Neuromuscular and Other Diseases of the Chest Wall

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Chapter 29

Neuromuscular and Other Diseases of the Chest Wall

Rendell w Ashton

Chapter Objectives

Identify pulmonary function test results seen in patients with neuromuscular disease.

List the potential respiratory complications associated with neuromuscular disease.

Identify the clinical signs and symptoms associated with respiratory muscle weakness.

Describe techniques for monitoring patients with respiratory muscle weakness.

Describe general respiratory care management of patients with respiratory muscle weakness.

Describe the clinical findings and treatment for each of the following neuromuscular disorders: Duchenne muscular dystrophy, myotonic dystrophy, polymyositis, myasthenia gravis, Lambert-Eaton syndrome, Guillain-Barré syndrome, unilateral diaphragmatic paralysis, amyotrophic lateral sclerosis, critical illness myopathy and polyneuropathy, spinal cord injury, stroke, traumatic brain injury, kyphoscoliosis, and flail chest.

The respiratory system comprises the lungs, which provide an interface between inhaled air and circulating blood, mediating gas exchange; the thoracic cage, which forms the structure of the ventilatory pump; and the muscles of respiration, which are linked to respiratory centers in the brainstem by nerves exiting the spinal column and whose action on the thoracic cage produces movement of air into and out of the lungs. Understanding the interactions between these components is essential to understanding how their dysfunction leads to disease. The neuromuscular organization of the components of the respiratory system is shown in Figure 29-1. Maintenance of normal ventilation depends critically on intact, functional components of the neuromuscular system, which contribute to breathing in three principal ways: (1) regulation of respiratory drive and rate, (2) control of the mechanics of ventilation, and (3) cough and other airway protection. Diseases that affect the brain, nerves, muscles, or thoracic cage can lead to respiratory failure or hypoxemia even if the lungs are normal. The pulmonary consequences of neuromuscular disease include the following:

FIGURE 29-1 The neuromuscular components of the respiratory system include elements of the cortex (which allow conscious alteration of breathing) and motor centers (which maintain upper airway tone). Brainstem structures receive input from peripheral oxygen, pH, and stretch receptors and generate automatic respiration. Efferent nerves carry central nervous impulses to the muscles of respiration through the phrenic and spinal nerves, which drive the muscles of respiration.

1. Dysregulation of respiratory drive or rate

• Hyperventilation

• Hypoventilation

• Central apnea

• Other pathologic breathing patterns (listed in Table 29-6)

2. Loss of strength or control of the mechanics of breathing

• Respiratory failure owing to excessive work of breathing

• Atelectasis leading to hypoxemia

• Secondary effects of chronic hypoxemia (e.g., cor pulmonale, pulmonary hypertension)

3. Loss of strength or control of the muscles that produce cough and protect the airway

Some systemic diseases that affect the neuromuscular system also cause interstitial lung disease, which can lead to considerable respiratory dysfunction (see Chapter 24). Respiratory failure, often associated with pulmonary infection, is a frequent cause of death in patients with neuromuscular disorders.

A thorough understanding of the physiology of ventilation and chest wall mechanics (see Chapters 10 and 18) is needed to help the reader understand how abnormalities of the upper airway, chest wall, diaphragm, and abdominal muscles cause disease. This chapter reviews major disorders of the neuromuscular and skeletal systems that affect breathing. Disorders are grouped according to which functional unit of the neuromuscular system is affected, focusing on pulmonary manifestations of these disease processes (Table 29-1).

TABLE 29-1

Locations at Which Several Neuromuscular Diseases Affect the Respiratory System

Location	Disease
Cortex and upper motor neurons	Stroke, traumatic brain injury
Spinal cord	Trauma, transverse myelitis, multiple sclerosis
Anterior horn cells (lower motor neurons)	ALS, spinal muscular atrophy, poliomyelitis and postpoliomyelitis
Peripheral nerves	GBS, critical illness polyneuropathy, Lyme disease
Neuromuscular junction	MG, LES, botulism
Muscle	DMD, polymyositis, acid maltase deficiency
Interstitial lung disease*	Polymyositis, dermatomyositis, tuberous sclerosis, neurofibromatosis

^*A category of systemic diseases that can affect neuromuscular function.

General Principles Relating To Neuromuscular Weakness Of The Ventilatory Muscles

This section describes the evaluation and testing of patients with suspected neuromuscular weakness of the respiratory muscles, regardless of the disease causing the weakness.

