Chest Wall and Respiratory Muscle Disorders

Jonathan D. Finder

Chest wall disorders range from cosmetic to life-threatening and are not uncommon in pediatric medicine. Disease states that lead to diffuse muscle weakness eventually involve the respiratory muscles and often lead to chest wall disorders secondarily. Respiratory “success” can be thought of as a balance between pump (respiratory muscles) and load (the resistance and compliance of the respiratory system).

ANATOMY AND PHYSIOLOGY

The physiology of the chest wall changes dramatically during the first year of life. The chest wall of the newborn is very compliant and cannot oppose the tendency of the lung to collapse it inward. Infants use a combination of a rapid respiratory rate, premature termination of the expiratory phase, and laryngeal braking to maintain an adequate volume of air (functional residual capacity) within the thorax. Over the course of the first year of life, there is significant decrease in chest wall compliance, leading to a respiratory rate that gradually slows. As the child ages, the relationship of the diaphragm to the chest wall changes as well, leading to a longer portion of the diaphragm involved in the “zone of apposition” (this refers to the portion of diaphragm in direct contact with the inner surface of the thoracic cavity). This creates a more efficient “piston” as the diaphragm muscle fibers shorten and increase the intrathoracic volume.

The main muscles of inspiration are the diaphragm and external intercostal muscles. The diaphragm acts as a piston and increases intrathoracic volume by displacing the abdominal contents downward. Contraction of the external intercostals increases the anteroposterior diameter of the thorax and aids in inspiration. In infants whose chest wall is overly compliant, the action of the intercostal muscles also opposes the tendency of the chest wall to buckle inward with the downward displacement of the diaphragm on inspiration. Accessory muscles of inspiration include the sternocleidomastoids, which lift the sternum and increase intrathoracic volume by a “bucket handle” mechanism. Other accessory muscles of inspiration include the scalenes, the shoulder girdle muscles, and serratus anterior.

During normal tidal breathing, expiration is passive, as the elastic recoil of the lung and chest wall return to their resting position (in which the two opposing forces are in balance). The accessory muscles of expiration are used for coughing, sneezing, and speech. Contraction of the internal intercostal muscles decreases the anteroposterior diameter of the thorax, aiding in expiration. The abdominal muscles aid in forceful expiration (such as coughing) by increasing intra-abdominal pressure, which is transmitted through the diaphragm to the pleural space.

PHYSIOLOGICAL ASSESSMENT

Physiological assessment of the respiration muscles can be performed by using a handheld manometer to measure maximum inspiratory and expiratory pressures. The sniff nasal inspiratory pressure (SNIP) has also been used to assess inspiratory muscle strength noninvasively. Twitch transdiaphragmatic pressure is another modality to measure diaphragm strength and requires magnetic stimulation of the phrenic nerve.

Fatigability is not often measured but can be a critical factor in whether a patient can sustain independent ventilation. The “tension time index” of the diaphragm reflects what proportion of maximum force is being used continuously. If the diaphragm is being used at its maximum capacity for a prolonged period of time, it will fatigue and fail.

Assessment of the chest wall also includes measurements of pulmonary function such as spirometry and plethysmography. In chest wall diseases, the forced vital capacity and total lung capacity tend to fall. Neuromuscular weakness also leads to decreased volumes, suggesting a restrictive defect but simultaneously leads to an elevation in the residual-volume-to-total-lung-capacity ratio (RV/TLC). Since an elevated RV/TLC and decreased forced vital capacity (FVC) can occur in obstructive diseases, it is useful to test respiratory muscle strength at the same time. Additional useful measurements include oxyhemoglobin saturation and end-tidal CO₂. CO₂ retention is not uncommon in restrictive chest wall diseases and can be an indication for close monitoring and consideration of ventila-tory assistance.

