Spinal Muscular Atrophy (SMA)

Original Editor - Evelin Milev Top Contributors - Evelin Milev and Meaghan Rieke

Introduction

Spinal Muscular Atrophy (SMA) is a genetic condition under the scope of the neuromuscular disorders. It is characterised by degeneration of alpha motor neurons in the spinal cord that affects the control of voluntary muscle movement. The disease is characterised as an autosomal recessive condition with prevalence of approximate 1 in 6-10,000 births affected by SMA with a carrier frequency of 1 in 35-70[1]. Classification of SMA type depends on the age of onset and the highest level of motor function achieved[1].

Clinically Relevant Anatomy

SMA is caused by deficiency of a motor neuron protein called SMN (Survival Motor Neuron). This protein is essential for normal motor function and the lack of it is caused by genetic flaws on chromosome 5 in the gene SMN1. The neighbouring SMN2 gene can compensate some of the functions of SMN1 and this is where some of the pharmaceutical companies trying to develop a drug which can enhance the effect of SMN2.

Mechanism of Injury / Pathological Process

Spinal Muscular Atrophy is an autosomal recessive condition due in most cases to the homozygous deletion of the SMN1 gene[2]. This means that both parents of the affected individual are only carriers of the affected gene. Therefore, they are not going to present with any symptoms of the disease and this is what makes SMA difficult to foreseen and apply preventable measures.

Clinical Presentation

Spinal Muscular Atrophy (SMA) is the second most common neuromuscular disorder of childhood. People affected by the mildest types of SMA have proximal weakness and impaired ambulation. Furthermore, fatigue is a symptom to present in almost every case of SMA which may also lead to impaired function and endurance. Current research in the area shows that there is good correlation between upper and lower limb function in patients with the disease. There are several types of SMA, which start at different ages and may present with various phenotype[3].

Types of SMA:
  • SMA type I - affects babies less than six months old and is the most severe type of the disease
  • SMA type II - develops in babies between 7 and 18 months old. This type is less severe than type I and most children survive into adulthood and can live long, fulfilling lives.
  • SMA type III - appears after 18 months of age and is the least severe type affecting children. SMA type III has been divided into two further sub-categories: SMA IIIa and SMA IIIb - according to the time when the first symptoms of the condition appears (if before or after 3 years of age).
  • SMA type IV - this type of SMA patients are diagnosed in adulthood and they present with only mild problems.

Diagnostic Procedures

Diagnoses could be made by prenatal screening or by gene panel investigation and/or muscle biopsy. In early stages the diagnose may be suspected due to symptoms like floppiness and muscular weakness. Children with type I SMA can present with lack of head control, minimal to absent anti-gravity movements and severe respiratory complications.

The first steps in diagnosing someone with SMA would be by taking a full clinical examination and family history. As mentioned above, a blood test might be required to look at the amount of creatine kinase (CK), an investigation to indicate if muscle damage has occurred. High levels levels of the CK in the blood is not damaging itself, but it is an important indicator of a muscle disorder condition. Further investigation will probably include a genetic testing as this is the most accurate way to diagnose if patient has Spinal Muscular Atrophy.


Outcome Measures

There are several outcome measures which can be used to detect changes in the natural history of the patients with SMA. These tools should be appropriate selected according to the age and severity of the disease.

Six-Minute Walking Test (6MWT)

The Six-Minute-Walking-Test can be safely performed in ambulant patients with SMA. It has been proven to detect fatigue-related changes in this population of patients and also correlates with other established outcome measures for patients with spinal muscular atrophy[4].

Revised Hammersmith Scale for SMA (RHS)

The RHS is predominantly used in patients with SMA type 2 and 3. In a combination with the WHO motor milestones the scale can be more sensitive towards the description of SMA phenotype. The RHS has been designed to capture wide range of of abilities across broad spectrum of SMA, from very young children to adolescents and adults[5].

WHO Developmental Milestones

The WHO scales aims to link the growth of the child and the motor development in one single reference. The final version of the protocol includes six items: "Sitting without support", "Hands-and-knees crawling", "Standing with assistance", "Walking with assistance", "Standing alone", and "Walking alone". The WHO provides an important information about child's gross motor development in different cultural settings[6].

Revised Upper Limb Module (RULM) for SMA

The RULM is specifically designed outcome measure for upper limb function in patients with Spinal Muscular Atrophy. The scale has shown good reliability and validity, which makes it a good choice for assessing arm function in children and adults with SMA[7].

