning nine degrees in latitude. A spring/summer peak of admissions for mania was found. The four regions showed marked, unexpected variability in seasonality. Regression analyses were performed to test the association of admissions for mania, in the month of admission and the previous month, with mean daily temperature, day length, hours of bright sunshine and mean relative humidity plus the rate of change of each of these variables.

Key words: Mania;

Seasonality;

Latitude:

Climate

Introduction Admission rates for mania have research advantages in studying the relationship bctwcen affectivc disorders and season, in that manic episodes are discrete, often lead to admission and most importantly, the time between onset and admission is relatively short. Winokur (1976) found 66% of admissions for mania occurred within 1 month of onset. Most studies of admission rates for mania have found a spring or summer peak (Symonds and

Address Wellington

for

correspondence:

School

South, New Zealand.

of Mrdicinc.

Professor P.O.

G.W.

Box 7343,

Mellsop, Wellington

Williams, 1976; Walter, 1977; Myers and Davies, 1978). However, others have found a bimodal distribution with peaks in spring and autumn (Frangos et al., 1980; Rihmer, 1980) or have not found any significant seasonal variation (Eastwood and Stiasny, 197X). In the southern hcmisphere, Parker and Walter (1082) found a spring peak in New South Wales, Australia, and Mulder et al. (1990) found a spring/ summer peak in New Zealand. If these findings arc true, the question becomes, through what variable or combination of variables dots season act. Consideration has been given to social and psychological factors, to day length (photoperiod) and temperature (Silverstone and Romans-Clarkson, 1989); rate of increase in bright sunshine and solar radiation

(Parker and Walter, 1982); rate of change in day length (Arendt, 1989); air ionisation, barometric pressure and relative humidity (Mawson and Smith, 19X1>. Myers and Davies (1978), in a large study in the United Kingdom, obtained their highest correlation between monthly admission rates for mania and current month’s temperature followed by previous month’s day length and hours of sunshine. Carney et al. (19X8), in a smaller study in Ireland, found the reverse with day length and hours of sunshine in the month of admission for mania being the closest correlates. Peck (1990) reanalysed the latter study’s data to conclude that hours of sunshine in the previous month was the significant correlate. To investigate the effect of latitude, Abe (196.3) correlated psychiatric admissions with latitude in Japan to find a spring peak which was later in timing with increasing latitude. Unfortunately, in Abe’s study it is not clear to what extent admissions for mania, as opposed to other psychiatric diagnoses, contributed towards the peak. Southern hemisphere studies may be valuable in teasing out the relative importance of variables such as public holidays, some of which occur at the same time of year in both hemispheres. Muldcr et al. (1990) analysed admissions for mania in New Zealand over a S-year period from 1980 to 19X4. Pooling data over this period they found a significant spring/summer peak which was not constant from year to year. Breakdown of their data by sex, age and admission status (first admission versus readmission) showed considerable variability in seasonality between subgroups with female first admissions in the Xl-49-year-old age group showing the greatest seasonality. Latitude was of interest for two reasons. Firstly, rate of change of day length not only changes with time of year being maximal in spring, but also increases with increasing latitude. If the spring/summer peak in admissions for mania does relate to rate of change of day length it could be hypothesised that with increasing latitude, i.e., in the southern hemisphere. the further south, the timing of the peak would be earlier and/or the magnitude of the peak greater. Secondly, it could be hypothesised that the spring/ summer peak for mania is representative of the

