Spinal Muscular Atrophy (SMA) is a rare, inherited neuromuscular disorder that affects the motor neurons responsible for controlling voluntary muscles. It leads to muscle weakness and progressive muscle wasting (atrophy) due to the loss of motor neuron...
**Spinal Muscular Atrophy (SMA): An Overview**
Spinal Muscular Atrophy (SMA) is a rare, inherited neuromuscular disorder that affects the motor neurons responsible for controlling voluntary muscles. It leads to muscle weakness and progressive muscle wasting (atrophy) due to the loss of motor neurons in the spinal cord and brainstem. SMA is caused by mutations in the **SMN1** (Survival Motor Neuron 1) gene, which plays a critical role in the survival and function of these motor neurons. Without sufficient production of SMN protein, motor neurons begin to deteriorate, leading to loss of muscle control.
### Types of SMA
SMA is classified into four main types based on the age of onset and severity of symptoms:
1. **SMA Type 1 (Werdnig-Hoffmann Disease):**
This is the most severe form and often appears within the first six months of life. Infants with Type 1 experience severe muscle weakness, difficulty breathing, swallowing, and are usually unable to sit or hold up their heads without assistance. Life expectancy is generally limited, although treatments are improving survival rates.
2. **SMA Type 2:**
Symptoms of Type 2 usually emerge between 6 to 18 months of age. Children with Type 2 can sit but often cannot stand or walk unaided. While life expectancy varies, many live into adulthood, though they may require extensive support and mobility aids.
3. **SMA Type 3 (Kugelberg-Welander Disease):**
This type manifests after 18 months of age and has a milder progression. Individuals with Type 3 can usually stand and walk, but as the disease progresses, they may lose the ability to walk and experience muscle weakness over time. Life expectancy is near normal, but physical limitations may increase.
4. **SMA Type 4:**
This is the mildest form of SMA, and symptoms typically appear in adulthood, usually after the age of 20. Muscle weakness progresses slowly, and individuals can live a normal lifespan with minimal mobility issues.
### Causes of SMA
SMA is a genetic disorder passed down in an autosomal recessive manner. This means that both parents must carry a mutated copy of the SMN1 gene for their child to inherit the disease. If a child inherits two faulty SMN1 genes, one from each parent, they will develop SMA. Carriers, who have only one mutated gene, typically do not show symptoms of the disorder.