Russell J. Miller and John Scott Parrish
NEUROMUSCULAR DISEASES
Neuromuscular disorders can affect multiple aspects of respiratory function. This heterogeneous group includes disorders affecting motor neurons (e.g., poliomyelitis, amyotrophic lateral sclerosis, Guillain–Barré syndrome [GBS]), neuromuscular junctions (e.g., myasthenia gravis, Eaton–Lambert syndrome, botulism), and skeletal muscles (e.g., muscle dystrophy, drug-induced myopathy, polymyositis). Although the pathophysiology of these diseases can vary greatly, the expected morbidity is relatively predictable and directly related to respiratory insufficiency. There are three major muscle groups required for ventilation: inspiratory, expiratory, and bulbar muscles. The inspiratory muscles, with primary responsibility for normal tidal breathing, include the diaphragm, external intercostals, and accessory muscles, such as the sternocleidomastoid and scalene, which are used in stressed inhalation. Expiration is normally a passive process, but forced expiration used with coughing, exercise, or spirometry testing is performed primarily by the abdominal and internal intercostal muscles. The bulbar muscles are responsible for cough, swallowing, and airway protection.
Hypoventilation
Hypoventilation in neuromuscular disorders is primarily related to respiratory muscle weakness as reflected by pulmonary function tests (PFT) demonstrating reduction in vital capacity (VC), tidal volume, and minute ventilation. Hypoventilation may be further complicated by stiffening of the chest wall leading to decreased thoracic compliance. With exertion, patients will often develop a rapid shallow breathing pattern as a mechanism to maintain minute ventilation as a result of reduced ability to generate adequate tidal volumes. Respiratory muscle fatigue from this pattern of breathing can result in air hunger and hypercapnia from increased dead space ventilation. Dyspnea at rest and air hunger are actually late findings in many of the neuromuscular diseases due to the patient’s adjustment to sedentary lifestyle associated with the underlying disorder. Pulmonary function testing is typically characterized by restriction, although the pattern varies depending on which muscles are affected (inspiratory and/or expiratory). VC can be reduced either due to a loss of inspiratory capacity (reduced total lung capacity), expiratory reserve volume (elevated residual volume), or both. Functional residual capacity (FRC), the endtidal lung volume representing the balance of static lung and chest wall elastic recoil, may be unaffected. The gold standard for evaluation of diaphragmatic strength is measurement of trans-diaphragmatic pressure, but this requires the use of an esophageal balloon, is somewhat invasive, and is not commonly used. Other less-invasive tests for assessing respiratory muscle weakness include the maximum inspiratory pressure (MIP) and maximum expiratory pressure (MEP). To perform these tests, the patient is encouraged to inspire or expire with maximal effort against an occluded external airway from residual volume (MIP) or from total lung capacity (MEP). The maximum pressure generated is recorded with a manometer. The MIP and MEP can be abnormal early in the course of disease, even when the static lung volumes are normal. These maneuvers, however, can be technically difficult. A normal MIP and MEP is useful in excluding clinically significant respiratory muscle weakness, but values are often falsely low and hence lack specificity. The VC obtained during routine, upright spirometry may miss or underestimate significant diaphragmatic weakness since restrictive changes are most pronounced in the supine position. Daytime hypercapnia (Pco2 > 45) is an ominous sign and usually predicts development of overt respiratory failure.
Chronic hypoventilation in patients with neuromuscular disease is typically treated with noninvasive ventilatory support. Although tracheostomy is a tempting means to aid daytime hypoventilation, the complications of chronic invasive ventilation often outweigh the benefits. These complications include increased risk of developing ventilator-associated infections and reduced quality of life. In patients with intact bulbar muscle function, inspiratory and expiratory muscle support can be provided with intermittent noninvasive ventilation. The most commonly used method of noninvasive positive pressure ventilation for daytime use is through a mouth piece attached to either a bi-level pressure or portable volume ventilator. Patients can receive ventilator-assisted breaths as needed by creating a lip seal around the mouth piece and making a “sipping” effort to trigger the ventilator. Alternatively, positive pressure ventilation can be administered with a nasal interface that provides the advantage of delivering continuous ventilation; however, mouth air leaks and skin breakdown are significant problems that can limit its effectiveness.
