Alzheimer Disease vs Dementia with Lewy Bodies

Why Lecanemab is correct

• The picture is early Alzheimer disease (AD): amnestic decline with an MMSE of 23/30 (mild dementia range) plus a confirmatory CSF biomarker profile of low Abeta42 and elevated total-tau and phospho-tau
• Lecanemab is an anti-amyloid-beta monoclonal antibody directed at soluble protofibrils and is FDA-approved for mild cognitive impairment or mild dementia due to AD with confirmed amyloid pathology
• In the CLARITY-AD trial it slowed clinical decline by roughly 27% versus placebo over 18 months, making it disease-modifying rather than purely symptomatic
• Eligibility requires confirmed amyloid pathology (CSF or amyloid PET) and a baseline MRI; donanemab is a similarly disease-modifying alternative

Why the others are wrong

• Donepezil — a cholinesterase inhibitor that improves symptoms by raising synaptic acetylcholine but does not alter amyloid pathology or the disease trajectory (right-concept-wrong-mechanism)
• Solanezumab — also an anti-amyloid monoclonal antibody, but it binds monomeric soluble Abeta rather than the protofibrils and deposited plaque that have to be cleared; it failed to slow decline in the EXPEDITION program and in the A4 trial and was never approved, so it does not modify the disease (right-class-wrong-target)
• Memantine — an NMDA-receptor antagonist used in moderate-to-severe AD for symptom control, not to slow underlying pathology (anchoring on AD label without matching mechanism)

Additional high-yield points

• Amyloid-related imaging abnormalities (ARIA-E edema, ARIA-H microhemorrhage) require serial MRI monitoring during anti-amyloid therapy
• APOE4 carriers (especially homozygotes) have higher ARIA risk, and concurrent anticoagulation raises ARIA-H/hemorrhage risk, so genotype and bleeding risk should be assessed before starting

Why Start donepezil is correct

• This vignette is classic dementia with Lewy bodies (DLB): fluctuating cognition, recurrent well-formed visual hallucinations, REM sleep behavior disorder, and parkinsonism with relatively preserved memory and impaired visuospatial testing (clock draw, pentagons).
• Cholinesterase inhibitors (donepezil, rivastigmine, galantamine) are first-line. DLB has profound cholinergic deficits and is among the most responsive dementias to these agents, which improve cognition and reduce visual hallucinations.
• Per McKeith 2017 DLB consensus and AAN guidance, distressing hallucinations are addressed first with a cholinesterase inhibitor and removal of offending drugs before any antipsychotic is considered.

Why the others are wrong

• Start haloperidol — a typical (high-potency) antipsychotic; strictly avoided in DLB because of severe neuroleptic sensitivity (rigidity, hyperthermia, autonomic instability, death).
• Start memantine — an NMDA antagonist used only as an adjunct in moderate-to-severe disease; it is not the first-line agent for cognition or hallucinations in DLB, where cholinesterase inhibitors are more effective.
• Start risperidone — a high-potency D2-blocking antipsychotic that carries the same life-threatening neuroleptic sensitivity risk in DLB; if pharmacotherapy is ever required for psychosis, pimavanserin or low-dose quetiapine/clozapine is preferred.