The prevalence of a disease identifies how many people are affected by a condition and with Lewy body dementias the prevalence is estimated to be 1.3 million Americans. Incidence, however, measures how many new cases of a disease are identified during a specific period of time. Only limited information is available about the incidence of Lewy body dementias, but it is widely accepted to be highly underdiagnosed and is the most frequently misdiagnosed form of dementia. A new study examines for the first time in North America the incidence of Lewy body dementias and its distribution by age and sex in the general population.
Researchers at Mayo Clinic reviewed the medical records of the population of Olmsted County, MN from January 1991 to December 2005, taking advantage of the unique resources of the Rochester Epidemiology Project. Dr. Rodolfo Savica and colleagues identified individuals who had documented two core symptoms of LBD (parkinsonism and dementia) and studied their medical records to further diagnose individuals with LBD.
The use of medical records to diagnose LBD in this study, including both dementia with Lewy bodies (DLB) and Parkinson’s disease dementia (PDD) has inherent limitations with defining incidence rates. For example, some cases of undiagnosed DLB have documented dementia but not other DLB symptoms like fluctuating cognition or sensitivity to neuroleptics, to support a DLB diagnosis. Those individuals were diagnosed as PDD and were not included in the incidence rates of DLB. Others may have minimal, undocumented parkinsonism and this study required the documented presence of both dementia and parkinsonism. For those with Parkinson’s disease, mild dementia may be overlooked or more significant dementia may not be documented if it is simply considered late stage Parkinson’s. Each example limits the ability to capture the full incidence of LBD. On the other hand, the case identification procedure in this study was extremely rigorous and strict; therefore only small numbers of LBD cases may have been missed. Given both the strengths and limitations, this study should be viewed as establishing the minimum incidence of LBD.
Researchers identified 542 incident cases of parkinsonism that met criteria for the study; of those, 64 cases were diagnosed by researchers examining their medical records as having DLB and 46 cases as having PDD. Two experts independently diagnosed each case, using a sub-set of 40 cases to ensure reliability of the case-finding procedure. The procedure resulted in the same diagnosis in 96.7 percent of cases.
The overall incidence rate of DLB in this study was 3.5 cases per 100,000 person years, which is lower than a previous study on Parkinson’s disease. Indeed the incidence rate of PD was 14.2 cases per 100,000 years, four times higher than DLB.
This relatively low incidence rate may be an indicator of a major difficulty in detecting and documenting other DLB symptoms. This may be a result of the time it took for DLB diagnostic criteria established in 1996 to become part of routine clinical practice when diagnosing dementia. Some DLB symptoms, like fluctuating cognition or REM sleep behavior disorder, would not have been widely screened for in the 1990’s and 200’s when making a differential dementia diagnosis.
The combined incidence of Lewy body dementias in this study was 5.9 cases per 100,000 person-years, with a higher incidence in men than women (7.1 vs 4.9). Incidence of DLB was twice as common in men than in women (4.8 vs. 2.2) and was nearly 4-fold more common in men at ages 70-79. Incidence of PDD was rather even in men and women with PDD (2.3 vs. 2.7), until the age of 80 when more men had PDD. Individuals with DLB had a higher frequency of hallucinations (62.5 percent vs. 20.0 percent) and fluctuating cognition (25.0 percent vs. 8.9 percent) than those with PDD.
The incident rate increased exponentially with increasing age both overall, and in men and women separately. DLB appears to affect individuals at a younger age than PDD. The incidence of DLB rose with age, from 10.3 for age 60-69, peaking at 44.5 in ages 70-79 and remaining high at 30.1 in those aged 80-99 years. Comparatively, PDD incidence increased more slowly with age, ranging from .06 in those 50-59, 3.4 in ages 60-69, 19.8 in ages 70-79, and a peak of 47 in persons aged 80-99.
Of the 542 cases identified, 19 percent of the 343 who have since died had an autopsy performed. Of those 65 autopsies, 31 were classified in this study as having DLB or PDD. 24 of those (77.4 percent) had autopsy-confirmed Lewy body pathology. The clinical and pathological diagnosis agreement in those with DLB was 94.1 percent, but only 57.1 percent for those with PDD.
This unique study reveals differences between DLB and PDD in age and sex, which hints of possible differences in the underlying causes. More research is needed to understand how broad the incidence of Lewy body dementias are and why it affects men and women differently.
This study was funded by the National Institute on Aging and was first published online in JAMA Neurology on September 16, 2013.
Savica R, Grossardt BR, Bower JH, Boeve BF, Ahlskog J, Rocca WA. Incidence of Dementia With Lewy Bodies and Parkinson Disease Dementia. JAMA Neurol. 2013; doi:10.1001/jamaneurol.2013.3579.