I’ll Take Mine Rare

Ataxia is often described as a rare disease. For a long time, I presumed the word rare was the casual use of an adjective, but it turns out that rare disease is a specific term that’s been around for many years.


In U.S. law, a rare disease is a disease that affects less than 200,000 people in the U.S. It’s a mystery to me why they did not specify a ratio rather than use an absolute number. In 2002, at the time of the law, that was 70 per 100,000 people, or 1 in 1,438. In 2014, that’s 63 per 100,000 people, or 1 in 1,585. (I’ll be using 317 million as the population of the U.S and 7.1 billion as the population of the world.)

Still, one wonders: how rare is rare, especially compared to other ailments?


Here are some terms to bear in mind:

  1. Prevalence: number of people living with a disease.
  2. Incidence rate: number of new cases in a given time period.
  3. Mortality rate: number of deaths in a given time period.

For these numbers to make sense, they must be given per population, such as per a geographic locale and/or age group. Incidence rate is sometimes given per annual live births for a given population.

Most rare diseases are genetic, chronic, degenerative, and incurable. People with a particular disease have the genetic defect their whole lives and are symptomatic for perhaps decades. For those degenerative diseases, prevalence is mentioned perhaps at the exclusion of the other measures. But bear in mind that there’s a relationship among these measures, approximately:

prevalence change = (incidence rate – mortality rate) x duration

The only way prevalence can stay constant is if the incidence rate and mortality rate are equal. From the “casual” data that exists out there, I don’t think it’s possible to accurately discern a precise account of the numbers for a specific disease.

Also bear in mind that the concept of disease itself is complicated. Many diseases aren’t fatal to healthy people, but even a mild disease can kill someone whose life is hanging in the balance. Also, you can have a fatal disease and die of something unrelated, such as murder or a car accident. Some diseases are 100% fatal if untreated and are curable (or become less than 100% fatal) if treated early, such as cervical cancer in women. Some diseases are incurable and 100% fatal, such as ataxia. Every variation exists.


Ataxia prevalence

Ataxia covers a lot of autosomal dominant variations (37 or so) and recessive variations (13 or so), as well as many acquired (nongenetic) types that occur twice as frequently as the hereditary types. I will focus here on what I have, which is autosomal dominant spinocerebellar ataxia (SCA).

Many sites give the prevalence of SCA as 1-5 per 100,000. Many others give the prevalence a little lower, at around 2-3 per 100,000. The only thing I can say is likely is that the number is in the low single digits per 100,000.

http://ataxia.org/pdf/2009_Managing_Ataxia_and_Managing_Your_Neurologist.pdf (removed from website)

Let’s say the prevalence of SCA is about 5 per 100,000 (but try to keep in mind that that’s an upper maximum). That’s 15,850 in the U.S. or 355,000 in the world. That indeed seems quite rare to me, in the U.S. anyway, but the question still is: how rare is rare? We need some comparisons.

(Let me briefly mention the prevalence of SCA3. This document says that the prevalence of SCA3 is about 15% of the SCA cases, which when combined with 1-5 per 100,000 is 1.5-7.5 per million.)

See here for my follow-up article on ataxia prevalence.

The upper extremes

What are the most prevalent fatal diseases? Heart disease and cancer are the leading killers in the U.S.

The prevalence of heart disease is about 80 million people in the U.S. That’s over 25,000 per 100,000 Americans, or about 1 in 4 (which is 5,000 times worse than ataxia). The mortality rate is about 1 million people annually in the U.S. That’s over 300 per 100,000 Americans, or about 1 in 300.


The prevalence of cancer (including survivors) is about 14.5 million people in the U.S. That’s over 4,500 per 100,000 Americans, or about 1 in 22 (which is 900 times worse than ataxia). The mortality rate is about half a million people annually. That’s over 150 per 100,000 Americans, or about 1 in 600.


Stop, you’re killing me

There are some causes of death where mortality rate is the only meaningful measure: murder, suicide, and fatal accidents (trauma). All three of these cases generally show up in top-15 lists of leading causes of death in the U.S. and account for upwards of 200,000 deaths per year (63 per 100,000, or affecting about 13 times as many people as those with ataxia).

The number of yearly motor vehicle deaths in the U.S. (10.3 per 100,000 in 2013), which is nearly identical to the number of gun-related deaths (10.5 per 100,000 in 2013) in the U.S., is of the same magnitude, though perhaps twice as many, as people living with ataxia.