CHEST WALL DISEASES

SCOLIOSIS, KYPHOSIS, AND KYPHOSCOLIOSIS

Scoliosis is a condition in which there is angulation of the spinal column in the lateral (coronal) plane; in kyphosis, the angulation is in the anteroposterior (sagittal) plane. A combination of these disorders is common, especially in more severely affected patients as the scoliosis develops a rotational deformity. This is referred to as kyphoscoliosis. Scoliosis can be primary and not related to any other identifiable cause (“idiopathic scoliosis”), or it can be secondary to congenital malformations of the vertebrae or ribs or related to neuromuscular weakness (such as in muscular dystrophy or spinal muscular atrophy). The scoliosis is generally named in the direction in which the primary curve is convex (eg, dextro-convex vs. levoconvex scoliosis). The convex side will have widening of the interspaces, while the concave side will show narrowing. Having a curve of less than 10% by Cobb angle (which is measured using a standing radiograph) is considered within normal limits. A curve greater than 10% by Cobb angle defines scoliosis.

Scoliosis can be mild and of primarily cosmetic concern, or it can be severe and progressive, which can become life-threatening. As the spinal column angulates, the intrathoracic volume decreases, leading to a restrictive defect. More severe scoliosis can lead to distortion of the normal thoracic structures, compromising the central airways and vascular structures.

Idiopathic scoliosis is common, occurring in approximately 3% of American children. A scoliotic curve of more than 20% occurs in 0.5% of the general population. Idiopathic scoliosis occurs almost exclusively in adolescents and is the most common form, comprising 80% to 85% of cases. Girls are affected much more commonly than boys. These patients generally have a much milder clinical course than those with scoliosis of a secondary cause. The US Preventive Services Task Force recently recommended against routine screening for idiopathic scoliosis in asymptomatic adolescents because no direct evidence has been found to demonstrate benefits of screening.¹ Treatment options for idiopathic scoliosis include bracing, exercise, and surgery. Bracing in idiopathic scoliosis remains quite controversial, and currently there is little evidence for efficacy, despite fairly widespread use. A 50-year prospective natural history of patients with untreated, late-onset (adolescent) idiopathic scoliosis² demonstrated no increased risk of death during adult years, in contrast to previous reports. Having a Cobb angle of greater than 50 degrees at skeletal maturity was associated with diminished pulmonary function. Only those with a Cobb angle of greater than 80 degrees experience shortness of breath with exertion. Back pain was more common in these patients than in the general population.

PECTUS EXCAVATUM AND CARINATUM

Pectus excavatum and carinatum are chest wall malformations involving the sternum. These disorders can be congenital or acquired. Pectus excavatum, also known as “funnel chest,” is a condition in which the sternum is misaligned and depressed relative to the ribs. The cause is usually unknown, although acquired pectus excavatum can occur in association with chronic obstructive lung disease. This disorder is relatively common, occurring in an estimated 1 in 300 to 400 births. Pectus excavatum is common in Marfan syndrome and has been described in association with Pierre Robin sequence and Coffin-Lowry syndrome and in a rare autosomal dominant form. Mild pectus excavatum does not lead to a significant physiological derangement and is considered a cosmetic deformity. This can be the case especially in girls, as breast development can accentuate the abnormality and create a “cross-eyed” appearance of the nipple. Moderate and severe pectus deformities will lead to a restrictive chest wall with diminished lung function. Alterations in cardiac function have been reported in adult patients undergoing strenuous exercise.

Surgical repair has recently included a minimally invasive approach (the Nuss procedure) in which a metal “pectus bar” is inserted under the sternum, forcing it forward. Although this is a shorter procedure and results in less scarring, it is associated with greater postoperative pain. This bar is generally kept in place for 2 years.³ Currently, a trial is in progress to attempt slow repair by using magnetic force. A magnet is implanted onto the sternum and an anterior chest orthosis with a magnet provides a very gradual anterior movement of the sternum.⁴

ASPHYXIATING THORACIC DYSTROPHY (JEUNE SYNDROME)

