Diagnosis: A Misunderstood Word

The Meaning of Diagnosis

The concept of diagnosis (the identification of the nature and cause of a disease, or injury) has a central role in medicine. It is a familiar concept, as we are accustomed to visiting our doctor with a problem, having some tests done, and coming away with a diagnosis and treatment. Yet for all of its familiarity, the medical concept of diagnosis does not mean what most people think it means, and the difference between the perception of diagnosis, and the reality, leads to confusion when the patient's problems have to do with mental illness.

Suppose, for example, that you visited your doctor with a case of strep throat, which is a sore throat caused by an infection of the bacteria Streptococcus pyogenes. Your doctor would ask questions about your symptoms, examine your throat, possibly order a throat culture or other test, provide a diagnosis of strep throat, and prescribe an antibiotic to fight the infection.

The strep-throat example contains all of the elements of a medical visit that patients take for granted, including a diagnosis (strep throat) that identifies the specific cause of the problem (its etiology, in medical terms), which makes possible the selection of a specific treatment that is known to work.

Now, contrast the neat picture of strep throat with that of meningitis, which is an inflammation of the membranes that cover the brain and spinal cord (the meninges). Meningitis is diagnosed based on symptoms such as neck stiffness, fever, headache, and so forth.

Superficially, the diagnoses of strep throat and meningitis appear similar: A doctor provides the diagnosis based on symptoms and tests. However, there is a very important difference between the two diagnoses.

In the case of strep throat, tests identify the specific cause of the illness (Streptococcus pyogenes), and the name of the diagnosis even reflects this cause. (I will call this an etiological diagnosis.)
In the case of meningitis, the specific cause is usually unknown (it can be any of numerous viruses or bacteria), and the name of the diagnosis reflects not the cause of the disease, but its primary effect, i.e. inflammation of the meninges. (I will call this a symptomatic diagnosis.)

These two examples reflect the fact that there are two types of diagnosis: One identifies the specific cause of an illness, while the other is essentially a name for a group of symptoms or effects. While the diagnoses may seem similar to the patient, their implications for choice and effectiveness of treatment are significant.

An etiological diagnosis enables the doctor to prescribe a treatment that addresses the specific cause of the illness.
A symptomatic diagnosis does not identify a specific cause, and so the only options available to the doctor are those that offer symptomatic relief (such as medicine to relieve pain, fever, and swelling), or which may treat the underlying cause successfully, but for reasons that are unknown (such as the treatment of malaria by quinine, whose effectiveness was clear centuries before its mechanism of action was understood).

Diagnosis and Mental Illness

The relevance of this discussion to mental illness is simple: Diagnoses of mental illness are of the symptomatic type, not the etiological type. In other words, the diagnosis of depression, bipolar disorder, or schizophrenia is essentially a label applied to a set of symptoms that commonly occur together. The diagnosis does not reflect the underlying cause of the disorder (its etiology), and usually does not even involve diagnostic techniques other than a review of the patient's symptoms and history.

The unfortunate truth is that the causes of serious mental illness are not known. The various theories proposed to explain them, which involve concepts such as neurotransmitter concentrations and receptor densities, probably have some degree of validity, but they cannot be tested in patients, and lack the specificity required to design treatements that precisely address the causes.

Critics of the pharmacological (medication-oriented) treatments of mental illness, such as the Church of Scientology, frequently claim that the inability to provide a etiological diagnosis for any mental illness invalidates both the diagnosis, and the justification for medications used to treat the illness. Their claims make superficial sense, as the cause of mental illness is unknown, but their conclusions are simply wrong.

In reality, most of the ills we encounter on a daily basis not only have unknown causes, but are treated without obtaining any kind of diagnosis. Consider, for example, the person who takes nonprescription pain medicine for an occasional headache, or a decongestant for the "common cold" (another symptomatic diagnosis). Few people refuse to seek symptomatic relief just because they cannot obtain an etiological diagnosis of their discomfort, and rightly so. The fact that the diagnosis of mental illness, and treatments available for it, are symptomatic in nature, does not make the treatments useless! Many people have discovered profound relief through effective psychiatric medications, and live much happier lives as a result.

The Future of Diagnosis

Yet as useful as the concept of diagnosis has been for medicine, the challenges raised by mental illness lead me to think that it is becoming less relevant. Given the central nature of diagnosis to medicine, this is a shocking concept. After all, how can we treat illnesses that we cannot diagnose?

The short answer is that we are already treating illnesses we cannot diagnose. The tidy categories of mental illness provided in diagnostic guidelines, such as the DSM-IV, often do not map neatly to the problems patients bring to their doctors. Just how useful is a diagnosis if three different doctors can diagnose the same patient as suffering from depression, anxiety, and bipolar disorder? How meaningful is a diagnosis of depression if the effective medication turns out to be an antipsychotic? While the diagnostic labels have some value, they clearly do not provide reliable guidelines to treatment. They are simply the best that we can do right now.

To the extent that we understand anything about the etiology of mental illness, this situation is not surprising. One thing that seems clear is that depression, schizophrenia, and other mental illnesses do not typically arise from simple causes, such as a lesion in one part of the brain, or a failure to produce enough of a particular biochemical. Instead, they arise from abnormalities of nerve-cell populations, neurochemistry, and electrical activities distributed throughout much of the brain and nervous system. If so, then there is no single, localized, identifiable cause for (say) depression, even in principle. As a result, no medication can simply cure the problem in the sense that an antibiotic can cure an infection. A true cure would have to restructure the brain and nervous system of the patient, in a fashion customized for each person, a capability that is far beyond the medical science of today.

If neither diagnosis nor treatment can truly address the reality of mental illness, then where does that leave us? Eventually, many decades in the future, I suspect that genuine cures will become available, but in the near term, we are limited to increasingly effective symptomatic treatment, which is still of great value. The challenge, then, is to find ways to optimize treatment for conditions that do not have precise diagnoses. I will take up that challenge in the next issue of this newsletter.

Conclusion

The popular conception of diagnosis as a precise description of an illness and its cause, which enables precise treatment, has always been inaccurate. It is true in many cases, and useful when true, but diagnoses are often labels applied to groups of symptoms, and do not address the specific cause of the illness.

Further, while diagnosis and treatment have long been partners in the practice of medicine, the usefulness of that partnership is weakening because of the particular challenges posed by mental illness. The concept of diagnosis is not likely to disappear, but alternative ways of thinking about illness are likely to become more useful in the future. In the meantime, those of us who live in the present can be grateful that symptomatic relief for mental illness, while imperfect, can provide tremendous benefits.

Kevin Thompson, Ph.D. is the author of Medicines for Mental Health: The Ultimate Guide to Psychiatric Medication. You can find information about treatments for depression, bipolar disorder, schizophrenia, and sexual problems on his Web site at www.MentalMeds.org.