Alzheimer's disease

The first readily identified symptoms of Alzheimer's disease are usually short-term memory loss and visual-spatial confusion. These initial symptoms progress from seemingly simple and often fluctuating forgetfulness and difficulty orienting oneself in space such as in a traffic lane while driving, to a more pervasive loss of short-term memory and difficulty navigating through familiar areas such as one's neighborhood, then to loss of other familiar and well-known skills as well as recognition of objects and persons.



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Since family members are often the first to notice changes that might indicate the onset of Alzheimer's they should learn the early warning signs and serve as informants during initial evaluation of patients clinically. Aphasia, disorientation and disinhibition often accompany the loss of memory. Alzheimer's disease (AD) may also include behavioral changes, such as outbursts of violence or excessive passivity in people who have no previous history of such behavior.

In the later stages of the disease, deterioration of musculature and mobility, leading to bedfastness, inability to feed oneself, and incontinence, will be seen if death from some external cause (e.g. heart attack or pneumonia) does not intervene. Once identified, the average lifespan of patients living with Alzheimer's disease is approximately 7-10 years, although cases are known where reaching the final stage occurs within 4-5 years or at the other extreme they may survive up to 21 years.


Stages and symptoms
Mild — In the early stage of the disease, patients have a tendency to become less energetic or spontaneous, though changes in their behavior often go unnoticed even by the patients' immediate family. This stage of the disease has also been termed Minor Cognitive Impairment (MCI), when the patient does not meet the criteria for a diagnosis of dementia.
Moderate — As the disease progresses to the middle stage, patients might still be able to perform tasks independently (such as using the bathroom), but may need assistance with more complicated activities (such as paying bills).
Severe — As the disease progresses from the middle to the late stage, patients will not be able to perform even simple tasks independently and will require constant supervision. They become incontinent of bladder and then incontinent of bowel. They will eventually lose the ability to walk and eat without assistance. Language becomes severely disorganized, and then is lost altogether. They may eventually lose the ability to swallow food and fluid, and this can ultimately lead to death.


Diagnosis

No medical tests are available to diagnose Alzheimer's disease conclusively pre-mortem. A definitive diagnosis of Alzheimer's disease must await microscopic examination of brain tissue which generally occurs at autopsy therefore Alzheimer's disease (AD) is primarily a clinically diagnosed condition based on the presence of characteristic neurological and neuropsychological features and the absence of alternative diagnoses. Determination of neurological characteristics is made utilizing patient history and clinical observation, while neuropsychological evaluation includes memory testing and assessment of intellectual functioning over a series of weeks or months. Supplemental physical testing, including blood tests and neuroimaging, is utilized to rule out other diagnoses. Psychological testing, to include screening for depression and a mini mental state examination, can be helpful in establishing the presence and severity of dementia. Although certain clues from history may suggest a diagnosis of vascular dementias instead of, or in addition to, AD, the ability of certain neuroimaging modalities such as SPECT to differentiate vascular type from Alzheimer disease types of dementias, appears to be superior to clinical exam (PMID 15545324). Differential diagnosis should also include dementia with Lewy bodies and frontotemporal dementia.

Interviews with family members and/or caregivers are also utilized in the initial assessment of the disease, as a patient with Alzheimer's may tend to minimize his or her symptoms, or may undergo evaluation at a time when his or her symptoms are less apparent, as quotidian fluctuations ("good days and bad days") are a common feature of the disease. Observations noting that a patient's good memory function decreases over time plays a critical role in the diagnosis of Alzheimer's.

Biochemical characteristics

Alzheimer's disease has been identified as a protein misfolding disease, or proteopathy, due to the accumulation of abnormally folded A-beta and tau proteins in the brains of AD patients. A-beta, also written Aβ, is a short peptide that is a proteolytic byproduct of the transmembrane protein amyloid precursor protein (APP), whose function is unclear but thought to be involved in neuronal development. The presenilins are components of a proteolytic complex involved in APP processing and degradation. Although amyloid beta monomers are soluble and harmless, they undergo a dramatic conformational change at sufficiently high concentration to form a beta sheet-rich tertiary structure that aggregates to form amyloid fibrils that deposit outside neurons in dense formations known as senile plaques or neuritic plaques, in less dense aggregates as diffuse plaques, and sometimes in the walls of small blood vessels in the brain in a process called amyloid angiopathy or congophilic angiopathy.

AD is also considered a tauopathy due to abnormal aggregation of the tau protein, a microtubule-associated protein expressed in neurons that normally acts to stabilize microtubules in the cell cytoskeleton. Like most microtubule-associated proteins, tau is normally regulated by phosphorylation; however, in AD patients, hyperphosphorylated tau accumulates as paired helical filaments that in turn aggregate into masses inside nerve cell bodies known as neurofibrillary tangles and as dystrophic neurites associated with amyloid plaques.

