認知症 l:はじめに

Dementia is an acquired and progressive decline in cognitive functions severe enough to interfere with daily living and social independence. Unlike normal aging, dementia involves impairment of memory plus at least one other cortical function such as language, calculation, spatial orientation, decision-making, judgment, or abstract reasoning. These deficits reflect underlying neurodegenerative or vascular processes that gradually disrupt neuronal networks essential for higher brain function.

Alzheimer disease is characterized by beta-amyloid plaques and neurofibrillary tangles. Beta-amyloid plaques are abnormal protein clumps that accumulate between nerve cells, disrupting cell-to-cell communication. Tau tangles consist of hyperphosphorylated tau protein that twists inside neurons, impairing nutrient transport and destabilizing the cell's internal structure. Together, these changes lead to progressive neuronal loss, especially in the hippocampus.

The hippocampus is crucial for forming and consolidating new memories. In Alzheimer disease, neuronal degeneration begins prominently in the hippocampus and medial temporal lobes, leading to progressive memory impairment. As the disease advances, degeneration spreads to association cortices, contributing to worsening cognitive decline across multiple domains beyond memory alone.

Vascular dementia results from ischemic injury to the brain caused by compromised cerebral perfusion and oxygenation. This may occur through large-vessel infarcts, multiple small-vessel lacunar strokes, or chronic microvascular disease. The resulting cognitive deficits often appear stepwise, corresponding to successive vascular events, and may include impaired attention, slowed processing speed, and executive dysfunction.

Dementia diagnosis requires impairment in at least one cortical function beyond memory, including language, calculation, spatial orientation, decision-making, judgment, abstract reasoning, executive function, and visuospatial skills. These deficits reflect disruption of neuronal networks across multiple brain regions. The specific pattern and severity of cognitive impairment depend on the underlying pathological process and affected brain areas.

Early recognition of dementia symptoms and underlying mechanisms is essential for appropriate management, risk factor modification, and supportive care planning. Dementia frequently involves mixed pathology, with both neurodegenerative and vascular contributions. Understanding whether cognitive decline stems from Alzheimer disease, vascular dementia, or both allows clinicians to tailor interventions and slow disease progression.

Beta-amyloid plaques are abnormal protein clumps composed of amyloid-β peptides that accumulate extracellularly between neurons. These plaques interfere with synaptic signaling, the process by which neurons communicate with each other. Additionally, plaques trigger local inflammatory responses that further damage surrounding neurons, contributing to the progressive neuronal loss characteristic of Alzheimer disease.