WHY RURAL DWELLING DOES NOT MATTER, BUT THE TYPE OF DEMENTIA DOES - A RESPONSE TO MORTIMER
SuvarnaAlladi, Additional Professor, Nizam's Institute of Medical Sciencesalladisuvarna@hotmail.com
Thomas H Bak, Edinburgh, UK; Tom C Russ, Edinburgh, UK; Mekala Shailaja, Hyderabad, India, ; Vasanta Duggirala, Hyderabad, India.
Submitted December 04, 2013
Dr. Mortimer raises the important issue of confounding variables, which are relevant to all observational studies. Our study controlled for them [1] to a higher degree than others have. We succeeded in eliminating the immigration confound. We also examined illiterate mono- and bilinguals separately and found an even larger difference than among literates. Following Dr. Mortimer's comments, we conducted an additional analysis, examining rural patients (n=149) separately and still found a difference between mono- and bilinguals (56.2(10.9) vs.60.7(9.6)years, p less than 0.01).
Our study was not a single disease study; our cohort comprised different types of dementia with different etiologies, symptomatologies and, importantly, ages of presentation. If the differences in the age of onset were due to lower life expectancy among monolinguals, we would expect to find the largest effect in those dementia types that present late: we found exactly the opposite. The largest difference was found in frontotemporal dementia, which presents earlier than other dementias. [2] In AD, presenting almost a decade later, the difference was smaller and in dementia with Lewy Bodies and mixed dementia, with the highest age at presentation, smallest. Future studies, seeking further to minimize potential confounding variables, will clarify these associations.
1. Alladi S, Bak TH, Duggirala V, et al. Bilingualism delays age at
onset of dementia, independent of education and immigration status.
Neurology 2013;81:1938-1944.
2. Ratnavalli E, Brayne C, Dawson K, Hodges JR. The prevalence of
frontotemporal dementia. Neurology 2002;58:1615-1621.
For disclosures, contact the editorial office at journal@neurology.org.
Dr. Mortimer raises the important issue of confounding variables, which are relevant to all observational studies. Our study controlled for them [1] to a higher degree than others have. We succeeded in eliminating the immigration confound. We also examined illiterate mono- and bilinguals separately and found an even larger difference than among literates. Following Dr. Mortimer's comments, we conducted an additional analysis, examining rural patients (n=149) separately and still found a difference between mono- and bilinguals (56.2(10.9) vs.60.7(9.6)years, p less than 0.01).
Our study was not a single disease study; our cohort comprised different types of dementia with different etiologies, symptomatologies and, importantly, ages of presentation. If the differences in the age of onset were due to lower life expectancy among monolinguals, we would expect to find the largest effect in those dementia types that present late: we found exactly the opposite. The largest difference was found in frontotemporal dementia, which presents earlier than other dementias. [2] In AD, presenting almost a decade later, the difference was smaller and in dementia with Lewy Bodies and mixed dementia, with the highest age at presentation, smallest. Future studies, seeking further to minimize potential confounding variables, will clarify these associations.
1. Alladi S, Bak TH, Duggirala V, et al. Bilingualism delays age at onset of dementia, independent of education and immigration status. Neurology 2013;81:1938-1944.
2. Ratnavalli E, Brayne C, Dawson K, Hodges JR. The prevalence of frontotemporal dementia. Neurology 2002;58:1615-1621.
For disclosures, contact the editorial office at journal@neurology.org.