Neuropathology

Both amyloid plaques and neurofibrillary tangles are clearly visible by microscopy in AD brains. At an anatomical level, AD is characterized by gross diffuse atrophy of the brain and loss of neurons, neuronal processes and synapses in the cerebral cortex and certain subcortical regions. This results in gross atrophy of the affected regions, including degeneration in the temporal lobe and parietal lobe, and parts of the frontal cortex and cingulate gyrus. Levels of the neurotransmitter acetylcholine are reduced. Levels of the neurotransmitters serotonin, norepinephrine, and somatostatin are also often reduced. Glutamate levels are usually elevated

Disease mechanism

Three major competing hypotheses exist to explain the cause of the disease. The oldest, on which most currently available drug therapies are based, is known as the "cholinergic hypothesis" and suggests that AD is due to reduced biosynthesis of the neurotransmitter acetylcholine. The medications that treat acetylcholine deficiency have served to only treat symptoms of the disease and have neither halted nor reversed it. The cholinergic hypothesis has not maintained widespread support in the face of this evidence, although cholingeric effects have been proposed to initiate large-scale aggregation leading to generalized neuroinflammation.

Research after 2000 includes hypotheses centered on the effects of the misfolded and aggregated proteins, amyloid beta and tau. The two positions differ with one stating that the tau protein abnormalities initiate the disease cascade, while the other states that amyloid beta (Aβ) deposits are the causative factor in the disease. The tau hypothesis is supported by the long-standing observation that deposition of amyloid plaques do not correlate well with neuron loss; however, a majority of researchers support the alternative hypothesis that Aβ is the primary causative agent.

The amyloid hypothesis is initially compelling because the gene for the amyloid beta precursor (APP) is located on chromosome 21, and patients with trisomy 21 (Down Syndrome) who thus have an extra gene copy almost universally exhibit AD-like disorders by 40 years of age. The traditional formulation of the amyloid hypothesis points to the cytotoxicity of mature aggregated amyloid fibrils, which are believed to be the toxic form of the protein responsible for disrupting the cell's calcium ion homeostasis and thus inducing apoptosis. A more recent and widely supported hypothesis suggests that the cytotoxic species is an intermediate misfolded form of Aβ, neither a soluble monomer nor a mature aggregated polymer but an oligomeric species. Relevantly, much early development work on lead compounds has focused on the inhibition of fibrillization, but the toxic-oligomer theory would imply that prevention of oligomeric assembly is the more important process or that a better target lies upstream, for example in the inhibition of APP processing to amyloid beta.

It should be noted further that ApoE4, the major genetic risk factor for AD, leads to excess amyloid build up in the brain before AD symptoms arise. Thus, Aβ deposition precedes clinical AD. Another strong support for the amyloid hypothesis, which looks at Aβ as the common initiating factor for Alzheimer's disease, is that transgenic mice solely expressing a mutant human APP gene develop first diffuse and then fibrillar amyloid plaques, associated with neuronal and microglial damage


Epidemiology

Alzheimer's disease is the most frequent type of dementia in the elderly and affects almost half of all patients with dementia. Correspondingly, advancing age is the primary risk factor for Alzheimer's. Among people aged 65, 2-3% show signs of the disease, while 25–50% of people aged 85 have symptoms of Alzheimer's and an even greater number have some of the pathological hallmarks of the disease without the characteristic symptoms. Every five years after the age of 65, the probability of having the disease doubles. The share of Alzheimer's patients over the age of 85 is the fastest growing segment of the Alzheimer's disease population in the US, although current estimates suggest the 75-84 population has about the same number of patients as the over 85 population

Prevention

Aging itself cannot be prevented, but the senescence of it can be mitigated. However, the evidence relating certain behaviors, dietary intakes, environmental exposures, and diseases to the likelihood of developing Alzheimer's varies in quality and its acceptance by the medical community. It is important to understand that interventions that reduce the risk of developing the disease in the first place may not alter its progression after symptoms become apparent. Due to their observational design, studies examining disease risk factors are often at risk from confounding variables. Several recent large randomized controlled trials—in particular the Women's Health Initiative—have called into question preventive measures based on cross-sectional studies. Some proposed preventive measures are even based on studies conducted solely in animals or in cell cultures but are not listed here.

Adults with damaged blood vessels in the brain or atrophy in their temporal lobe are more likely to develop Alzheimer's disease. It is known that blood vessel damage in the brain is more likely to occur in patients with high blood pressure, high cholesterol or diabetes. Therefore, prevention of these conditions can lower the risk of developing Alzheimer's, as well as heart attack and stroke.

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