ALZHEIMER'S DISEASE: CAUSES, BIOLOGICAL MECHANISMS
ALZHEIMER'S DISEASE: CAUSES, BIOLOGICAL MECHANISMS, SYMPTOMS AND DIAGNOSIS, TREATMENTS
Alzheimer's disease is the most common neurodegenerative disease in the elderly. The increase in the average lifespan due to improved living conditions is partly one of the reasons which explain the increase in the number of people affected by this pathology. Today, it is estimated that 900,000 people suffer from Alzheimer's type dementia in France, and 35 million sufferers worldwide. While its occurrence before age 65 is rare (0.5%), its frequency is 2 to 4% once past this age. Then, it increases proportionally with it, to exceed 15% at age 80. This disease affects more and more women (1 in 4 women and 1 in 5 men after the age of 85).
CAUSES AND BIOLOGICAL MECHANISMS
The so-called “familial” hereditary forms of Alzheimer’s disease are very rare and represent less than 1% of cases. They appear before the age of 60. Symptoms sometimes begin as early as the thirties or forties. The disease is not a hereditary disease in the strict sense in 99% of cases but genetic predisposition factors, i.e. increasing the risk of developing the disease, have been identified. For example, being a carrier of a specific allele of the ApoE gene (ApoE4), which is involved in several neuronal protection mechanisms, increases the risk of developing the disease (3 to 15 times depending on whether you are a carrier of one or two APOE4 alleles). However, this allele is neither sufficient nor necessary to develop the disease since some people carrying this allele will not be sick while non-carriers will develop the disease. so-called “environmental” factors have been associated with Alzheimer’s disease, such as chronic lack of sleep or taking certain psychotropic drugs, such as benzodiazepines, which can increase the risk of developing the disease. Recently, a list, although still incomplete, of risk factors for Alzheimer's disease was published. Factors to consider include excessive alcohol consumption, head injuries, air pollution, low education, high blood pressure, hearing problems, smoking, obesity, depression, physical inactivity, diabetes and social isolation. The degeneration of neurons which occurs in Alzheimer's disease is the result of the concomitant progression of two types of lesions: on the one hand the abnormal accumulation outside the cells of ß-amyloid peptides (or A-peptides beta or Aß peptides) leading to the formation of “amyloid plaques” also called “senile plaques”, and on the other hand the abnormal accumulation of the TAU protein in neurons leading to their degeneration.
SYMPTOMS AND DIAGNOSIS
Memory loss is often the first symptom of Alzheimer's disease that helps guide the diagnosis. Then, executive function disorders, temporo-spatial orientation disorders occur, then gradually disorders of language (aphasia), writing (dysorthography), movement (apraxia), behavior, mood disorders (anxiety, depression, irritability). The fact that the patient is not aware of his deficits (anosognosia) but that it is those close to him who report the disorders is in itself a diagnostic criterion. The diagnosis is clinical and today uses effective diagnostic tests such as a complete neuropsychological evaluation of cognitive functions, imaging examinations such as MRI and glucose PET which highlight the areas of the brain in suffering, and finally the lumbar puncture which is capable of showing the biological signs of the disease, that is to say the presence of abnormal deposits of amyloid protein and tau protein.
TREATMENTS
The aim of treating Alzheimer's disease today is to slow the progression of the disease and allow the patient and those around them to adapt to the disabilities. The care is therefore multidisciplinary. Unfortunately, there is currently no treatment that directly addresses the causes and mechanisms at the origin of the disease. Two types of drugs (acetylcholinesterase inhibitors and memantine) are validated and are used around the world. They strengthen the brain circuits and in certain cases stabilize the clinical picture of the disease. The lesions responsible for the disease To date, the causes of Alzheimer's disease remain poorly understood.
On the other hand, it is established that even before the appearance of the first symptoms, neurons are affected by two types of lesions: amyloid plaques which are found between neurons, and neurofibrillary degeneration which is found at the inside the neurons. These two lesions correspond to clumps of proteins that form during the normal aging process. However, in Alzheimer’s type diseases, these proteins accumulate in much greater quantities Amyloid plaques Amyloid plaques, or senile plaques, are formed by the abnormal accumulation of a protein called “β-amyloid”. These plaques are deposited between nerve cells located in the gray matter of the cerebral cortex, causing a dysfunction of the connections between neurons.
