Gender and remission of mental illness.

Schimmele, Christoph M. ; Wu, Zheng ; Penning, Margaret J. 等

Gender differences in the prevalence of mental illness are well documented, (1-6) but whether gender also influences the timing of remission is unclear. Do the factors that contribute to a higher prevalence of illness among females also translate into a gender gap in the remission of illness? There is a good rationale to anticipate that gender is a factor in remission. For example, the literature suggests that gender dissimilarities in response to depression could lead to differences in the alleviation, complication or persistence of symptoms. (7-13) In addition, gender differences in the clinical features of illness could also influence remission. Yet, the literature provides inconsistent conclusions and largely focuses on mood disorders.

One group of studies argues that gender has a non-significant effect on remission. For example, Benedetti et al. (14) investigated whether gender influences the course of bipolar disorder. Their research showed that gender is non-significant in terms of the reduction of symptoms and number of recurrences of bipolar disorder. Benedetti et al. remarked that the effect of gender observed in other studies could be a result of ignoring dissimilarities in medical treatment. However, gender does not moderate the effect of pharmacological treatment for depression, according to Grubbe Hildebrandt et al. (15) Their research demonstrates that, given equivalent therapies, gender is a non-significant factor in the post-treatment outcomes of depression.

Other studies indicate that gender has a significant effect. (7-13) Riise and Lund (13) demonstrated that depressed females have a higher risk of depression at long-term follow-up. The authors observed that the risk of chronic depression among females increased after baseline level of illness had been adjusted for. Riise and Lund's findings could reflect a higher rate of relapse among females, not a lower remission rate, but they at least demonstrated that depression is a more persistent condition for females and that gender can influence long-term prognosis. Other studies confirm that females experience higher rates of chronic depression and relapse. (9-11)

Bland et al. (16) examined remission of several psychiatric disorders in a Canadian population and observed potential gender patterns that warrant the present research. However, their attention was directed toward age patterns of remission, and their analysis did not report whether there were statistically significant differences in gender patterns of remission. That said, Bland et al. presented some interesting gender-specific findings in 1-year remission rates. Their cross-tabulations suggest that there could be a female disadvantage. In general, about 26% of females achieve remission within 1 year, in contrast with 40% of males. Bland et al. also suggested that this female disadvantage is loaded on specific illnesses rather than representing a generalized effect.

The present research examines the effect of gender on the timing of remission (length of medical treatment) for mental illness. The analysis considers remission of a wide spectrum of mental disorders. Almost all previous studies have focused on a particular disorder or class of disorders, and most targeted depression. Hence, this research is an improvement on previous studies because it considers the general and disorder-specific effects of gender on remission and controls for the effects of comorbid conditions. Another limitation of previous studies is the reliance on data from a particular source (e.g., a specific psychiatric practice). Our data represent an entire population under treatment for a clinical mental illness in British Columbia. These data reduce the influence of potential site-specific biases, because they consist of hundreds of different treatment sites and service providers.

METHODS

Sample

Our analysis used longitudinal data from the British Columbia Linked Health Database (BCLHD), which includes datasets on physician and specialist visits, hospitalizations and hospital separations, and tertiary and extended care. (17) The datasets are linked to Medical Service Plan (MSP) records. The MSP is a single-payer medical insurance plan that conforms with the Canada Health Act, guaranteeing universal and comprehensive medical coverage for all "medically necessary" hospitalizations, outpatient treatments and extended care. About 95% of BC residents are enrolled in the MSP. (18) Persons with a diagnosis of a clinical mental illness (ICD-9 diagnostic codes 290-314) are eligible to receive public direct health care, as provided through provincial/ regional agencies, private practitioners and hospitals. The MSP does not offer comprehensive coverage for milder conditions (e.g., subsyndromal depression), but general practitioners often treat these, and such treatment is also an MSP-billable service.

The target population was all BC residents aged 18 and older who started treatment for an ICD-9 diagnosis of mental illness in 1990. The study followed a 10% random sample of these patients from 1990 to 2001. It excluded cases missing core variables (e.g., care episode and gender). The final study sample consisted of 5,118 female and 2,470 male patients, and a total of 10,137 care episodes (cases). These cases were complete for patients with admission dates in 1990 and discharge dates before or in 2001. The information for ongoing (censored) cases was unavailable.

Variables

Our dependent variable was length of treatment for a mental illness. It was time-invariant at the episode level but time-variant at the individual level when the respondent experienced multiple, non-concurrent care episodes (separate cases) during the period of observation. A "care episode" refers to MSP-billable contact with a health care professional, and it represents the formal diagnosis of illness and commencement of treatment. We subtracted the date of discontinuation of treatment from the date of first contact to measure the timing of remission (symptom resolution) in terms of days of treatment.

