14.4 Mild Cognitive Impairment, Dementia & Its Subtypes
Key Takeaways
- Dementia in the elderly is a named Geriatric Syndromes subsection, and dementia also appears under disorders of cerebral function in Neurology.
- Evaluation of new cognitive impairment includes thyroid-stimulating hormone, vitamin B12 and structural neuroimaging to exclude reversible causes.
- Dementia with Lewy bodies features fluctuating cognition, visual hallucinations, parkinsonism and REM sleep behavior disorder.
- Antipsychotics, particularly first-generation agents, can cause severe and potentially fatal sensitivity reactions in dementia with Lewy bodies.
- Amyloid-directed monoclonal antibodies require confirmation of amyloid pathology and monitoring for amyloid-related imaging abnormalities.
Last updated: August 2026
Cognitive impairment and palliative care represent core competencies in internal medicine. Clinicians must accurately distinguish insidious neurodegenerative dementias from acute reversible delirium, navigate novel disease-modifying therapies and their safety monitoring, deliver rigorous symptom relief for patients with serious life-limiting illnesses, and uphold patient autonomy through proactive advance care planning.
1. Neurocognitive Disorders: MCI vs. Dementia & Reversible Workup
Diagnostic Distinction: MCI vs. Major Neurocognitive Disorder
- Mild Cognitive Impairment (MCI):
- Objective cognitive decline in >=1 cognitive domain (complex attention, executive function, learning and memory, language, perceptual-motor, or social cognition), typically >=1.5 standard deviations below age- and education-matched norms.
- Preserved Functional Independence: The patient remains fully independent in Instrumental Activities of Daily Living (IADLs) (managing finances, taking medications, driving, cooking), although tasks may take more time or require compensatory strategies.
- Clinical Course: Converts to dementia at a rate of 10% to 15% per year (compared to 1-2% per year in the general older adult population).
- Major Neurocognitive Disorder (Dementia):
- Significant cognitive decline in >=1 cognitive domain.
- Functional Dependence: Cognitive deficits interfere with independence in everyday activities, requiring assistance with complex IADLs (and eventually basic ADLs).
- Deficits do not occur exclusively in the context of delirium and are not better explained by another mental disorder (e.g., major depressive disorder).
Validated Cognitive Screening Instruments
| Screening Tool | Administration Time | Scoring & Clinical Cutoffs | Diagnostic Pearls & Limitations |
|---|---|---|---|
| Montreal Cognitive Assessment (MoCA) | 10-12 minutes | Total score 0-30.<br/>< 26 / 30 indicates cognitive impairment.<br/>(+1 point if <=12 years formal education). | Superior sensitivity (90%) for MCI and early dementia compared to MMSE. Rigorously tests executive function (trail making, clock drawing), visuospatial skills, and abstraction. |
| Mini-Mental State Examination (MMSE) | 8-10 minutes | Total score 0-30.<br/>< 24 / 30 indicates cognitive impairment.<br/>- 20-23: Mild dementia<br/>- 10-19: Moderate dementia<br/>- < 10: Severe dementia | Heavily weighted toward orientation and language; insensitive to early MCI and frontal/executive dysfunction; substantial educational and cultural bias. Proprietary copyright restrictions. |
| Mini-Cog | 3 minutes | Total score 0-5:<br/>- 3-word registration (0 pts)<br/>- Clock Drawing Test (CDT) (normal = 2 pts, abnormal = 0 pts)<br/>- 3-word delayed recall (1 pt per word; 0-3 pts)<br/>Score 0-2 = Positive screen for dementia. | Rapid, highly sensitive bedside screen. Unaffected by educational level or language barriers. Highly effective for initial ambulatory screening. |
Mandatory Laboratory & Neuroimaging Workup for Reversible Causes
Before confirming a primary neurodegenerative dementia, secondary and potentially reversible etiologies must be systematically excluded:
- Serum Thyroid-Stimulating Hormone (TSH): Excludes clinical hypothyroidism.
- Serum Vitamin B12 (Cobalamin): If borderline (200-400 pg/mL), check Serum Methylmalonic Acid (MMA) and Homocysteine; elevated MMA confirms tissue-level B12 deficiency requiring intramuscular cyanocobalamin repletion.
- Complete Metabolic Panel (CMP): Evaluates serum calcium (hyperparathyroidism/hypercalcemia), sodium (hyponatremia), renal function (uremia), and liver enzymes (hepatic encephalopathy).
- Complete Blood Count (CBC): Evaluates for severe anemia, hematologic malignancies, or occult infection.
- Targeted Serologies (When Clinically Indicated): Rapid Plasma Reagin (RPR) / Treponemal antibodies for neurosyphilis; HIV antibody/antigen; toxicological screens.
- Structural Brain Neuroimaging (Non-Contrast Head CT or Brain MRI):
- Mandatory in all patients with new cognitive decline.
- Rules out Normal Pressure Hydrocephalus (NPH) (triad of magnetic / apraxic gait disturbance, urinary urgency/incontinence, and cognitive decline; ventriculomegaly with Evans index >0.3 out of proportion to sulcal enlargement; treated with large-volume LP tap test and ventriculoperitoneal shunt).
- Rules out chronic subdural hematoma, intracranial neoplasms (meningioma, metastasis), multi-infarct vascular lesions, and establishes patterns of regional cortical atrophy (e.g., bilateral hippocampal volume loss in Alzheimer disease).
