Small numbers with significant clinical and public health implications
Yesterday’s Mayo Clinic’s press release said:
The study . . . reports that 296 of the 1,450 study participants developed MCI [mild cognitive impairment], an incidence rate of 6.4 percent per year overall.
Among men, the incidence rate was 7.2 percent, compared with 5.7 percent per year for women.
People with MCI have mild problems with thinking and memory that do not interfere with everyday activities, although their forgetfulness is often apparent to them and their friends and family. While not everyone with MCI develops dementia, an estimated 5 to 10 percent do.
© 2012 Mayo Clinic, Mild Cognitive Impairment is Common, Affects Men Most, Newswise (25 January 2012)
The Mayo Clinic went on to say that mild cognitive impairment is characterized by difficulty in learning, remembering, problem-solving, decision-making, and coping with complex activities.
A wit might observe that, on an absolute scale, this characterizes all of humanity.
R.O. Roberts, Y.E. Geda, D.S. Knopman, R.H. Cha, V.S. Pankratz, B.F. Boeve, E.G. Tangalos, R.J. Ivnik, W.A. Rocca, and R.C. Petersen, The incidence of MCI differs by subtype and is higher in men: The Mayo Clinic Study of Aging, Neurology, doi: 10.1212/WNL.0b013e3182452862 (published online before print, 25 January 2012)
In 2004, the research team chose a sample of people in Olmsted County, Minnesota, who were aged from 70 to 89. They evaluated each member according to accepted neurological, neuropsychological, and Clinical Dementia Rating scales.
Then, they repeated this testing at 15 month intervals, using examiners who did not know how the people had tested in the prior evaluations.
From the abstract:
Among 1,450 subjects who were cognitively normal at baseline, 296 developed MCI [mild cognitive impairment].
The age- and sex-standardized incidence rate of MCI was 63.6 (per 1,000 person-years) overall, and was higher in men (72.4) than women (57.3) and for [amnestic] aMCI (37.7) than [non-amnestic] naMCI (14.7).
The incidence rate of aMCI was higher for men (43.9) than women (33.3), and for subjects with ≤12 years of education (42.6) than higher education (32.5).
The risk of naMCI was also higher for men (20.0) than women (10.9) and for subjects with ≤12 years of education (20.3) than higher education (10.2).
© 2012 R.O. Roberts, Y.E. Geda, D.S. Knopman, R.H. Cha, V.S. Pankratz, B.F. Boeve, E.G. Tangalos, R.J. Ivnik, W.A. Rocca, and R.C. Petersen, The incidence of MCI differs by subtype and is higher in men: The Mayo Clinic Study of Aging, Neurology, doi: 10.1212/WNL.0b013e3182452862 (published online before print, 25 January 2012)
Translating this terminology
Translated, these numbers mean that — if you clump both kinds of cognitive impairment together — men (aged 70-89 in one Minnesota county) are 26 percent more likely than women to develop mild cognitive impairment each year.
The 26 percent comes from dividing 72.4 by 57.3.
Both men and women are 256 percent (2.56 times) more likely to develop simple forgetfulness than the other components of cognitive impairment. Meaning, presumably, that though we might forget things, if reminded, we can still do complex tasks or calculations.
This 256 percent came from dividing 37.7 by 14.7.
Educationally, advancing beyond high school appears to reduce the risk of forgetfulness (amnestic impairment) by 31 percent in both sexes.
The 31 percent comes from dividing 42.6 by 32.5.
Similarly, advancing beyond high school also appears to reduce the risk of the other components of cognitive impairment by 99 percent.
The 99 percent comes from dividing 20.3 by 10.2.
As to gender differential, men were approximately 32 percent more likely to develop forgetfulness than women and 83 percent more likely to develop difficulties with learning and coping with complex tasks.
Women might say, “So, what else is new?”
The 32 percent comes from dividing 43.9 by 33.3.
The 83 percent comes from dividing 20.0 by 10.9.
A caution regarding the education finding
I do not consider this education-founded spectrum to be particularly meaningful insofar as its implied effects on public health or education policy go. We do not yet know whether ending school in 12th grade or below says something about us individually, or whether identical brains that go beyond K-12 are favorably impacted by doing so.
When overall risk was split into its forgetfulness and non-forgetfulness components, men’s risks for each were 32 and 83 percent higher than women’s. Yet, the study reports only a 26 percent enhanced total risk for men.
How did that happen?
The answer probably lies in the team’s definitions of incidence and risk. And whether the authors were consistent in measuring both on an annual basis.
This ambiguity may be a good example of the unnecessary confusion that medical studies create, when scientists do not properly define their terms or, worse, use them in overlapping, but conceptually conflicting ways in their findings.
I wrote about the bane of sloppy scientific and medical language, here.
A caveat about exaggerating what these results mean at the individual level
The numbers involved in the Minnesota study are small and should not be especially worrisome for individuals. “Only” 6.4 percent of this aged sample developed mild cognitive impairment annually.
That means that my 32 and 83 percent calculations are not as negatively big as they might seem, when they are considered at the level of one man’s individual risk for developing mild cognitive impairment each year.
But at the population level the implications are large
Even seemingly small gender-differentials like these, distributed among a large national population, means that public health and medicine are going to need to develop means to deal with noticeably larger numbers of impaired men than women.
Women, I suspect are smiling, “Do you think you’re you telling me something that I don’t already know?”