Pathophysiology and Pulmonary Function Testing

Weakness of the respiratory muscles leads to the inability to generate or maintain normal respiratory pressures. Pulmonary function testing in patients with neuromuscular weakness typically reveals a restrictive ventilatory defect even if the lungs are normal. Vital capacity (VC), forced expiratory volume in 1 second (FEV₁), and total lung capacity (TLC) are decreased. Residual volume (RV) is normal or decreased, and diffusing capacity corrected for alveolar volume is normal or near-normal but has been reported to be decreased.¹ Comparison of spirometric results obtained with the patient in seated and supine positions can be useful in showing that orthopnea is caused by neuromuscular weakness. A decrease in VC or FEV₁ of greater than 20% when a patient moves from the seated to the supine position suggests diaphragmatic weakness (Table 29-2 and Figure 29-2). The inability to generate normal respiratory pressures is reflected in a decreased maximal inspiratory pressure (PImax). Expiratory muscle weakness is characterized by a decreased maximal expiratory pressure (PEmax).

TABLE 29-2

Pulmonary Function Testing Results from a Patient With Profound Diaphragm Weakness

	Predicted Value	Lower Limit of Normal	Sitting Position	% of Predicted	Supine Position	% Change from Sitting
FVC	4.42	3.55	1.85	42	0.89	−52
FEV₁	3.36	2.62	1.51	45	0.68	−55
FEV₁/FVC	75.88	66.20	81.75	108	75.76	−7
TLC	6.53	4.92	4.21	64
RV	2.10	1.34	2.39	114
DLCO	24.93	16.67	17.16	69
DLCO/VA	3.88	2.38	5.57	144
PImax	110.58	75.02	18.46	17
PEmax	207.29	140.04	26.52	13

FIGURE 29-2 Normal flow-volume loop compared with loops from a patient with neuromuscular weakness, showing characteristic ventilatory restriction, which worsens when the patient is placed in the supine position.

Arterial blood gases in the setting of a rapid, shallow breathing pattern may show a decreased PaCO₂, although progressive inspiratory muscle weakness leads to hypoventilation and hypercapnia. Hypoxemia can occur in patients unable to take deep breaths and may be caused by microatelectasis, which leads to ventilation/perfusion mismatching within the lung and a resulting decrease in PaO₂ (Figure 29-3). Chronic hypoxemia in this situation may be protective against acute respiratory muscle failure. Experimental data suggest that when hypoxemia is chronic, it may increase diaphragm muscle endurance.² Hypoventilation that occurs with progressive neuromuscular disease may be a protective mechanism that avoids acute respiratory muscle fatigue. However, when hypoxemia is acute, it potentiates respiratory muscle fatigue, hastening respiratory failure.³

FIGURE 29-3 Atelectasis as a mechanism of hypoxemia in patients with respiratory muscle weakness. , Ventilation/perfusion.

Rule of Thumb

Neuromuscular weakness of the respiratory muscles may be present before any substantial decrease in VC or FEV₁ is noticed. Values of PEmax may be decreased by 50% or more before any decrement in VC or FEV₁ is noticed.

Clinical Signs and Symptoms

In the early stages of neuromuscular disease, patients with respiratory muscle weakness initially report exertional dyspnea and fatigue. As the disease process progresses, patients may complain of orthopnea or symptoms of cor pulmonale. These symptoms occur because the muscles involved with respiration can no longer generate or maintain normal ventilation. The response to hypoxemia and respiratory drive, measured with airway occlusion pressure (P0.1), is preserved in most patients with neuromuscular weakness.⁴ Because these patients do not have the strength to take deep breaths, they increase minute ventilation by increasing respiratory rate and adopting a rapid, shallow breathing pattern, which uses less respiratory muscle strength but provides less efficient ventilation. Patients with poor inspiratory muscle function (especially diaphragm weakness) may have marked orthopnea and prefer to sleep in a seated position. They may also experience a decline in the volume and power of voice or voice quality. Progressive muscle weakness can progress to the point where adequate ventilation is no longer maintained, and hypercapnia occurs.

Mini Clini

Consider Neuromuscular Weakness When a Patient Complains of Dyspnea

Problem

A 27-year-old woman was referred to the pulmonary clinic for persistent shortness of breath for 2 months, along with a sore throat and a hoarse voice. The symptoms had begun with a viral syndrome including sore throat, myalgias, and fever, progressing to cough and chest discomfort. All symptoms except her shortness of breath and hoarseness had resolved. She complained of being particularly short of breath while lying down and not being able to lie down on her left side at all because she felt she could not breathe in that position. She also described running out of breath in the middle of sentences and of her voice having a hoarse, “airy” quality. She had been evaluated previously, including a normal chest x-ray and spirometry, and had been told her dyspnea was due to anxiety. No other diagnosis was made. What clues in her history might suggest a pulmonary disease, and what additional testing could help identify it?

