Introduction
Spinal muscular atrophy is a common neuromuscular disorder that causes bilateral weakness and atrophy of the proximal muscles. The respiratory muscles are the intercostal muscles, and the diaphragm is relatively spared. As a result of the muscle weakness and atrophy, the affected patient has impaired cough reflex, and the airway secretions are not cleared, which translates into recurrent respiratory infections, poor chest wall, and lung growth, which will lead to restrictive lung disease (decline in the total volume of air that the lungs can hold), nocturnal hypoventilation (shallow breathing at night, occurs as a complication of neuro-muscular disorders) and eventually into respiratory failure.
How Is Respiratory Function Monitored?
Systematic monitoring of respiratory muscle function is necessary for patients with spinal muscular atrophy to foresee any respiratory complications, like respiratory failure, and provide appropriate clinical care accordingly.
Systematic monitoring is done in a clinical setup with non-invasive procedures, such as:
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Vital capacity (the maximum amount of air one can expire after maximum air intake).
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Peak expiratory flow (maxim speed with which air is expired).
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Maximal static pressure (the measurement of maximum mouth pressure against an occluded airway).
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Sniff nasal inspiratory pressure (pressure measured through an occluded nostril while sniffing through the other nostril).
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Peak cough flow (it is the maximum airflow during a cough).
All the procedures mentioned above are simple and easily available, thus, are commonly used. In young children, invasive tests are employed to check for the performance of respiratory muscles because of poor cooperation and extreme respiratory muscle weakness. For detecting nocturnal hypoventilation, sleep studies are performed, and nocturnal gas exchange is monitored overnight. When the strength of respiratory muscles falls below 50 % or if the patient suffers from repeated respiratory infections, training is given for assisting cough so that the secretions are expectorated from the airways. Non-invasive ventilation (NIV) should be established in children with isolated cases of nocturnal hypoventilation. Pediatric management of respiratory involvement in spinal muscular atrophy involves a multidisciplinary (respiratory and neurology) approach for the best care of children.
What Is Spinal Muscular Atrophy (SMA)?
Spinal Muscular Atrophy is a common neuromuscular disorder characterized by a progressive decline in motor function. The incidence of SMA is one in 6000 to one in 10,000 live births and is the leading cause of mortality in toddlers due to genetic reasons. It is associated with muscle weakness and atrophy that results from the progressive destruction of specialized neurons that controls voluntary movements. SMA is divided into five major types based on the age when the symptoms begin to appear; they are:
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Type 0.
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Type 1.
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Type 2.
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Type 3.
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Type 4.
Of these, type 0 is the most severe form, with the onset of disease before birth, and type 4, is the mildest form, where the symptoms appear in adulthood only.
How Does Spinal Muscular Atrophy Affect the Respiratory System?
Spinal muscular atrophy types 0, 1, and 2 are commonly associated with severe respiratory problems in children and are the main cause of morbidity and mortality among children with these disease types.
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SMA affects the intercoastal muscles and results in their weakness and atrophy, leaving the diaphragm to manage the role of the main breathing muscle single-handedly. As a result, the patient will have difficulty in breathing. The lung and the chest wall will be underdeveloped, and a restrictive lung will be prone to recurrent and severe respiratory infections. Due to weakness in the muscle, the cough reflex will be poor, accumulating secretions from the airways, increasing the risk of infection and hypoventilation.
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Muscle weakness will also lead to another serious complication known as aspiration pneumonia. It is a lung infection that occurs due to the entry of food or liquid into the lungs because of the poor coordination of deglutition with the closure of the airway as a result of throat muscle weakness.
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Hypoventilation and difficulty in breathing worsen at night. When a patient lies down, most muscles will relax, and the contents in the abdomen are pushed up against the diaphragm. Since the diaphragm acts as the main breathing muscle in SMA, the patient will face additional difficulty taking deep breaths when lying down, which may lead to sleeping difficulties, such as obstructive sleep apnea.
How to Manage Respiratory Concerns in Spinal Muscular Atrophy (SMA)?
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Respiratory muscle weakness and atrophy are significant problems in SMA. It is the commonest cause of mortality in SMA types 0,1 and 2 but not the only cause. Due to respiratory muscle weakness, air movement into and out of the lungs does not occur smoothly and results in general adverse health. Respiratory muscle weakness will display signs such as headaches, insomnia, tiredness, and frequent yawning in the daytime due to sleep deprivation, poor concentration, difficulty lying down flat, lung infections, and eventually respiratory failure and heart damage.
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Appropriate ventilation is required for an SMA patient with respiratory muscle weakness. The invention of portable ventilation devices in recent years has made it easier for infants with SMA and has helped extend life to a great extent for some. Assisted ventilation is also used in various SMA forms in children and adults.
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Invasive procedures are used when sufficient ventilation is not feasible through non-invasive techniques. Tracheostomy is one such procedure to assist in ventilation. In this procedure, a surgical hole is created in the trachea, or the windpipe, and a tube are inserted through which air is delivered under pressure. Once the patient gets adapted to the tracheostomy tube, they can talk, eat and drink comfortably with the tube.
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Another thing that needs to be taken care of is the excess secretions in the respiratory passage; they need to be cleared with a mechanical device to prevent the onset of infections. One such device is an insufflator- that will help clear the secretions from the airway, mimicking a natural cough mechanism. It first creates a positive pressure in the airway and then immediately reverses to negative pressure expectorating the secretions. A high-frequency Chest Wall Oscillation Device is another mechanical clearance aid. It is made up of a vest worn around the chest that inflates and deflates rapidly. It will create vibrations in the chest resulting in mini coughs, which will help expectorate the secretions.
Conclusion
Respiratory involvement in spinal muscular dystrophy plays an important role in morbidity and mortality in children. A multidisciplinary pediatric approach involving a team of pulmonologists, neurologists, physiotherapists, and experienced home non-invasive ventilation (NIV) assistance is necessary to obtain optimal respiratory care. A regular and systematic evaluation of the functioning of respiratory muscles is crucial for effectively managing possible respiratory complications.
