Respiratory Muscle Weakness in Neuromuscular Diseases

Introduction:

Respiratory muscle weakness is one of the most severe complications for patients with neuromuscular disorders. The incidence of this condition may vary with the underlying diseases. However, most patients with neuromuscular diseases such as amyotrophic lateral sclerosis (a neuromuscular disease that weakens the muscles and impairs physical function) may die from chronic respiratory failure.

What Are the Muscles of Respiration and Cough?

The primary muscles involved in the respiratory action include:

• The inspiratory muscles are the diaphragm, external intercostals, trapezii, sternocleidomastoids, and scalenes.

• Expiratory muscles include rectus abdominis, external and internal obliques, transverse abdominis, and internal intercostals.

• Upper airway muscles include the tongue, lips, palate, glottis, pharynx, and larynx.

What Is the Mechanism of Cough?

A cough has three sequential phases, which include:

Inspiratory Phase:

• A rapid and large tidal volume (amount of air that moves in and out of the lungs during each respiratory cycle) is generated through an open glottis in this phase.

• The weakness in the inspiratory muscles decreases the tidal volume inspiration and restricts or limits the volume and flow of gas during the expiratory phase.

• The opening of the glottis is restricted due to the weakness of the inspiratory muscles.

Compressive Phase:

• The compressive phase presents with the closure of the epiglottis, which results in the increase of positive intrathoracic pressure required for expectoration.

• The inadequate closure of the glottis is due to the weakness of the upper respiratory muscles.

• The inspiratory phase is less effective, and a poor compressive degree is observed due to inadequate glottis closure.

Expiratory Phase:

• In the expiratory phase, the glottis opens, which allows high peak expiratory flow due to the sudden release of the gas.

• The compression of the airway dislodges the mucus adhering to the lining of the airways.

• The weakness of the expiratory muscles controls the increase in positive intrathoracic pressure, which reduces the peak expiratory flow and the compression of the airways.

What Are the Clinical Manifestations of Respiratory Muscle Weakness in Neuromuscular Disease?

The clinical manifestations of respiratory muscle weakness present with various signs and symptoms, and these include:

• Inadequate or Hypoventilation:

Inadequate ventilation during sleep is the major clinical manifestation in patients with chronic respiratory muscle weakness. It is also referred to as nocturnal hypoventilation. If the condition is more severe or acute, the symptoms of hypoventilation can also present during waking hours.

• Nocturnal Hypoventilation-

This type of hypoventilation may present with various symptoms, which are caused due to bulbar dysfunction and decreased inspiratory muscle actions during rapid eye movement (REM) sleep. The symptoms include:

• Daytime hypersomnolence.

• Headache, particularly in the morning.

• Fatigue.

• Choking during sleep

• Impaired cognition.

• Insomnia (rarely).

• Awake Hypoventilation-

Inadequate ventilation during waking hours is referred to as awake hypoventilation. The symptoms include:

• Orthopnea (sensation of breathlessness).

• Dyspnea (shortness of breath).

• Use of accessory respiratory muscles.

• Thoracoabdominal paradox (during inspiration, the abdomen is moved inwards and becomes worse in the supine position)

• Rapid shallow breathing.

The primary cause of daytime hypoventilation is the weakness of the inspiratory muscles. It leads to a decrease in the tidal volume and increases respiratory frequency to balance and maintain alveolar ventilation.

• Bulbar Dysfunction-

• It is caused due to the dysfunction of the upper airway muscles of the tongue, lips, palate, pharynx, larynx, and glottis.

• A patient may have these symptoms acutely with aspiration and chronically with nocturnal hypoventilation.

• The symptoms include:

• Dysphagia (difficulty in swallowing).

• Dysarthria (speech disorder due to weakness of speech muscles).

• Drooling.

• Facial weakness.

• Poor mastication.

• Abnormal secretion clearance.

• Protrusion of tongue.

• Nasal speech.

• Ineffective Cough:

• The weakness of inspiratory, expiratory, and upper airway muscles results in a poor cough.

• Patients with poor cough may predispose to pneumonia, retention of secretions, and aspiration.

What Are the Various Diagnostic Approaches to Respiratory Muscle Weakness in Neuromuscular Disorders?

