Motor neuron disease (MND) is a neurodegenerative disease characterized by a progressive loss of upper and lower motor neurons leading to muscle atrophy and paralysis. Patients with this condition usually die as a result of respiratory failure.
The term MND often encompasses amyotrophic lateral sclerosis (ALS) and its related diseases, such as primary lateral sclerosis and progressive muscular atrophy, which form a very heterogeneous group of diseases affecting the first and/or second motor neuron and are of either sporadic or hereditary nature.
Within this group, ALS accounts for 85% of cases of MND. Most ALS patients have no family history of this condition (85-90%), which is why it is mostly considered a sporadic disease.
The onset of clinical symptoms of dysphagia, weight loss, and respiratory failure must be controlled as of the time of diagnosis of the disease in order to establish a nutritional treatment and, therefore, be able to have a positive impact on the patient’s survival. Malnutrition is known to be an important predictor of mortality in this type of patient. A weight loss greater than 5-10%, both at the time of diagnosis and throughout the course of the disease, is associated with a 30-50% increase in the risk of death. It has been estimated that there is a 14-30% increase in the likelihood of death per every 5% of weight loss. A body mass index (BMI) below 18.5 kg/m² also increases the relative risk of death, and the risk of dying increases by 9-20% per every one-point decrease in the BMI.
Amyotrophic lateral sclerosis is considered a minority disease due to its low prevalence ranging between 4.1 and 8.4 affected persons per 100,000 inhabitants. However, one cannot forget that new cases are constantly being diagnosed. The incidence of this disease ranges between 0.6 and 3.8 new cases per 100,000 inhabitants/year.
This heterogeneity in both the incidence and prevalence of ALS is also observed with other variables such as sex and age. Thus, the incidence is between 1.3 and 1.5 times higher in men compared with women. It is also a known fact that ALS is very rare before the age of 40, and that its average age of onset ranges from 58 to 63 in the sporadic form and from 40 to 60 in the familial form.
This heterogeneity in the incidence and prevalence of ALS may also be a consequence of different diagnostic criteria, different clinical practices, and different ways of recording cases. Nevertheless, an important cause of this heterogeneity can be attributed to the interrelationship between genetic and non-genetic factors. The non-genetic factors can include lifestyle variables (smoking, antioxidants intake, exercise, and body mass index), medical conditions (head trauma, metabolic diseases, cancer, and inflammatory diseases), as well as occupational and environmental exposures (β-methylamino-L-alanine [BMAA], viral infections, electromagnetic fields, metals, and pesticides).
ALS is currently an incurable disease and is fatal as of disease onset. Only 10% of patients survive 5 years after the initial diagnosis. The overall mortality rate associated with this disease in Spain is estimated at 1.49 per 100,000 inhabitants, with a slightly higher rate among men and a peak in the 60-69 age group.
The number of ALS-related deaths in Spain is so high that it is almost identical to that of the new cases diagnosed annually. This results in a stabilization of the prevalence of ALS at very low levels compared with other diseases, which, despite having the same incidence, are associated with a much higher survival rate.