Vann Jones & O'Brien's meta-analysis of epidemiological studies (Vann Jones & O'Brien, Reference Vann Jones and O'Brien2013) reported incidence (0.57–1.4 cases/1000 person-years), and prevalence (3.8–4.5% of all new diagnoses) for dementia with Lewy bodies (DLB). The authors do an excellent job explaining the complexities underlying the significant data variation in the source reports. I would like to expand upon their arguments as to why the true incidence and prevalence may be significantly higher than the reported ranges.

First, as Vann Jones & O'Brien note, under-diagnosis is common: the sensitivity of the clinical diagnosis of DLB is only 32%, whereas specificity is 95% (Nelson et al. Reference Nelson, Jicha, Kryscio, Abner, Schmitt, Cooper, Xu, Smith and Markesbery2010). Clinicians in these expert centers suspected DLB, or dual Alzheimer's/DLB pathology in 8.1% of 2861 cases, yet 14.3% of the cases had significant Lewy pathology at autopsy (Nelson et al. Reference Nelson, Jicha, Kryscio, Abner, Schmitt, Cooper, Xu, Smith and Markesbery2010). DLB may be further under-diagnosed outside of specialist dementia settings, where the clinical syndrome is less well-known. Against this we must factor in referral bias. Atypical cases are also more likely to undergo autopsy, and thus enrich the pathology data with atypical findings.

Second, the prevalence increases by 40–300% when possible DLB is included (Vann Jones & O'Brien Reference Vann Jones and O'Brien2013), and when populations in secondary referral centers are examined.

Third, the ‘1-year rule’ arbitrarily divides these Lewy body diseases: subjects with cognitive impairment within a year of the onset parkinsonism are diagnosed with DLB, whereas those whose cognitive problems begin after 1 year are labeled Parkinson's disease dementia (PDD; McKeith et al. Reference McKeith, Dickson, Lowe, Emre, O'Brien, Feldman, Cummings, Duda, Lippa, Perry, Aarsland, Arai, Ballard, Boeve, Burn, Costa, Del Ser, Dubois, Galasko, Gauthier, Goetz, Gomez-Tortosa, Halliday, Hansen, Hardy, Iwatsubo, Kalaria, Kaufer, Kenny, Korczyn, Kosaka, Lee, Lees, Litvan, Londos, Lopez, Minoshima, Mizuno, Molina, Mukaetova-Ladinska, Pasquier, Perry, Schulz, Trojanowski and Yamada2005; Aarsland et al. Reference Aarsland, Londos and Ballard2009). DLB sits at the cusp between movement disorder and behavioral neurology specialty clinics, and referral patterns may dictate diagnosis. Movement disorder specialists underestimate cognitive deficits (Hely et al. Reference Hely, Reid, Adena, Halliday and Morris2008) and dementia specialists overlook parkinsonism (Schneider et al. Reference Schneider, Arvanitakis, Bang and Bennett2007). Yet 42% of incident PD cases have mild cognitive impairment (Yarnall et al. Reference Yarnall, Breen, Duncan, Barker and Burn2013) and 31% of all PD patients are demented (Aarsland et al. Reference Aarsland, Zaccai and Brayne2005), almost triple the rate of controls (de Lau et al. Reference de Lau, Schipper, Hofman, Koudstaal and Breteler2005).

Finally, DLB diagnosis can stand or fall on the basis of this core criterion: ‘spontaneous features of parkinsonism’. An unanswered question is this: how much parkinsonism is ‘enough’ parkinsonism? More research is needed.