I have often heard that schizophrenia, bipolar illness and some other mental disorders are caused by a chemical imbalance in the brain. Can you explain why blood tests, MRI’s and PET scans are not routinely used to confirm or rule out a mental illness?

Given the effort that scores of researchers have put into studying the brains of patients with mental disorders, such as schizophrenia, during the last 20 years, it is reasonable to expect that by now we should have laboratory tests for such illnesses. Every clinician that treats patients with such illnesses wishes such tests existed so we could make the diagnosis more quickly and with more confidence. However, the results of our research have shown us perhaps too much about the complexity of the brain and the subtlety of the impact of mental disorders on the brain.

 Studies of brain structure and function using tools like magnetic resonance imaging (MRI) and positron emission tomography (PET) have detected differences between persons with and without schizophrenia. However, these differences have been small, often smaller than differences that ordinarily occur among healthy persons. For example, the hippocampus, a brain area related to memory, appears to be 5% smaller on average in patients with schizophrenia, while among healthy persons its size can vary by 50% or more. Also, not all patients with a given mental disorder may show such an abnormality.

In fact, some experts believe that there may be many forms of schizophrenia, and that different brain abnormalities occur in each one of them. Finally, blood tests have not been useful because it is hard to understand what is going on in the billions of different nerve cells in the brain by measuring the byproduct of the brain’s thousands of chemical signals in the blood. A wise man once said that trying to detect a mental disorder by measuring chemicals in the blood is like trying to tell what the President had for breakfast by sampling the Potomac River.

Research goes on. Every day we are improving the quality of techniques for scanning the brain to assess its structure and function. By making these tools more sensitive, we should be able to distinguish persons with mental disorders from healthy persons and to distinguish among persons with different mental disorders. Eventually, we may be able to use such tests to make the diagnosis of a mental disorder when symptoms are very few or confusing. No one would accept a situation where we could not diagnose hypertension until someone had a stroke. Hopefully, one day we will be able to diagnose mental disorders and treat them before those who have them, and their families, have to go through the anguish of a psychotic episode or suicide attempt.

John Csernansky, MD
Metropolitan St. Louis Psychiatric Center

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