For our purposes, a psychiatrist grouped the numerous specific diagnoses of mental illness contained in BCLHD data into nine distinct classes of illness: 1) alcohol/substance abuse, 2) delirium, 3) psychoses, 4) mood disorders, 5) anxiety disorders, 6) adjustment disorders, 7) dementia, 8) conditions needing counseling (e.g., bereavement, relationship difficulties, school-related problems) and 9) other disorders (e.g., sexual disorders, sleep disorders, pain disorders). This categorical variable was introduced to control for the effect of type of illness and the effect of comorbid illness on the timing of remission. Table 1 presents the definitions and descriptive statistics for all selected variables.

The regression models included controls for several other control variables. The analysis controlled for age, marital status, Aboriginal status, geographic location and socio-economic status, which have well-established effects on the prevalence of mental illness, remission of illness and access to services. (5,16,19,20)

Statistical model

The generalized estimating equations (GEE) method (marginal models) was used to estimate average group-level (male-female) differences in length of treatment. Length of treatment was measured as a discrete count variable (days of treatment), which was assumed to follow the Poisson distribution. Because care episodes at the patient level are sequential, we treated them as repeated measurements in the longitudinal design. The GEE method is well suited for analyzing repeated measurements. (21,22) The GEE model allows the number and spacing of the repeated measurements to vary among individuals. It assumes that observations for each individual are correlated, though observations among individuals are assumed to be independent. We assumed that the correlation is constant (exchangeable) between any two observation times and used an exchangeable correlation model. The GEE models were estimated using the GEN-MOD procedure in SAS (Statistical Analysis System) version 9.1.

RESULTS

Table 1 presents a bivariate examination of gender differences in remission from mental illness. Length of treatment (symptom resolution) serves as a proxy for remission. These initial results illustrate an important and encouraging finding: gender appears to be a non-significant factor in remission from mental illness in general. The average length of treatment is about 208 days for females and 203 days for males, a small but non-significant difference in the timing of remission. This finding implies that gender disparities in mental illness do not complicate treatment or prolong remission. Hence, even though gender still could influence responses to mental illness, this does not seem to have an effect on the duration of illness.

Table 1 also illustrates the gender-specific frequencies of the types of mental illnesses that were treated. In general, mood disorders accounted for a large proportion of all treatment received and represented the most prevalent illness in this respect. About 24% of females and 21% of males were treated for a mood disorder. This represents a significant difference at the p<0.001 level. There are similar gender differences in the treatment of anxiety and adjustment disorders. About 13% of females and 7% of males were treated for an anxiety disorder, and 17% of females and 13% of males were treated for an adjustment disorder. Again, these differences are significant at the 0.001 level. There are also significant gender differences in the treatment of alcohol/substance dependencies, psychoses, delirium and dementia, as a higher proportion of males than females were treated for these illnesses.

As Table 2 suggests, gender differences in remission could depend on the type of illness. The average length of treatment for mood disorders was 264 days for females and 223 days for males, a difference of about 6 additional weeks of treatment for females. The length of treatment for females also appears to be longer for anxiety disorders (18 additional days), adjustment disorders (45 additional days) and other illnesses. These findings warrant concern, and the objective of the subsequent regression analysis was to determine whether the differences contributed to significant overall or illness-specific gender differences in the timing of remission.

Table 3 presents GEE results for the effects of gender and other selected variables on remission. Model 1 considers the effect of gender alone. There was no gender difference according to this model, which indicates that, on average, the timing of remission is not longer for females than for males. In consideration of the gender differences in length of treatment shown in Table 2, Model 2 estimates the effect of type of illness on remission and controls for comorbidities. In comparison with counseling-needed conditions, all illnesses except addictions showed increased length of treatment. However, type of illness is a non-significant factor in terms of gender differences in the timing of remission. Model 3 introduces demographic and socio-economic variables to those considered previously. A modest but significant (p<0.001) gender difference emerges in this model, which demonstrates that gender is indeed a determinant of remission, net of demographic and socio-economic characteristics.

Table 4 explores whether there are gender differences in remission in terms of specific types of illness. Although we observed a small general difference in the timing of remission between females and males, this aggregated analysis could mask important gender differences in remission from specific illness. As Table 4 shows, however, the general pattern observed in Table 3 is accurate. There are no gender differences in the timing of remission of specific illnesses, and it is only in cumulative terms that a gender difference in remission is observable.