In 1955, Jeune and colleagues described this disorder, which is characterized by skeletal malformations, including a small, narrow rib cage with shortened ribs; associated lung hypoplasia; and renal disease. Short-limb dwarfism, pelvic and phalangeal abnormalities, polydactyly, and hepatic disorders are possible associated manifestations. Asphyxiating thoracic dystrophy is an autosomal recessive disorder linked to 15q13. This diagnosis can be made antenatally on ultrasound or at birth, based on the characteristic findings of a narrow and bell-shaped thoracic cage with short horizontal ribs and flaring of the costochondral junctions. These findings are associated with a fixed chest wall and result in restrictive lung disease because of the mechanical limitations of the chest. These children are at risk for recurrent atelectasis, pneumonia, and impaired lung growth. Many patients die soon after birth from respiratory failure, but milder cases may be managed by long-term mechanical ventilation. Other forms of skeletal dysplasia (Jarcho-Levin, achondroplasia, Ellis-van Creveld syndrome, etc) are also associated with a restrictive defect and can be similar in presentation. Vertically expandable prosthetic titanium rib placement has been used for treating children with thoracic dystrophy. The US Food and Drug Administration approved this device under the Humanitarian Device Exemption program in 2005 for treatment of “thoracic insufficiency syndrome.”⁵

NEUROMUSCULAR DISORDERS

Primary disease of the muscle or nerve can have significant effects on chest wall function. The two most common neuromuscular disorders in children are Duchenne muscular dystrophy and spinal muscular atrophy. Other pediatric neuro-muscular disorders that can result in weakness of the respiratory muscles and lead to secondary chest wall disorders include spinal cord injury, Guillain-Barré syndrome, and other muscular dystrophies (Becker, congenital, limb-gird muscular dystrophy). Neuromuscular disorders that involve the respiratory musculature lead to three kinds of complications: recurrent pneumonia because ability to cough is lost; ventilatory failure during sleep and eventually during awake time; and development of a restrictive chest wall due to a combination of chest wall rigidity and scoliosis. The combination of respiratory muscle weakness and chest wall rigidity limits even mechanical support of breathing and coughing. Newer approaches to managing respiratory muscle weakness in these disorders have changed the natural history of these diseases and have improved outcomes.

DUCHENNE MUSCULAR DYSTROPHY

Duchenne muscular dystrophy (see Chapter 572) is caused by a mutation in the dystrophin gene and leads to a loss of the protein dystrophin, which links the sarcolemmal cytoskeleton to the extracellular matrix. This loss creates an increase in membrane fragility and repetitive injury to the muscle cells, which causes scarring and progressive weakness. Duchenne muscular dystrophy is X-linked and occurs in approximately 1 in 3000 boys. Although the majority of involvement is in the skeletal muscle, cardiac muscle involvement is common. Although there is no cure, systemic corticosteroids have been shown to slow the progression of the muscle disease. Loss of ambulation tends to occur at the end of the first decade of life. After the patient loses ambulation, the respiratory complications of this disease begin. These complications include scoliosis, loss of effective cough, obstructive apnea, and the development of ventilatory failure.

Scoliosis is less common in patients who are treated with systemic corticosteroids. Bracing has been shown to be ineffective, and surgical correction with rod placement is indicated once the Cobb angle reaches 30 degrees.

Obstructive sleep apnea has been reported prior to the development of respiratory insufficiency during sleep; this is probably related to a combination of obesity and upper airway muscular weakness.⁶

Lung function should be monitored, especially after loss of ambulation, with spirometry, lung volumes, and respiratory muscle strength measurements. The usual pattern is restrictive lung disease with increased residual volume secondary to weaker muscles of expiration compared to inspiration. These patients are unable to actively expire to lung volumes below functional residual capacity.

When the vital capacity falls below 30% of predicted value, there is a high incidence of respiratory failure in sleep.⁶ Elevation of the end-tidal pCO₂ level to 45 mm Hg and higher while awake also predicts respiratory insufficiency in sleep.⁷ Eventually, daytime respiratory failure occurs. Each of these stages in the Duchenne patient can be effectively managed in a noninvasive manner.⁸ Mechanical insufflation-exsufflation has been proven effective and safe in this population to prevent pneumonia. Noninvasive positive-pressure ventilation with a mask during sleep and a mouthpiece during the day can be used to support ventilation, avoiding the need for a tracheotomy.

