Causes
Three major competing hypotheses exist to explain the cause of the disease. The oldest, on which most currently available drug therapies are based, is the cholinergic hypothesis, which proposes that AD is caused by reduced synthesis of the neurotransmitter acetylcholine. The cholinergic hypothesis has not maintained widespread support, largely because medications intended to treat acetylcholine deficiency have not been very effective. Other cholinergic effects have also been proposed, for example, initiation of large-scale aggregation of amyloid, leading to generalised neuroinflammation.
In 1991, the amyloid hypothesis postulates that amyloid beta (Aβ) deposits are the fundamental cause of the disease. It is a compelling theory because the gene for the amyloid beta precursor (APP) is located on chromosome 21, and people with trisomy 21 (Down Syndrome) who thus have an extra gene copy almost universally exhibit AD by 40 years of age. Also APOE4, the major genetic risk factor for AD, leads to excess amyloid buildup in the brain before AD symptoms arise. Thus, Aβ deposition precedes clinical AD. Further evidence comes from the finding that transgenic mice that express a mutant form of the human APP gene develop fibrillar amyloid plaques and Alzheimer's-like brain pathology. An experimental vaccine was found to clear the amyloid plaques in early human trials, but it did not have any significant effect on dementia.
Deposition of amyloid plaques does not correlate well with neuron loss.This observation supports the tau hypothesis, the idea that tau protein abnormalities initiate the disease cascade. In this model, hyperphosphorylated tau begins to pair with other threads of tau. Eventually, they form neurofibrillary tangles inside nerve cell bodies. When this occurs, the microtubules disintegrate, collapsing the neuron's transport system. This may result first in malfunctions in biochemical communication between neurons and later in the death of the cells.
Pathophysiology
Neuropathology
Alzheimer's disease is characterised by loss of neurons and synapses in the cerebral cortex and certain subcortical regions. This loss 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.
Both amyloid plaques and neurofibrillary tangles are clearly visible by microscopy in brains of those afflicted by AD. Plaques are dense, mostly insoluble deposits of amyloid-beta protein and cellular material outside and around neurons. They continue to grow into insoluble twisted fibres within the nerve cell, often called tangles. Although many older individuals develop some plaques and tangles as a consequence of ageing, the brains of AD patients have a greater number of them in specific brain regions such as the temporal lobe.
Biochemistry
Alzheimer's disease has been identified as a protein misfolding disease (proteopathy), caused by accumulation of abnormally folded A-beta and tau proteins in the brain. Plaques are made up of small peptides, 39–43 amino acids in length, called beta-amyloid (also written as A-beta or Aβ). Beta-amyloid is a fragment from a larger protein called amyloid precursor protein (APP), a transmembrane protein that penetrates through the neuron's membrane. APP is critical to neuron growth, survival and post-injury repair. In Alzheimer's disease, an unknown process causes APP to be divided into smaller fragments by enzymes through proteolysis. One of these fragments gives rise to fibrils of beta-amyloid, which form clumps that deposit outside neurons in dense formations known as senile plaques.
In Alzheimer's disease, changes in tau protein lead to the disintegration of microtubules in brain cells.
AD is also considered a tauopathy due to abnormal aggregation of the tau protein. Every neuron has a cytoskeleton, an internal support structure partly made up of structures called microtubules. These microtubules act like tracks, guiding nutrients and molecules from the body of the cell to the ends of the axon and back. A protein called tau stabilises the microtubules when phosphorylated, and is therefore called a microtubule-associated protein. In AD, tau undergoes chemical changes, becoming hyperphosphorylated; it then begins to pair with other threads, creating neurofibrillary tangles and disintegrating the neuron's transport system.
Disease mechanism
Exactly how disturbances of production and aggregation of the beta amyloid peptide gives rise to the pathology of AD is not known. The amyloid hypothesis traditionally points to the accumulation of beta amyloid peptides as the central event triggering neuron degeneration. Accumulation of aggregated amyloid fibrils, which are believed to be the toxic form of the protein responsible for disrupting the cell's calcium ion homeostasis, induces programmed cell death (apoptosis). It is also known that Aβ selectively builds up in the mitochondria in the cells of Alzheimer's-affected brains, and it also inhibits certain enzyme functions and the utilisation of glucose by neurons.
Various inflammatory processes and cytokines may also have a role in the pathology of Alzheimer's disease. Inflammation is a general marker of tissue damage in any disease, and may be either secondary to tissue damage in AD or a marker of an immunological response.
Genetics
While the rare, early-onset form of Alzheimer's disease is mainly explained by mutations in three genes, the most common form has yet to be explained by a purely genetic model. The APOE gene is the strongest genetic risk factor for Alzheimer's discovered so far, but its presence is far from explaining all occurrences of the disease.
Less than 10% of AD cases occurring before 60 years of age are due to autosomal dominant (familial) mutations, which therefore represent less than 0.01% of all cases. These mutations have been discovered in three different genes: amyloid precursor protein (APP) and presenilins 1 and 2. Most mutations in the APP and presenilin genes increase the production of a small protein called Abeta42, which is the main component of senile plaques.
Most cases of Alzheimer's disease do not exhibit familial inheritance, but genes may act as risk factors. The best known genetic risk factor is the inheritance of the ε4 allele of the apolipoprotein E (APOE). This gene is implicated in up to 50% of late-onset sporadic Alzheimer's cases. Geneticists agree that numerous other genes also act as risk factors or have protective effects that influence the development of late onset Alzheimer's disease. Over 400 genes have been tested for association with late-onset sporadic AD. One example is a variant of the reelin gene that may contribute to Alzheimer's risk in women.
