Motor neuron diseases (MNDs) are a group of neurodegenerative disorders that selectively affect motor neurons, the cells which control voluntary muscles of the body. According to ICD-11, the following disorders are counted among motor neuron diseases: amyotrophic lateral sclerosis (ALS), progressive bulbar palsy (PBP), pseudobulbar palsy, progressive muscular atrophy (PMA), primary lateral sclerosis (PLS), and monomelic amyotrophy (MMA), as well as some rarer variants resembling ALS. Motor neuron diseases affect both children and adults. While each motor neuron disease affects patients differently, they all cause movement-related symptoms, mainly muscle weakness. Most diseases seem to occur randomly without known causes, but some forms are inherited. Studies into these inherited forms have led to discoveries of various genes (e.g. SOD1) that are thought be important in understanding how the disease occurs. Symptoms of motor neuron diseases can be first seen at birth or can come on slowly later in life. Most diseases worsen over time; while some diseases shortening one's life expectancy (e.g. ALS), others do not. Currently, there are no approved treatments for the majority of motor neuron disorders, and care is mostly symptomatic.
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