A 72-year-old woman is referred to neurology with a 3-year history of progressive cognitive decline and motor symptoms. She presents with bradykinesia, rigidity, and postural instability. Her daughter reports visual hallucinations of people and animals, which are vivid and recurrent. Cognitive examination reveals fluctuating attention and poor performance on visuospatial tasks. She has REM sleep behaviour disorder confirmed by her husband, who reports her acting out dreams at night. Brain MRI shows mild generalized atrophy with no focal lesions. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Lewy Body Dementia (LBD) ### Core Diagnostic Features Present **Key Point:** LBD is defined by the **triad of cognitive decline + parkinsonism + visual hallucinations**, with REM sleep behaviour disorder as a strong supporting feature. This patient exhibits all cardinal features: | Feature | Present | Diagnostic Weight | |---------|---------|-------------------| | Cognitive decline (fluctuating) | Yes | Core criterion | | Visual hallucinations (vivid, recurrent) | Yes | Core criterion | | Parkinsonism (bradykinesia, rigidity, postural instability) | Yes | Core criterion | | REM sleep behaviour disorder | Yes | Highly suggestive | | Visuospatial impairment | Yes | Typical in LBD | | Normal/mild MRI findings | Yes | Typical (no focal atrophy) | ### Diagnostic Criteria: McKeith Consensus Criteria **High-Yield:** LBD diagnosis requires: 1. **Cognitive decline** (progressive, prominent in attention and visuospatial domains) 2. **Two or more core features:** - Visual hallucinations (typically complex, well-formed) - Parkinsonism - REM sleep behaviour disorder - Fluctuating attention/consciousness This patient meets all criteria. ### Pathophysiology: Alpha-Synuclein Pathology **Clinical Pearl:** Lewy bodies (intracytoplasmic inclusions of alpha-synuclein) accumulate in: - Substantia nigra → parkinsonism - Cortex → hallucinations and cognitive decline - Brainstem → REM sleep behaviour disorder (loss of REM atonia) ### Why Visuospatial Impairment Occurs Alpha-synuclein deposition in visual association cortex (parietal and occipital regions) causes: - Impaired visual processing and perception - Visuospatial dysfunction (worse than memory loss early) - Predisposition to visual hallucinations ### Temporal Sequence: Cognitive vs. Motor Onset ```mermaid flowchart TD A[Lewy body dementia presentation]:::outcome --> B{Symptom onset order?}:::decision B -->|Cognitive first, then motor| C[Dementia with Lewy bodies<br/>DLB variant]:::action B -->|Motor first, then cognitive| D[Parkinson disease dementia<br/>PDD variant]:::action C --> E[Cognitive decline precedes<br/>parkinsonism by ≥1 year]:::outcome D --> F[Parkinsonism precedes<br/>cognitive decline by ≥1 year]:::outcome ``` **Key Point:** In this case, cognitive decline preceded motor symptoms, making this **Dementia with Lewy Bodies (DLB)** rather than Parkinson disease dementia. ### Distinguishing LBD from Parkinson Disease Dementia | Feature | DLB (This Case) | PDD | |---------|-----------------|-----| | Cognitive decline onset | First | After motor symptoms (≥1 yr) | | Hallucinations | Early, prominent | Later | | Parkinsonism severity | Mild to moderate | Prominent, early | | Visuospatial impairment | Marked | Mild | | REM sleep disorder | Common | Less common | ### Why Other Diagnoses Are Wrong **Mnemonic: "CHAP" for LBD vs. others:** - **C**ognitive fluctuation (not steady decline as in AD) - **H**allucinations (visual, not tactile or olfactory) - **A**lpha-synuclein pathology (not tau or amyloid-beta) - **P**arkinsonism (not dystonia or spasticity) [cite:Harrison 21e Ch 452; McKeith et al. Lancet Neurol 2017]