Is Alzheimer’s The Most Common Cause of Dementia?

Alzheimer’s disease and other causes of dementia impact more than 55 million people worldwide, with nearly 10 million new cases diagnosed every year. Alzheimer’s is the most common cause of dementia, a general term for memory loss and a decline in mental abilities that interfere with an individual’s ability to complete Activities of Daily Living (ADLs). Alzheimer’s disease accounts for roughly 60-80% of all dementia cases. Other causes of dementia include vascular dementia, Lewy body dementia, Parkinson’s disease dementia, and Huntington’s disease.

Alzheimer’s is characterized by a gradual decline in neurocognitive function, meaning there is a decline in both cognitive and functional aspects of brain health. Earlier stages of the disease may be more biased towards cognitive impairment with small magnitudes of functional impairment. In the later stages of the disease, both cognitive and functional impairments are typically present and can significantly impact an individual’s ability to complete ADLs. According to researchers, Alzheimer’s disease is not caused by a single factor, but rather develops from multiple factors, including genetics, lifestyle, and environment. 

Neuropathological hallmarks, or neuropathological changes that occur within the brain of an individual with Alzheimer’s disease, include beta-amyloid plaques, tau hyperphosphorylation, neurofibrillary tangles, and neuronal and synaptic loss.

Below, we detail the clinical and pathological aspects of common causes of dementia.

Common Causes of Dementia

While Alzheimer’s is the most common cause of dementia, there are other causes of dementia with varying pathologies. Let’s take a look into some of the other common causes of dementia.

Vascular Dementia

Vascular dementia is the second most common cause of dementia and is caused by conditions that reduce blood flow to the brain, such as brain hemorrhages and strokes. Insufficient blood flow can cause damage to brain cells, eventually killing them. However, brain changes linked to vascular dementia can also be caused by any condition that causes damage to blood vessels anywhere in the body. Individuals with vascular dementia experience a decline in thinking, memory, judgment, reasoning, planning, and other cognitive abilities. 

Symptoms of vascular dementia can vary significantly depending on the severity of blood vessel damage as well as the part of the brain that is impacted. Approximately 5-10% of dementia cases are attributed to vascular dementia alone. However, vascular dementia is more common as part of mixed dementia. Individuals with mixed dementia may experience changes to the brain associated with Alzheimer’s, vascular dementia, and Lewy body dementia, though the most common form of mixed dementia is Alzheimer’s and vascular dementia.

Lewy Body Dementia

Lewy body dementia, also referred to as dementia with Lewy bodies, is believed to be the third most common cause of dementia. 

Lewy body dementia is a progressive dementia and is clinically characterized by a decline in thinking, reasoning, and independent function. Individuals with Lewy body dementia may also experience visual hallucinations and changes in attention and alertness. Lewy bodies, which are protein deposits made of abnormal filaments composed of alpha-synuclein, develop in nerve cells within the regions of the brain responsible for memory, thinking, and motor control. While alpha-synuclein is found naturally in the brain, its function is not yet fully understood. An estimated 5% of individuals living with Lewy body dementia show evidence of Lewy body dementia alone; however, most people with Lewy body dementia also have Alzheimer’s disease pathology.

Many individuals with Lewy body dementia experience functional impairments, such as rigid muscles, hunched posture, tremors, and difficulty walking. 

Parkinson’s Disease Dementia

Parkinson’s disease dementia is a decline in cognitive function that develops in many individuals with Parkinson’s disease, a progressive nervous system disorder caused by genetics and environmental triggers that affects movement. Similar to how many individuals with Lewy body dementia and Alzheimer’s disease may develop functional impairments, many individuals with Parkinson’s disease develop cognitive impairments. 

According to the Alzheimer’s Association, a large study found that about three-quarters of people living with Parkinson’s for more than 10 years will develop dementia. As changes in the brain caused by Parkinson’s disease spread, cognitive abilities become impaired. The main neuropathological hallmark associated with Parkinson’s disease and Parkinson’s disease dementia is the presence of Lewy bodies. 

The overlap in symptoms between Lewy body dementia and Parkinson’s disease dementia suggests a link between the two diseases and their pathologies. The two diseases may be connected to the same underlying abnormalities in how the brain processes alpha-synuclein. 

Huntington’s Disease

Huntington’s disease is a rare hereditary disease that causes progressive degeneration of nerve cells in the brain. Huntington’s causes changes in the central area of the brain, resulting in a broad range of cognitive and functional impairments. Functional impairments may include involuntary writhing movements, slow or abnormal eye movements, impaired posture, gait, balance, and difficulty speaking, while cognitive impairments may include difficulty organizing and prioritizing tasks, lack of flexibility and impulse control, changes in mood, difficulty learning new things, and a slow processing speed.

Huntington’s disease is caused by a single defective gene of chromosome 4. As this genetic defect is dominant, only one copy of the gene will cause an individual to develop Huntington’s.