Clustering of physical health multimorbidity in 68,392 people with severe mental illness and matched comparators: a lifetime prevalence analysis of United Kingdom primary care data
Authors: Launders Naomi , Hayes Joseph F , Price Gabriele , Osborn David PJ 
Published: May 2021

Objective

To investigate the clustering of physical health multimorbidity in people with severe mental illness (SMI) compared to matched comparators.

Design

A cohort-nested analysis of lifetime diagnoses of physical health conditions.

Setting

Over 1,800 UK general practices (GP) contributing to Clinical Practice Research DataLink (CPRD) Gold or Aurum databases.

Participants

68,392 adult patients with a diagnosis of SMI between 2000 and 2018, with at least one year of follow up data, matched 1:4 to patients without an SMI diagnosis, on age, sex, GP, and year of GP registration.

Main outcome measures

Odds ratios for 24 physical health conditions derived using Elixhauser and Charlson comorbidity indices. We controlled for age, sex, region, and ethnicity; and then additionally for smoking status, alcohol and drug misuse and body mass index. We defined multimorbidity clusters using Multiple Correspondence Analysis and K-Means cluster analysis and described them based on the observed/expected ratio.

Results

Patients with a diagnosis of SMI had an increased odds of 19 of 24 physical health conditions and had a higher prevalence of multimorbidity at a younger age compared to comparators (aOR: 2.47; 95%CI: 2.25 to 2.72 in patients aged 20-29). Smoking, obesity, alcohol, and drug misuse were more prevalent in the SMI group and adjusting for these reduced the odds ratio of all comorbid conditions. In patients with multimorbidity (SMI cohort: n=22,843, comparators: n=68,856), we identified six multimorbidity clusters in the SMI cohort, and five in the comparator cohort. Five profiles were common to both. The “hypertension and varied multimorbidity” cluster was most common: 49.8% in the SMI cohort, and 56.7% in comparators. 41.5% of the SMI cohort were in a “respiratory and neurological disease” cluster, compared to 28.7% of comparators.

Conclusions

Physical health multimorbidity clusters similarly in people with and without SMI, though patients with SMI develop multimorbidity earlier and a greater proportion fall into a “respiratory and neurological disease” cluster. There is a need for interventions aimed at younger-age multimorbidity in those with SMI.

Summary box

What we already know

People with severe mental illness have higher rates of a range of physical health conditions, including cardiovascular disease, diabetes, and chronic obstructive pulmonary disease (COPD), and a higher mortality rate

Despite growing attention to disease clustering and profiles of multimorbidity in the general population, there is a lack of evidence regarding multimorbidity clustering in people with SMI.

What this study adds

Profiles of multimorbidity in people with SMI are similar to the general population, but multimorbidity occurs earlier in those with SMI, with a higher proportion of multimorbid patients defined by clusters of respiratory and neurological disease; services and research should focus on early multimorbidity to decrease the mortality gap, as should commissioners and policy makers.

People with schizophrenia appear to have lower prevalence of recorded disease for cancer, hypertension, cardiac arrhythmias, valvular disease, and rheumatoid and collagen disease than people without SMI despite high levels of risk factors for these conditions; which requires further investigation to determine whether this is a true effect, or reflects under-diagnosis or inequity in access to healthcare.