What Is a Rare Disease, Exactly?

I am Dr Marshall Summar, Chief of Genetics and Metabolism at Children's National Medical Center.

A question that I am often asked is: What exactly is a rare disease? We hear about them in the media. We hear about all the new drug therapies being developed, but there is a lot of confusion over what exactly we are talking about.

There is a practical definition, and what I would call an "official designation." The US Food and Drug Administration (FDA) defines a rare disease as occurring in less than 200,000 people in the United States. That means that as few as five people, or as many as 200,000, could have a rare disease. Obviously, such conditions as asthma or diabetes wouldn't qualify as rare diseases because many more than 200,000 people are affected with these conditions.

Some rare diseases are better known. Down syndrome is at the upper range of prevalence for a rare disease. Phenylketonuria (the metabolic disorder that we screen newborns for) is also considered a rare disease.

But there are other rare diseases that you might not think about when we say "rare disease." Many cancers are developing into rare diseases. With increased genetic technology and diagnosis, what were once common groups of cancers are subdividing into smaller and smaller groups. For example, prostate cancer, which is one of the most common cancers, is subdividing into smaller groups as we apply DNA technology and better biomarkers to the disease. It is no longer one large group, but is now many small groups.

How is this relevant clinically? It allows us to better target therapies for these diseases. One lesson we have learned from the rare disease world is that each patient benefits from having an individualized treatment plan. This is certainly being applied in the cancer field, but it will be applied to other fields as well.

When we start looking at hypertension, we find that there are smaller groups comprising patients with different kidney diseases or vascular diseases. Hypertension may be subtyped into increasingly smaller groups of patients.

It reminds me of the war on cancer that we started in the 1960s. We were going to attack cancer as one big group of diseases, but we found that it is actually a "divide and conquer strategy." As we divide diseases into smaller groups, it is easier to target therapies for specific conditions. Rare diseases are developing the therapeutic model that will eventually be applied to the general field of medicine.

Rare diseases affect a lot of patients. The current estimates from the National Institutes of Health and others are that there are more than 7000 rare diseases that affect 8%-10% of the population. It is a really difficult number to prove, because many aren't reported and we don't have a good central reporting mechanism. It is, however, a good working number.

In your practice, you probably have many patients with some form of a rare disease. Some of these diseases cause few or no problems, but some may cause many problems.

The other thing to remember is that the subject of rare diseases is not just children anymore. In the classic rare disease model, we think of a newborn with a serious condition who goes to see a rare disease specialist and is never heard from in the common field of medicine again. We are going to find that patients with a lifetime of good health may manifest a rare disease late in life. We may see that a commonly reported disease actually has a rare disease component. Eventually, the term "rare disease" will probably fade away, and it will start to merge with either "precision medicine" or "personalized medicine," as this field evolves going forward.

I hope this information is helpful to you when you start thinking about your patients, but also when you start hearing about rare diseases and the advances that are taking place in the field.

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Cite this: What Is a Rare Disease, Exactly? - Medscape - Nov 24, 2014.