Background: Current understanding of comorbidities associated with acquired brain injury (ABI) and the effects on post-acute ABI outcomes is poor.

Objective: To describe the prevalence, severity and patterns of comorbidity for the pre-injury, acute and post-acute phases, and to examine the effects of post-acute comorbidities on functional independence and length of stay (LOS) at discharge from post-acute care.

Methods: Retrospective whole-population cohort study of n=1,011 individuals with traumatic (TBI) or non-traumatic brain injury (NTBI), or eligible neurologic conditions admitted to a post-acute neurorehabilitation and disability support service in Western Australia (WA) between 1991 and 2020. Comorbidities were ascertained from internal electronic medical records and linked hospital and emergency department data from the WA Data Linkage System. We measured comorbidities across 14 body systems using the Cumulative Illness Rating Scale (CIRS) and Elixhauser Comorbidity Index (ECI), and functional independence with the UK Functional Independence Measure and Functional Assessment Measure (UK FIM+FAM). We used multilevel mixed-effect regression models to determine the effects of comorbidity on post-acute outcomes.

Results: NTBI was the most common diagnosis (54%), followed by TBI (34%) and neurologic conditions (10%). Pre-injury comorbidities were present in over half the cohort. Comorbidity prevalence increased significantly from 57% to 84% (∆+27%) and severity (mean ECI score) increased significantly from 2.1 to 13.8 (∆+11.7) between pre-injury and the acute phase and remained elevated at admission to post-acute services (82%, mean ECI score 7.3). Psychiatric comorbidity was the most prevalent (56%) and was associated with significantly poorer functional outcomes at discharge and an increase in LOS of 6.5 months. Genitourinary, musculoskeletal, eye, ear nose and throat, and renal comorbidities also had significant effects on post-acute outcomes.

Conclusions: ABI has a long-term impact on multiple body systems. Identification and management of comorbidities is critical to maximise functional outcomes and reduce the cost of post-acute care.