SMN protein is produced inadequately leading to severely compromised bodily functions, or physical disability.

There have been reports in the Indian news media recently about the plight of infants afflicted by Spinal Muscular Atrophy (SMA), a genetic neuromuscular disorder. One of the recognised remedies – Zolgensma (Onasemnogene Abeparvoves-Xioi), an FDA-approved gene therapy medicine, costing INR 16 crore – is possibly the most expensive injection across the world. 

While further research is needed to understand the long-term effects of current therapies, experts underscore the importance of early detection, including newborn screening, to ensure efficient treatment. SMA is the most common genetic cause of infant mortality. 

Let’s look at what happens within the body in this progressive disorder. We know that muscle movement is controlled by nerve cells, also referred to as neurons. Neurons are of three types –motor neurons that control all voluntary muscle movements, sensory neurons that transmit information from the outside world to the brain, and interneurons that link motor and sensory neurons.

The term atrophy refers to the condition of shrinking. Muscles get atrophied when they do not receive stimulus from the motor neurons. Since most motor neurons are located in the spinal cord region, the bodily condition in which muscles are atrophied is referred to as SMA.

The most prevalent form of SMA is caused by a mutated or missing survival motor neuron genes 1 (SMN1). The corresponding protein – survival motor neuron (SMN) protein –is essential for good health and smooth functioning of motor neurons. In SMA condition, SMN protein is produced inadequately leading to severely compromised bodily functions, or physical disability.

The disorder has been categorised into four types. Type 1 is usually evident before six months of age, and the affected children exhibit reduced movement and chronic shortening of muscles or tendons. Type II is noticed between six and 18 months of age, and afflicted children are unable to stand or walk without help and experience respiratory difficulties. Type III, also known as Kugelberg-Welander disease, is observed after 18 months of age, wherein movement is restricted, with abnormalities such as curvature of the spine. SMA Type IV develops after 21 years of age, and mild to moderate leg muscle weakness is observed.

Some symptoms are common to all types of SMA. Weakness because of skeletal muscle wasting is severe in the chest and muscles of upper leg and arm as compared to muscles of the hand and feet. In addition to approved drugs, physical therapy, occupational therapy, and rehabilitation may help improve posture, and joint mobility.

A blood test can help detect mutations, or deletions of SMN1 gene. In addition, one may need an electromyography to record electrical activity of muscles, nerve conduction velocity studies to ascertain nerve’s capacity to send electrical signal, and muscle biopsy to diagnose neuromuscular disorders.

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