CONCLUSION

This study examined gender differences in remission from a spectrum of mental illnesses. The analysis indicates that more females received treatment for an illness than males. This higher rate of treatment could correspond to a greater need among females, but it could also involve an underutilization of service among males. A greater proportion of females received treatment for mood disorders, anxiety disorders and adjustment disorders. A greater proportion of males received treatment for addictions, delirium and psychoses. There are also gender differences in average length of treatment for each type of illness considered. In several instances, females appear to need a longer course of treatment. Although our regression analysis confirms that the timing of remission is somewhat longer for females it provides no clear explanation for this finding. However, a possible candidate is marital status, for there are a disproportionate number of single (never married) males in our sample. Prior research demonstrates that singlehood represents a remission disadvantage. (23) To test this conjecture (unreported analysis), we removed marital status from Model 3 in Table III. As anticipated, gender becomes non-significant in the revised model (p=0.10), suggesting that this demographic characteristic (more single males) suppresses a slight comparative disadvantage in remission among females. These conclusions are limited inasmuch as these data include only individuals treated for serious clinical illnesses and thus may not be generalizable to less severe disorders.

Acknowledgements: The authors acknowledge Chi Zheng and Ruth Kampen for their research assistance and the Canadian Institutes of Health Research, Institute of Neurosciences, Mental Health, and Addiction, for financial support.

Received: February 9, 2009

Accepted: August 6, 2009

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Christoph M. Schimmele, MA, Zheng Wu, PhD, Margaret J. Penning, PhD

Department of Sociology, University of Victoria, BC

Correspondence and reprint requests: Christoph M. Schimmele, P.O. Box 3050 STN CSC, Cornett A333, 3800 Finnerty Rd., Victoria, BC V8W 3P5, E-mail: chrissch@uvic.ca.

Table 1. Definitions and Gender Differences
in the Variables Used in the Analysis:
Canadian Adults (Age 18+), 1990

Variable                Definition                           Females

Care episode in days    Length of treatment in               207.6
                        days (range 0-4,178 days)
Type of illness
  Alcohol/substance     Dummy indicator (1 = yes, 0 = no)      2.4%
  Delirium              Dummy indicator (1 = yes, 0 = no)      3.6%
  Psychoses             Dummy indicator (1 = yes, 0 = no)      4.1%
  Mood disorders        Dummy indicator (1 = yes, 0 = no)     24.0%
  Anxiety disorders     Dummy indicator (1 = yes, 0 = no)     12.5%
  Adjustment            Dummy indicator (1 = yes, 0 = no)     16.5%
    disorders
  Dementia              Dummy indicator (1 = yes, 0 = no)      5.1%
  Other conditions      Dummy indicator (1 = yes, 0 = no)     16.0%
  Counseling-needed              Reference group              15.9%
    conditions
Age                      Age at diagnosis (range 18-105)      47.44
Marital status
  Single                Dummy indicator (1 = yes, 0 = no)     17.9%
  Separated/divorced    Dummy indicator (1 = yes, 0 = no)     23.3%
  Widowed               Dummy indicator (1 = yes, 0 = no)     16.2%
  Married/cohabiting             Reference group              42.7%
Aborginal               Dummy indicator (1 = yes, 0 = no)      3.8%
Rurality
  Urban fringe          Dummy indicator (1 = yes, 0 = no)      3.7%
  Rural fringe          Dummy indicator (1 = yes, 0 = no)      7.0%
  Urban areas outside   Dummy indicator (1 = yes, 0 = no)     15.5%
    CMAs/CAs ([dagger])
  Rural areas outside   Dummy indicator (1 = yes, 0 = no)      9.8%
    CMAs/CAs ([dagger])
  Urban core                     Reference group              63.9%
Work outside home       Dummy indicator (1 = employed full/   28.8%
                             part time, 0 = otherwise)
Household income           Household income in decile          4.98
                                   (range: 1-10)              5,118
N

Variable                              Males                 p value *

Care episode in days                 202.5                   0.576

Type of illness
  Alcohol/substance                    7.0%                   <0.001
  Delirium                             5.8%                   <0.001
  Psychoses                            8.3%                   <0.001
  Mood disorders                      20.6%                    0.001
  Anxiety disorders                    7.4%                   <0.001
  Adjustment                          12.8%                   <0.001
    disorders
  Dementia                             6.4%                    0.019
  Other conditions                    17.5%                    0.089
  Counseling-needed                   14.2%                    0.064
    conditions
Age                                  49.48                    <0.001
Marital status
  Single                              28.8%                   <0.001
  Separated/divorced                  20.6%                    0.009
  Widowed                              8.2%                   <0.001
  Married/cohabiting                  42.4%                    0.828
Aborginal                              3.7%                    0.954
Rurality
  Urban fringe                         3.9%                    0.807
  Rural fringe                         7.0%                    0.962
  Urban areas outside                 17.4%                    0.038
    CMAs/CAs ([dagger])
  Rural areas outside                 10.2%                    0.572
    CMAs/CAs ([dagger])
  Urban core                          61.5%                    0.040
Work outside home                     29.2%                    0.673

Household income                      5.04                     0.359
                                     2,470
N

* p values are obtained from bivariate
logit models of gender and each of the
explanatory variables.

