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Day 43: Bipolar Disorder - The “who” and the “why”

Welcome to day two of our theme on bipolar disorder. Today we are going to go one level deeper and overview some epidemiology, risk factors, and the course and prognosis of bipolar disorder.

Today’s Content Level: Beginner, Intermediate


•Prevalence = proportion of individuals who have a condition at a particular period of time

•Incidence = proportion or rate of individuals who newly develop a condition during a period of time

•The overall lifetime prevalence of bipolar spectrum disorders is around 2.4%. 1

Bipolar I

  • Prevalence: lifetime prevalence of bipolar I is somewhere between 1-2%.

  • Gender: women and men are about equally affected.

  • Age: average age of onset is 18-21 years old. A bimodal distribution of the incidence of bipolar has been suggested and studies have found two peaks in age of onset at 15–24 years and at 45–54 years.

  • Race: prevalence is similar across racial groups, however, high-income countries have twice the rate of low-income countries (1.4% versus 0.7%).

  • Suicide risk: much higher risk of suicide than general population. 33–50% of patients attempt and 10–19% die by suicide. Particularly elevated risk during depressive and mixed episodes and those with comorbid anxiety. 2

  • Mortality risk: associated with mortality rate 2-3x higher than the general population. Also among the 20 leading causes of disability worldwide. 3

Bipolar II

  • Prevalence: unclear, with some studies showing greater prevalence than bipolar I and others showing lower prevalence.

  • Gender: may be slightly more common in women.

  • Age: similar to above, onset usually before age 30.

  • Race: no ethnic differences seen.

  • Frequently misdiagnosed as unipolar depression and thereby inappropriately treated.


•Biological, environmental, psychosocial, and genetic factors are all important.


  • Bipolar I has the strongest genetic link of the major psychiatric disorders.

  • First-degree relatives of patients with bipolar disorder are 10 times more likely to develop the illness.

  • Lifetime risk is 5-10% with a first degree relative and 40-70% with a monozygotic twin.

Other known risk factors include:

  • Childhood maltreatment: correlation with physical, sexual, and emotional abuse. Known risk factor for many psychiatric conditions.

  • Substance use: frequently comorbid but also hypothesized to have causality in both directions. Substances implicated include cannabis, opioids, cocaine, sedatives and alcohol.

  • Some evidence for prenatal infections (strongest for T. gondii).


•Overall bipolar disorder has a worse prognosis than MDD in regards to number of episodes as well as morbidity and mortality.

Untreated manic episodes generally last several months.

•Once an individual has a manic episode there is roughly a 90% chance they will have a repeat mood episode within 5 years. This could be a manic, hypomanic, or depressive episode. 5

•The lifetime course of the disorder is typically chronic with multiple relapses. As the disease progresses episodes may occur more frequently.

•Maintenance treatment with mood stabilizing medications between episodes helps to decrease the risk of recurrent episodes.

•Bipolar II disorder also tends to be chronic and requires long-term treatment. There is less data on bipolar II but is estimated to have better overall prognosis.


Bipolar disorders are highly co-morbid with other psychiatric disorders as well as general medical conditions. There are multiple reasons for this including shared genetic and environmental vulnerabilities, potential causal relationships, and consequences of treatment.

  • Psychiatric: anxiety disorders are common (generalized anxiety disorder, panic disorder, & social anxiety disorder), ADHD, and any disruptive or impulse-control disorders (IED, ODD, conduct). Substance use use disorder (eg, alcohol use disorder) occurs in roughly half of patients.

  • Medical: irritable bowel syndrome, metabolic syndrome, migraines, and asthma are shown to be more common in this population.

•Also, keep in mind that within psychiatry it is difficult to have consistent reliability and low inter-rater variability in regards to our diagnoses. As one of our readers (and a trusted colleague) pointed out... "we do our best to elicit an accurate and detailed history and identify symptom patterns but it is a complex process prone to imprecision."


I hope today's lesson provided a nice overview about the "who" and the "why" of bipolar disorder. In our next session we will have a review quiz that covers the first two lessons of bipolar disorder as well as the last few posts in the depressive disorder theme.

Resources for this post include Pocket Psychiatry, Kaplan and Sadock synopsis of psychiatry, DSMV, and First Aid For the Psychiatry Clerkship as well as the articles referenced in the post. I highly recommend adding these to your psych library.

Bullet Psych is an Amazon Associate and we receive a small commission if you use our links for the purchase of our recommended resources.

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Marcus Hunt
Marcus Hunt
Nov 27, 2020

Thanks for the comment, Kyle. You're right!


Kyle Jeanne Gray
Kyle Jeanne Gray
Nov 27, 2020

Thanks again for another great post. Re: reasons co-morbid diagnoses, would also add the unreliability and interrater variability of our diagnoses in general. We do our best to elicit an accurate, detailed history and identify symptom patterns but it's a complex process prone to imprecision.

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