SMA

Spinal Muscular Atrophy (SMA)

Spinal Muscular Atrophy (SMA) is a genetic disorder characterized by weakness and wasting (atrophy) in muscles used for movement (skeletal muscles). It is caused by a loss of specialized nerve cells, called motor neurons, in the spinal cord and the part of the brain that is connected to the spinal cord (the brainstem). Motor neurons control muscle movement. The loss of these cells leads to weakness and atrophy in the muscles used for activities such as crawling, walking, sitting up, and controlling head movement. Severe forms of SMA can affect the muscles used for breathing and swallowing.

Classification

SMA is classified into several types based on the age of onset and the severity of symptoms:

• SMA Type 0: The most severe form, evident before birth.
• SMA Type 1: Also known as Werdnig-Hoffmann disease, it is the most common and severe type, with symptoms appearing before 6 months of age.
• SMA Type 2: Intermediate form, with symptoms appearing between 6 and 18 months of age.
• SMA Type 3: Also known as Kugelberg-Welander disease, it is a milder form with symptoms appearing after 18 months of age.
• SMA Type 4: Adult-onset form, with symptoms appearing in adulthood.

Genetics

SMA is primarily caused by mutations in the SMN1 gene, which provides instructions for making a protein that is essential for the survival of motor neurons. The severity of SMA is influenced by the number of copies of the SMN2 gene, which produces a similar protein but in smaller amounts.

Symptoms

The symptoms of SMA vary depending on the type and severity of the condition. Common symptoms include:

• Muscle weakness and atrophy
• Poor muscle tone (hypotonia)
• Difficulty breathing and swallowing
• Delayed motor milestones (e.g., sitting, standing, walking)

Diagnosis

Diagnosis of SMA typically involves a combination of clinical evaluation, genetic testing, and electromyography (EMG) to assess the health of muscles and the nerve cells that control them.

Treatment

While there is no cure for SMA, several treatments are available to manage symptoms and improve quality of life:

• Nusinersen (Spinraza): An antisense oligonucleotide that increases the production of the SMN protein.
• Onasemnogene abeparvovec (Zolgensma): A gene therapy that delivers a functional copy of the SMN1 gene.
• Risdiplam (Evrysdi): A small molecule that increases the production of the SMN protein.

Prognosis

The prognosis for individuals with SMA varies widely depending on the type and severity of the condition. Early diagnosis and treatment can significantly improve outcomes and quality of life.

Related Pages

• Motor neuron disease
• Genetic disorder
• Muscular dystrophy
• Neuromuscular disease