Dementia and Neurocognitive Disorders

Etiology and Clinical Context

Definition and etiologic spectrum

Dementia is a clinical syndrome of acquired decline in cognitive function sufficient to interfere with independence; major neurocognitive disorder is a contemporary diagnostic term (Ehret & Chamberlin, 2026). Alzheimer disease is a common cause, but vascular cognitive impairment, Lewy body disease, frontotemporal degeneration, and mixed pathology require distinction.

Epidemiologic context and interpretation

Neurodegenerative cognitive disorders become more frequent with age, but younger-onset dementia occurs and may suggest a different mix of genetic or frontotemporal mechanisms (Ehret & Chamberlin, 2026). Alzheimer, Lewy body, vascular, and frontotemporal disorders have different presenting-domain patterns and risk contexts. Vascular risk factors, family history, functional baseline, and progression rate are essential to interpreting suspected dementia.

2026 epidemiology: distinguish a population estimate from an individual prognosis

The Alzheimer’s Association estimated that 7.4 million U.S. adults aged 65 years and older were living with clinical Alzheimer dementia in 2026, approximately 11% of that age group (about one in nine). This modeled national prevalence estimate is specific to clinical Alzheimer dementia, not all dementia phenotypes, and does not establish an individual diagnosis. Age, functional trajectory, presenting cognitive domain, comorbid cerebrovascular disease, and collateral history remain necessary for diagnosis (Alzheimer’s Association, 2026).

Pathophysiology

Alzheimer disease pathobiology

Amyloid-beta accumulation and abnormal phosphorylated tau pathology are associated with synaptic dysfunction, neuroinflammation, and neuronal degeneration. Medial temporal/hippocampal networks are often affected early, accounting for episodic memory impairment, followed by broader association-cortex dysfunction. Biomarker positivity alone must be interpreted in the clinical context (Shakkottai & Lomen-Hoerth, 2019; Ehret & Chamberlin, 2026).

Proteinopathy and treatment boundaries

Abnormal amyloid-beta processing and aggregation, pathologic tau accumulation, neuroinflammation, and synaptic degeneration contribute to Alzheimer disease pathology (Shakkottai & Lomen-Hoerth, 2019). Lewy body disease involves abnormal alpha-synuclein aggregation and dysfunction of affected neural circuits. Frontotemporal degeneration encompasses several protein pathologies, including tau and TDP-43, rather than one uniform disease. Symptomatic cognitive agents, caregiver support, safety planning, and function-centered care have different indications across syndromes. Anti-amyloid therapy is restricted to clinically and biomarker-defined Alzheimer populations and requires product-specific assessment of amyloid-related imaging abnormalities and other treatment risks.

Distinguish non-Alzheimer neuropathology by first affected network

In Alzheimer disease, early medial temporal and association-network dysfunction often produces prominent episodic memory encoding and consolidation difficulty (Shakkottai & Lomen-Hoerth, 2019). Dementia with Lewy bodies is associated with alpha-synuclein pathology and may feature fluctuating cognition, recurrent well-formed visual hallucinations, REM sleep behavior disorder, visuospatial deficits, and spontaneous parkinsonism. Behavioral-variant frontotemporal dementia may begin with progressive disinhibition, loss of empathy, compulsive behaviors, or altered food preferences with comparatively less prominent early episodic memory impairment; tau- and TDP-43-related pathology are important molecular groups. Vascular cognitive impairment reflects the burden and distribution of infarction, small-vessel injury, hemorrhage, and network disconnection; executive slowing and gait disturbance may predominate. Mixed pathologies are common in older adults, so a characteristic syndrome does not prove a single underlying proteinopathy.

A rapidly progressive cognitive syndrome over weeks or months requires a different workup from gradual multi-year decline. Consider medication toxicity, autoimmune encephalitis, infection, neoplastic disease, rapidly progressive prion illness, metabolic failure, and seizures in addition to neurodegeneration. New fluctuation and inattention in an established dementia patient should prompt investigation for delirium or acute illness rather than attribution to inevitable disease progression (Shakkottai & Lomen-Hoerth, 2019).

