Bipolar disorder, formerly known as manic depression, is a mental health condition characterized by extreme shifts in mood and energy levels. A person with bipolar disorder experiences alternating episodes of mania (elevated mood) and depression (low mood). These mood episodes cause significant disturbances in daily life and functioning.
Bipolar disorder is considered a brain disorder, as it is associated with changes in brain structure and function. This raises an important question – can bipolar disorder be detected through brain imaging scans? In particular, are there identifiable patterns of brain activity or anatomy that can serve as biomarkers for bipolar disorder?
Researchers have been investigating whether different types of brain scans, such as MRI, CT, PET and fMRI, can reveal biomarkers that distinguish bipolar brains from healthy brains. A biomarker is a measurable indicator that can objectively identify a medical condition. If imaging biomarkers for bipolar disorder exist, brain scans could potentially be used to aid diagnosis and measure treatment response.
In this article, we will examine the evidence on brain imaging studies in bipolar disorder and evaluate whether the current technology has advanced enough to reliably detect this condition through neuroimaging.
Brain Structural Changes
Bipolar disorder has been associated with several structural brain changes visible on imaging scans. MRI and CT scans can detect subtle differences in the size, shape and integrity of brain regions between bipolar patients and healthy controls.
Ventricle Enlargement
One of the most consistent structural findings is enlargement of the ventricles in the brain of bipolar patients. The ventricles are fluid-filled cavities within the brain. MRIs show that the lateral and third ventricles are significantly larger in both manic and depressed bipolar patients compared to healthy individuals. However, ventricle enlargement is not specific to bipolar disorder. It is also found in other psychiatric conditions like schizophrenia.
Reduced Cortical Thickness
MRI scans reveal widespread reduction in gray matter thickness and gray matter volume in different parts of the cerebral cortex in patients with bipolar disorder. The decreases are most prominent in the prefrontal and temporal cortical regions. Cortical thinning is postulated to reflect diminished synapses, dendrites and neuropil.
White Matter Hyperintensities
White matter hyperintensities are lesions that appear as bright spots on MRI scans. They are more prevalent in people with bipolar disorder compared to the general population. However, they are also frequently found in healthy elderly adults. The significance of these hyperintensities is still unclear.
Overall, structural neuroimaging studies show there are detectable brain changes in bipolar disorder that involve both gray and white matter. However, the alterations are quite subtle and nonspecific to bipolar illness.
Functional Brain Changes
Functional imaging techniques like fMRI, PET and SPECT scans measure activity in different brain regions. They can identify patterns of altered brain function associated with bipolar disorder.
Abnormal Prefrontal Activity
Functional studies consistently show dysfunction in the prefrontal cortex in bipolar patients. During mania, the prefrontal cortex is underactive, failing to regulate emotions and impulses. Depression is linked to overactivity in the prefrontal cortex. Normalizing prefrontal function is an important goal of bipolar treatment.
Limbic System Alterations
The limbic system regulates emotions and mood. Hyperactivity in the amygdala and other limbic structures underlies mood elevation in mania. Depressed bipolar patients exhibit lowered activity in the amygdala and subgenual prefrontal cortex. Limbic hyperactivation may also occur during sleep loss, triggering manic relapses.
Disrupted Brain Networks
Functional connectivity studies using fMRI reveal disrupted communication between different brain networks involved in emotional processing and cognitive control in bipolar disorder. This includes altered connectivity within prefrontal networks as well as between prefrontal and limbic regions. The mood disturbances in bipolar disorder may arise from decoupling of normally integrated functional brain networks.
Overall, there are identifiable functional brain changes associated with manic and depressive phases of bipolar disorder. However, similar patterns of brain activity changes are observed in other psychiatric conditions like major depression.
Limitations of Current Findings
While neuroimaging studies have revealed several structural and functional brain changes linked to bipolar disorder, there are limitations to these findings:
Lack of Specificity
Brain changes found in bipolar patients, such as enlarged ventricles, cortical thinning and prefrontal dysfunction, overlap with changes seen in other psychiatric disorders like schizophrenia and major depression. No distinctive neuroimaging biomarkers specific to bipolar disorder have been identified so far.
Heterogeneity of Bipolar Disorder
Bipolar disorder is highly heterogeneous between patients in onset, clinical course, mood cycling patterns, presence of psychosis and response to medication. Neuroimaging findings are also highly variable between individual bipolar patients. Identifying a universal brain signature for such a heterogeneous condition is challenging.
Effect of Medication
Most neuroimaging studies are conducted on medicated bipolar patients. Many medications used to treat bipolar disorder can themselves alter brain structure and activity. This makes it difficult to separate disease effects from medication effects on imaging findings.
Confounding Health Factors
Brain changes in bipolar patients may be influenced by commonly co-occurring factors like substance abuse, obesity, cardiovascular disease and anti-psychotic use, rather than arising from bipolar disorder per se. Large, carefully controlled studies are needed to account for these confounds.
Small Sample Sizes
Most studies involve small samples, fewer than 30 patients. Larger, multi-site studies with greater statistical power are essential to establish robust and reliable imaging biomarkers for bipolar disorder.
Ongoing Research
While current neuroimaging techniques have significant limitations in bipolar disorder, research is ongoing to develop more sophisticated scanning technologies and analytic methods to identify reliable imaging biomarkers. Some active research directions include:
Multimodal Imaging
Combining multiple scanning modalities like MRI, fMRI and DTI to provide complementary structural and functional data may allow more definitive conclusions to be drawn about brain changes in bipolar disorder.
Machine Learning Approaches
Advanced machine learning algorithms applied to multimodal neuroimaging data may be able to discern imaging patterns that distinguish bipolar disorder from other conditions more accurately than traditional statistical methods.
Genetic Neuroimaging
Imaging genetics explores links between imaging findings and risk genes for bipolar disorder. Identifying brain changes associated with specific genetic variants may reveal biological mechanisms underlying bipolar pathophysiology.
Early Stage Studies
Studying patients in the early stages of bipolar disorder allows brain changes related to disease onset and progression to be delineated while minimizing confounding factors.
Treatment-Related Changes
Assessing how brain changes in bipolar patients are modified by different medications and psychotherapies may elucidate the mechanisms through which treatments exert their effects.
Conclusion
In summary, bipolar disorder is associated with structural and functional brain changes detectable through modern neuroimaging techniques. However, current research has not yet identified distinctive imaging biomarkers that can accurately and reliably diagnose this condition or differentiate it from other psychiatric disorders with shared clinical features. Ongoing advances in neuroimaging and data analytics hold promise to improve diagnosis and management of bipolar disorder by establishing brain-based biosignatures. But more research in larger, well-defined patient cohorts will be essential to realize these goals. While neuroimaging provides insights into the neural underpinnings of bipolar illness, clinical assessment remains the gold standard for diagnosis.
