Five Types of Spinal Muscular Atrophy

Spinal muscular atrophy is a congenital condition, which causes the victim’s skeletal muscles to become increasingly weaker as they age. This disorder leads to a gradual loss of motor neurons and tends to affect the muscles nearest to the body’s center more than the distal ones. This paper covers the five types of this disease that are characterized by different degrees of severity and the age at which the first symptoms are seen.

Etiology and Physiopathology

Spinal muscular atrophy is caused by abnormalities in the structure and number of SMN1 and SMN2 genes, respectively. SMN1 is the gene responsible for the production of the survival motor neuron protein that prevents the degeneration of motor neurons controlling the body’s muscles (Farrar et al., 2017). SMN2 also builds smaller amounts of said protein in different versions, all but one of which are not functional.

In most cases, spinal muscular atrophy tends to be induced by a mutation in the SMN1 gene that hinders the production of the SMN protein. With a lack of this vital protein, motor neutrons begin to die, and the transmission of signals from the brain and the spinal cord to the skeletal muscles becomes weaker and less reliable. Consequently, the muscles do not contract the way they are supposed to, and movement becomes increasingly difficult. What is more, because the muscles do not perform enough work, they start to waste away, further impeding movement.

The SMN2 gene is not the root of the disorder, but it is a significant factor in determining the severity of the symptoms and the age-related decline in muscle functionality. This gene can partially counteract the effects of the defective SMN1 gene by producing small amounts of the SMN protein. Since the protein produced by the SMN2 gene is often unusable, having more copies of the gene increases the chances of offsetting the adverse effects of the disease. Most people possess either one or two copies of this gene, which equates to spinal muscular atrophy type 0 or I. Having tree copies usually correlates with type II, and having more means type III or type IV. It is also important to note that there are other factors that affect the severity, many of which have not been discovered yet.

Types and Clinical Manifestations

Type 0 is the most severe of the five, and luckily it is also the rarest. The symptoms of type 0 spinal muscular atrophy can be distinguished even during pregnancy, as the fetuses with this condition tend to be less active. The newborns have difficulty breathing and commonly die from respiratory failure at an extremely young age of less than a month (Wadman et al., 2017). Certain heart deformities are also associated with type 0.

Type I, while being almost as deadly, is also much more common than type 0. The symptoms can be identified at birth or not later that in a few months. This type is mainly characterized by the infant’s inability to support their own weight when sitting or move their head in a controlled manner (Wadman et al., 2017). A particular feature of type I spinal muscular atrophy is the patient’s bell-shaped torso that does not allow the lungs to operate at full capacity. The disease can also manifest in the form of hindered swallowing, which has its own set of consequences. The most prevalent cause of death for type I is also respiratory failure.

Type II has its first symptoms emerge at the age of 6 to 12 months. While affected children are unable to walk or stand without assistance, they can sit on their own before the condition becomes more severe (Wadman et al., 2017). Other effects include potentially lethal respiratory problems, tremors, and scoliosis. The average life expectancy with type II spinal muscular atrophy is about 20 to 30 years.

Spinal muscular atrophy type III manifests itself in early childhood and is significantly less severe. At a young age, infected individuals experience general weakness but retain the ability to stand and walk normally (Wadman et al., 2017). With age, physically demanding tasks become harder to perform to the point where a wheelchair may be required. This form of the disorder is not life-threatening, and most of those affected by it do not have a reduced lifespan.

Finally, type IV is the least severe form of the disease that generally only begins to take effect at the age of 20 to 30 years. It has a very slight form of most of the normal symptoms of spinal muscular atrophy: breathing problems, general weakness, and respiratory problems (Wadman et al., 2017). Since the negative effects are so mild, type IV does not affect the patient’s life expectancy.

Medical Management

In regards to the medical management of spinal muscular atrophy, the main challenge is dealing with respiratory problems. New patients need to be tested for cough and breathing effectiveness as soon as possible. This is vital because, in many cases, the cough of an infected individual can be too weak to clear their airways from secretions that hinder ventilation (Mercuri et al., 2018). If such problems are discovered, the person may need various forms of respiratory care, such as cough assistance and noninvasive ventilation.

It has also been noted that respiratory issues often indicate the presence of nutrition problems. Depending on the severity of spinal muscular atrophy, patients can experience undernutrition or overnutrition. Those who cannot sit without support struggle with reaching healthy growth, while patients with less severe forms of the disease easily become overweight. Despite being opposite to each other, both these conditions can be lethal and require careful monitoring of the patient’s status and developing an optimized nutrition plan. This includes weight and length control, as well as a constant detailed analysis of the subject’s diet.

With undernutrition, there are cases when swallowing problems cause growth failure, and intervention in the form of feeding tubes is needed (Mercuri et al., 2018). Overnutrition, which leads to decreased mobility and is generally a risk factor, can usually be aided without surgery, by means of various bowel regulating agents.

Finally, a crucial part of managing spinal muscular atrophy is communication. Families of patients need to be in regular contact with the physician assigned to them. The doctor should start by explaining the nature of this genetic disorder, its types, the way its symptoms develop with age, and the dangers of living with it. After that, there needs to be a continuous discussion about the family’s attitude toward various forms of treatment. The importance of clarifying all the ethical issues at the beginning cannot be understated, as any lapse in understanding could have devastating consequences.


Farrar, M. A., Park, S. B., Vucic, S., Carey, K. A., Turner, B. J., Gillingwater, T. H.,… & Kiernan, M. C. (2017). Emerging therapies and challenges in spinal muscular atrophy. Annals of neurology, 81(3), 355-368.

Mercuri, E., Finkel, R. S., Muntoni, F., Wirth, B., Montes, J., Main, M.,… & Bertini, E. (2018). Diagnosis and management of spinal muscular atrophy: part 1: recommendations for diagnosis, rehabilitation, orthopedic and nutritional care. Neuromuscular Disorders, 28(2), 103-115.

Wadman, R. I., Stam, M., Gijzen, M., Lemmink, H. H., Snoeck, I. N., Wijngaarde, C. A.,… & van der Pol, W. L. (2017). Association of motor milestones, SMN2 copy and outcome in spinal muscular atrophy types 0–4. J Neurol Neurosurg Psychiatry, 88(4), 365-367.

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NursingBird. "Five Types of Spinal Muscular Atrophy." March 12, 2022.