DLB is dementia within the first year of presentation where the motoric Parkinsonian symptoms may be present or emerge later. Whereas PDD is the normal continuum of PD in most patients (reported prevalence 4–93% averaging at 40%). The cumulative incidence of dementia increases with age and disease duration and reaches 80% by 80 years.
On SPECT, MIBG or DAT scans there is no difference between the two but neuropsychiatric issues are more common in DLB and motoric parkinsonism is greater in PDD.
In terms of pathology, Aβ plaques and neurofibrillary tangles (NFT) are commoner in DLB and temporal lobe structures are also targeted in DLB whereas it is limbic in PDD. Striatal dopamine loss is greater in PDD as there is a longer duration of illness, but cholinergic deficit is greater in DLB.
The typical presentation of DLB with visual hallucinosis and relatively mild memory impairment rarely presents diagnostic difficulties. Parkinsonian motor signs are often present at first consultation. The difficult cases are those presenting “blind” ie. no visual hallucinations and no motoric Parkinsonian features.
Visual hallucinosis is typically hierarchical, initially visual abberation, then insects, animals and finally people. It is often dead relatives or strangers— less commonly living relations. Sometimes the visions speak to the patient; more often they talk amongst themselves and often say nothing at all.
Difficult diagnostic cases can be indistinguishable clinically from Alzheimer’s disease or even Creutzfeldt-Jakob disease (CJD) in rapid progressive cases. A FP-CIT scan is particularly important as a diagnostic aid in these cases.
If the visions talk, the auditory hallucination is often not of a persecutory nature. Fluctuating consciousness is typical—sometimes prolonged periods of deep sleep— pseudo-encephalopathic. Autonomic dysfunction can be a prominent feature and severe neuroleptic sensitivity may occur. Myoclonus as seen in CJD is rare. Parkinsonism when seen is often akinetic-rigid in type and shows relatively poor levodopa response.
Visual recognition memory can differentiate DLB and PDD. One study found that despite global similarities in cognitive performances between patients, important differences were observed. In particular, DMS‐48, a test of visual object recognition memory and visual storage capacity, was poorer in DLB patients.1
In PDD, the normal progression of PD in most is Braak stage IV onwards. Risk factors include older age, male, depression, psychosis and mild cognitive impairment. Because it occurs on a background of PD, diagnosis is usually not an issue and specialist tests are less often required.
For a diagnosis of PDD, two core features must be present: a diagnosis of PD and PD developed prior to the onset of dementia. PDD can be temporally distinguished from DLB by the “one-year rule”; in PDD, motor symptoms develop at least one year before development of dementia, while in DLB, the motor symptoms occur no more than one year prior to the onset of dementia and frequently after the onset of dementia.
1. Mondon K, et al. J Neurol Neurosurg Psychiatry 2007; 78(7): 738–41