SPINAL MUSCULAR ATROPHY

Spinal muscular atrophy (SMA) is a congenital disorder of the anterior horn cell. Clinical features of SMA include hypotonia; muscle weakness, including extensive involvement of the intercostal muscles; muscle atrophy; and fasciculations. Muscle weakness is symmetric, greater proximally, and typically more profound in the legs than the arms (see Chapter 570).

SMA is classified into three types (SMA type I, type II, and type III), which are best distinguished by the age of onset and clinical course. SMA type I (Werdnig-Hoffmann disease or severe infantile SMA) is the most severe disorder and generally presents before 6 months of age with profound hypotonia and weakness, swallowing dysfunction, and tongue fasciculations; the disease generally results in respiratory insufficiency and death by 2 years of age unless long-term mechanical ventilation is employed.

SMA type II (intermediate or chronic infantile SMA) has an estimated incidence of 1:15,000 to 1:25,000 live births. Children with SMA type II may initially achieve normal motor milestones, but these are lost during the first 2 years of life. Median age of death is 12 years unless long-term mechanical ventilation is employed. Children with SMA type II may not be able to sit independently. Weakness may be static for long periods, with progression of weakness during intercurrent illness or immobilization. Bulbar musculature is generally intact. The onset of SMA type III (Kugelberg-Welander or mild SMA) is usually between 2 and 17 years of age; these children are usually able to stand and walk unaided.

Clinical examination, laboratory evaluation (including normal serum creatine kinase), electromyogram, and muscle biopsy were used in the diagnosis of SMA. The discovery of the gene in 1995 resulted in diagnosis by gene mutation screening alone in 95% of cases. Laboratory data demonstrate subtle differences in the SMA types. Therefore, Dubowitz uses functional status to distinguish the types (ie, in severe SMA, children are unable to sit; in intermediate SMA, children are able to sit unsupported but cannot stand or walk unaided; with mild SMA, children are able to stand and walk unaided).⁹

Unlike Duchenne muscular dystrophy, SMA I and II is characterized by early and severe involvement of the intercostal muscles. As a result, there is greater chest wall involvement in SMA patients. These individuals tend to develop a bell-shaped chest wall, as the action of the diaphragm, which tends to cause an indrawing of the compliant chest wall of the infant during inspiration, is not balanced by opposing action of the intercostal muscles. A pectus excavatum deformity occurs simultaneously. Bach has shown that positive-pressure ventilation has remodeled the chest wall and normalized the chest wall’s shape.¹⁰

Management of children with SMA includes aggressive respiratory, nutritional, and orthopedic management.¹¹ Children with SMA are at risk for nocturnal hypoventilation. Evaluation includes assessment of lung function with spirometry, lung volumes, respiratory muscle strength measurements, and pulse oximetry. Hypoventilation is best evaluated by nocturnal polysomnography with multiple channel measurements, including end-tidal CO₂ and pulse oximetry. Hypoventilation is managed using respiratory support such as noninvasive positive-pressure ventilation while sleeping. During viral respiratory infections, aggressive airway clearance of increased secretions is critical to the child’s survival, using assisted coughing techniques (mechanical insufflation-exsufflation).

Progressive scoliosis is managed initially with bracing to facilitate sitting, although improper bracing can actually impair ventilation. Scoliosis may require surgical correction. These interventions contribute significantly to quality of life.

The option of noninvasive ventilation versus tracheostomy and long-term mechanical ventilation should be discussed with patients and family members well before the onset of respiratory failure.

