Dementia: A High-Yield USMLE Review
Dementia is a clinical syndrome—not a single disease—defined by decline from a previous level of function, impairment in two or more cognitive domains, and interference with daily activities. On the exam, your job is to classify the subtype (Alzheimer's, vascular, frontotemporal, DLB) and, critically, to exclude reversible causes before labeling a patient with a degenerative dementia.
Pathophysiology
Different subtypes converge on progressive cognitive impairment through distinct mechanisms. In Alzheimer's, extracellular amyloid-β plaques and intracellular neurofibrillary tangles of hyperphosphorylated tau drive synaptic loss and neuronal death—hippocampal and temporoparietal involvement explains the early amnestic and visuospatial deficits. Vascular dementia results from cerebrovascular disease—multiple cortical/subcortical infarcts or small-vessel white matter disease—so cognition drops in a stepwise fashion tied to each ischemic event. FTD reflects frontal/temporal lobe atrophy from tau (Pick bodies) or TDP-43 inclusions, producing behavioral rather than amnestic features.
Presentation
- Alzheimer's disease: gradual, progressive short-term memory loss—forgetting recent conversations, repeating questions—with word-finding difficulty, getting lost in familiar places, and poor judgment/inability to manage finances; personality is preserved early
- Frontotemporal dementia: personality and behavioral change—inappropriate social behavior, apathy, new obsessions—with memory relatively preserved early (unlike AD)
- Vascular dementia: stepwise decline with each stroke, focal neurological signs, and a subcortical pattern of slowed processing, executive dysfunction, and gait disorder; memory less prominent than in AD
- Dementia with Lewy bodies: parkinsonism, formed visual hallucinations, fluctuating cognition, REM sleep behavior disorder, and dementia developing before or within one year of motor symptoms
- Late-stage disease across types: apraxia, agnosia, incontinence, and total dependence
- Cognitive domains affected span memory, executive function, language, visuospatial ability, and social cognition/behavior
Diagnosis
- Clinical diagnosis of Alzheimer's: gradual onset, progressive memory loss, and exclusion of other causes
- MRI: hippocampal and generalized atrophy in AD; infarcts, white matter hyperintensities, and lacunes in vascular dementia; frontal/temporal atrophy in FTD
- PET: amyloid PET and FDG-PET showing temporoparietal hypometabolism in Alzheimer's
- CSF in Alzheimer's: decreased Aβ42 with increased total tau and phospho-tau
- Rule out reversible causes before diagnosing degenerative dementia: B12 deficiency, hypothyroidism, normal pressure hydrocephalus, depression, medications, syphilis, HIV, and structural lesions
Management
- Alzheimer's: cholinesterase inhibitors (donepezil, rivastigmine, galantamine) for symptomatic benefit, and memantine (NMDA antagonist) for moderate-to-severe disease—these treat symptoms but do not modify disease progression
- Anti-amyloid therapies (aducanumab, lecanemab): reduce amyloid but clinical benefit remains controversial
- Vascular dementia: secondary stroke prevention and control of vascular risk factors
- DLB: avoid antipsychotics if possible; if a neuroleptic is truly needed, use quetiapine or clozapine given severe neuroleptic sensitivity
High-yield
- Alzheimer's is the most common cause of dementia (60–70%) and risk increases dramatically with age
- Pathologic hallmark of AD = extracellular amyloid-β plaques + intracellular neurofibrillary tangles of hyperphosphorylated tau
- "Alzheimer's = memory loss; FTD = personality/behavioral change"—this distinction guides workup and prognosis
- Vascular dementia = stepwise decline + focal signs; FTD is the second most common dementia in patients under 65 (onset in 50s–60s)
- DLB "1-year rule": dementia before or within 1 year of parkinsonism (vs. Parkinson's disease dementia, which develops years after motor symptoms)
- DLB patients have severe neuroleptic sensitivity—antipsychotics can cause severe parkinsonism, rigidity, and death
Pitfalls
- Failing to rule out reversible/treatable causes (B12 deficiency, hypothyroidism, NPH, depression, medications, syphilis, HIV, structural lesions) before diagnosing a degenerative dementia
- Giving standard antipsychotics to a DLB patient—this can precipitate severe parkinsonism and NMS-like deterioration; use quetiapine or clozapine only if necessary
- Mislabeling behavioral change as psychiatric disease or as Alzheimer's when early memory preservation with prominent personality change points to FTD
- Assuming AD-type prominent memory loss in vascular dementia, where memory may be less affected than executive function and gait
- Confusing dementia with delirium—delirium can be superimposed on dementia, and the cognitive baseline must anchor interpretation of acute changes
- Over-crediting anticholinergic and other culprit medications, which can worsen cognition and mimic or exacerbate dementia in older patients
Don't just memorize Dementia — practice reasoning through it on branching cases where your decisions shape the patient.