Alzheimer Disease

Detailed Overview

The pathophysiology involves the cleavage of amyloid precursor protein (APP) into neurotoxic amyloid-beta (Aβ42) peptides, which aggregate into plaques. Concurrently, hyperphosphorylation of the microtubule-associated tau protein causes neuronal cytoskeleton collapse, forming neurofibrillary tangles. This dual accumulation disrupts synaptic communication, notably depleting acetylcholine levels in the basal nucleus of Meynert. As the disease advances, patients experience progressive aphasia, apraxia, and agnosia. Clinically, it poses a profound burden, necessitating comprehensive multidisciplinary care and caregiver support.

Epidemiology & Demographics

Accounts for 60-80% of dementia cases. Incidence increases exponentially with age, affecting ~5% of individuals over 65 and up to 30% over 85. More common in females (approximately 2:1 ratio), largely attributed to longer life expectancy.

Etiological Mechanism

Sporadic forms account for >95% of cases, heavily influenced by the Apolipoprotein E epsilon 4 (APOE-ε4) allele. Early-onset familial Alzheimer disease (<5% of cases) is caused by autosomal dominant mutations in the APP gene on chromosome 21, Presenilin 1 (PSEN1) on chromosome 14, and Presenilin 2 (PSEN2) on chromosome 1.

Primary Causes

Primary causes are genetic mutations (APP, PSEN1, PSEN2) for early-onset and complex genetic/environmental interactions for late-onset.

• Age: The greatest known risk factor, with risk doubling every 5 years after age 65.
• APOE-e4 Genotype: Heterozygotes have a 3-fold increased risk; homozygotes have a 15-fold increased risk for sporadic AD.
• Cardiovascular Disease: Hypertension, dyslipidemia, and type 2 diabetes independently exacerbate amyloid clearance and vascular damage.
• Traumatic Brain Injury: History of severe or repeated concussion (e.g., CTE overlap) accelerates amyloid deposition.

Misfolding and aggregation of Aβ42 peptides into extracellular plaques trigger microglial activation and neuroinflammation. Intracellularly, hyperphosphorylated tau proteins dissociate from microtubules, self-assembling into paired helical filaments known as neurofibrillary tangles. This leads to profound loss of cholinergic neurons, particularly in the basal forebrain, and glutamatergic excitotoxicity.

Characteristic Clinical Presentation

• Anterograde Amnesia: Inability to form new memories, often the earliest and most prominent symptom.
• Aphasia: Difficulty finding words (anomia) progressing to severely reduced speech output.
• Apraxia: Inability to perform learned motor tasks (e.g., buttoning a shirt) despite intact motor function.

Physical Examination Signs

• Impaired delayed recall on Montreal Cognitive Assessment (MoCA) or Mini-Mental State Examination (MMSE).
• Frontal release signs (e.g., snout reflex, grasp reflex) in advanced stages.
• Anosmia (loss of smell) frequently precedes cognitive decline.

• Aspiration Pneumonia: Due to progressive dysphagia; the leading cause of death in late-stage AD.
• Malnutrition and Dehydration: Caused by loss of appetite, dysphagia, and inability to recognize food.

Diagnostic Criteria & Guidelines

Clinical diagnosis based on NIA-AA criteria requires insidious onset and progressive cognitive decline. Definitive diagnosis traditionally required autopsy, but in vivo diagnosis is now supported by biomarkers: low Aβ42 and elevated phosphorylated tau (p-tau181) in CSF, or positive amyloid/tau PET scans.

Differential Diagnosis

• Vascular Dementia
• Lewy Body Dementia
• Frontotemporal Dementia
• Pseudodementia (Major Depressive Disorder)

Laboratory Tests & Biomarkers

• CSF Aβ42: Decreased (< 500 pg/mL) due to sequestration in brain plaques.
• CSF Phosphorylated Tau: Elevated (> 60 pg/mL) reflecting active neurodegeneration.
• Vitamin B12 / TSH: Ordered to rule out reversible causes of dementia; expected to be normal in primary AD.

Imaging Modalities & Findings

• MRI Brain:
• FDG-PET Scan:

• Preclinical
  Biomarker changes (amyloid positivity) present without detectable cognitive symptoms.
• Mild Cognitive Impairment (MCI)
  Measurable cognitive deficit (e.g., MMSE 21-25) not significantly interfering with activities of daily living (ADLs).
• Dementia Stage
  Significant functional impairment (MMSE < 20); terminal phase involves bedbound state and inability to swallow.

Second-Line & Adjunctive Therapy

NMDA receptor antagonists for moderate to severe disease. Memantine 5 mg PO daily, titrated weekly by 5 mg increments to 10 mg PO BID. Can be used as monotherapy or combined with AChEIs (e.g., Namzaric). Disease-modifying monoclonal antibodies (Lecanemab 10 mg/kg IV every 2 weeks) for early AD with confirmed amyloid pathology.

Surgical & Procedural Management

No surgical management indicated for primary Alzheimer pathology. Gastrostomy tubes (PEG) are generally NOT recommended in advanced dementia due to lack of survival benefit and risk of complications.

Recommended Lifestyle Changes

• Aerobic exercise 150 minutes/week to maintain functional status.
• Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) diet.
• Implementation of advance directives and establishment of power of attorney early in the disease course.

Patient Counseling & Advice

Counsel families on driving cessation when navigating becomes impaired. Discuss caregiver burnout and refer to local support groups and respite care resources.

Follow-Up & Monitoring Schedule

Reassess cognitive status every 6 months using tools like MMSE or MoCA. Monitor weight, nutritional status, and functional independence (ADLs/IADLs).

Preventive Strategies

Strict management of mid-life cardiovascular risk factors (BP < 130/80 mmHg). Continuous cognitive engagement and high educational attainment contribute to cognitive reserve.

Average life expectancy is 4 to 8 years following diagnosis, though some may live up to 20 years. Prognosis is uniformly fatal, typically concluding with infectious complications like pneumonia.