The diagnosis of respiratory muscle weakness in neuromuscular disorders is performed with various tests, and these include:

• Pulmonary Function Testing-

The pulmonary function testing includes:

• Spirometry Findings:

  • The typical finding is the restriction pattern in patients with respiratory muscle weakness in spirometry.

  • There is a reduced forced expiratory volume in one second, and vital and forced necessary capacities are observed.

  • In the case of expiratory muscle weakness, the expiratory reserve volume (amount of extra air exhaled during forceful breath out) is reduced, and residual volume (volume of air that remains in the lungs after forceful expiration) is increased.

  • The value of vital capacity is often marked as an indicator of respiratory muscle weakness.

  • The essential capacity refers to the maximum volume of gas expelled from the full inspiration, except the individual exhales with maximal speed and effort. In cases of concurrent obstruction, the vital capacity records are higher in relation to the degree of obstruction.

• Vital Capacity in the Upright and Supine Position or Reduced Forced Vital Capacity:

  • In the supine position, performing pulmonary function tests is challenging.

  • A supine forced vital capacity or vital capacity that is more than ten percent but lower than an upright forced vital capacity or vital capacity indicates the diagnosis of respiratory muscle weakness.

  • A decrease in a vital capacity greater than 30 percent in the supine position compared to the upright posture is a marker of isolated or disproportionate bilateral diaphragm weakness and is more reliable with the maximal inspiratory pressure in this condition.

  • The vital capacity usually falls late in the neuromuscular disease progression, whereas the maximal inspiratory pressure falls earlier and corresponds with the disease progression.

• Preserved Diffusing Capacity for Carbon Monoxide (Dlco) -

  • Diffusing capacity for carbon monoxide is typically preserved in patients with neuromuscular weakness, with no coexisting pulmonary parenchymal, vascular disease or obstructive disease resulting in the ventilation or perfusion mismatch.

• Reduced Maximum Voluntary Ventilation-

  • Maximum voluntary ventilation refers to the volume of air that is maximally or rapidly inhaled and exhaled within one minute.

  • This value is usually reduced in patients with respiratory muscle weakness and is most accurate when measured during exercise.

• Respiratory Muscle Strength Testing:

Patients with suspected respiratory muscle weakness, maximum expiratory pressure (MEP) and maximum inspiratory pressure (MIP), and sniff nasal inspiratory pressure (SNIP) are performed.

• Maximal Inspiratory and Expiratory Pressure (MIP and MEP) -

  • Maximal inspiratory pressure is reduced in patients with inspiratory muscle weakness.

  • In patients with expiratory muscle weakness, maximal expiratory pressure is reduced.

• Sniff Nasal Inspiratory Pressure -

  • In patients with inspiratory muscle weakness, the sniff nasal inspiratory pressure is typically lower than expected.

  • It is especially useful in patients with bulbar dysfunction who have difficulty creating a seal with the mouthpiece for performing pulmonary function tests.

  • It is not indicated in patients with hyperinflation due to severe airway obstruction.

• Other Tests:

• Sniff Esophageal and Gastric Pressure -

  • Gastric pressure is measured using a pressure catheter in the stomach following maximal cough efforts, which helps in analyzing expiratory muscle strength.

  • Sniff esophageal pressure is best indicated in patients with hyperinflation due to obstructive airway disease. It helps to measure overall inspiratory muscle strength.

• Arterial Blood Gas Analysis-

  • It helps to determine the presence of daytime hypercapnia.

  • If hypercapnia is present, it is necessary to evaluate whether it is acute or chronic. It is an important investigation in patients with respiratory muscle weakness.

• Sleep Study -

  • In patients with sleep-related hypoventilation or sleep-related symptoms are diagnosed, respiratory muscle weakness is confirmed through a sleep study.

  • Most patients with neuromuscular disease experience hypoventilation during sleep.

Conclusion:

Patients with neuromuscular disorders are at an increased risk of respiratory complications during the disease. Therefore, early recognition and a multidisciplinary approach should be performed in order to provide the best optimal treatment. Non-invasive positive pressure ventilation and advanced cough management strategies are the most standard care in neuromuscular disease as they help improve sleep, survival, and quality of life.