‘rccovcry’ phase of seasonal affective disorder (SAD or ‘winter depression’ as first defined by Rosenthal et al., 1984). A proportion of patients with SAD have manic or hypomanic episodes in the spring/summer following winter depression (Rosenthal and Wehr, 1987). As there is some evidence that the prevalence and duration of SAD increase with increasing latitude (Potkin et al., 1986; Lingjaerdc et al.. 19X6), if the spring/ summer peak for mania has some relationship with SAD, the peak could be expected to be late] and/or of greater magnitude with increasing latitude. This study aimed to more firmly establish the seasonality of mania in the southern hemisphere by expanding on the earlier study by Mulder ct al. (lY900, by analysing admission rates in New Zealand over a 9-year period. The second aim was to again examine the correlation of the admission rates for mania with climatic variables in the month of admission and the previous month. considering the lack of consistent findings in the literature. Lastly, the study aimed to investigate the effect of latitude. To summarisc, the study aimed to dcterminc: (1) If a spring/ summer peak for admissions for mania in New Zealand could bc dcmonstratcd over a O-year period (1979-19873. (2) If admission rates for mania varied in a predictable relation to: (a) day length; (b) recorded hours of bright sunshine; (c) mean daily tcmpcrature; (d) mean relative humidity; (e) rate of change of each of these. (3) If any dcmonstratcd seasonal pattern differed by latitude in either the timing or magnitude of the spring/ summer peak. Method Admission datu These were provided by the National Health Statistics Ccntre. Department of Health. They made available the total of admissions per month for each of the years 1979-1987 inclusive, with a diagnosis of mania (ICD-9 296.0 manic-depressive psychosis, manic type. and ICD-9 296.3 manic-depressive psychosis, circular type currently manic). Data were provided for the whole of New Zealand and for each of four Ilospital

1.53

Board regions: (Christchurch) Climatic

Auckland, Wellington, and Otago (Dunedin).

FREQUENCY

Canterbury

600

data 300

The monthly recorded hours of bright sunshine, mean daily temperature, mean relative humidity were obtained from the New Zealand Meteorological Service’s yearly publications of meteorological observations (N.Z. Met. S. Misc. Pub. 109 1979-1987). Data were used from the recording sites: Auckland City (36” 51’ South); Kelburn, Wellington (41 o 17’ S); Christchurch Airport (43 o 29’ S) and Musselburgh, Dunedin (45 o 54’ S). Data on day length for each of the four regions were obtained from sun tables provided by the National Observatory of New Zealand.

200

lo:: JAN

J

MAR

APR

MAY

JUN

JUL

ADMlSSlON

Fig.

I.

Monthly

admission Zealand,

AUG

SEP

OCT

NOV

DEC

MONTH

frequency 1979-1987.

for

mania

in New

and Fig. 1). The influence of the seasonal cycle on admission underwent considerable variation from year to year. In some years it was not apparent at all, while in other years (e.g., 1983) it was present as a strong trend. It can be seen in Fig. 1 that November and January were found to bc the peak months. The curve for the latter part of the year (from June to November) shows a gradual increase that could be readily attributed to seasonality. However, from December to March the admission rate becomes erratic and passes through a series of peaks and troughs indicating that in these months the seasonal influence has been modified by other factors. The seasonality of admission frequency varied widely between the Hospital Boards chosen. Canterbury demonstrated an even, regular seasonal cycle which was highly significant. Admissions in Auckland were also influenced by a seasonal cycle but to a lesser degree. Wellington and Otago

Statistical analysis Seasonality was examined using the test for seasonality developed by Walter and Elwood (1975). A series of multiple regression analyses were employed to ascertain the most relevant climatic variable influencing admission frequency. To avoid multicollinearity problems separate regression models were used for climatic data relating to the month of admission and the month prior to admission. The distribution underlying admission frequency is Poisson, and thus the square root transformation was used to ensure valid F-tests. Results Pooling the admission data for the total 9-year period did show significant seasonality (Table 1 TABLE

FEB

1

MONTHLY 1Y7Y-1Y87

FREQUENCY

OF ADMISSIONS

FOR

MANIA

AND TEST

FOR

SEASONALITY

BY AREA

OF DOMICILE,

Region

Jan

Feb

Mar

Apr

May

Jun

JUI

Aug

Sep

Ott

Nov

Dee

Total

Auckland * Wellington (NS) Canterbury (Christchurch) Otago (NS) (Dunedin) New Zealand total * *

157 68 79 46 500

100 45 61 34 376

127 73 47 44 442

104 42 80 36 392

105 46 65 3’) 410

98 5h 61 38 391

110 64 75 48 429

118 58 72 41 441

112 50 79 34 429

121 46 94 33 442

136 62 95 39 491

105 4X 76 40 417

1393 658 884 472 5 160

**

* P < 0.05; * * P < 0.001; * * * P < 0.005; NS, not significant.

I’ERC‘ENTAGE SONAI.

C‘ONTRIUUTION

VARIATIONS

OF

IN ADMISSIONS

7.X’