Discussion
Throughout the world, the prevalence of diabetes mellitus continues to increase with predictions that the 2045 worldwide prevalence will reach 783 million.24 Within the cohort of people with SMI, most antipsychotic pharmacotherapy contributes to weight gain and insulin resistance. International studies have found that the prevalence of diabetes is two to three times higher in those with SMI than that observed in the general population.16 In the Australian setting, we have confirmed a significantly higher prevalence of diabetes in the SMI population than that seen in the general Australian population. In the category aged 18–44 years, diabetes prevalence in the ccCHiP population is 16.7% (>10 times higher than the age-matched, general Australian population). In the 45–54 years of age category, diabetes prevalence in the ccCHiP population is 25.9% (>6 times higher than the age-matched, general Australian population) and in the 55–64 years of age category, diabetes prevalence in the ccCHiP population is 31.7% (>3 times higher than the age-matched, general Australian population).
Within the ccCHiP cohort alone, we found that diabetes was significantly associated with older age; elevated WtoHt; presence of a family history of diabetes; maternal ethnic background being Aboriginal/Torres Strait Islander, Indian or Middle Eastern compared with Caucasian/Mediterranean, and being on clozapine or olanzapine±other antipsychotic medication compared with not using any antipsychotic medication. Comparable risk factors are prevalent in high-risk psychosis groups globally, reinforcing the notion that the diabesity ‘pandemic’ is even more accentuated in those with SMI.
In this study, treatment with olanzapine±other antipsychotic medication was associated with lower odds of having diabetes than those on other antipsychotic medication only. This unexpected finding was in contrast to the increased odds of diabetes observed in those on treatment with clozapine±other antipsychotic medication. One possible explanation for the discordance between olanzapine and clozapine may be driven by prescriber behavior. We observe that many prescribers in our local area are acutely aware of the orexigenic and diabetogenic effects of olanzapine and are therefore more proactive in the metabolic management of patients commenced on treatment with olanzapine. This certainly needs to be explored in future research in similar cohorts.
A significant concern for the ccCHiP service is a relatively high prevalence of undiagnosed diabetes at the time of first clinic visit. From the literature, it is estimated that up to 70% of cases with diabetes in people with SMI are undiagnosed.25 In our clinic population, over 15% of those with diabetes mellitus were newly diagnosed at the time of their first visit. Over 40% of those newly diagnosed with diabetes at the time of their first visit were aged less than 45 years.
These data support a more proactive screening approach in the young adult SMI population. It is now well recognized that early detection and assertive treatment of diabetes can be very useful in achieving better long-term outcomes.26 The high rate of undiagnosed diabetes provides a strong argument for more intensive screening for diabetes within the SMI population. It appears that screening for diabetes should start at least two decades earlier for those with SMI. Such screening also provides an opportunity to identify those who have impaired glucose tolerance or pre-diabetes. From a pragmatic point of view, screening with fasting glucose and an HbA1c level would appear to be a reasonable strategy to adopt.
Another concern for clinicians at the ccCHiP clinic is that of treatment inertia in the management of diabetes for the target cohort. For those with a pre-existing diagnosis of diabetes, >40% were not at the recommended HbA1c target (<7%) at the time of their ccCHiP visit. On review of pharmacotherapy use, it is apparent that metformin, gliclazide and insulin were the most commonly prescribed antihyperglycemic agents during the study period. In view of high levels of suboptimal glycemic control at the time of a patient’s initial ccCHiP clinic appointment, diabetes treatment intensification appears currently inadequate within the local community who have SMI. In this context, a ccCHiP-like service is well placed to provide diabetes management support to primary care and local community mental healthcare teams. The relative underuse of newer, antihyperglycemic pharmacotherapies (namely, SGLT2 inhibitors and GLP-1 receptor agonists) is most likely explained by the national pharmaceutical benefit scheme prescribing restrictions that were in place for these agents during the study period (2014–2019). Moving forward, there is certainly a role for services like ccCHiP to support an increased uptake of these metabolically favorable treatments in partnership with local general practitioners.
Strengths and limitations
Our study is notable for a number of strengths. The ccCHiP clinic has provided clinical services to >1400 individuals with SMI over the past 5 years, and thus this study captured data from a substantial number of individuals with SMI who reside in the Sydney Local Health District.
This study provides the first systematically collected data on cases of schizophrenia in our catchment area. The question of representativeness arises. Given the currently accepted point prevalence rate for the schizophrenias (0.28%), the catchment population size serviced (700 000) and the known number and diagnoses of persons in public psychiatric care (2200; ~65% schizophrenia spectrum), and given public clinics are the predominant setting for the treatment of SMI, we estimate that about three-quarters of suitable patients are referred to ccCHiP. There is no significant difference (age and gender) between our cohort of people with SMI and the Australian SMI population not referred to the ccCHiP Clinical Service.27 Further, referral criteria are not based on anthropometric or pathology criteria, but rather a mandate that all new patients with psychosis are assessed within a year and then between 12 and 24 months in periodic follow-up. This suggests that the ccCHiP cohort is representative of those that are managed in public psychiatric settings.
The use of a hierarchical approach to diabetes identification incorporating both HbA1c and fasting glucose data, in addition to self-report and review of pharmacotherapy, is another strength. Previous Australian studies have primarily relied on fasting glucose measurements to estimate diabetes/pre-diabetes prevalence, and in some studies, simply a self-report. Evidently, these previous studies may have resulted in a less accurate estimation than the more comprehensive approach employed in this study. It is also important to note that the ccCHiP service prospectively collects clinical data at each patient visit to the clinic, and thus missing data are kept to a minimum. In addition, the comprehensive data collected by the ccCHiP service has allowed insights into pharmacotherapy use within the cohort of patients with co-morbid SMI and diabetes. In turn this provided an opportunity to quantify the degree of diabetes treatment inertia currently faced by this high-risk population.
It is also important to recognize the study’s limitations. Data presented are collected from a secondary referral service and thus may be subject to a referral bias. Nevertheless, our criteria used for referring individuals to ccCHiP clinics accommodated all people with a diagnosis of ‘psychosis’ which is reasonably representative of those in the Central Sydney public mental health population, as described previously. Referrals are accepted from all local healthcare providers; however, the majority of referrals are received from local community mental health services and a minority are received from within-area general practitioners. Individuals with SMI managed in the private health sector are relatively under-represented. We also acknowledge that the criteria used to identify individuals with diabetes within the ccCHiP clinic population, while more comprehensive than those used in previous Australian studies, are imperfect. Ideally, to confirm a new diagnosis of diabetes, one would normally request a second confirmatory fasting blood test. This is usually performed by the patient’s GP, and the ccCHiP clinic does not have direct access to the results of confirmatory testing that may have been performed by general practitioners.