Clinical Manifestations

Clinical phenotypes and differentiation

Alzheimer disease often begins with progressive episodic memory deficits (Shakkottai & Lomen-Hoerth, 2019). Vascular cognitive impairment may feature executive and processing-speed dysfunction with cerebrovascular burden. Dementia with Lewy bodies is associated with cognitive fluctuations, recurrent visual hallucinations, spontaneous parkinsonism, and REM sleep behavior disorder. Frontotemporal syndromes can begin with behavioral, personality, or language changes. Delirium differs by acute onset, impaired attention, and fluctuation, although both may coexist.

Diagnostic Evaluation

Evaluation

Establish baseline and rate of change, functional impact, collateral history, medication and substance exposure, depression/sleep symptoms, and neurologic findings (Ehret & Chamberlin, 2026). Use cognitive testing, selected reversible-cause laboratory studies, and structural imaging as appropriate. Specialist assessment and selected fluid or imaging biomarkers may clarify etiology and treatment eligibility.

Mild versus major neurocognitive disorder: function changes the diagnosis

Mild neurocognitive disorder denotes objectively demonstrable decline in one or more cognitive domains with substantially preserved independence, sometimes requiring greater effort or compensatory strategies (American Psychiatric Association, 2022; Ehret & Chamberlin, 2026). Major neurocognitive disorder involves decline sufficient to interfere with independence in everyday activities. The DSM-5-TR evaluates attention, executive function, learning and memory, language, perceptual-motor abilities, and social cognition; major impairment affects independence, whereas mild impairment can be accommodated without surrendering it. A single screening score cannot establish either diagnosis in isolation. Assessment of finances, medication management, cooking, driving, appointments, personal care, and collateral history can reveal disability not obvious during a brief office interaction. Delirium can coexist with pre-existing cognitive impairment and must be evaluated when symptoms fluctuate or attention acutely deteriorates.

Testing: interpret functional impairment, screens, and biomarkers

Cognitive screening instruments such as the Montreal Cognitive Assessment or Mini-Mental State Examination describe performance on sampled domains; education, language, hearing, vision, test environment, and baseline functioning influence results. A low score is not a disease-specific biomarker, and a near-normal score does not exclude early frontotemporal or executive syndromes. Obtain functional history from the patient and an appropriate informant, review medication effects, screen depression and sleep contributors, and evaluate for metabolic or structural mimics when indicated. Neuropsychological testing is useful when brief screening and real-world function disagree or phenotype is atypical (Ehret & Chamberlin, 2026).

Amyloid and tau biomarkers may increase diagnostic confidence in selected cases, especially when contemplating amyloid-targeting treatment, but test eligibility, pretest probability, discordant findings, and consequences of a positive result must be discussed (Ehret & Chamberlin, 2026). A positive amyloid marker does not by itself establish that amyloid disease explains every symptom. Decisions about anti-amyloid medication require patient-specific evaluation of current indications, MRI exclusion criteria, monitoring for amyloid-related imaging abnormalities, anticoagulation/bleeding considerations, and individualized benefit-harm assessment.

Clinical decision: a cognitive test is not the whole diagnosis

A 68-year-old with new disinhibition, altered eating behavior, and progressive interpersonal change but relatively preserved day-to-day recall raises a frontotemporal phenotype; asking only about memory risks delayed recognition. A 74-year-old with visual hallucinations, nocturnal dream enactment, and marked day-to-day fluctuation requires assessment for Lewy body disease and careful medication-risk planning. A 79-year-old with longstanding cognitive impairment who abruptly stops maintaining attention during an infection requires urgent assessment for superimposed delirium. The first domain affected, tempo, functional loss, and reversible contributors determine the next diagnostic step.

Differential Diagnosis

Reversible contributors and diagnostic pitfalls

History should establish longitudinal decline and change in instrumental and basic daily activities using collateral information (Ehret & Chamberlin, 2026). Evaluate medication burden, sleep disorders, depression, thyroid dysfunction, vitamin B12 deficiency, and other clinically indicated causes rather than presuming that age-related forgetfulness is dementia. Rapidly progressive impairment, focal deficits, new seizures, or pronounced fluctuation call for a different workup. Delirium can superimpose on established dementia and should not be dismissed as baseline confusion.

