26 April 2023
Zingel et al. reported an association between psoriasis and dementia and concluded that patients with psoriasis should be advised about the relationship between psoriasis and the development of dementia . With a sample size of over 20,000 patients and over 175,000 patient-years of follow up, the study had considerable power for detecting small differences in relative risk. They found that psoriasis was associated with a statistically significant 1.2-fold higher risk of developing dementia. Relative risk can falsely amplify the perceived association between two variables; absolute risk is needed to assess the clinical significance of the association .
The incidence of dementia was 15 per 1,000 patient-years in those with psoriasis and 11.9 per 1,000 patient-years in those without psoriasis, a difference of 3.1 per 1000 patient-years. Even assuming that this difference is due to psoriasis and not some confounding variable, it is not clear that this risk would warrant screening psoriasis patients for dementia or warning patients of an increased risk. There is no evidence that such screening would be cost effective, and advising patients about the small increased risk of dementia might cause more distress or other harms that outweigh any benefit.
A causal relationship between psoriasis and dementia remains unclear. In a 2020 cohort study, after controlling for comorbidities such as chronic liver disease and chronic obstructive pulmonary disease, psoriasis was not a risk factor for dementia . In a 2021 study comparing cognitive test scores and brain MRI markers, there was no significant correlation between psoriasis and dementia risk .
It may be good to be reticent about drawing conclusions about what “should” be done based on relative risk differences. Absolute risks and cost-benefit studies may be needed before recommending screening and counseling measures.
Rithi John Chandy1, Aditi Chokshi1, Steven R. Feldman1,2,3,4
1Center for Dermatology Research, Department of Dermatology, Wake Forest University School of Medicine, Winston-Salem, NC, USA
2Department of Pathology, Wake Forest University School of Medicine, Winston-Salem, NC, USA
3Department of Social Sciences & Health Policy, Wake Forest University School of Medicine, Winston Salem, NC, USA
4Department of Dermatology, University of Southern Denmark, Odense, Denmark
 Zingel R, Jacob L, Smith L, Konrad M, Kostev K (2023) Association between psoriasis and dementia: a retrospective cohort study. J Alzheimers Dis Rep 7, 41-49.
 Noordzij M, van Diepen M, Caskey FC, Jager KJ (2017) Relative risk versus absolute risk: one cannot be interpreted without the other. Nephrol Dial Transplant 32 (Suppl_2), ii13-ii18.
 Wu C-Y, Hu H-Y, Chou Y-J, Li C-P, Chang Y-T (2020) Psoriasis is not a risk factor for dementia: a 12-year nationwide population-based cohort study. Arch Dermatol Res 312, 657-664.
 Pezzolo E, Mutlu U, Vernooij MW, Dowlatshahi EA, Gisondi P, Girolomoni G, Nijsten T, Ikram MA, Wakkee M (2021) Psoriasis is not associated with cognition, brain imaging markers, and risk for dementia: the Rotterdam Study. J Am Acad Dermatol 85, 671-680.
Response to Rithi John Chandy and colleagues
We thank Rithi John Chandy and colleagues for their letter. The authors correctly write that the absolute dementia risk increase in the psoriasis cohort compared to the healthy population is small and may not warrant screening psoriasis patients for dementia or warning patients of an increased risk. Such screening may not be cost-effective, and advising patients about the small increased risk of dementia might cause distress that would outweigh any potential benefit. We absolutely agree with this point.
Indeed, we mentioned in the last sentence of our discussion that patients with psoriasis should be monitored closely by physicians and should be advised about the possible relationship between the disease and the risk of developing dementia. Nevertheless, the treating physician is always responsible for deciding which investigations should be performed. Physicians often use treatment guidelines published by medical societies on the basis of many existing studies. If an association between psoriasis and dementia is only rarely identified in such studies and if the increase in risk is as small as that shown in our study, medical societies will not recommend screening psoriasis patients for dementia. In other words, we do not believe that one isolated article will have a major impact on the decisions made by physicians.
Rebecca Zingel, Karel Kostev
Epidemiology, IQVIA, Frankfurt, Germany