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Last reviewed February 27, 2013

I have often heard it said that the number of ways to express a concept tells you how important that concept is. If that's true, the unknown must be quite important in medical practice. If you are a health care professional and want to describe a condition or situation that develops for no particular reason and has no identifiable cause, consider the words we have to say that:

Idiopathic — Meaning "a disease arising from itself" (from the Greek, idios meaning "own" and pathos meaning "disease"), this term is applied to a number of conditions, including idiopathic thrombocytopenic purpura (also called immune thrombocytopenic purpura), a disorder of unknown cause in which one type of blood cells, the platelets, are attacked by the immune system, causing these blood cells to fall to dangerously low levels.

Essential — This is most commonly used to describe hypertension (high blood pressure), and it applies to the vast majority of hypertension cases, which have no clear underlying explanation. Rarely (perhaps 5% of the time), high blood pressure is due to a specific, recognized disorder such as a narrowed artery near the kidney or excessive amounts of a hormone called aldosterone.

Primary — As opposed to "secondary," this term usually is used for a symptom or disease that is not associated with any other disease. For example, "primary Raynaud's" describes the 90% of people with this condition who have no associated rheumatic disease; the other 10% have rheumatological disorders such as lupus or scleroderma.

Cryptogenic — This is most commonly used to describe cirrhosis that develops for no identifiable reason. This is an example of a term that has largely been supplanted by advances in medical knowledge: Most cases of "cryptogenic cirrhosis" turn out to be due to fatty liver disease (also called NASH or non-alcoholic steatohepatitis)or hepatitis C, a virus only discovered in the last 20 years.

Idiosyncratic — This is usually used in reference to an unexpected and unpredictable reaction to a drug, such as an allergic reaction.

Fever of unknown origin (FUO) — As the name implies, this commonly used term describes people who have a fever but no recognized reason for it.

And there are still other terms to describe disorders of unknown cause. The remarkable thing is that not only are there many ways to say the same thing, but there are many basic conditions for which a cause has not yet been identified.

Why So Many Names?

A combination of factors probably has contributed to the multiple ways to say the same thing. Many of these conditions were named long ago and the name simply stuck. Others serve a useful purpose in segregating one category of illness from another, as is true with the notion of "primary" vs. "secondary," or simply in dividing the known from the unknown. It is also probably true that having a label for an illness provides some degree of comfort and security. Essential hypertension probably sounds better to the doctor and the patient than "elevated blood pressure of unknown cause."

Appreciating the Unknown

It surprised me to learn in medical school how much is unknown, and it surprises many of my patients when they realize it. The amount we do not know is almost surely underestimated and underappreciated.

While there is often a great deal of information about disease associations — those features that tend to go together — that's far from recognizing a cause. For example, we know quite well that obesity is associated with (is a risk factor for) type 2 diabetes; but why some people who are overweight develop diabetes while others do not is far from clear. Even with the recent identification of genes that increase the risk of disease, most of the time not everyone with that gene develops the disease. The reason for this is unknown most of the time. Similarly, we usually cannot explain why some people respond well to a particular treatment while another person with the same condition does not.

The Bottom Line

With so much unknown, perhaps it should be no wonder that there are so many ways to say "I don't know." Acknowledging what is not known and asking for a "translation" of terms that are unfamiliar will help to demystify medical information you may read or hear. And demystifying medical information is a major step toward understanding what your doctor is saying. Just don't be surprised if the ultimate explanation turns out to be a shrug of the shoulders.

Robert H. Shmerling, M.D., is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

More What Your Doctor Is Saying Articles

Sometimes It's 'I Don't Know'

Why So Many Names?

Appreciating the Unknown

The Bottom Line