Discussion

This patient had already been evaluated and had been told that her dyspnea was psychologic in nature because her chest x-ray and standard pulmonary function testing were normal. Spirometry was within the normal range of values, but when repeated in the supine position, FEV₁ decreased by 41%. Lung volumes were normal except for a slight elevation of residual volume. Diffusing capacity was also normal. A fluoroscopic sniff test, in which her diaphragms were visualized in real time as she performed a simple sniff maneuver, showed paradoxical motion of the right hemidiaphragm. She was diagnosed with neuralgic amyotrophy, a rare neuromuscular condition affecting the phrenic nerve, which is often triggered by a viral syndrome and which usually improves gradually with time.

The important clue that led to the diagnosis of neuromuscular weakness was her complaint of orthopnea, especially the inability to breathe comfortably on one side or the other. When one diaphragm is weak or paralyzed, a patient often cannot breathe when lying on the opposite side because this prevents the good side from compensating for the weak side. When the details of her history were recognized as classic symptoms of diaphragm weakness, testing to establish the diagnosis was straightforward.

Problem

What physical findings may suggest respiratory distress in a patient with diaphragmatic weakness?

Discussion

Patients whose diaphragmatic strength is inadequate to meet their ventilatory needs may use accessory muscles of inspiration. The sternocleidomastoid, intercostal, and scalene muscles all may be found active in the setting of respiratory distress. Use of these muscles in the setting of a weak or paralyzed diaphragm can lead to cephalad movement of the diaphragm during inspiration that is accompanied by paradoxical inward movement of the abdomen during inspiration (which is called paradoxical breathing). The presence of these signs in this patient should prompt evaluation of ventilatory adequacy and the need for ventilatory support.

Rule of Thumb

When a patient complains of immediate shortness of breath on lying down, especially if one side is worse than the other, the problem is diaphragm weakness until proved otherwise.

Monitoring and Assessing Patients With Muscle Weakness for Respiratory Insufficiency

If respiratory muscle weakness progresses and cannot be stopped, the eventual result is respiratory failure. The onset of respiratory failure is acute or chronic depending on the time course of the disease process and the circumstances of the patient. When this progression toward respiratory failure is noted, careful follow-up and monitoring of symptoms and pulmonary function are necessary for assessment of the need for mechanical ventilation.

Monitoring the ventilatory function of a patient with neuromuscular weakness can involve repeated measurement of inspiratory pressure, VC, and arterial blood gas values. Depending on the condition, the need for mechanical support can be signaled by reaching either a critical value on testing or an overall clinical condition that does not allow unassisted ventilation to continue. Additional measurements that have been suggested to be more indicative of early respiratory insufficiency, at least in amyotrophic lateral sclerosis (ALS), include maximal sniff nasal inspiratory force ⁵ and nocturnal oximetry.⁶

At least two caveats to this general approach should be mentioned. First, patients with myasthenia gravis having an acute myasthenic crisis may have normal test results within minutes of acute ventilatory failure because of the nature of the disorder.⁷ Second, the need to protect the upper airway from secretions and aspiration may not be clearly reflected in results of pulmonary function tests, which evaluate only the mechanical function of the ventilatory pump. Neuromuscular weakness may not manifest uniformly in all muscle groups. Ventilation may be only moderately reduced in patients with gross oropharyngeal dysfunction leading to aspiration. Patients with ventilatory weakness can have a high risk of acute respiratory failure if upper respiratory tract infection or pneumonia develops. In these patients, inability to clear secretions can increase the work of breathing; the results are muscle fatigue, hypoventilation, and acute respiratory failure.

Patients with significant weakness of the respiratory muscles can have a high risk of respiratory failure when any pulmonary process increases the work of breathing. Pulmonary edema, pneumonia, and mucous plugging are examples of clinical conditions that can precipitate respiratory failure rapidly in patients with significant neuromuscular weakness. Such patients may need observation of their respiratory status when they are in the hospital with these conditions. Although the underlying disease may not have progressed to the point that these patients need continual or routine ventilatory support, that support may be critical in the setting of acute, exacerbating illness.

Nocturnal oximetry or formal sleep testing with polysomnography may be suggested in some clinical settings when patients have cor pulmonale, sleep disturbance, or excessive daytime somnolence that is otherwise unexplained.

