By Eleanor McDermid, Senior medwireNews Reporter

Patients with bipolar disorder who are insulin resistant are likely to also be refractory to mood-stabilising treatment, report researchers.

Martin Alda (Dalhousie University, Halifax, Nova Scotia, Canada) and team note that their cross-sectional study could not prove that insulin resistance actually causes bipolar treatment resistance.

“Experimental studies will be required to determine whether normalisation of glucose metabolism will affect course and treatment outcomes among patients with bipolar disorder”, they write in The British Journal of Psychiatry.

“We may find that unless we identify and treat underlying insulin resistance in patients with refractory bipolar disorder, these patients may remain unwell.”

On testing, 32.2% of the 121 patients in the study proved to have insulin resistance and 21.5% had Type 2 diabetes, with about 40% of these patients being unaware of their status. Patients with diabetes or insulin resistance had higher body mass indices (BMIs) than those without and more frequently had dyslipidaemia, hypertension and cardiovascular disease.

The clinical course of bipolar disorder was significantly associated with the presence of diabetes or insulin resistance, with these features reducing the likelihood of patients having an episodic course, defined as achieving full remission of at least 2 months’ duration, without residual symptoms, between mood episodes.

Specifically, 50.0% of patients with Type 2 diabetes and 48.7% of those with insulin resistance had a chronic (non-episodic) course, compared with 27.3% of euglycaemic patients, giving about a threefold increased risk after accounting for age and gender. The association persisted after further adjustment for lifetime and current antipsychotic use and BMI.

Similarly, insulin resistance and diabetes were associated with a threefold increased likelihood of rapid cycling.

Furthermore, patients with impaired glucose metabolism had an increased likelihood of being resistant to lithium treatment. In all, 36.7% and 36.8% of patients with insulin resistance and diabetes, respectively, failed to respond to treatment, compared with just 3.2% of those with normal glucose metabolism, giving a greater than eightfold increased risk. Again, the associations were independent of age, gender, BMI and antipsychotic use.

The researchers note that insulin resistance and diabetes were equally strongly linked to treatment resistance.

“This is important, as there are no recommendations to screen for insulin resistance, even in patients whose illness is treatment refractory, even though insulin resistance appears to be as clinically significant as type 2 diabetes in patients with bipolar disorder”, they say.

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