Diagnosis
Alzheimer's disease is usually diagnosed clinically from the patient history, collateral history from relatives, and clinical observations, based on the presence of characteristic neurological and neuropsychological features and the absence of alternative conditions. Advanced medical imaging with computed tomography (CT) or magnetic resonance imaging (MRI), and with single photon emission computed tomography (SPECT) or positron emission tomography (PET) can be used to help exclude other cerebral pathology or subtypes of dementia. Assessment of intellectual functioning including memory testing can further characterise the state of the disease. Medical organisations have created diagnostic criteria to ease and standardise the diagnostic process for practicing physicians. Sometimes the diagnosis can be confirmed or made at post-mortem when brain material is available and can be examined histologically.
Diagnostic criteria
The National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association (now known as the Alzheimer's Association) established the most commonly used diagnostic criteria for Alzheimer's disease. These criteria require that the presence of cognitive impairment, and a suspected dementia syndrome, be confirmed by neuropsychological testing for a clinical diagnosis of possible or probable AD. A histopathologic confirmation including a microscopic examination of brain tissue is required for a definitive diagnosis. Good statistical reliability and validity have been shown between the diagnostic criteria and definitive histopathological confirmation. Eight cognitive domains are most commonly impaired in AD—memory, language, perceptual skills, attention, constructive abilities, orientation, problem solving and functional abilities. These domains are equivalent to the NINCDS-ADRDA Alzheimer's Criteria as listed in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) published by the American Psychiatric Association.
Diagnostic tools
Neuropsychological screening tests can help in the diagnosis of AD. In them patients have to copy drawings similar to the one shown in the picture, remember words, read or sum.
Neuropsychological tests such as the mini-mental state examination (MMSE), are widely used to evaluate the cognitive impairments needed for diagnosis. More comprehensive test arrays are necessary for high reliability of results, particularly in the earliest stages of the disease. Neurological examination in early AD will usually provide normal results, except for obvious cognitive impairment, which may not differ from standard dementia. Further neurological examinations are crucial in the differential diagnosis of AD and other diseases. Interviews with family members are also utilised in the assessment of the disease. Caregivers can supply important information on the daily living abilities, as well as on the decrease, over time, of the person's mental function. A caregiver's viewpoint is particularly important, since a person with AD is commonly unaware of his own deficits. Many times, families also have difficulties in the detection of initial dementia symptoms and may not communicate accurate information to a physician. Supplemental testing provides extra information on some features of the disease or is used to rule out other diagnoses. Blood tests can identify other causes for dementia than AD[4]—causes which may, in rare cases, be reversible. Psychological tests for depression are employed, since depression can either be concurrent with AD or be the cause of cognitive impairment.
When available as a diagnostic tool, SPECT and PET neuroimaging are used to confirm a diagnosis of Alzheimer's in conjunction with evaluations involving mental status examination. The ability of SPECT to differentiate Alzheimer's disease from other possible causes in somebody already known to be suffering from dementia, appears to be superior to attempts to diagnose by mental testing and history. A new technique known as PiB PET has been developed for directly and clearly imaging beta-amyloid deposits in vivo using a tracer that binds selectively to the Abeta deposits. Another recent objective marker of the disease is the analysis of cerebrospinal fluid for amyloid beta or tau proteins. Both advances have led to the proposal of new diagnostic criteria.
Prevention
Intellectual activities such as playing chess or regular social interaction have been linked to a reduced risk of AD in epidemiological studies, although no causal relationship has been found.
Global studies of measures to prevent or delay the onset of AD have often produced inconsistent results. At present, there appears to be no definitive evidence to support the belief that any particular measure is effective in preventing AD. However, epidemiological studies have proposed relationships between certain modifiable factors, such as diet, cardiovascular risk, pharmaceutical products, or intellectual activities among others, and a population's likelihood of developing AD. Only further research, including clinical trials, will reveal whether, in fact, these factors can help to prevent AD.
The components of a Mediterranean diet, which include fruit and vegetables, bread, wheat and other cereals, olive oil, fish, and red wine, may all individually or together reduce the risk and course of Alzheimer's disease. Several vitamins such as B12, B3, C or folic acid have been found in some studies to be related to a reduced risk of AD but other studies indicate that they do not have any significant effect on the onset or course of the disease and may have important secondary effects. Curcumin from the curry spice turmeric has shown some effectiveness in preventing brain damage in mouse models.
Although cardiovascular risk factors, such as hypercholesterolemia, hypertension, diabetes, and smoking, are associated with a higher risk of onset and course of AD, statins, which are cholesterol lowering drugs, have not been effective in preventing or improving the course of the disease. However long-term usage of non-steroidal anti-inflammatory drug (NSAIDs), is associated with a reduced likelihood of developing AD in some individuals. Other pharmaceutical therapies such as female hormone replacement therapy are no longer thought to prevent dementia. A 2007 systematic review concluded that there was inconsistent and unconvincing evidence that ginkgo has any positive effect on cognitive impairment, and a recent study concludes that it has no effect in reducing the rate of AD incidence.
Intellectual activities such as reading, playing board games, completing crossword puzzles, playing musical instruments, or regular social interaction may also delay the onset or reduce the severity of Alzheimer's disease. Bilingualism is also related to a later onset of Alzheimer's.
Some studies have shown an increased risk of developing AD with occupational exposure to magnetic fields, intake of metals, particularly aluminium, or exposure to solvents. The quality of some of these studies has been criticised, and other studies have concluded that there is no relationship between these environmental factors and the development of AD.
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