Neurofibrillary tangles Neurofibrillary degeneration corresponds to an abnormal accumulation of filaments inside the neuron. The protein causing this dysfunction is called “Tau protein”. Neurofibrillary degeneration progressively leads to cellular disorganization and then neuronal death. The death of neurons occurs above all in the hippocampus, one of the essential regions of memory, and in the associative cortex, which allows different functions to be linked together. Neuronal death will result in atrophy of certain brain areas, that is to say a reduction in brain volume. Genetic susceptibility factors Very rare, familial or hereditary forms represent less than 1% of patients suffering from Alzheimer's disease. This purely genetic form is characterized by a very early onset of symptoms (generally around age 50) and by so-called autosomal dominant transmission (half of each generation is affected). Non-familial and sporadic forms represent more than 99% of patients and generally develop after age 65. Although not hereditary, these sporadic forms appear to have a genetic predisposition. The ApoE4 gene is the most important risk factor for sporadic Alzheimer's disease. Of the three ApoE gene variants (ApoE2, ApoE3, and ApoE4), the ApoE4 variant is associated with an increased risk of Alzheimer's disease. However, the presence of ApoE4 is neither necessary nor sufficient for the development of the disease. This means that a person who does not carry the ApoE4 gene can still develop Alzheimer's disease and that an individual carrying the ApoE4 gene will not necessarily have it. Risk factors associated with Alzheimer's disease Alzheimer's disease is a multifactorial pathology whose appearance results from the interaction between a genetic background and risk factors. The latter refers to the notion of probability, namely that its presence statistically increases the risk of developing the disease without being the cause (for example tobacco greatly increases the risk of lung cancer, however a long-term smoker may not never develop it). Age is the main proven risk factor with a prevalence that doubles every 5 years from the age of 65 (2% after 65, 15% after 80). Women are also more at risk than men: 60% of patients are women. This is partly explained by greater longevity, but there are probably specific genetic factors that remain to be explored. Other risk factors are now well established: low level of education; Cardiovascular risk factors: untreated high blood pressure, stroke, high cholesterol, diabetes, overweight, obesity; environmental factors (tobacco, alcohol, pollution, certain medications, etc.) sleep disorders; Certain less frequently cited risk factors are increasingly documented: chronic inflammation of the body (recurrent and persistent increase in white blood cells) is linked to a shrinkage of the brain areas involved in Alzheimer's disease; a history of head trauma with loss of consciousness lasting more than 5 minutes would favor an early onset of symptoms of the disease due to weakening of the brain; Mood disorders such as chronic stress or depression are also linked to Alzheimer's disease. Furthermore, an unbalanced diet, lack of physical activity and stimulating intellectual activity could also be associated with an increased risk of Alzheimer's disease. Conversely, regular sports practice, particularly aerobics or the intake of certain nutritional elements (rich in antioxidant substances) could have a protective effect.
What is Alzheimer’s disease?
It was in 1906 that Aloïs Alzheimer discovered the disease that bears his name. This condition causes a progressive disappearance of neurons in regions of the brain that manage certain abilities, such as memory, language, reasoning or even attention. Cells lose their functions and then die. By disappearing, the neurons can no longer effectively program a certain number of actions, memory loss, behavioral problems and other symptoms then appear. Result: certain faculties are altered and gradually reduce the person’s autonomy. But although Alzheimer's disease appears more often in older people, it is not a normal consequence of aging. Generally, Alzheimer's disease appears to be correlated with several risk factors. Among the main identified causes of Alzheimer's disease, we can note a sedentary lifestyle, cardiovascular diseases, and even loneliness. Contrary to what we can still read in certain journals, no serious scientific study has been able, to date, to confirm the hypothesis of a cause and effect link between absorption of aluminum (through containers food, kitchen utensils or hygiene products) and Alzheimer's disease.
How does Alzheimer’s disease affect the brain?
Alzheimer's disease is often associated with memory loss. Indeed, it is the neurons located in the region of the hippocampus, the seat of memory, which are first affected by the phenomenon of degeneration. But that's not all. As it progresses, the disease affects other areas of the brain, complicating the ability to communicate, to do several things at the same time or to carry out daily activities. This is why the presence of a family or professional caregiver alongside the sick person is generally necessary. Alzheimer's disease causes two types of damage to the central nervous system; the dysfunction of a protein essential to neurons called “Tau” and the appearance of so-called “senile” plaques due to another protein “amyloid beta”. Little by little, these lesions multiply and invade the upper regions of the brain. The disease then becomes more and more visible. It is generally at this stage that the diagnosis is made. Is it a hereditary disease? Studies show that in 99% of cases, Alzheimer's disease is not hereditary. This means that for one in 100 sick people, the disease is caused by a defective gene inherited from one of the parents. In this case, the disease most often develops at an earlier age: Research carried out by Inserm indicates that 10% of cases of Alzheimer's disease in patients under 65 are rare hereditary familial forms of disease. At what age can you get Alzheimer's disease? Alzheimer's disease appears most often after the age of 65, where it affects approximately 3% of this age group. It is increasing rapidly to reach more than 20% of the population aged over 80. But more than 65,000 people under the age of 65 in France suffer from Alzheimer's disease or a related disease. Often forgotten in discussions about illness, they encounter specific problems linked to the impact on professional and family life and access to support systems...
Source: Dementia in Europe Yearbook 2019 –
Estimating the prevalence of dementia in Europe How does Alzheimer’s disease progress?
The course of the disease varies greatly from one patient to another. It depends in particular on the age of the sick person at the time of diagnosis, their general state of health, the level of medical care they receive and the appearance or development of other pathologies. There are three stages of Alzheimer's disease: Light stage: Around 25% of the hippocampus decreases in volume and the link between short-term and long-term memory becomes more difficult. The cognitive deficit is subtle: the patient has benign forgetting of names or recent events which intensify over time. Moderate stage: Other areas of the brain are affected, causing problems with behavior, gestures, language and recognition. The person with Alzheimer's needs help with certain activities (moving, managing their budget, cooking, etc.). Severe stage: The lesions progress and retrieval of information is almost impossible: past events and information disappear from memory. Brain cell failure is significant. The patient, suffering from dementia, has lost his autonomy for almost all the actions of his daily life. What is the life expectancy of a person with Alzheimer's? The life expectancy of a person with Alzheimer's disease varies from eight to twelve years from the time the diagnosis is made. But this is just an average. It is important to consider the age of the person at the time of diagnosis. Do we die from Alzheimer's disease? Alzheimer's disease cannot be the direct cause of death. But indirect complications, notably infections, reduce the life expectancy of people who suffer from them. Swallowing disorders, for example, increase the risk of developing pneumonia, which is one of the most common causes of death in people with Alzheimer's disease. Is there a treatment for Alzheimer's disease? To date, there is no treatment capable of curing or slowing the progression of Alzheimer's disease. Certain medications, including Donepezil (Aricept), Rivastigmine (Exelon), Galantamine (Reminyl), and Memantine (Ebixa), can nevertheless be administered to the patient to alleviate the cognitive symptoms of the disease.
Comments
Post a Comment