Airway Clearance and Secretions
Respiratory infections in patients with neuromuscular disorders are common and result from a combination of inability to generate adequate cough and recurrent aspiration. Coughing, an essential protective mechanism that helps to clear mucus-trapped particles and maintain patent airways, produced by a short, deep inspiration followed by forceful exhalation against a transiently closed glottis. Neuromuscular weakness can impair cough through several mechanisms. Reduced lung expansion and/or force of exhalation results in inability to generate the intrathoracic pressures necessary to achieve adequate airway clearance. Patients with isolated bulbar dysfunction will have impaired cough due to inability to adequately clear upper airway secretions and protect the airway from aspiration. Patients with abnormal cough may not be symptomatic until they develop respiratory infection. Peak cough flow can be measured easily with a peak flow meter connected to a mouthpiece. A peak cough flow of less than 160 L/minute has been shown to correlate with extubation failure in patients with neuromuscular disease. In non-ventilated patients, baseline peak cough flow less than 270 L/minute is associated with a reduction in peak cough flow to less than 160 L/minute during respiratory infections. Abdominal thrust-assist maneuvers performed by caregivers can aid in airway clearance in patients with expiratory muscle weakness. With inspiratory muscle weakness breath stacking maneuvers can help patients obtain inflation volumes necessary to generate an adequate cough. A variety of commercial devices are also available to aid in airway clearance by augmenting mechanical insufflation and exsufflation. These devices include the “pneumobelt,” which provides intermittent abdominal pressure, and mechanical in-exsufflators (MI-E), which simulate a normal cough by delivering positive-pressure insufflation followed by expulsive exsufflation through a nasal mask. In patients with bulbar dysfunction, these simple therapies are of limited value, and tracheostomy may be indicated when persistent hypoxemia develops due to inability to clear airway secretions.
Sleep-Related Problems
Sleep-related breathing problems are common in patients with neuromuscular diseases and generally precede awake manifestations of overt respiratory failure. Normal rapid eye movement (REM) sleep results in reduced activity of the intercostal and accessory muscles of respiration causing a drop in minute ventilation with an associated decrease in oxyhemoglobin saturation and rise in carbon dioxide. Causes of this physiologic sleep-induced hypoventilation include loss of a “wakefulness drive” (from reduced chemoreceptor and mechanoreceptor responsiveness) and concurrent pharyngeal muscle relaxation, which increases upper airway resistance. Patients with neuromuscular impairment are at risk for upper airway obstruction (obstructive sleep apnea), exaggerated sleep-related hypoventilation, or both, depending on the relative strength of the pharyngeal dilator muscles and diaphragm. Patients with diaphragmatic paralysis tend to hypoventilate during sleep, while patients with predominantly bulbar weakness are more likely to develop upper airway obstruction. In these patients, nocturnal hypercapnia often precedes and predicts the development of daytime hypercapnia and subsequent ventilatory failure.
Predictors of nocturnal hypoventilation in patients with neuromuscular diseases include daytime sleepiness (assessed with standard questionnaires such as the Epworth Sleepiness Scale) and daytime supine inspiratory VC of less than 60% predicted. It should be noted, however, that in 15% of patients with neuromuscular disease unsuspected nocturnal hypoventilation can be found during polysomnography. Use of nocturnal positive pressure ventilation in patients with neuromuscular disease has been shown to produce sustained daytime effects including normalization of Pco2 and improved medical quality of life and survival. It is not completely clear why noninvasive nocturnal ventilation has such profound effects. Proposed mechanisms include improved ventilatory mechanics, resting fatigued muscles, and improvement of the blunted chemoreceptor response to hypercapnia, which occurs secondary to sleep deprivation.
Guillain–Barré Syndrome
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