What is rare? and how many people have Primary Orthostatic Tremor?
Is OT present at a rate of 1 per 100,000 people for a total of 70,000 people throughout the world,
or 1 in a million for a total of 7,000 people worldwide?
Question - What is a Rare Disorder?
The definition changes depending upon where you live.
USA - A Rare disease is defined in the USA as fewer than 200,000 Americans at any given time. (approximately the population of Boise, Idaho or Richmond, Virginia)
Calculated for the current population of the US (324 million) a rare disease would effect roughly 617 people per million
EU - A Rare disease is defined in the European Union (EU) as fewer than 245,000 people (approximately the population of Nantes, France or Wolverhampton, UK) at any given time. Or, defined as rare when it affects less than 1 in 2000 citizens.
Calculated for the current population (508 million) a rare disease would effect 393 people per million
The population in the world today exceeds 7 billion. Where does OT fit into this picture?
Orphanet** is a consortium of approximately 40 countries, coordinated by the French INSERM team. Among the many tasks of this group are to gather information on research. And in the document - Orphanet Report Series - Diseases listed by decreasing prevalence or number of published cases they determined the prevalence of 2000 rare diseases. Included in the document is a very reasonable statement and something that we should keep in mind when considering any statistics.
The exact prevalence rate of each rare disease is difficult to assess from the available data sources...Therefore, these estimates are an indication of the assumed prevalence but may not be accurate.There a 2 parts to the Orphanet report. The first list contains diseases that are more prevalent and therefore Orphanet was able to estimate how many people are effected by the disease per 100,000. This group which we will call Group 1, is a list includes 600 of the more "common" rare diseases. The least common disease in this list is a rare tumor that effects 25 people per 100,000. This would be equal to 81,000 people in the USA. But OT is not in Group 1 because it is less prevalent and as a result less data is available.
The second part of the Orphanet report, Group 2 contains rare diseases with less prevalence and too little data to estimate how many people as a percentage of the population. This second list is where Primary Orthostatic Tremor falls. They have determined that from the beginning of the 20th century until now, there have been 391 published cases of Primary Orthostatic Tremor, worldwide. This number, 390, doesn't tell us how many people have OT but it does give us some clues as to where it falls amongst other rare diseases. A Mayo Clinic retrospective research study published in 2016 included all 184 OT patients from 1976-2013. As a result this may change the number used by Orphanet in the future. As reported in US News, Mayo Clinic locations see roughly 500,000 people each year. Many of these may be repeat patients, but even so, 500,000 people each year compared to 184 people with OT over a 37 year span does lead us to an idea of the rarity of OT.
The lowest ratio of people in the Group 1 that has adequate information and are therefore more prevalent, is 25 people per 100,000. Since OT is in the Group 2 we might conclude that OT is then below the ratio of 25 people per 100,000. Within Group 2 which includes OT, it ranks relatively high. Of the approximate 1130 diseases in this group, OT is 32nd, based upon the number of individuals included in published research papers.
OT is in the group of ultra-rare diseases with a prevalence of less than 25 per 100,000 ... but how much less?
To make this a little more approachable, we will consider the population of the USA and how many people may have OT. The least prevalent disease in the Group 1 would have roughly 81,000 people in the USA. But OT is in Group 2 with less prevalence and roughly 1/3 the way down this list. This might indicate no more than 53,000 people in the US may have OT. And that number is likely to be very far off. This group is not using the same methods of the more prevalent diseases in group 1 so it may be 53,000 or 4,000 people, or less. And considering that most specialists have only known 1-6 patients with OT over a career, the "guess-estimate" of people with OT continues to drop. To put the rarity of OT into perspective, it is estimated that in the USA there are 7 million people with Essential Tremor, and 0.5 - 1 million people with Parkinson's Disease.
- USA population 324 million:
Essential Tremor - 7 million (USA) - SOURCE
Parkinson's Disease - 1 Million (USA) - SOURCE
Orthostatic Tremor - is it less than 53,000 worldwide (not just the US) or possibly as low as 7,000 people ... 1 in a million?
note: 1 in a million would only be 324 people in the US. My guess is this number does fall short, and the ratio is larger simply based on the 1,200 people that have registered on this website. Even though they are not all from the US, and clearly only a small portion of people have registered. It is easy to remember the phrase 1 in a million, but it's more likely that it is somewhere between 2-20 people per million. That's a wide range but it is only a guess.
What about the number of people that are misdiagnosed or undiagnosed?
As with many rare diseases, mis-diagnosis or no diagnosis is a problem. Primary Orthostatic Tremor is not unique with this challenge. Accurate diagnosis would in theory lift all of the numbers of people with rare diseases within the Orphanet report.
I know that it's disappointing to have read this far without an absolute number, but hopefully we have a better understanding of how rare Orthostatic Tremor is.
Lack of data leads to plenty of guessing, and this article has it's share. Do you have any thoughts about the prevalence of OT? please click reply and share your ideas
Many thanks to Orphanet* and Inserm for the very helpful report on rare disease prevalence.
**Orphanet is led by a consortium of 35 countries (and 5 contact points) , coordinated by the French INSERM team. National teams are responsible for the collection of information on expert centres, medical laboratories, ongoing research and patient organisations in their country and translation in their language if funding available.