What is the most likely diagnosis for an 86‑year‑old man with a 10‑month history of fluctuating cognitive impairment, visual hallucinations, rapid eye movement (REM) sleep behavior disorder (including hitting his partner in sleep), intermittent daytime sleepiness, slow gait with falls, normal clock drawing but recall of only 2 of 3 words on the Mini‑Cog, cerebral atrophy on computed tomography, type 2 diabetes mellitus, and well‑controlled depression treated with metformin and sertraline?

Diagnosis: Lewy Body Dementia

This patient has Lewy body dementia (DLB), answer b, based on the classic tetrad of fluctuating cognition, visual hallucinations, REM sleep behavior disorder (hitting partner during sleep), and parkinsonism (slow gait, falls). 1, 2

Core Clinical Features Present

This 86-year-old man demonstrates all four core clinical features that define probable DLB:

• Fluctuating cognition is evident by his pronounced variations in attention and alertness—intermittently drifting off to sleep during meals and conversations, which represents the characteristic fluctuations that can occur over minutes to hours 1

• Recurrent visual hallucinations are present, specifically well-formed, detailed hallucinations of his deceased relative, which is the hallmark visual hallucination pattern in DLB involving people 1, 2

• REM sleep behavior disorder (RBD) is demonstrated by him hitting his partner during sleep and falling out of bed at night, representing the acting out of dreams due to lack of normal muscle paralysis during REM sleep 1, 2

• Parkinsonism is shown by his slow gait and recurrent falls, even without tremor (tremor is not required for the diagnosis) 1

Why Not the Other Diagnoses

Alzheimer Dementia (Option a) is Incorrect

• The National Institute on Aging-Alzheimer's Association guidelines explicitly state that probable AD dementia should NOT be diagnosed when core features of DLB are present, including visual hallucinations, parkinsonism, and cognitive fluctuations 1

• Visual hallucinations are not a core feature of AD, and when present early and prominently, they strongly suggest alternative pathology 1

• The early appearance of visual hallucinations and RBD strongly favors DLB over AD 1

• Medial temporal atrophy on imaging is typical of AD, but this patient shows only general cerebral atrophy, and DLB characteristically shows relative preservation of medial temporal lobe structures 3

Vascular Dementia (Option c) is Incorrect

• Vascular dementia does not explain the constellation of visual hallucinations, parkinsonism, and REM sleep behavior disorder that characterize this patient's presentation 1

• The absence of stepwise cognitive decline, stroke history, or cerebrovascular events makes primary vascular dementia unlikely 1

• While this patient has diabetes (a vascular risk factor), the clinical phenotype is dominated by DLB features, not vascular pathology 1

Normal Pressure Hydrocephalus (Option d) is Incorrect

• NPH presents with the classic triad of gait disturbance, urinary incontinence, and cognitive impairment, but does not cause visual hallucinations or RBD 4

• CT shows cerebral atrophy, not the ventricular enlargement out of proportion to sulcal atrophy that characterizes NPH 4

Clinical Implications for Management

Recognition of DLB rather than AD is crucial because management differs significantly:

• Cholinesterase inhibitors (rivastigmine or donepezil) are first-line treatment for both cognitive symptoms and visual hallucinations in DLB, and they are actually more effective in DLB than in AD 2, 3, 5

• Traditional antipsychotics must be absolutely avoided due to severe neuroleptic sensitivity that can double or triple mortality rates in DLB patients 2, 5

• If antipsychotic treatment becomes absolutely necessary for severe hallucinations, only atypical antipsychotics at the lowest possible doses should be considered, though even these carry significant risk 3

• Levodopa can be used cautiously for motor symptoms if needed, but dopaminergic agents may worsen hallucinations 1

Common Pitfalls to Avoid

• Do not dismiss visual hallucinations as simply "behavioral and psychological symptoms of dementia" (BPSD) in AD—their early prominence points to DLB 1

• Do not attribute RBD symptoms to "restless sleep" or "nightmares"—this is a core diagnostic feature of DLB that often precedes cognitive symptoms by years 1, 2

• Do not overlook the significance of fluctuating attention—the intermittent drowsiness during conversations is not depression or medication effect, but a cardinal feature of DLB 1

• Mixed pathology (DLB + AD) occurs in over 50% of DLB cases, but the clinical phenotype determines management, and this patient's phenotype is clearly DLB-dominant 1