Spinal muscular atrophy

Spinal muscular atrophy
Synonyms	Autosomal recessive proximal spinal muscular atrophy, 5q spinal muscular atrophy
Pronounce
Field	Neurology
Symptoms	Progressive muscle weakness
Complications	Scoliosis, joint contractures, pneumonia
Onset
Duration
Types	Type 0 to type 4
Causes	Mutation in SMN1
Risks
Diagnosis	Genetic testing
Differential diagnosis	Congenital muscular dystrophy, Duchenne muscular dystrophy, Prader-Willi syndrome
Prevention
Treatment	Supportive care, medications
Medication	Nusinersen, onasemnogene abeparvovec
Prognosis	Varies by type
Frequency	1 in 10,000 people
Deaths

Bell-shaped torso characteristic of spinal muscular atrophy

Diagram illustrating autosomal recessive inheritance

Spinal muscular atrophy (SMA) constitutes a collection of neuromuscular disorders, hallmarked by the degeneration of motor neurons and ensuing progressive muscle wasting. This condition is a notable genetic cause of mortality in infants, though the onset and severity of symptoms can considerably vary between different types of SMA.

Classification and Symptoms

SMA can manifest at different stages of life, from infancy to adulthood, and its types are primarily distinguished by their onset and severity. All types, however, lead to progressive muscle weakness, typically affecting the arm, leg, and respiratory muscles initially. Additional complications can include difficulty swallowing, scoliosis, and joint contractures. The observable muscle twitching is often a key symptom associated with this disorder.

Genetic Basis

The genetic basis of SMA resides in a defect in the SMN1 gene, which encodes for survival motor neuron (SMN) protein, essential for the survival of motor neurons. The disorder is inherited in an autosomal recessive manner, meaning both copies of the gene in each cell must have mutations for the disease to manifest. The absence of functional SMN protein leads to the loss of motor neurons, thereby inhibiting the transmission of signals from the brain to the skeletal muscles.

Diagnosis

Diagnosis of SMA typically begins with a thorough clinical evaluation of the symptoms, followed by confirmatory genetic testing to identify mutations in the SMN1 gene.

Treatment and Management

The management of SMA involves a multidisciplinary approach that includes physical therapy, nutritional support, and in severe cases, mechanical ventilation. Recently, new therapeutic approaches have emerged. One such treatment, nusinersen, is an antisense oligonucleotide injected into the spinal fluid, which has shown to slow the disease progression and enhance muscle function. Moreover, a gene therapy named onasemnogene abeparvovec has received approval in the US for treating children under 24 months.

Prognosis

Prognosis for individuals with SMA is highly dependent on the type of SMA. Some forms of the disease may reduce life expectancy to a few months, while others lead to mild muscle weakness but do not significantly affect life expectancy.

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