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Abstract
Neuroepidemiology is a special branch of epidemiology that addresses the distribution and determinants of neurological diseases in specified population, and the application of this study to the control of neurological problems.
The main goals of neuropidemiological studies are to identify the causes and contributory factors of neurological disorders (including genetic and environmental risks), and to assess the effectiveness of interventions in defined populations.
Epidemiological studies are done to verify certain hypothesis through different sequential steps to reach definite statistically significant conclusion weather concordance or discordance with the tested hypothesis.
Many difficulties face neuroepidemiologists to perform well established accurate studies. This includes the fact that epidemiologists are not familiar with neurological diseases and neurologists are not familiar with epidemiological approaches. Also there is absence of the international standards for training in the specialty of neurology as well as for principles of epidemiology. Beside other several methodological problems such as problems for the choice of an appropriate study design and complex sampling techniques. In addition there are problems in case identification and developing an acceptable case assessment instrument for large scale studies and difficulties in data collection and presentation procedure. Regarding the neurological practice it is more difficult because the actual time of neurological disease onset often may not be easily determined with intermittent symptoms and signs that occur in some neurological diseases. So, the potential for missing cases is greatest especially with absence of good diagnostic tests. Meanwhile many neurological diseases are relatively rare and most neurological diseases are not reportable with few disease registries.
As regard neuroepidemiology in Egypt, there are many efforts for development of extensive health care system and research capacities. In addition, there is an increased awareness of need to neuroepidemiological research and training program for physicians and public health officials. However, recorded studies in the field of neuroepidemiology are scanty in Egyptian literatures; Assiut University has contributed greatly in this effort.
Recently, with the advancement of molecular techniques; the increasing sophistication of statistical methods; and the emergence of the field of molecular epidemiology, the field of genetic epidemiology has developed much, especially in the diseases of the nervous system aiming to cover a broad spectrum of research, ranging from studies of disease aggregation in families to studies of the specific molecular origin of a disorder and to elucidate the role of genetic factors and their interaction with environmental factors in the occurrence of disease in populations and these goals can be performed through genome screening and candidate gene study.
Among the most important examples of neurological diseases that need epidemiological surveillance are multiple sclerosis, Dementia and motor neuron disease.
Regarding multiple sclerosis Epidemiological studies show that it is a disease of young adults, common in women than in men, with incidence rates in most high-risk areas ranging from 1 to 10 cases per 100,000. According to etiological studies, the existence of a strong but still unidentified environmental determinant(s) of MS is highly supported. Less certain is whether this is an infectious or a noninfectious agent. Several aspects in the epidemiology of MS are consistent with the hypothesis that MS is an autoimmune reaction to infection with one or more microbes, and there is strong, albeit non-conclusive, evidence that EBV and other viruses is involved in the etiology of MS. This hypothesis does not exclude a role of noninfectious factors, such as cigarette smoking and diet.
Familial studies indicate that the aggregation of MS within families is clearly due to genetic and not to environmental factors. The risk of MS among genetically predisposed individuals appears to be over 100-fold higher than in the general population, yet the responsible genes have not been identified. It is now recognized that two or more genes interact in increasing the risk of MS; only large investigations and pooling of data from different studies will provide the power needed to detect these epistatic effects.
Epidemiological studies of MS face many difficulties, one of them is that often a person experiences subtle symptoms over a period of months to years before the disease is suspected, the variability of symptoms and signs, no single clinical feature or diagnostic test is sufficient for the diagnosis of MS, and a large number of conditions can be confused with MS or causing picture mimicking MS especially in young adults. In addition, there are several sets of published criteria for diagnosis of multiple sclerosis.
In Egypt a study was done byAbdulghani 2002 who found that MS is not rare in Egypt with expected prevalence is about 20 per 100,000 and expected number of patients reaching 15,000-20,000.
Regarding the epidemiology of dementia and AD, the epidemiology has increased rapidly over the last two decades, with increase the prevalence of about 2% between ages 65 and 74, 8% from 75 to 84, and 30% for ages 85 and over. Many risk factors have been identified, both genetic and environmental. The appearance of clinical symptoms of dementia in life depends on many risk and protective factors, some of which are under genetic control, such as genes for apolipoprotein E4 allele and its association with AD. In addition, one of the earliest risk factors identified is a family history of dementia which increases the risk for AD two to four fold, with risk increasing with the number of affected relatives. Also it is clear that in some very rare forms of AD single gene mutations occur (chromosomes 1, 14, and 21, with 1 being the most common).
Many environmental risk factors which precipitate and aggravate the pathological process of AD are discovered such as socioeconomic status in early life, level of education, adult socioeconomic conditions, lifestyle, head trauma, exposures to neurotoxins (aluminum, solvents and electromagnetic fields), smoking, and alcohol. Recent studies showed that the presence and severity of cognitive impairment in AD are affected by the presence of vascular lesions with high rate of cognitive decline among persons with high systolic blood pressure, atherosclerosis of the internal carotid artery, and diabetes mellitus.
Difficulty to obtain accurate studies on demented patients are due to many factors like the presence of more than one definition of dementia/dementia subtypes that can produce up to 10-fold variation in prevalence estimates. In addition, the non-standardized use of cognitive tests, missed cases of dementia such as institualized patients, ignorance of family members about the symptoms of dementia and refusal of patients and caregivers to follow up are factors that contribute to the problem.
Egyptian epidemiological studies which were done in Assiut University in 1998 show that the crude prevalence ratio (case per 100 populations over the age of 60) is 4.5 Prevalence ratios for dementia subtypes were 2.2 for AD, 0.95 for multi-infarct dementia, 0.55 For mixed dementias and 0.45 for secondary dementias. Age-specific prevalence tends to be doubled every 5 years. Occupation, level of education and residence did not affect the prevalence or severity of dementia. Comparison with other studies suggests that dementia of all types is as frequent in Assiut governorate as elsewhere.
Regarding motor neuron disease epidemiology especially amyotrophic lateral sclerosis, prevalence is approximately three times the incidence rate, or about 6 per 100,000 with bad prognosis that almost all patients with ALS die of their disease within 3 to 5 years. The cause of ALS is not known, and scientists do not yet know why ALS strikes some people and not others. There are several research studies investigating possible risk factors that may be associated with ALS. Genetic studies suggested that approximately 20% are due to mutations in the gene for super oxide dismutase type 1 (SOD-1) and approximately 10% of cases are familial, usually with dominant inheritance.
MND epidemiological studies are scanty due to the absence of definitive test for ALS. In addition clinical symptoms can mimic many other medical conditions, making the diagnosis of ALS difficult meanwhile the onset of ALS may be so subtle that the symptoms are frequently overlooked. Beside other methodological difficulties these make a country-wide study of the frequency of MND especially amyotrophic lateral sclerosis difficult to be done.