Also note that when considering communicable diseases subject to epidemics and pandemics, outbreaks of almost any size will make international news. For example, there’s been a recent Ebola outbreak in West Africa. News of hundreds and then thousands of deaths has been reported on daily, and work on a vaccine has been feverish (no pun intended). It’s interesting to note and to ponder how seriously the deaths of these people (and their quantities) are taken, though bear in mind that a threat of a pandemic is vastly different from concerns over a centuries-old genetic anomaly.

Warning: these numbers are bogus

What I’d like to know is where the prevalence of ataxia fits—in order—into a complete list of fatal diseases, in particular how many other diseases have the same prevalence (10? 100?). But that information doesn’t seem to be available, nor would it or could it be particularly accurate. If only considering time frame nebulosity, no two statements of statistics are or can be consistent. And there’s no homogeneousness across geographical regions and age groups. And some people don’t even know they have a particular disease. There are a few numbers here or there that are overwhelming enough to make a point, but that’s about it.

There is a disease that is incurable, communicable, and can affect billions of people in a year. Sound terrifying? Well, it’s the most widespread disease in the world. Wait—what? Why aren’t we all dead? Because I’m talking about the common cold, which is incurable but goes away by itself after about 10 days. The point here is that there’s an endless number of ways to present data. It’s perhaps impossible to find abundant data that are agendum-free.

Some “same family” comparisons

Moving on. Now that we know how SCA compares to the most prevalent fatal diseases out there, we may wonder how SCA compares to other diseases that are genetically or symptomatically similar. Here are a few comparisons (from above: assuming the prevalence of SCA is about 5 per 100,000, or 15,850 people in the U.S., which is a high estimate).

Cerebral palsy
Alzheimer’s disease
Parkinson’s disease
Multiple sclerosis
Muscular dystrophy
Huntington’s disease
Adrenoleukodystrophy (ALD)
  • https://en.wikipedia.org/wiki/Adrenoleukodystrophy
  • http://www.malacards.org/card/adrenoleukodystrophy
  • Prevalence in the U.S.: given as 1-9 per 100,000, but more exact numbers seem to be unknown. Occurs only in boys. It’s autosomal recessive, so both parents must have the defect to cause the disease, and it seems to be better known how many carry the defect vs. how many have the disease.
  • This is a rare disease, with a prevalence possibly similar to SCA, but probably higher. It affects boys only.
  • This is the first genetic brain disease to get a gene-therapy treatment past human trials—not a cure, of course, but a disease-modifying treatment.
Amyotrophic lateral sclerosis (ALS, or Lou Gehrig’s disease)
Friedreich’s ataxia
  • https://en.wikipedia.org/wiki/Friedreich%27s_ataxia
  • Prevalence in the U.S.: 6,340-9,510 people; 2-3 per 100,000.
  • This is a rare disease. The prevalence of FA is greater than any one of the SCAs, yet all the SCAs combined have a greater prevalence than FA. Still, this makes FA the most common form of genetic ataxia. FA is a severe childhood disease and as such is considered far worse than SCA.

Rare diseases come roaring back

According to Wikipedia, there are upwards of 7,000 rare diseases. These diseases affect about 30 million in the U.S. (9,464 per 100,000; 1 in 11) and about 350 million in the world (4,930 per 100,000; 1 in 20). The collective group of everyone with a rare disease is quite large.


It is interesting to stop and think about that for a moment. Whereas it takes a city-sized population of 100,000 to encounter a small handful of people with any of 30+ different kinds of autosomal dominant ataxia, and a much larger city of millions to encounter someone with your same genetic SCA defect, it only takes a small room-sized population of 11 or so to encounter someone with a rare, possibly fatal, disease.

If you consider rare diseases individually, you might guess that there wouldn’t be abundant resources devoted to one particular disease. But diseases can be grouped such that advances related to one disease apply to the whole group. And in fact, some rare diseases such as ataxia get a fair amount of attention.

There’s financial incentive and relaxation of some restrictions for companies working on drugs for rare diseases in the U.S. In this context, it’s also handy to be aware of the terms orphan drug and orphan disease (the latter being a synonym for rare disease):

Leave a Reply

Your email address will not be published. Required fields are marked *