Understanding Spinal Muscular Atrophy — Causes, Symptoms And Diagnosis Of SMA Genetic Disorder

SMA is a hereditary disease that progressively destroys motor neurons. The nerve cells in the brain stem and spinal cord control essential skeletal muscle activity.

August is Spinal Muscular Atrophy awareness month

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By Dr Mary Iype

Spinal muscular atrophy (SMA) is the leading genetic cause of infant death. For every 10,000 live births, one infant dies from spinal muscular atrophy. SMA is a hereditary disease that progressively destroys motor neurons. The nerve cells in the brain stem and spinal cord control essential skeletal muscle activity such as speaking, walking, breathing, and swallowing degenerate, leading to muscle weakness and atrophy. 

What Are The Causes & Symptoms Of This Disease?

The "survival motor neuron," a protein made by the SMN1 gene, is crucial for normal nerve activity and the regulation of different muscle groups. A mutation in this gene ending in no copies of the SMN1 gene causes SMA. Severe muscle weakness is the rule and the condition is relentlessly progressive, in the majority. People who inherit an aberrant SMN1 gene or who lack both copies of the SMN1 gene are more likely to develop SMA.

There is a range of symptoms associated with SMA, from mild to severe. Symptoms of SMA vary depending on the age of onset. The disease severity correlated directly with a younger age of onset.  When the child becomes symptomatic even at birth, the disease is severe and results in death invariably. Onset in the first six months of life results in a phenotype with a maximum lifespan of two years. The common symptoms experienced by children with spinal muscular atrophy:

1. Breathing problems: Infants are more likely to experience SMA-related breathing issues, which can include shortness of breath, underdeveloped or weak lungs, coughing and shallow breathing during sleep.
2. Feeding problems: A child's capacity for safe eating and drinking can be impacted by weak muscles in the throat. Children with SMA tend to choke on food and aspirate food and drink more frequently. Eating becomes an ordeal and the child often pretends that he has no appetite. As a result, the child may end up malnourished.
3. Weakness of muscles: When SMA is identified in infancy, the muscles of the shoulders and legs are among the first to be affected. It is the muscles in the lower limbs that become weak first and remain weakest. Additionally, spinal muscular atrophy frequently hinders a baby's ability to sit up straight, walk, and achieve other motor development milestones. Over time, muscle weakness typically gets worse. People who can walk when they are young may become non-ambulant as age advances.
4. Scoliosis: Scoliosis, or an uneven curvature of the spine, is a frequent occurrence in persons who are born with SMA. Because the muscles that support the spine ordinarily aren't strong enough to keep it in place, scoliosis develops. The child’s posture and mobility may be affected by scoliosis. It may also cause pain or numbness.
5. Respiratory infections: The ineffective cough and the weak thoracic muscles added to the weak throat muscles that lead to aspiration increase the chances of contracting pneumonia and other respiratory infections. Babies with early-onset SMA frequently have a short life span up to a few months or years due to respiratory problems and infections.

Diagnosis

Spinal muscular atrophy can be diagnosed using one or more of the following tests:

1. Blood test: Creatine kinase levels will be mildly elevated, unlike muscle disorders where creatine kinase will be markedly elevated.
2. Genetic test: This blood test looks for SMN1 and SMN 2 gene copies. The test used is Multiplex Ligation-dependent Probe Amplification. It is 95% accurate as a diagnostic technique. 
3. Electromyogram (EMG): This test gives clues regarding whether the condition is SMA.
4. Muscle biopsy: Rarely a sample of your muscles can be taken for testing. A little bit of muscle tissue is removed during this surgery, and it is sent to a lab for analysis. The muscle biopsy result also may help in diagnosing the disease. Today muscle biopsy is done only if a genetic test is not available.

The symptomatic management of SMA includes physical therapy, occupational therapy, modification of the consistency of food, monitoring of respiratory function and intervening when required, scoliosis monitoring and intervention, and use of orthotics and adaptive equipment.

A person with SMA can lead a long and active life with the assistance of medication, physical therapy, and other methods. Some may require intensive medical care, while people with milder variants of SMA can expect to live as long as someone without the disease.

The author is Pediatric Neurologist, Professor & HOD, Medical College, Trivandrum