Age and Gender as Factors of Pressure Sensitivity of Pain-Free Persons: Are They Meaningful?

Abstract

Purpose: Prior findings suggest that women and elderly persons are more sensitive to pressure than men and younger persons; however, the magnitudes of these differences are substantially inconsistent. We answered the question whether the higher sensitivity of women and elderly persons is quantitatively meaningful. Specifically, we investigated if it is large enough to hamper the diagnosis, classification and follow-up of pain conditions by clinicians.

Materials and methods: From each age stratum (18-20, 21-30, 31-40, 41-50, 51-60, 61-70, 71-80, and >80 years), 40 pain-free women and 40 pain-free men were recruited. They rated the intensity of pressure of ten Newtons over ten seconds on an analogue zero to ten rating scale. The pressure was applied on their middle fingers and ear lobes with a threshold algometer. Centile curves visualized the sex- and age-dependent fluctuation of pressure sensitivity.

Results: Over the entire age range from 20 to 80 years, the median curves fluctuated within the interval of less than two points. The distance between the median curves of men and women was also less than two points. On the average, the median difference was half a point on the finger (p = 0.249) and the ear lobe (p = 0.083).

Conclusion: Less than two points is below the minimal clinically important difference for a zero to ten analogue pain rating scale; differences smaller than one point are even below the resolution of the scale. Sex differences and age fluctuations of pressure sensitivity are negligible.

Keywords: age; pain; sensitivity; sex.

Figure 1

Interview: The personnel interviewed the participants and recorded their gender, their age, and the intensity with which they perceived a standard pressure stimulus on their middle fingers and on their ear lobes. “0” was no sensation at all; “10” was the strongest imaginable pain.

Figure 2

Participant flow: All participants provided full data.

Figure 3

Pressure sensitivity on the middle finger in 648 healthy individuals. Provided that our sample is representative, 25% of the pain-free individuals are below the lower quartile, 25% are between the median and the adjacent quartile, and 10% are beyond the 90^th centile. The middle lines are our estimates, and the 95% confidence bands indicate their statistical precision. Given that they are narrower than one measurement unit, the statistical uncertitude is smaller than the measurement uncertitude. Thus, the statistical power is sufficient. The median increase in pressure sensitivity starts with the age of 40 years and reaches its top difference of one measurement unit at the age of 75 years. The absolute minimal clinically important difference for 0 to 10 numeric rating scales is at least two units, and the relative minimal clinically important difference is 30%. In other words, the increase from 40 to 75 years is relatively relevant to its onset but absolutely, one point is too little to be meaningful for patients.

Figure 4

Pressure sensitivity on the ear lobe in 648 healthy individuals. Provided that our sample is representative, 25% of the pain-free individual are below the lower quartile, 25% are between the median and the median and the adjacent quartile, and 10% are beyond the 10^th and 90^th centile. The middle lines are our estimates, and the 95% confidence bands indicate their statistical precision. Given that they are narrower than one measurement unit, the statistical uncertitude is smaller than the measurement uncertitude. Thus, the statistical power is sufficient. The median of pressure sensitivity decreases from the age of 20 years to a minimum at 30 years and increases to its top difference of 1.2 measurement units at the age of 76 years. The absolute minimal clinically important difference for a zero to ten numeric rating scales is at least two measurement units, and the relative minimal clinically important difference is 30%. In other words, the increase from 30 to 76 years is neither relatively nor absolutely meaningful for the perception of patients.

Figure 5

Pressure sensitivity on the finger in 347 women compared to 301 men. Provided that our sample is representative, the medians split the individuals of the respective gender in two equally large groups. The middle lines are our estimates, and the 95% confidence bands indicate their statistical precision. Due to the smaller number of individuals, the gender-specific confidence bands are wider than the overall confidence bands. However, they remain narrower than one unit. Thus, the gender-specific statistical power is sufficient, too. The difference between men and women fluctuates between zero and 0.6 units. This difference is below the individual perception of one unit and clearly below the minimal clinically important difference of two units. It is below statistical significance, too, as the 95% confidence bands overlap. The median increase in pressure sensitivity in men starts to increase with the age of 40 years and reaches its maximum difference of half a measurement unit at the age of 70 years. In women pressure sensitivity increases from 20 years to 32 years reaching a first peak followed by a slight dip and a second peek with 76 years with a maximum difference of 1.2 measurement units.

Figure 6

Pressure sensitivity on the ear lobe in 347 women compared to 301 men. Provided that our sample is representative, the medians split the individuals of the respective gender in two equally large groups. The middle lines are our estimates, and the 95% confidence bands indicate their statistical precision. Due to the smaller number of individuals, the gender-specific confidence bands are wider than the overall confidence bands. They are wider than one unit. The difference between men and women ranged between zero and 1.6 measurement units. The largest difference in pressure sensitivity between men and women is between 40 and 50 years. During this period, the sensitivity of both sexes increases. The absolute minimal clinically important difference for zero to ten numeric rating scales is at least two measurement units, and the relative minimal clinically important difference is 30%. In other words, the difference between women and men is not meaningful.