Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy (SMA) affects many systems in the body and can cause a number of complications. These include complications affecting the musculoskeletal system such as joint contracture, scoliosis, hip dislocations, and bone fractures, complications caused by dysphagia such as dehydration, malnutrition, and aspiration pneumonia, as well as heart failure and metabolic and nutritional issues.^1,2

Complications Affecting the Musculoskeletal System

The muscle weakness and atrophy caused by the degeneration of motor neurons in SMA can lead to a number of secondary complications such as contracture, scoliosis, hip dislocations, and bone fractures. 

Contracture

Depending on the type of SMA, contracture can form even before birth (as in the case of SMA type 0).¹ Contracture may also affect patients later, especially at the hips and knees once the ability to walk is lost. In patients with SMA type 2, it affects both the lower (hips, knees, and ankles) and upper extremities (shoulders, elbows, and wrists). The loss of range of motion usually worsens as the disease progresses and the patient ages.^3,4

Scoliosis

Scoliosis is one of the main complications affecting patients with SMA. It is directly linked to the severity of the condition, with almost all patients with SMA type 1 and 2. Scoliosis is less common in SMA type 3. 

Scoliosis is the result of weakness in the muscle of the back that normally supports the spine. It develops over a number of years and may continue to worsen after a patient has stopped growing.

Scoliosis itself can cause other complications, including breathing difficulties and subluxation.⁵

Hip Dislocations

Hip dislocations mainly affect patients with SMA who are unable to walk. It develops most rapidly in patients with SMA type 1. In a study by Coomer and colleagues, all patients with SMA type 1 developed hip dislocations by age 4.^6,7 

Patients with SMA types 1, 2, and 3 experience a decrease in acetabular depth ratio as they age. The acetabular index values are also higher than normal in all 3 types of disease.⁷

Bone Fractures

Bone fractures are common amongst SMA patients regardless of disease type. They are caused by low bone mineral density and mostly affect the femur. Bone mineral density is further reduced over time in patients with all types of SMA, making fractures more likely as the disease progresses. There is no difference between SMA types for the age at which the first fracture occurs, but it tends to be earlier in patients with SMA type 1.⁸

Complications Caused by Dysphagia

Patients with SMA often experience dysphagia, which can cause a number of complications, including dehydration, malnutrition, and aspiration pneumonia.

Dysphagia is the result of weakness in the muscles of the mouth and throat. Because the disease affects the bulbar motor neurons, which are involved in controlling chewing, speaking, and swallowing, the disease influences all these activities.

Dysphagia can prevent a patient from swallowing liquids, solids, or their own saliva. If they are not able to get enough nutrients or water into their system because of this, they may become dehydrated or malnourished.

Dysphagia can also cause the aspiration of liquids or food into the airways and increase the risk of pneumonia.⁹

Heart Failure

While it was long thought that SMA only affected the skeletal muscles, recent studies have indicated it may also affect the heart. However, it is not known how common cardiovascular defects are among SMA patients.

Studies on mouse models have shown SMA causes structural changes in the heart as well as impaired left ventricular function and lower heart rate. It is not clear whether similar abnormalities also occur in the heart of SMA patients, but based on these findings, it is important to monitor heart function in patients with SMA.¹⁰

Metabolic and Nutritional Issues

SMA can cause metabolic and nutritional issues affecting lipid and glucose metabolism and causing abnormal levels of vitamins.

Research has shown that patients with severe SMA have abnormal levels of fatty acid oxidation metabolites, such as dicarboxylic aciduria and esterified carnitine. Other consequences of altered lipid metabolism include increased urine dicarboxylic acids, abnormal excretion of ethylmalonic acid from urine organic acids, low serum carnitine levels, and high urinary acylcarnitines.

The reduced muscle protein in SMA can also lead to low alanine, which is a substrate of glycogenesis, so glucose metabolism can be altered and lead to hypoglycemia.

Levels of vitamin E, K, B, D, and calcium can also be altered in patients with SMA possibly due to the role of the SMN protein in bone remodeling and vitamin D and calcium uptake.¹¹

Reviewed by Kyle Habet, MD on 7/1/2021

References

1. Spinal muscular atrophy. MedlinePlus. May 25, 2021. Accessed June 10, 2021.
2. Spinal muscular atrophy. Cleveland Clinic. April 26, 2021. Accessed June 10, 2021.
3. Fujak A, Kopschina C, Gras F, Forst R, Forst J. Contractures of the lower extremities in spinal muscular atrophy type II. Descriptive clinical study with retrospective data collection. Ortop Traumatol Rehabil. 2011;13(1):27-36. doi:10.5604/15093492.933792
4. Fujak A, Kopschina C, Gras F, Forst R, Forst J. Contractures of the upper extremities in spinal muscular atrophy type II. Descriptive clinical study with retrospective data collection. Ortop Traumatol Rehabil. 2010;12(5):410-9.
5. Scoliosis in spinal muscular atrophy. Spinal Muscular Atrophy UK. Accessed June 10, 2021.
6. Granata C, Magni E, Merlini L, Cervellati S. Hip dislocation in spinal muscular atrophy. Chir Organi Mov. 1990;75(2):177-84.
7. Coomer W, Gerk A, Kwok C, et al. Progression of hip instability in children with spinal muscular atrophy. Developmental Medicine & Child Neurology. 2020;62(3):4-134.
8. Wasserman HM, Hornung LN, Stenger PJ, et al. Low bone mineral density and fractures are highly prevalent in pediatric patients with spinal muscular atrophy regardless of disease severity. Neuromuscul Disord. 2017;27(4):331-337. doi:10.1016/j.nmd.2017.01.019
9. Mwanza N. Difficulty swallowing with spinal muscular atrophy. 2021. My SMA team. Accessed June 10, 2021.
10. Nationwide Children’s Hospital. Spinal muscular atrophy may also affect the heart, study finds.  Science Daily. August 11, 2010. 
11. Li Y, Chen T, Wu Y, Tseng Y. Metabolic and nutritional issues associated with spinal muscular atrophy. Nutrients. 2020;12(12):3842. doi:10.3390/nu12123842

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Ozge Ozkaya, MSc, PhD

Özge’s background is in research; she holds a MSc. in Molecular Genetics from the University of Leicester and a PhD. in Developmental Biology from the University of London. Özge worked as a bench scientist for six years in the field of neuroscience before embarking on a career in science communication. She worked as the research communication officer at MDUK, a UK-based charity that supports people living with muscle-wasting conditions, and then a research columnist and the managing editor of resource pages at BioNews Services before joining Rare Disease Advisor.