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General introduction   |   9

                 Motor neuron disease

                 Motor neuron diseases (MNDs) are fatal diseases that lead to loss of upper and lower
                 motor neurons and can affect all voluntary muscles, and include amyotrophic lateral
                 sclerosis (ALS), progressive muscular atrophy (PMA) and primary lateral sclerosis (PLS).
                 The presentation of symptoms and the rate of disease progression varies greatly between   1
                 MND’s. ALS is the most prevalent and aggressive MND, affecting both the upper and lower
                 motor neurons, with an average survival of 2 to 4 years from disease onset. However some
                 patients may survive for more than 10 years. In patients with PMA only the lower motor
                 neurons are affected, and the average survival is 5 years from disease onset, while PLS only
                 affects the upper motor neurons and is not fatal in most cases. In a subgroup of patients,
                 PMA and PLS can develop into ALS, generally within the first years after diagnosis. 3–9

                 The initial symptoms of MND’s may occur in the arms or legs, known as a spinal onset, or
                 show in difficulties with speech and swallowing, known as a bulbar onset. As the disease
                 progresses, patients will experience progressive loss of motor function, which inhibits
                 daily activities, such as walking, writing, getting dressed and eating. Symptoms generally
                 start focal (in one body part), and will spread to other parts of the body.  Besides physical
                 impairments, cognitive impairments and psychological issues, such as frontotemporal
                 dementia and feelings of hopelessness, are also common in patients with MND. 10,11  Over
                 time, patients with MND will develop respiratory muscle weakness, which will lead to
                 respiratory  failure,  and  consequently,  death  in  most  patients. 12,13   Out  of  all  respiratory
                 muscles, weakness of the diaphragm is the main cause of respiratory impairment, as it
                 inhibits adequate inhalation, and leads to shortness of breath (dyspnea). During sleep,
                 diaphragm weakness will lead to nocturnal hypoventilation, resulting in abnormally high
                 carbon dioxide levels in the blood, known as hypercapnia.  Patients with prolonged
                 hypercapnia may experience sleep-related complaints (e.g. restless sleep, nightmares,
                 morning headaches) and daytime symptoms (e.g. excessive fatigue, daytime sleepiness),
                 which can cause distress and negatively affect quality of life. 15,16

                 Multidisciplinary care

                 As a result of the complexity and heterogeneity of MND, patients require specialized care
                 at a multidisciplinary ALS clinic, provided by a team consisting of at least a neurologist,
                 (rehabilitation) physician, physical therapist, occupational therapist, speech and language
                 therapist, dietician and psychologist.  Due to disease progression, patients are generally
                 monitored every 3 months during a visit to a multidisciplinary clinic.  In the Netherlands
                 there are 25 certified multidisciplinary ALS clinics spread across the country, but in most
                 countries the density of specialist centers is lower. In between clinic visits, care at home is
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