Management / Interventions

Spinal Muscular Atrophy (SMA) is a severe genetic condition which requires precise diagnosis and extensive physiotherapy treatment in order to protect the muscles from rapid deterioration and development of contractures. The management of SMA must be as a part of a broad multi-disciplinary team which should include rehabilitation, spinal management, orthopaedics, nutritional and gastrointestinal management. Recently, it has been stated that SMA might be a multi-organ disease and more detailed examination should be performed. Further recommendations have been made on pulmonary management and acute care issues in the severe forms of spinal muscular atrophy[8].

Physiotherapy

  • Assessment of the patient with neuromuscular disease and particularly with SMA is of a great importance. Looking at baseline function, joint range and power will assist the physiotherapist to follow on the progression of the condition.
  • Orthotics
  • Splinting
  • Taping
  • Management of contractures
  • Exercise and activity

Respiratory Care

Many of the children and adults with Spinal Muscular Atrophy will be dependant on pulmonary management due to loss of muscle function. When a patient with SMA has a respiratory failure they need to be transferred on non-invasive positive pressure ventilation (NIV). In order, this to be implemented in the best possible way, a respiratory physiotherapist should be involved in the assessment and the management of pulmonary complications[9].

Airway clearance (chest physio) is best administered with the combination of Cough Assist and this should be the primary airway clearance therapy for all SMA patients with respiratory illness.

Suctioning is a critical part of the treatment and should be used in all patients with excessive secretions or in those with an ineffective cough.

The high frequency chest wall oscillation (Vest) is another option for managing secretions. However, there is no evidence that the Vest improve airway clearance and secretions.

Non-invasive positive pressure ventilation (NIV) should be used for respiratory failure or to prevent chest wall distortion.

Continuous positive airway pressure (CPAP) should be used only when NIV is not tolerated or in the treatment of chronic respiratory failure[9].

Differential Diagnosis

Neuromuscular conditions:

  • Duchenne Muscular Dystrophy
  • Charcot-Marie-Tooth
  • Amiotrophic Lateral Sclerosis

Resources and Useful Links

SMA REACH UK - Information Page

Spinal Muscular Atrophy Support UK

References

  1. 1.0 1.1 Darras BT, Markowitz JA, Monani UR, De Vivo DC. Chapter 8—Spinal Muscular Atrophies. Neuromuscular Disorders of Infancy, Childhood, and Adolescence (Second Edition). San Diego: Academic Press; 2015. p. 117–45
  2. Mercuri E, Bertini E, Iannaccone ST. Childhood spinal muscular atrophy: controversies and challenges. Lancet Neurol. 2012;11(5):443–52. pmid:22516079
  3. Spinal muscular atrophy/ nhs.uk/conditions/spinal-muscular-atrophy-sma
  4. Montes, J., et al. (2010). "Six-Minute Walk Test demonstrates motor fatigue in spinal muscular atrophy." Neurology 74(10): 833-838.[1]
  5. Ramsey D, Scoto M, Mayhew A, Main M, Mazzone ES, Montes J, et al. (2017) Revised Hammersmith Scale for spinal muscular atrophy: A SMA specific clinical outcome assessment tool. PLoS ONE 12(2): e0172346. doi:10.1371/journal. pone.0172346
  6. Wijnhoven, T. M., et al. (2004). "Assessment of gross motor development in the WHO Multicentre Growth Reference Study." Food Nutr Bull 25(1 Suppl): S37-45.
  7. Mazzone, E. S., Mayhew, A. , Montes, J. , Ramsey, D. , Fanelli, L. , Young, S. D., Salazar, R. , De Sanctis, R. , Pasternak, A. , Glanzman, A. , Coratti, G. , Civitello, M. , Forcina, N. , Gee, R. , Duong, T. , Pane, M. , Scoto, M. , Pera, M. C., Messina, S. , Tennekoon, G. , Day, J. W., Darras, B. T., Vivo, D. C., Finkel, R. , Muntoni, F. and Mercuri, E. (2017), Revised upper limb module for spinal muscular atrophy: Development of a new module. Muscle Nerve, 55: 869-874. doi:10.1002/mus.25430
  8. Mercuri E, Finkel RS, Muntoni F, et al. Diagnosis and management of spinal muscular atrophy: Part 1: Recommendations for diagnosis, rehabilitation, orthopedic and nutritional care. Neuromuscul Disord. 2017 Nov 23
  9. 9.0 9.1 Finkel, R. S., et al. (2018). "Diagnosis and management of spinal muscular atrophy: Part 2: Pulmonary and acute care; medications, supplements and immunizations; other organ systems; and ethics." Neuromuscul Disord 28(3): 197-207.