[Epidemiology, clinical spectrum of ALS and differential diagnoses].

Amyotrophic Lateral Sclerosis (ALS) is the most common motor neuron disease in adults. Its incidence in France is estimated at 2.5 per 100,000 population and its prevalence between 5 and 8 per 100,000 inhabitants. Good prognostic factors are age of early onset, a longer time to diagnosis, initial damage to the spinal onset, early management of undernutrition and restrictive respiratory failure. The diagnosis of ALS is primarily clinical and is based on the evidence of involvement of the central motor neuron and peripheral neuron (NMP) in different territories or spinal or bulbar. The EMG confirms the achievement of NMP, shows the extension to clinically preserved areas and allows to exclude some differential diagnoses. The clinical spectrum of ALS is broad: conventional forms beginning brachial, lower limb or bulbar onsets, rarer forms to start breathing, pyramidal forms, forms with cognitive and behavioural impairment. In 5-10% of cases, ALS is familial. In 15% of cases, it is associated with frontotemporal degeneration rather than orbito-frontal type. The main differential diagnoses are guided by the clinic: combining pure motor neuropathy with or without conduction block, post-polio syndrome, cramp-fasciculation syndrome, myasthenia gravis, paraneoplastic syndromes, Sjögren syndrome, retroviral infections, some endocrine disorders, some metabolic diseases, genetic diseases (Kennedy and SMA) and inclusion body myositis.