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2. Dementia Subtypes & Novel Amyloid-Directed Monoclonal Antibodies
Comparative Overview of Primary Dementias
| Dementia Subtype | Primary Neuropathology | Clinical Hallmark Features | First-Line Pharmacotherapy & Key Board Rules |
|---|---|---|---|
| Alzheimer Disease (AD)<br/>(60-80% of cases) | Extracellular Amyloid-beta ($A\beta_{42}$) plaques and intracellular Neurofibrillary Tangles (NFTs) composed of hyperphosphorylated Tau protein. Begins in entorhinal cortex and hippocampus. | Insidious onset with early episodic short-term memory impairment (repeating questions, misplacing items, impaired delayed recall); progresses to executive dysfunction, word-finding difficulty (anomia), visuospatial disorientation, and loss of insight. | 1. Cholinesterase Inhibitors (Donepezil 5-10 mg daily, Rivastigmine 4.6-13.3 mg/24h patch, Galantamine 8-24 mg daily): Indicated for mild, moderate, and severe AD.<br/>2. NMDA Receptor Antagonist (Memantine 5-20 mg daily): Indicated for moderate-to-severe AD (MMSE <20).<br/>3. Amyloid Monoclonal Antibodies (Lecanemab, Donanemab): Indicated for early symptomatic AD. |
| Dementia with Lewy Bodies (DLB)<br/>(15-20% of cases) | Intracytoplasmic Lewy bodies composed of aggregated alpha-synuclein in neocortex, limbic structures, and substantia nigra. | Core Diagnostic Tetrad:<br/>1. Fluctuating cognition and attention (profound day-to-day variations in alertness)<br/>2. Recurrent, detailed, well-formed visual hallucinations (animals, people)<br/>3. REM Sleep Behavior Disorder (RBD) (dream enactment, thrashing during sleep)<br/>4. Spontaneous Parkinsonism (rigidity, bradykinesia, gait changes developing <=1 year of cognitive decline ["1-year rule"]). | Cholinesterase Inhibitors (Donepezil, Rivastigmine) are first-line for cognitive and behavioral symptoms.<br/>CRITICAL BOARD CONTRAINDICATION: Severe Neuroleptic Sensitivity. Typical antipsychotics (Haloperidol) and potent atypicals cause catastrophic rigidity, irreversible parkinsonism, autonomic instability, and increased mortality. If psychosis requires therapy, use low-dose Quetiapine or Pimavanserin (5-HT2A inverse agonist). |
| Frontotemporal Dementia (FTD / Pick Disease)<br/>(5-10% of cases) | Frontal and temporal lobar degeneration with tau inclusions (Pick bodies) or TDP-43 inclusions. | Younger onset (Age 50-65).<br/>- Behavioral variant (bvFTD): Early, profound personality change, disinhibition (socially inappropriate remarks, shoplifting), apathy, loss of empathy, hyperorality (compulsive carbohydrate craving, binge eating), and repetitive/stereotyped behaviors.<br/>- Language variant (PPA): Progressive loss of speech fluency or word comprehension.<br/>Visuospatial skills and episodic memory are preserved early. | Symptomatic only: SSRIs (Sertraline, Citalopram) or Trazodone to manage impulsivity, compulsions, and agitation.<br/>Cholinesterase inhibitors and memantine are INEFFECTIVE and may worsen behavioral agitation. |
| Vascular Dementia (VaD)<br/>(10-20% of cases) | Large-vessel cortical infarcts, multiple lacunar infarcts, or extensive subcortical ischemic microvascular white matter disease (Binswanger disease). | Stepwise / abrupt cognitive deterioration temporally linked to clinical strokes, or insidious executive dysfunction (processing speed, planning, organizing) exceeding memory loss. Accompanied by focal neurologic deficits (pseudobulbar affect, hyperreflexia, hemiparesis, gait apraxia). | Aggressive secondary cardiovascular risk reduction: Blood pressure control (target SBP <130 mmHg per SPRINT-MIND), high-intensity statin (Atorvastatin 80 mg), antiplatelet therapy (Aspirin), glycemic control, and smoking cessation. |
| Prion Disease (Creutzfeldt-Jakob [CJD]) | Misfolding of normal cellular prion protein ($PrP^C$) into pathological scrapie form ($PrP^{Sc}$). | Rapidly progressive dementia over weeks to months, accompanied by myoclonus (especially startle-induced), cerebellar ataxia, visual disturbances, and terminal akinetic mutism. | Universally fatal (median survival 4-6 months). Palliative care.<br/>Diagnostic Triple:<br/>1. Brain MRI (DWI): Cortical ribboning and hockey-stick / pulvinar sign in basal ganglia/thalamus.<br/>2. CSF RT-QuIC assay (>95% sensitive/specific) and elevated 14-3-3 / tau protein.<br/>3. EEG: Periodic sharp wave complexes (PSWCs) at 1 Hz. |
Test Your Knowledge
A 74-year-old man is brought to the outpatient neurology clinic by his daughter due to a 9-month history of progressive cognitive decline and unusual behaviors. His daughter notes that on some days he is relatively alert and conversational, while on other days he is profoundly confused, drowsy, and stares blankly into space for hours. Over the past 4 months, he has described recurrent, detailed, colorful visual hallucinations of children and small animals playing in his living room when no one is present. His sleep has become severely disrupted over the past 2 years, characterized by thrashing, punching, and yelling during nightmares. Neurological examination reveals masked facies, bilateral cogwheel rigidity at the wrists, a resting hand tremor, and a slow, shuffling gait with reduced arm swing. A brain MRI shows mild diffuse cortical atrophy without focal infarction or hydrocephalus. What is the most likely diagnosis, and which of the following medications is strictly contraindicated due to the risk of life-threatening adverse reactions?
A
B
C
D