Bipolar I and Bipolar II Disorders in Apparently Healthy Adults: A Critical Review from Population Prevalence to Prevention
Jacques Forwah Ndeh *
Department of Hematology and Blood Transfusion Sciences, Faculty of Clinical Sciences, University of Calabar Teaching Hospital, Calabar, Cross River State, Nigeria.
Emem Samuel Edung
Emergency Department, Basildon and Thurock University Hospital, London NHS Trust Foundation, United Kingdom.
Blessing Festus Arimie
Department of Psychiatry, NES Healthcare / Priory Group, Peterborough, United Kingdom.
Odion Judith Erhagbe
Department of Quality, Sygen Pharmaceutical Ltd. Ota, Ogun State, Nigeria.
Ukeme Samuel Edung, Bosah
Department of English and Literature Studies, University of Uyo, Akwa Ibom State, Nigeria.
Ezekpeazu-Chike
Department of Mental Health, Southwestern University, Dunstable, United Kingdom.
Adekemi Angelina Ibijola
Department of Psychiatry, NES/Priroy Hospital, Kneesworth, Birmingham, United Kingdom.
Unakalamba Adaobi Sandra
Department of General Adult Psychiatry, North London NHS Foundation Trust, Broxbourn, United Kingdom.
Akib Abba
Department of Internal Medicine, Murtala Muhammad Specialist Hospital, Kano, Kano State, Nigeria.
Catherine Chinwendu Obijuru
Department of Psychiatry, Manchester, United Kingdom.
Ilevbare Martina Olohirere
Department of Psychiatry, Medimax Hospital, Abuja, Nigeria.
Immaculata Chiamaka Adindu-Nwankwo
Department of Medicine, University of Greater Manchester, United Kingdom.
Jane Ulifo Falade
Department of General Medicine, Mashi Hospital and Specialist Clinics, Kaduna, Nigeria.
Chatt Usaini
Department of Internal Medicine, Genomics Medicals and Multi-specialist, Walsall, UK.
Nsikak Emmanuel Ekwere
Department of Rehabilitation Psychiatry, NHS Lothian, Scotland.
Bukunade P. Adenusi
Department of Psychiatry, Lancashire South Cumbria Foundation Trust, Preston, United Kingdom.
Euna Kenechi Onyia
Department of Neuropsychiatry, Enugu State University of Science and Technology, Nigeria.
Odiong Okon Odiong
Department OF Psychiatry, NHS Lothian, Edinburgh, Scotland.
Ikechukwu Collins Nnaoma
Department of Psychiatry, NES/ Priory Hospital, Hayes Grove, Sheffield, United Kingdom.
Karen Okorie
Department of General Medicine / Acute Medicine, Midyork Teaching Trust, NHS Harrogate, United Kingdom.
Abasiubong Emmanuel Abasiubong
Department of Emergency, General Hospital Suwair, Al Jouf, Saudi Arabia.
Nworgu Chizaramekpere Ihechukwukwuru
Department of Psychiatry, Federal Neuropsychiatric Hospital, Enugu, Nigeria.
Mary Ibhade Jibiro
Department of Psychiatry, University of Port Harcourt Teaching Hospital, Rivers State, Nigeria.
Ibe Chibuike Leonard
Department of Psychiatry, North East London NHS Foundation Trust, Broxbourne United Kingdom.
Ejike Emmanuel Ogbodo
Department of Paediatrics, Royal Berkshire NHS Foundation Trust, Reading, United Kingdom.
Oghenerukevwe Anthony Ovadje
Department of Family Medicine, University of Port Harcourt, River State, Nigeria.
Benjamin Essien
Department of Hematology and Blood Transfusion Sciences, Faculty of Clinical Sciences, University of Calabar Teaching Hospital, Calabar, Cross River State, Nigeria.
Bassey Okon Bassey
Department of Hematology and Blood Transfusion Sciences, Faculty of Clinical Sciences, University of Calabar Teaching Hospital, Calabar, Cross River State, Nigeria.
Otti Chidiedere Joel
Department of Obstetrics and Gynecology, University of Nigeria Teaching Hospital, Utuku-Oxxllx, Enugu State Nigeria.
Ewa Anthony Obi
Department of Family Medicine, University of Calabar Teaching Hospital Calaba, Cross River State Nigeria.
Idiege Idiege Omang
Department of Surgery, University of Calabar Teaching Hospital Calabar, Cross River State Nigeria.
Akaba Kingsley Onoridea
Department of Hematology and Blood Transfusion Sciences, Faculty of Clinical Sciences, University of Calabar Teaching Hospital, Calabar, Cross River State, Nigeria.
Nnaji Chimuanya Joseph
Department of Surgery, Nnandi Azikiwe University Teaching hospital, Nnewi, Anambra State, Nigeria.
Immaculate Ihuoma Ekeagba
WORCACCCE UNION GROUP Integrated Healthcare Sciences, Technological Development and Training and Innovative Research Foundation (WUGIHSTTAIRF), P.O Box 45 Bamenda, North West Region, Cameroon.
Abeshi Sylvester Etenikang
Department of Obstetrics and Gynecology, University of Calabar Teaching Hospital, Calabar, Cross River State, Nigeria.
*Author to whom correspondence should be addressed.
Abstract
Background: Bipolar I disorder and bipolar II disorder are recurrent mood disorders that usually begin in late adolescence or early adulthood, carry excess mortality from suicide and cardiovascular disease, and are often missed because the presenting complaint is depression. Many adults who meet criteria in population surveys have never received a bipolar diagnosis, so the boundary between apparent health and illness is a central but neglected issue.
Purpose and Scope: This critical narrative review examines both disorders from the perspective of adults who appear healthy when encountered in surveys, primary care or screening. It appraises evidence on prevalence, aetiology, clinical features, diagnostic criteria, pathophysiology, management and prevention, asking throughout whether conclusions drawn from bipolar I disorder transfer legitimately to bipolar II disorder.
Approach: Peer-reviewed literature published from January 2000 to August 2026 was identified through structured searches of biomedical and multidisciplinary indexes, citation tracking and authoritative classification and guideline documents, and was appraised qualitatively for design, measurement validity, subtype disaggregation and consistency.
Principal Findings: Community prevalence estimates vary several-fold, largely because lay-administered interviews and self-report screeners detect hypomania poorly and criteria have shifted between classification editions. Under-recognition and diagnostic delay are substantial, especially for bipolar II disorder, although over-diagnosis also occurs. Familial and polygenic liability is the strongest established risk factor, whereas most environmental exposures rest on observational and frequently low-quality meta-analytic evidence. Bipolar II disorder is not a mild variant: its depressive burden, chronicity and suicide risk approach those of bipolar I disorder. Mechanistic models involving reward sensitivity, circadian instability, immune activation and neuroprogression remain largely correlational. Lithium and several antipsychotics have the strongest maintenance evidence, but randomised evidence specific to bipolar II disorder is sparse and of very low certainty.
Implications: Prevention is at present secondary and tertiary rather than primary. Structured enquiry about hypomania in adults presenting with depression, cautious antidepressant use, attention to sleep, substances and the postpartum period, and early specialised care are better supported than population screening.
Conclusion: Progress depends on subtype-specific trials, prospective validation of risk states and population studies using clinician-validated diagnoses.
Keywords: Bipolar II disorder, hypomania, community epidemiology, underdiagnosis, screening accuracy, lithium, early intervention, neuroprogression