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DLB vs PDD 1-Year Rule — MRCPsych Paper B MCQ

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HardOld Age PsychiatryDLB vs PDD 1-Year RuleMRCPsych Paper B

A 74-year-old man develops bradykinesia and rigidity that meet diagnostic criteria for Parkinson's disease. Six months later, neuropsychological assessment identifies mild executive impairment, but he remains independent in everyday activities. Eighteen months after the onset of parkinsonism, progressive cognitive decline begins to interfere with managing medication and finances and meets criteria for dementia. Which classification follows the operational 'one-year rule'?

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Correct answer: B — Parkinson's disease dementia, because functional dementia began more than one year after established parkinsonism

Explanation lettering: D = shown as A · A = shown as D

B is correct. The operational rule is based on the onset of the dementia syndrome, not the first subtle cognitive deficit. DLB is diagnosed when dementia precedes, accompanies, or begins within one year of parkinsonism; PDD is used when dementia develops more than one year after the onset of established Parkinson's disease. Here, the impairment at six months did not compromise independence and therefore represents mild cognitive impairment rather than dementia. Dementia began at 18 months, supporting PDD. A incorrectly equates mild cognitive impairment with dementia. C and D apply incorrect two- and five-year thresholds. E is incorrect because dopamine-transporter imaging may support a Lewy body disorder but does not distinguish DLB from PDD; their separation remains an arbitrary clinical timing convention.

Reference: McKeith IG et al. Diagnosis and management of dementia with Lewy bodies: Fourth consensus report of the DLB Consortium. Neurology. 2017;89:88–100. https://pubmed.ncbi.nlm.nih.gov/28592453/