The incidence of spinal muscular atrophy worldwide varies from about 1 in 4,000 births to around 1 in 16,000 births, with 1 in 7,000 and 1 in 10,000 commonly quoted for Europe and the US respectively.Outcomes in the natural course of the disease vary from death within a few weeks after birth in the most acute cases to normal life expectancy in the protracted SMA forms. The introduction of causative treatments in 2016 has significantly improved the outcomes. Medications that target the genetic cause of the disease include nusinersen, risdiplam, and the gene therapy medication onasemnogene abeparvovec. Supportive care includes physical therapy, occupational therapy, respiratory support, nutritional support, orthopaedic interventions, and mobility support. Classification
5q SMA is a single disease that manifests over a wide range of severity, affecting infants through adults. Before its genetics was understood, its varying manifestations were thought to be different diseases – Werdnig–Hoffmann disease when young children were affected and Kugelberg–Welander disease for late-onset cases.In 1990, it was realised that these separate diseases formed a spectrum of the same disorder. Spinal muscular atrophy was then classified into 3–5 clinical types based either on the age of symptom onset or on the maximum motor function achieved. Currently, the consensus is that the phenotype of spinal muscular atrophy spans a continuum of symptoms without clear delineation of subtypes. However, the traditional classification, outlined in the table below, is still used today both in clinical research and sometimes, controversially, as a criterion of access to therapies.
For convenience, care-focused publications classify patients into "non-sitters", "sitters" and "walkers" based on their actual functional status. Motor development and disease progression in people with SMA is usually assessed using validated functional scales – CHOP-INTEND (The Childrens Hospital of Philadelphia Infant Test of Neuromuscular Disorders) or HINE (Hammersmith Infant Neurological Examination) in infants; and either the MFM (Motor Function Measure) or one of several variants of the HFMS (Hammersmith Functional Motor Scale) in older patients. The eponymous label Werdnig–Hoffmann disease (sometimes misspelled with a single n) refers to the earliest clinical descriptions of childhood SMA by Johann Hoffmann and Guido Werdnig. The eponymous term Kugelberg–Welander disease named after Erik Klas Hendrik Kugelberg (1913–1983) and Lisa Welander (1909–2001), who first documented the late-onset form and distinguished it from muscular dystrophy. Very rarely used Dubowitz disease (not to be confused with Dubowitz syndrome) is named after Victor Dubowitz, an English neurologist who authored several studies on the intermediate SMA phenotype. Signs and symptoms
The symptoms vary depending on the SMA type, the stage of the disease as well as individual factors.