Mini Clini

Assessment of a Patient With Neuromuscular Weakness

Problem

A 50-year-old man with ALS is admitted to the hospital because of right lower lobe pneumonia. The patient is moderately hypoxemic, with PO₂ of 68 mm Hg on room air. ALS was diagnosed 3 years previously, and he has had progressive worsening of dyspnea since then. These symptoms first occurred with mild exertion and then with supine position, which the patient has noticed in the last 1 or 2 months. A recent measurement of VC at the physician’s office was 35 ml/kg. The patient has noticed recent difficulty with swallowing and frequent coughing at meals. What features in the patient’s history may be relevant with regard to management?

Discussion

This patient has a disease that is associated with respiratory compromise, which progresses slowly in most cases, in contrast the acute case described here. All patients with ALS ultimately have respiratory insufficiency. The earliest symptom of neuromuscular weakness in the respiratory muscles is exertional dyspnea, which this patient has had for some time. A more significant finding is orthopnea, which is highly suggestive of diaphragmatic weakness. Patients with significant diaphragmatic weakness prefer the upright seated position, which allows the abdominal contents to shift toward the feet and allows unimpeded diaphragmatic descent. Although the patient may not have had a critically low VC recently, this value is low. Additional loading of already compromised ventilatory machinery can lead to fatigue and frank respiratory failure. It is important to recognize that the underlying neuromuscular weakness coupled with pneumonia may predispose this patient to respiratory fatigue and subsequent respiratory failure. The history of difficulty with feeding may suggest that the pneumonia is related to aspiration. The location of pneumonia in the lower lobe also favors the diagnosis if the aspiration occurs when the patient is seated.

Management of Respiratory Muscle Weakness

Respiratory insufficiency and failure to clear secretions are the major consequences of inspiratory and expiratory muscle weakness. Treatment of these patients involves consideration of mechanical ventilation via face mask or other noninvasive interfaces or via tracheostomy. Although often overlooked, therapies to augment secretion clearance and assist with cough are important in these patients (see Chapters 39 and 40). Used together, these interventions can decrease hospitalizations secondary to respiratory complications in patients with neuromuscular disease.⁸ These measures may also be useful in delaying or preventing the need for intubation or tracheostomy, although severe bulbar muscle weakness may limit a patient’s ability to avoid invasive ventilatory support.⁹ In addition to these interventions, general rehabilitation focusing on aerobic conditioning, muscle strengthening, and respiratory muscle training can often delay the need for ventilatory support and improve the overall quality of life for patients with muscle weakness.¹⁰

Noninvasive ventilation is being used increasingly for short-term and intermittent ventilatory support of patients with neuromuscular disease.¹¹ Acute deterioration, such as during a pneumonia, and surgical procedures such as gastrostomy tube insertion are situations where noninvasive ventilation is safe and effective if used carefully.^9,12 If a patient needs long-term ventilatory support on an intermittent basis, such as at night only, noninvasive ventilation may be appropriate.¹³ Decisions to begin mechanical ventilation in some patients with neuromuscular weakness have varied greatly among physicians¹⁴ and may be motivated by many different patient factors.¹⁵ No uniform guidelines exist for the use of long-term mechanical ventilation for patients with neuromuscular weakness. However, it is considered a standard option for patients who have reached the point of respiratory failure. Starting a patient on long-term mechanical ventilation requires careful planning and consideration of various issues related to respiratory care in alternative settings. These issues have been addressed in consensus statements^16,17 and are discussed in Chapter 51.

Diaphragm pacing in patients with spinal cord injury has been described for some time using direct stimulation of an intact phrenic nerve to contract the diaphragm and produce negative intrathoracic pressure and inspiration.¹⁸ This technique usually requires a thoracotomy, with its associated risks and high cost, and carries some risk of phrenic nerve injury. These objections have led to the development of an alternative system for diaphragm pacing using direct pacing of the diaphragm employing intramuscular electrodes implanted laparoscopically.¹⁹ When the electrodes can be placed in the diaphragm muscle at an electrophysiologically mapped motor point, the result is often elimination of the need for mechanical ventilation in patients with spinal cord injury and delay in the need to start mechanical ventilation in patients with ALS and other neuromuscular diseases.²⁰

Mini Clini

Care of a Patient With Neuromuscular Weakness

Problem

A 45-year-old man has myotonic dystrophy. He has progressive dyspnea that has increased particularly in the last year. PCO₂ determined from arterial blood gas analysis is 55 mm Hg. VC is 45% of the predicted value. The patient has no underlying pulmonary disease. Cough is decreased, but the patient maintains adequate control of secretions. The patient sleeps in a seated to semirecumbent position. What interventions are indicated for this patient?

