In this study, we established the prevalence and incidence of comorbidity in patients with T2D. We found that 84.6% of the patients with newly diagnosed T2D in a primary-care population had at least one chronic comorbid disease at the time of their diagnosis, and both concordant and discordant comorbidity were common. Incidence density rates after diabetes diagnosis showed that rates of chronic comorbidity further increased after diabetes onset. This study clearly showed the heterogeneity of this primary-care population with T2D in terms of comorbidity.
Relation to other studies
The prevalence of comorbidity in patients with T2D in this study was similar to [14, 37] or higher than [12, 13] those of previous studies. The number of comorbid diseases considered in a study contributes to any prevalence estimate [7, 38], and our work had the largest number. The relatively high prevalence of comorbidity we found is more pronounced when one considers that we investigated a primary-care population including all adult T2D patients, as opposed to studies that included only patients over 65 years of age [14, 37] or those requiring inpatient diabetes treatment . Patients with discordant comorbidity outnumbered those with concordant comorbidity, a finding similar to earlier CMR-based research on comorbidity in patients with heart failure as the index disease . Diabetes is not necessarily causally related to additional diseases, but co-existing chronic diseases may interfere with diabetes management in several ways [14–20]. These results encourage us to reflect on the general lack of attention to (discordant) comorbidity in evidence-based diabetes guidelines .
Patients with comorbidity may prioritize one condition over another, and experience overwhelming effects of an individual disease [21, 40]. A recent study showed that physician-experienced complexity of patients with diabetes increased with prevalent discordant comorbidity, but not with concordant comorbidity, implying that improvement in diabetes management could be made merely by focusing on patient-centred rather than disease-specific interventions . Patient-centred management is exactly what GPs prioritize in the management of multimorbidity ; however, the current tendency is to incentivise disease-specific instead of holistic care, thereby counteracting patient-centred approaches [43–46]. The extent of chronic comorbidity in patients with T2D, as shown in the current study, urges an approach of complementing disease-specific strategies with a personalized, generalist approach for the management of patients with multimorbidity [6, 42].
Strengths and limitations of the study
To our knowledge, this study is the first to describe the development of chronic comorbidity over time in patients with T2D. We were able to identify comorbidity diagnosed before diabetes diagnosis. Selection of patients with a diagnosis of T2D and all comorbidity data were based on the most reliable source, that is, physician diagnoses, rather than patient self-report [17, 37] or extraction of medication prescriptions .
Diabetes in this study served as an example of a common chronic disease with standardized management plans. The objectives of this exploratory study were to establish the prevalence rates of a range of chronic comorbid conditions and their development over time. Given the nature of the CMR database, comparing comorbidity data in our diabetes population with a control group with another index disease, such as osteoarthritis, would have been possible. However, this would have distracted from the intended epidemiologic description of chronic comorbidity in T2D patients. This study did not aim to quantify the comorbidity rate in patients with T2D compared with patients with other chronic diseases, or to compare the rates within specific subgroups of patients with T2D or at different time periods within the study. Considering the large number of comorbid conditions and clusters studied, such comparisons would have resulted in numerous statistically significant differences or interactions of uncertain clinical relevance. Instead, the current epidemiologic description may lead to more detailed exploration of specific conditions or subgroups for future research.
The particular strengths of the study are that the diabetes population we studied was unselected, and that we did not restrict comorbidity only to prevalent or concordant chronic diseases. Our data reflect the total burden of chronic comorbidity in patients with T2D in general.
Currently, no universally accepted definition of 'chronic diseases' is available. Within any definition, personalization of the concept of chronicity to the individual patient level is preferred, although often not attained [34, 48]. An Australian primary-care code set applied the same criteria for chronicity as we did . However, by adding the distinction of conditional chronicity based on physician-assigned ongoing episodes, we were able to personalize chronicity in our analyses. For diseases from which patients may recover (for example, depression), or for diseases with either episodic or chronic courses (for example, asthma, gout), we consider our classification comes closer to the correct description of chronicity than would a list with invariable chronic diseases.
Comments on specific comorbid diseases
Concordant comorbidity (that is, CVDs) showed the highest prevalence and incidence density rates. Although this cluster contained a large number of diseases, the main explanation for the high rate is the concordance with T2D. Care-related factors will have added to this finding. For instance, a GP will be more attentive for T2D in a patient who has had a myocardial infarction. The suggestion that presence of a disease enhances attention for other diseases [49, 50] might be particularly the case for concordant combinations.
For discordant combinations also, care-dependent factors might contribute to the high rates of comorbidity. There was an evidently increased incidence of cataract in the year after diabetes diagnosis. The reason for this may be that screening for diabetic retinopathy resulted in earlier diagnosis of, or otherwise unobserved cataract diagnoses. Moreover, people might not raise certain issues until they visit their doctor for other health problems; such restraints can contribute to a higher incidence of conditions such as incontinence in the first years after diabetes diagnosis. These examples illustrate that despite the high rate of comorbidity reported, our results may still be an underestimation, as the comorbidity data refer to disease episodes truly experienced by patients and presented to their GP.
Musculoskeletal diseases have an antagonistic effect on physical exercise, which is part of the recommended treatment for diabetes . Around 30% (3/10) patients with T2D had musculoskeletal disease at time of diabetes diagnosis, and of those unaffected, an additional 32 new cases per 1,000 patient-years at risk followed during the next 10 years. These are substantial figures, which are higher than chronic musculoskeletal diseases in the overall CMR population , and these cases are likely to interfere with diabetes management.
Diabetes treatment focuses on prevention of complications . The presence of a malignancy may overshadow the importance of co-existing diabetes, and thus treatment priorities may alter. Dutch researchers found that patients with diabetes who had cancer received less aggressive cancer treatment than those without diabetes .
Parallel to the reluctance of GPs to prescribe interventions for depression in patients with comorbidity , Dutch GPs might be conservative in 'adding' a chronic mental disease diagnosis after a diagnosis of diabetes. Including less prevalent diseases in our study enabled us to localize a heterogeneous group of patients with chronic comorbidity who possibly have difficulties in self-managing their diabetes. One in 25 patients had chronic psychosis, obsessive compulsive disorder, phobia, schizophrenia, dementia, mental retardation, or Down's syndrome when diagnosed with diabetes. A 'standard' approach to diabetes would often not respond to these patients' abilities or needs.