GUILLAIN-BARRÉ SYNDROME

Guillain-Barré syndrome is an acute inflammatory ascending demyelinating disease of the peripheral nervous system. The vast majority of affected individuals have a history of recent nonspecific illness, but the syndrome is also associated with cytomegalovirus and Epstein-Barr virus infection. The ascending paralysis and weakness progresses, typically over hours to several days. Involvement of the diaphragm and intercostal muscles results in hypoventilation, and tidal breathing may occur well below the patient’s usual functional residual capacity. Progressive weakness involving the cranial nerves may result in laryngeal and vocal cord dysfunction with resulting upper airway obstruction. Respiratory support is required when hypoventilation occurs or when the individual cannot clear lower airway secretions. Approximately 15% of children with Guillain-Barré syndrome will require mechanical ventilation for respiratory failure. Serial monitoring of vital capacity and respiratory muscle function is critical. Treatment includes intravenous immunoglobulin, corticosteroids, and aggressive respiratory support. Recovery ranges from days to months and is usually complete.¹²

DIAPHRAGMATIC PARALYSIS

Unilateral and bilateral paralysis of the diaphragm is rare in pediatric patients. However, it may be overlooked during the evaluation of infants with unexplained hypoxemia, atelectasis, or tachypnea. In the neonatal period, diaphragm paralysis is most frequently caused by complications during delivery or following thoracic or neck surgery. The phrenic nerve fibers arise from C3, C4, and C5. The superficial location of the phrenic nerve in the neck and within the thorax contributes to the risk of injury during delivery. During cardiovascular surgery, the phrenic nerve may be injured by cooling, stretching, or accidental direct injury. In addition, high-cervical-spine trauma or tumors can result in diaphragm paralysis, and neuromuscular disease may be complicated by diaphragm paralysis.

The neonate is at much greater risk of respiratory compromise from diaphragm paralysis. Older children and adults tolerate unilateral diaphragm paralysis well. Diaphragm paralysis should be considered in any infant who has unexplained respiratory distress with tachypnea and hypoxemia, especially after a difficult delivery, or in a child following thoracotomy who fails to wean from mechanical ventilation. Chest radiographs demonstrating an elevated hemidiaphragm or elevation of both diaphragms are suspect for diaphragm paralysis. Diagnosis is made by ultrasound or fluoroscopy, which shows very little movement with breathing or shows paradoxic (upward) movement of the diaphragm with inspiration. Transcervical nerve stimulation may be helpful in determining whether the phrenic nerve is intact.

Following thoracic or neck surgery, recovery may occur in up to 90% of patients over about 6 months if the phrenic nerve is intact. Infants may require positive-pressure ventilation, either invasively or noninvasively, until the diaphragm recovers. In individuals with permanent unilateral diaphragm paralysis, diaphragm plication may be helpful. Management options include respiratory support until the diaphragm recovers and surgical plication if recovery is incomplete. Indications for plication include persistent respiratory failure and paradoxical cephalad movement of the affected diaphragm during inspiration. This surgery can be performed via thoracotomy or video-assisted thoracoscopic surgery.¹³ In individuals with irreversible bilateral diaphragm paralysis (eg, spinal cord injury or neuromuscular disease), respiratory support with mechanical ventilation will be required. Older children and adult patients tolerate the diaphragm limitation relatively well, and they may require only nocturnal respiratory support.

MALFORMATION OF THE DIAPHRAGM

EVENTRATION

Eventration of the diaphragm is a marked elevation of the diaphragm. This is usually congenital but may be acquired after phrenic nerve injury from surgery or birth trauma. The diaphragm is thin and fibrous rather than muscular. This defect is more common on the left than on the right and is more common in boys than in girls. Symptoms may be similar to those of a diaphragmatic hernia (tachypnea, dyspnea, retractions, and cyanosis). In others, the symptoms are less severe. Some cases are asymptomatic or are recognized because of recurrent pneumonia in the poorly ventilated ipsilateral lung. Diagnosis is made by noting unilateral decreased ventilation on physical examination; by chest radiograph demonstrating the eventration; or by fluoros-copy, which shows paradoxic movement of the affected portion of the diaphragm. Patients with persistent symptoms are treated initially with intubation and ventilator support. Definitive treatment consists of plication of the affected diaphragm (as described above for diaphragm paresis). Asymptomatic cases are treated conservatively.¹⁴

REFERENCES

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