What Are Neuromuscular Disorders and Why Do They Cause Hypoventilation?
Neuromuscular disorders are conditions that affect the function of muscles due to problems with the nerves. When these problematic nerves affect the muscles responsible for respiration, the result is hypoventilation. Hypoventilation is breathing that is too slow to meet the needs of the body.
Respiration is a complex voluntary process that depends on the coordination between the muscles of respiration and the control center of the brain (medulla oblongata). The muscles responsible for respiration are-
The Diaphragm - It is a large dome-shaped muscle located just below the lungs and heart. The contraction and expansion of this muscle are essential to pull and push the air in and out of the lungs.
The Rib Cage Muscles - These comprise the internal and external intercostal muscles, which are essential for expiration and inspiration, respectively.
The Abdominal Muscles - These are accessory muscles responsible for the expiration (letting out air); some of them are the rectus abdominis, external oblique, internal oblique, etc.
The nerves that innervate the muscle mentioned above are-
Phrenic Nerve - It is a mixed nerve that originates from the spinal nerves of the neck and provides the primary motor supply to the diaphragm.
Vagus Nerve - It is the major sensory channel front the lungs to the brain. It controls pulmonary function and regulates respiration by providing sensory feedback from the lungs to the brain.
Posterior Thoracic Nerves - Three nerve roots (T3, T4, T5) of the thoracic spine innervate into the chest wall and aid in respiration.
In certain neuromuscular disorders, the nerves mentioned above do not function properly, leading to respiratory muscle weakness, which in turn causes hypoventilation.
What Are the Neuromuscular Disorders That Cause Hypoventilation?
The neuromuscular conditions that have been known to cause respiratory muscle weakness are-
Amyotrophic Lateral Sclerosis - It is a neurological condition caused due to mutations (alteration) in the SOD1 gene along with environmental factors like infection, physical trauma, diet, etc.
The condition primarily affects the nerve cells responsible for controlling voluntary muscle movement. Early symptoms of the condition include muscle weakness and stiffness; over time, the individual will lose the ability to speak, move, and breathe.
Myasthenia Gravis- It is an autoimmune condition, which means the immune system, which is supposed to protect the body from foreign objects, mistakenly attacks itself. The condition causes weakness in the skeletal muscles (those responsible for breathing, walking, speaking, etc.) that worsens over periods of activity and improves after periods of rest.
Occasionally, the muscles responsible for breathing might weaken to the point where the affected individual will need a ventilator to help them breathe. This condition is known as a myasthenic crisis.
Gillian-Barrie Syndrome- Also an autoimmune disorder, but this one causes inflammation of the nerves leading to muscle weakness, paralysis, and other associated symptoms. Emergency symptoms include temporary respiratory arrest, difficulty in breathing and swallowing, drooling, fainting, and feeling light-headed.
Multiple Sclerosis (MS)- It is a chronic central nervous system disorder characterized by multiple areas of nerve demyelination (damage to the myelin sheath, which is protecting covering around the nerves) that impair nerve conduction.
Respiratory dysfunction frequently occurs in individuals with advanced MS and can manifest as hypoventilation, respiratory failure, sleep-disordered breathing, etc.
Duchenne’s Muscular Dystrophy (DMD)- It is a genetic disorder characterized by progressive muscle degeneration and weakness due to alterations of a protein called dystrophin that helps keep muscle cells intact.
Muscle weakness is the primary symptom, first affecting the lower external muscles, followed by the upper external muscles. Later on, the heart and the respiratory muscles are affected as well. As the condition progresses, it will lead to impaired pulmonary function, eventually leading to hypoventilation and respiratory failure.
Polymyositis- It is a group of muscle diseases characterized by progressive muscle inflammation and predominantly muscle weakness. Patients typically will have early respiratory difficulty, motor dysfunction, and dysphagia (difficulty in swallowing).
Mitochondrial Myopathies- These are a group of neuromuscular conditions caused by damage to the mitochondria (a small energy-producing structure in the cell). Respiratory failure in mitochondrial myopathies occurs at the later stages of the disease and is associated with the deterioration of respiratory muscle weakness.
How Is Hypoventilation Managed in Neuromuscular Disorders?
In patients with neuromuscular disorders suspected of having respiratory muscle weakness, treatment begins with performing objective testing, which can include pulmonary function tests (PFTs), tests for respiratory muscle strength, and arterial blood gas analysis. Diagnostic testing helps identify patients who need specific therapies.
Patients with respiratory muscle weakness will be placed on any of the following therapies based on the severity of the conditions-
Medication- Several drugs can be used to treat hypoventilation, most of them producing the desired effect by stimulating the central respiratory drive or by reversing the effects of other medications that can depress the central respiratory drive.
A few examples are- bronchodilators such as beta-agonists (e.g., Albuterol), anticholinergic agents (e.g., Atropine), and methylxanthines (e.g., Theophylline). Theophylline also improves the diaphragm muscle contractility and stimulates the respiratory center.
Non-Invasive Ventilation (NIV)- It is considered in patients with neuromuscular weakness and acute respiratory failure. In NIV, ventilatory support will be given through the patient's upper airway using a mask or a similar device.
Invasive Ventilation- It is preferred for patients with chronic respiratory failure and sometimes with acute respiratory failure. This procedure requires access to the trachea through a surgically placed endotracheal tube.
It helps stabilize patients with hypoxemic and hypercapnic respiratory failure, decreases inspiratory work of breathing, and redistributes blood flow from exercising respiratory muscles to other tissues.
Most of the patients with respiratory failure will be admitted to the intensive care unit (ICU), where they will be provided with the following additional supportive measures apart from ventilation-
Sedation and analgesia.
Venus thromboembolism (blood clots in the veins) prophylaxis.
Prevention of ventilatory-assisted pneumonia.
Apart from the above-mentioned management techniques, therapies that aid in addicted cough and strengthening respiratory muscles can also help the patient to a certain extent, but they do not reverse the progressiveness of the underlying neuromuscular condition.
Hypoventilation is an associated symptom of some neuromuscular conditions. The respiratory muscle weakness caused by these conditions, along with bulbar dysfunction and weak cough, will lead to frequent aspiration, lung infections, and respiratory failure and contribute to morbidity and mortality in these patients. Early recognition of patients at risk for respiratory complications is important for the provision of appropriate care, which will delay disease-associated morbidity and mortality.