Did you know that Toronto is the most ethnically diverse city in the world?  Besides, it’s a pretty big place, with about 6 million people in its metropolitan area.  Besides that, Toronto is home to one of the top PSP research institutions in the world, the Rossy Centre at the University of Toronto, and Canada has universal, free medical insurance, which removes financial impediments to diagnosis and treatment.  That’s why Toronto is a great place to answer the question as to whether the prevalence of PSP is uniform across groups of different ethnic backgrounds.

Dr. Blas Couto and colleagues have done just that, reporting their results in the current issue of Parkinsonism and Related Disorders.  They tabulated ethnicity for 135 patients with PSP living in the officially designated Toronto area and seen at their center as patients from 2019 to 2023.  The group proved unusual in that only 4.4% had the PSP-Parkinsonism variant, compared with figures elsewhere of around 25% to 40%.  The group with PSP-Richardson syndrome comprised 68% rather than the usual 45% to 55% and the other variants gave the expected percentages.

The ethnicities they considered were actually geographical areas, not exactly race, whatever that is. (“Race” has no scientific definition, anyway.)  The categories were — and this is directly copied/pasted from the paper:

• East and southeast Asia, including China and Pacific Islands such as Philippines
• Southern Asia, including India, Pakistan and middle east countries
• Africa
• South America, Central America and Mexico
• West Indies, including Guyana, Haiti, Bahamas, Lesser Antilles such as Barbados, Trinidad and Tobago, Dominica, Grenada, Saint Kitts, Antigua and Barbuda, Santa Lucia
• Europe, Australia and North America, excluding Mexico.

The analysis compared the frequencies of these demographics to those from the census for people aged 65 and older living in the Toronto metro area. 

The result was that the southern Asia group was moderately over-represented among the patients with PSP.  That group accounted for 11.5% of the general population but 25.2% of the PSP population at the researchers’ center.  That was statistically significant at the <.001 level, meaning that the chance of its being a false-positive fluke are less than 1 in 1,000.

They also compared the six groups to one another with regard to the PSP subtypes, finding the same southern Asia group to include more PSP-progressive gait freezing (17.7%) and PSP-corticobasal syndrome (14.7%) than the European-derived patients (6.4% and 9.5%, respectively). 

Couto et al mention the possibility of some sort of genetic effect, but the literature offers no clues as to what that might be, and they cite three previous papers from the UK showing no difference in PSP prevalence between whites and southern Indians there.  Could something in the food or water in Toronto affect Asians disportionately?  The title of this post offers an unserious possibility, but you get the idea.

Chin-stroking on that aside, now is when the rest of us try to poke holes in the findings.  Here are my efforts:

• Do the ethnic percentages in their PSP practice or in medical institution as a whole accurately reflect those of the Toronto area?  That would be easy to measure.  I ask because in the US in recent years, the medical profession has acquired a disproportionate representation of people of southern Asian background.  Could that group therefore trust academic physicians more and seek their care more readily than do other ethnic groups? 
• The patient mix of a highly specialized practice like that at the Rossy PSP Centre is subject to the referral habits of outside neurologists.  Neurologists who feel less comfortable with the atypical Parkinsonisms may be more likely to refer patients.  Perhaps that applies to a few neurologists practicing in heavily southern Asian neighborhoods in the Toronto area.
• Despite the universal availability of free medical care in Canada, racially-based disparities in health and care access do exist there.  Couto et all cite this article. 
• This is a univariate comparison, meaning that it didn’t correct for a hypothetical effect of other health issues that might be more common in the southern Asian population.  Not that I know what those might be.
• Although the analysis age-matched the PSP group with the general population by confining itself to the over-65 group, that may not have been enough. Perhaps working-age people immigrating from southern Asia brought elderly parents with them more often than did those immigrating from elsewhere, thereby skewing the over-65 group towards the 80s and 90s and increasing the measured prevalence of an age-related disease like PSP.

An intriguing finding.  Hopefully this paper will stimulate others to dig deeper. That would be a victory for any scientific paper.