([dagger]) CMA, census metropolitan  area;
CA, census agglomeration

Table 2. Average Length of Treatment
(in Days) by Type of Illness and Gender:
Canadian Adults (Age 18+), 1990 *

                          Female          Males

Type of Illness        Mean     SD     Mean     SD

Alcohol/substance      141.9   262.4   107.0   210.6
Delirium               185.3   275.4   218.4   315.1
Psychoses              571.3   722.8   558.2   810.1
Mood disorders         264.0   424.5   223.1   342.6
Anxiety disorders      205.6   337.9   187.5   293.0
Adjustment disorders   174.9   310.3   129.3   212.4
Dementia               237.6   299.3   195.8   272.1
Other conditions       145.3   259.6   154.4   281.9
Counseling needed      132.1   272.4   141.7   342.7
  conditions
N                      5,118           2,470

* Self-weighted data

Table 3. Generalized Estimating Equations
for Effect of Gender and Selected Explanatory
Variables on Length of Treatment, by Model:
Canadian Adults (Age 18+), 1990-2001

Variable                       Model 1        Model 2       Model 3

Female (1 = yes)                -0.003       0.060          0.124 ***
Type of illness
  Alcohol/substance               -         -0.025         -0.045
  Delirium                        -          0.368 ***      0.274 ***
  Psychoses                       -          1.297 ***      1.144 ***
  Mood disorders                  -          0.647 ***      0.618 ***
  Anxiety disorders               -          0.389 ***      0.399 ***
  Adjustment disorders            -          0.188 **       0.236 ***
  Dementia                        -          0.450 ***      0.360 ***
  Other conditions                -          0.080          0.078
  Counseling-needed
  conditions ([dagger])
Age                               -            -            0.038 ***
Age square (x 100)                -            -           -0.030 ***
Marital status
  Single                          -            -            0.382 ***
  Separated/divorced              -            -           -0.072
  Widowed                         -            -           -0.057
  Married/cohabiting
    ([dagger])
Aboriginal (1 = yes)              -            -           -0.223 *
Rurality
  Urban fringe                    -            -            0.070
  Rural fringe                    -            -           -0.016
  Urban areas outside             -            -            0.089
  CMAs/CAs
  ([double dagger])
  Rural areas outside             -            -           -0.114 *
  CMAs/CAs
  ([double dagger])
  Urban core ([dagger])
Work outside home
  (1 = yes)                       -            -           -0.168 ***

Household income                  -            -           -0.025 ***
Intercept                     5.317 ***      4.846 ***      3.911 ***
Log likelihood             10778255       10778496       10778579
[DELTA] Log likelihood                       240.7 ***       83.4 ***
d.f.                              -              8           12

* p<0.05, ** p<0.01, ***
p<0.001 (two-tailed test)

([dagger]) Reference category

([double dagger]) CMA,
census metropolitan area;
CA, census agglomeration

Table 4. Generalized Estimating Equations of Effect of Selected
Explanatory Variables on Length of Treatment, by
Gender: Canadian Adults (Age 18+), 1990-2001

Variable                       Females       Males       p value

Type of illness
  Alcohol/substance           -0.006        -0.085        0.716
  Delirium                     0.231 *       0.293        0.757
  Psychoses                    1.149 ***     1.080 ***    0.689
  Mood disorders               0.662 ***     0.539 ***    0.364
  Anxiety disorders            0.429 ***     0.349 *      0.616
  Adjustment disorders         0.306 ***     0.070        0.112
  Dementia                     0.446 ***     0.206        0.204
  Other conditions             0.102         0.008        0.530
  Counseling-needed
    conditions ([dagger])
Age                            0.044 ***     0.026 **     0.110
Age square (x 100)             0.000 ***     0.000 *      0.157
Marital status
  Single                       0.358 ***     0.384 ***    0.821
  Separated/divorced          -0.020        -0.219 **     0.047
  Widowed                     -0.024        -0.092        0.614
  Married/
    cohabiting ([dagger])
Aboriginal (1 = yes)          -0.153        -0.413 *      0.232
Rurality
  Urban fringe                 0.077         0.046        0.864
  Rural fringe                -0.136         0.211        0.030
  Urban areas outside          0.082         0.107        0.803
    CMAs/CAs
      [double dagger]
  Rural areas outside         -0.077        -0.193 *      0.330
    CMAs/CAs
      ([double dagger])
  Urban core ([dagger])
Work outside home             -0.116 *      -0.257 ***    0.118
  (1 = yes)
Household income              -0.023 **     -0.029 **     0.683
Intercept                      3.832 ***     4.318 ***    0.107
Log likelihood               7279438       3499159

* p<0.05, ** p<0.01, *** p<0.001
(two-tailed test)

([dagger]) Reference category

([double dagger]) CMA, census
metropolitan area; CA, census
agglomeration