Subtype-specific diagnostic reasoning

Typical Alzheimer disease begins with impaired episodic learning and recall before later language, visuospatial, and executive decline, although atypical variants occur (Shakkottai & Lomen-Hoerth, 2019). Dementia with Lewy bodies suggests fluctuation, recurrent well-formed visual hallucinations, spontaneous parkinsonism, and REM sleep behavior disorder. Frontotemporal presentations may begin with progressive behavioral disinhibition, apathy, altered social cognition, compulsive behavior, or primary language impairment, sometimes with relatively spared memory early. Vascular cognitive impairment may produce executive slowing, gait changes, focal signs, and imaging evidence of infarcts or white matter disease; progression need not be strictly stepwise. Mixed pathology is common in older adults. Biomarkers and imaging can increase diagnostic confidence in selected cases but do not supersede history, examination, or functional evaluation.

Treatment and Management

Treatment boundaries, risk, and prognosis

Cholinesterase inhibitors support symptomatic treatment in appropriately diagnosed Alzheimer and some Lewy body phenotypes, while memantine has a role in selected more advanced cognitive impairment; benefits are modest and adverse effects require monitoring (Ehret & Chamberlin, 2026). Antipsychotics may pose disproportionate risks in Lewy body disorders and dementia more broadly. Vascular risk reduction addresses ongoing brain injury rather than directly removing amyloid or tau. Progressive functional and behavioral burdens require safety planning, swallowing and nutrition review, caregiver assessment, advance care planning, and assessment of potentially reversible contributors. Anti-amyloid treatment should occur through a specialist-supported pathway with documented eligibility, biomarker confirmation, MRI safety evaluation, adverse-effect surveillance, and shared decision-making.

Treatment mechanisms and the boundary of available evidence

Cholinesterase inhibitors are symptomatic treatments that enhance available cholinergic transmission in selected Alzheimer disease presentations; they do not replace lost neurons or reverse the underlying proteinopathy. Memantine modulates NMDA-receptor signaling and is principally used in more advanced disease. Treatment selection must include adverse effects, functional goals, caregiver capacity, and whether the clinical phenotype is consistent with the intended indication (Ehret & Chamberlin, 2026).

Disease-directed antiamyloid treatments require a distinct pathway from symptomatic drugs: identify the appropriate clinical stage, confirm amyloid pathology using an accepted method, and evaluate MRI-based edema and hemorrhage risk before considering treatment. Amyloid-related imaging abnormalities can require interruption or discontinuation. Eligibility, anticoagulation precautions, MRI surveillance, and expected treatment benefit must be assessed against the specific product labeling and the patient’s goals (Ehret & Chamberlin, 2026).

Disease-directed therapy and imaging safety

Anti-amyloid monoclonal antibodies have an indication in appropriately selected patients with early symptomatic Alzheimer disease after confirmation of amyloid pathology; they are not substitutes for evaluation of other causes of cognitive decline. Treatment can cause amyloid-related imaging abnormalities, including edema and hemorrhage. Selection requires shared discussion of expected benefit, apolipoprotein E genotype-related risks, contraindications, and scheduled MRI surveillance. In August 2025 the FDA recommended an additional MRI before the third lecanemab infusion to detect early ARIA (U.S. Food and Drug Administration [FDA], 2025). Donanemab is likewise used at the mild cognitive impairment or mild dementia stage studied in trials, not indiscriminately for advanced dementia (FDA, 2024).

Current lecanemab monitoring: The July 2026 U.S. prescribing information requires a recent baseline brain MRI and additional MRIs after 1, 2, 3, and 6 months of treatment. The first follow-up corresponds approximately to before the third intravenous dose or fifth weekly subcutaneous dose. Review each MRI before the next scheduled administration, and evaluate possible amyloid-related imaging abnormalities (ARIA) promptly when new neurologic symptoms occur. Monitoring is therefore defined by treatment duration and route; the August 2025 FDA communication describes the earlier intravenous schedule, not every currently authorized regimen (U.S. prescribing information, revised July 2026).

