Alzheimer's disease is a progressive neurodegenerative disease of the brain and the most common underlying cause of dementia in older adults. It produces a characteristic pattern of pathological changes involving abnormal amyloid-beta accumulation, tau pathology, synaptic dysfunction, neuronal injury and progressive loss of brain networks. Clinically, Alzheimer's disease usually begins insidiously and progresses over time, commonly affecting episodic memory early in its typical presentation before impairing additional cognitive domains such as language, orientation, executive function and visuospatial processing. Alzheimer's disease is distinct from the broader clinical syndrome of dementia: a person can have Alzheimer's disease before they meet criteria for dementia, while dementia can result from many other diseases.
The biological hallmarks of Alzheimer's disease include amyloid plaques and neurofibrillary tau pathology. Amyloid-beta is produced from the amyloid precursor protein through enzymatic processing, and in Alzheimer's disease certain amyloid-beta species can aggregate and form extracellular deposits. Tau is a protein normally involved in stabilising neuronal microtubules, but abnormal tau can become hyperphosphorylated and form aggregates within neurons. Tau pathology is closely associated with neuronal dysfunction and spreads through interconnected brain regions in characteristic patterns. The amyloid and tau processes interact with synaptic dysfunction, neuroinflammation, vascular factors and neuronal degeneration. Modern Alzheimer's research therefore treats the disease as a complex biological process rather than as a simple consequence of one type of protein accumulation.
Typical Alzheimer's disease often initially affects the hippocampus and adjacent medial temporal structures that are important for episodic memory. As pathology progresses, it involves increasingly extensive association networks of the cerebral cortex. This anatomical progression helps explain the clinical sequence in many patients: difficulty learning new information can appear before more widespread impairments in language, reasoning, orientation and visuospatial abilities. However, Alzheimer's disease is heterogeneous. Some individuals develop atypical presentations in which language, visual processing or executive abilities are initially more prominent than memory. The distribution of pathology and individual differences in brain organisation contribute to these variations, so no single symptom profile applies to every person with Alzheimer's disease.
Alzheimer's disease progresses through clinical stages that can broadly be described as preclinical disease, mild cognitive impairment due to Alzheimer's pathology, and Alzheimer's dementia, although clinical terminology and staging systems vary. In the preclinical phase, biological evidence of Alzheimer's pathology may exist before measurable symptoms. Mild cognitive impairment can involve objectively detectable decline that does not yet substantially prevent independent everyday functioning. Alzheimer's dementia occurs when cognitive impairment becomes severe enough to interfere with independence. Progression rates vary considerably between individuals, and stages should not be interpreted as rigid chronological categories. Some people experience a relatively gradual course, while others decline more rapidly or show periods in which symptoms appear relatively stable.
The symptoms of Alzheimer's disease extend beyond memory. People may have difficulty finding words, following complex instructions, planning tasks, recognising objects or people, navigating familiar environments, managing finances or adapting to unfamiliar situations. As disease advances, dependence on others increases and communication can become increasingly impaired. Behavioural and psychological symptoms can include apathy, agitation, anxiety, sleep disturbance, delusions or hallucinations, although their occurrence and severity vary. Physical functioning is usually affected later in the disease, but gait, swallowing and other abilities can eventually become impaired as degeneration becomes more widespread. These changes reflect progressive disruption of multiple interconnected neural systems rather than a simple worsening of one memory mechanism.
Diagnosis increasingly incorporates biological evidence as well as clinical assessment. A clinician may evaluate cognitive symptoms and functional changes while considering alternative causes through medical history, examination, blood tests and brain imaging. Biomarkers can provide evidence of amyloid and tau pathology through cerebrospinal-fluid analysis or specialised PET imaging, and blood-based biomarkers are increasingly being developed and implemented for appropriate clinical contexts. Biomarker positivity indicates biological evidence relevant to Alzheimer's disease but must be interpreted within the clinical context. Alzheimer's pathology can coexist with other brain diseases, particularly vascular and Lewy body pathology, so the presence of Alzheimer's biomarkers does not necessarily mean that every symptom is caused exclusively by Alzheimer's disease.
Treatment does not currently eliminate established Alzheimer's disease, but management can address symptoms, risks and quality of life, while disease-modifying therapies targeting amyloid have changed the therapeutic landscape for selected patients with appropriate disease stage and biomarker confirmation. Symptomatic medicines such as cholinesterase inhibitors can provide modest cognitive or functional benefits for some people, while memantine may be used in particular stages or circumstances.
Anti-amyloid monoclonal antibodies have been developed to reduce amyloid burden and slow aspects of clinical decline in appropriately selected people with early symptomatic Alzheimer's disease, but they have eligibility requirements, monitoring needs and clinically important risks such as amyloid-related imaging abnormalities. Treatment decisions therefore depend on diagnosis, disease stage, biomarker status, medical circumstances and informed discussion with clinicians.
Alzheimer's disease is influenced by multiple risk factors rather than one universal cause. Increasing age is the strongest risk factor for late-onset disease, while family history and genetic variation can modify risk. APOE ε4 increases susceptibility but is neither necessary nor sufficient for developing Alzheimer's disease. Rare inherited mutations in APP, PSEN1 and PSEN2 can cause autosomal-dominant early-onset Alzheimer's disease and account for a small proportion of cases.
Cardiovascular and cerebrovascular health also matter because vascular injury can interact with Alzheimer's pathology and increase cognitive vulnerability. Research into prevention and treatment continues to examine vascular health, amyloid, tau, inflammation, synaptic dysfunction and other biological pathways. Alzheimer's disease is consequently best understood as a progressive, biologically defined neurodegenerative disorder whose clinical expression reflects interactions among pathology, brain networks, genetics, vascular factors and individual resilience.
