Mental retardation can complicate the clinical course and outcome of bipolar disorder. How mental retardation affects the inpatient care of bipolar disorder warrants further investigation.

Information regarding demographic characteristic, pre-admission use of outpatient services, medical co-morbidities and indices of inpatient health resources use (length of admission, hospitalization expenses and use of psychotrophic medications) of all individuals diagnosed with bipolar disorder and mental retardation first admitted between 2000 and 2010 was extracted from a total population claims database in Taiwan and compared with those admitted during the same period due to bipolar disorder. Confounding factors affecting health utilization, including age, differences in hospital payment standard and medical cormobidity, were controlled by multivariate analysis.

451 and 13,513 bipolar patients with and without mental retardation were identified during the study period. For the index admission, bipolar individuals with mental retardation were younger, had longer hospital stay with higher total expenditures, and tended to be transferred for continual inpatient treatment. They also received smaller dosage of antipsychotics, lithium and benzodiazepines. Although the number of medical co-morbidity did not differ, the prevalence of hypertension and metabolic disturbances was lower among bipolar individuals with mental retardations.

The diagnosis of mental retardation was indeed associated with longer inpatient hospitalization and increased total cost of hospitalization expenses, despite being younger, with less metabolic imbalance and receiving less psychotrophic medications. Implications for the long-term course of bipolar disorder need to be confirmed by longitudinal follow-up studies.