Agitation associated with Alzheimer dementia: confirm the indication and the cause

On April 30, 2026, the FDA approved dextromethorphan-bupropion (Auvelity) for agitation associated with dementia due to Alzheimer disease in adults. This scheduled therapy is not indicated for as-needed treatment of agitation and is not an anti-amyloid disease-modifying treatment; its initiation and titration schedule differs from that used for major depressive disorder. Evaluate pain, delirium, infection, adverse medication effects, unmet needs, and environmental triggers before attributing a change in behavior to Alzheimer disease alone. The 2026 prescribing information carries a boxed warning about suicidal thoughts and behaviors in pediatric and young adult patients, lists seizure disorder, current or prior bulimia or anorexia nervosa, and abrupt discontinuation of alcohol or certain sedatives among its contraindications, and warns about blood-pressure elevation, mania or hypomania, clinically important drug interactions, seizure risk, and hyponatremia. Screening should also consider concurrent dextromethorphan- or bupropion-containing drugs. Person-centered nonpharmacologic interventions remain essential (U.S. Food and Drug Administration, 2026a, 2026b).

Complications and Red Flags

Functional safety and care transitions

Care plans must address the practical consequences of cognitive decline, including medication administration, driving and home safety, caregiver capacity, advance care planning, and treatment of vascular risk factors. Identify who can provide collateral history, recognize functional change, and help coordinate follow-up. Medication mechanisms and eligibility requirements are addressed separately below (Ehret & Chamberlin, 2026).

Clinical Reasoning Priorities

Applied case: the first impaired domain matters

A patient repeatedly forgets recent conversations and medications over several years and gradually needs help with finances; alertness remains stable. A memory-predominant major neurocognitive disorder is plausible, and loss of independence distinguishes this from mild impairment. New recurrent visual hallucinations, marked cognitive fluctuation, REM sleep behavior disorder, and spontaneous parkinsonism suggest Lewy body disease. Abrupt inattention during acute infection may instead represent superimposed delirium. These patterns also influence medication-risk counseling.

Related YourDNP Guides

For related clinical reasoning, see Delirium and Medical-Cause Neuropsychiatric Syndromes and Parkinson Disease.

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References

Alzheimer’s Association. (2026). 2026 Alzheimer’s disease facts and figures. Alzheimer’s & Dementia, 22(4), e71345. https://doi.org/10.1002/alz.71345

American Psychiatric Association. (2022). Diagnostic and statistical manual of mental disorders (5th ed., text rev.). American Psychiatric Association Publishing.

Ehret, M. J., & Chamberlin, K. W. (2026). Alzheimer disease. In M. A. Chisholm-Burns, P. M. Malone, J. M. Kolesar, K. C. Lee, P. B. Bookstaver, & K. R. Matthias (Eds.), Pharmacotherapy principles & practice (7th ed.). McGraw Hill.

Shakkottai, V. G., & Lomen-Hoerth, C. (2019). Nervous system disorders. In G. D. Hammer & S. J. McPhee (Eds.), Pathophysiology of disease: An introduction to clinical medicine (8th ed.). McGraw-Hill Education.

U.S. Food and Drug Administration. (2024, July 2). FDA approves treatment for adults with Alzheimer’s disease. https://www.fda.gov/drugs/news-events-human-drugs/fda-approves-treatment-adults-alzheimers-disease

U.S. Food and Drug Administration. (2025, August 28). FDA to recommend additional, earlier MRI monitoring for patients with Alzheimer’s disease taking Leqembi (lecanemab). https://www.fda.gov/drugs/drug-safety-communications/fda-recommend-additional-earlier-mri-monitoring-patients-alzheimers-disease-taking-leqembi-lecanemab

U.S. Food and Drug Administration. (2026a). Auvelity (dextromethorphan hydrobromide/bupropion hydrochloride) extended-release tablets: Prescribing information (corrected 2026 prescribing information). https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/215430Orig1s009%2C%20s018correctedlbl.pdf

U.S. Food and Drug Administration. (2026b, April 30). FDA approves first non-antipsychotic drug to treat agitation associated with dementia. https://www.fda.gov/news-events/press-announcements/fda-approves-first-non-antipsychotic-drug-treat-agitation-associated-dementia