Discussion

The patient has a disease that can result in respiratory insufficiency. VC is decreased, and arterial carbon dioxide levels are increased. These factors are consistent with hypoventilation secondary to neuromuscular weakness. The patient has dyspnea on exertion and orthopnea. All these factors suggest that mechanical ventilation should be considered.

Noninvasive positive pressure ventilation (NIPPV) may be a reasonable first choice in the care of this patient. The patient’s mental status and bulbar function are intact. He has no significant problems with secretions. (Important factors for successful application of NIPPV are discussed in Chapter 45.) Use of a nasal mask with a biphasic positive airway pressure unit may be instituted and titrated to patient tolerance. (Most pressure or volume-cycled ventilators can be used to deliver NIPPV and invasive ventilation.) A time-cycled backup rate can be set on some ventilators to facilitate ventilation of patients who may inadequately trigger the ventilator. If the ability to clear secretions becomes compromised, cough augmentation strategies should be implemented. As long as bulbar function—swallowing and secretion management—are maintained, noninvasive ventilation is a reasonable choice for ventilatory support.

Rule of Thumb

Patients with neuromuscular weakness who require noninvasive ventilation generally prefer a low expiratory pressure (2 to 3 cm H₂O) with a significantly higher inspiratory pressure (7 to 15 cm H₂O).

Specific Neuromuscular Diseases

Disorders of the Muscle (Myopathic Disease)

Primary muscle disease can decrease the ability of a normal neural impulse to generate effective muscle contraction. Some commonly recognized myopathies include Duchenne muscular dystrophy (DMD), myotonic dystrophy, and polymyositis. Table 29-3 presents a more complete list of myopathic diseases associated with ventilatory dysfunction.

TABLE 29-3

Myopathic Diseases With Associated Respiratory Dysfunction

Muscular Dystrophies	Myopathies
DMD	Congenital myopathies
Myotonic dystrophy	Nemaline rod myopathy
Facioscapulohumeral muscular dystrophy	Centronuclear myopathy
Limb-girdle dystrophy	Metabolic myopathies
Oculopharyngeal dystrophy	Acid maltase deficiency
	Mitochondrial myopathies (Kearns-Sayre syndrome)
	Inflammatory myopathies
	Polymyositis
	Dermatomyositis
	Hypothyroid-related and hyperthyroid-related myopathies
	Endocrine myopathies
	Steroid-induced myopathies (including critical illness myopathy)
	Miscellaneous myopathies
	Electrolyte disorders (e.g., hypophosphatemia, hypokalemia)
	Rhabdomyolysis
	Periodic paralysis
	Postneuromuscular blockade myopathy

Duchenne Muscular Dystrophy and Becker Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a genetic muscle-wasting disorder caused by mutations in the dystrophin gene.²¹ Because it is an X-linked recessive disorder, it affects mostly males. The diagnosis is made when a dystrophin mutation is found in DNA from peripheral white blood cells or when dystrophin is found to be absent or abnormal in muscle biopsy tissue.

DMD manifests early in life with proximal muscle weakness that leads to a waddling gait, exaggerated lumbar curvature (lordosis), and frequent falls. Most affected children need a wheelchair by 12 years of age. Death generally occurs by 20 years of age, usually as a result of declining respiratory muscle strength and subsequent infection. Becker muscular dystrophy, a milder form of DMD, also is associated with dysregulation of the dystrophin gene and manifests later in life.

Other systemic effects of DMD include scarring of the left ventricle and decreased bowel motility (intestinal pseudoobstruction). The progressive decline in respiratory function in patients with DMD parallels limb weakness and typically occurs at the point of wheelchair dependence. Respiratory weakness is primarily due to loss of muscle strength and leads to a lower PImax at all lung volumes than is present in healthy persons.

Progressive scoliosis is associated with DMD and can contribute further to respiratory insufficiency. Many patients undergo spine fusion surgery, and often the procedure allows greater comfort and ease in maintaining an upright posture. Although no randomized trials have proven any benefit of surgery on pulmonary function,²² observational data suggest that the rate of respiratory decline is slower after fusion surgery.²³

Obstructive sleep apnea has been described in a significant proportion of patients with DMD, prompting some authors to recommend formal polysomnography in patients with symptoms of obstructive sleep apnea or at the stage of becoming wheelchair-bound.²⁴ Institution of positive pressure ventilation (PPV) is a decision most patients face at some point in the disease. The point at which to begin different stages of ventilatory support depends on both test results and clinical condition. Nocturnal PPV is usually indicated when FVC reaches 30% of predicted with signs of hypoventilation.^25,26

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