4 Respiratory Implications of Abnormal Development of the Spine
Abnormal development of the spine can cause significant scoliosis, kyphosis, or lordosis, resulting in body deformities that can be distressing to patients and their families. The more serious threat to long-term health is the adverse effect of abnormal development of the spine on pulmonary function. This is well documented for curves exceeding 90 degrees, which cause severe restrictive lung disease, but not well understood for lesser curves; knowledge regarding their long-term effect on pulmonary health, whether they are treated or untreated, is almost completely lacking. Pulmonary function is an important determinant of long-term survival. Increased rates of mortality, mostly resulting from pulmonary failure, have been seen in patients with untreated infantile scoliosis beginning at the age of 20 years, with a rise in mortality rates to fourfold above normal by the age of 60 years. 1 The purpose of this chapter is to outline the biomechanical principles of respiration, which are related to the spine and its normal development; to discuss and critique the available metrics for the clinical and imaging assessment of pulmonary function and spine deformity; and to summarize the known effects of abnormal development of the spine on respiratory biomechanics and function.
4.1 Normal Pulmonary and Thoracic Function
The normal spine is the posterior pillar of support for the thorax, which is the anatomical combination of the rib cage and the thoracic spine, with the diaphragm as its base. The thorax provides structural protection for the vital organs of the chest, including the heart, lungs, and great vessels, and the rib cage provides stabilization for the thoracic spine. However, the most important role of the thorax is its role as the engine of respiration, in which it generates a rhythmic expansion of both lungs during breathing through the downward contraction of the diaphragm and posterior lateral expansion of the rib cage by intercostal muscle action.
The volume and flow rate of the air moved during the respiratory cycle by the efforts of the thoracic respiratory engine are easily measured in a cooperative individual through spirometry, and pulmonologists collectively term the multiple values obtained by complex analysis of these two variables pulmonary function. One could argue that this term is a misnomer because the act of respiration is totally dependent on the biomechanical expansion of the thorax during inspiration and expiration. So, one is actually measuring for the most part thoracic function with spirometry, but it is doubtful that this long-standing misconception will ever be changed.
Pulmonary function testing produces dozens of test values, but the result that most orthopedists focus on is the forced vital capacity (FVC), expressed both as a raw score, liters of air exhaled with maximum effort after deep inspiration, and as FVC percent predicted, a percentage value derived by comparison with normal values of FVC as a function of height. The latter value is usually most emphasized in the surgical literature, and most surgeons assume the FVC percent predicted “score” reflects comprehensive pulmonary function, with an FVC percent predicted of 100% graded as normal. Statistical increases in FVC percent predicted after surgery are usually interpreted as indicators of success. Unfortunately, pulmonary function testing is complex and inherently subject to variability, and it should be used as a metric, especially in children, with caution.
The standards for pulmonary function testing results and the methodology of testing are governed by the American Thoracic Society, and a normal FVC percent predicted can vary from 80% to 100% predicted. This variability is mostly due to testing methodology and the ability of the patient to cooperate consistently with performing the maximum inhale / maximum exhale spirometry maneuver needed to produce the flow / volume curve for FVC. Mild to moderate restrictive lung disease has values varying from 80% to 50% predicted, and restrictive lung disease is considered severe when the FVC is less than 50% predicted. Testing methodology requires three FVC maneuvers, with the average of these used for the final result. Patient cooperation and motivation are critical for the successful reproduction of these FVC maneuvers. Routine spirometry is not recommended for patients younger than 6 years of age because of the inability of young children to cooperate with the maximum inhale / maximum exhale maneuver. These younger children, however, can be assessed with passive approaches, such as infant pulmonary function testing, although such techniques require special equipment and are generally not available at most institutions. For those patients older than 6 years of age, motivation may become an issue for accuracy of the test, with highly motivated children using hyperkinetic “trick maneuvers” to maximize their results, while those with less motivation may have low scores as a result of their half-hearted efforts. With children, the respiratory therapist who is a charismatic coach tends to get the best efforts from patients, whereas others may be unable to coax maximum cooperation from children. Awareness of this inherent variability of the results of pulmonary function testing in children should provide a basis for caution in the interpretation of pulmonary function test differences that may be statistically valid but in fact may be influenced by the “cooperation factor.”
Conceptually, the FVC is a measure of the “emergency reserves” an individual can bring to bear when maximum respiration needed, such as in running a race or surviving an episode of acute pneumonia. In the act of maximum respiration, both the primary and accessory muscles of respiration are fully engaged in taking the deepest breath possible followed by maximum expiration, raising the respiratory rate to the highest sustainable level. In contrast, regular quiet breathing is an almost effortless act, very energy efficient, usually not even noticeable to the individual. The volume of air exhaled during normal, quiet breathing is termed tidal volume. Restrictive lung disease is present when the FVC is significantly decreased, with almost all authors agreeing that an increased risk of mortality from multiple causes long term is associated with restrictive lung disease, but the issue is not quite entirely clear. Numerous adult survivors of Jarcho–Levin syndrome, despite extremely severe restrictive lung disease in childhood, have been reported, 2 and is unclear what protective mechanism lets them survive with such a low vital capacity.
Physiologically, maximum respiration requires a lot of energy, and alveoli in the superior areas of the lung are recruited, increasing blood flow to the capillary beds, in order to provide maximum oxygenation. Excursion of the regular muscles of respiration is augmented by recruitment of the accessory muscles, including the scalene muscles and sternocleidomastoid muscles. In the FVC maneuver in adults, the diaphragm produces 80% of the total volume exhaled and the rib cage expansion provides the remaining 20%; the diaphragm / rib cage FVC ratio for children is unknown. Maximum respiration depends on maximum expansion and compression of the appropriate thoracic volume for the underlying lungs, appropriate for age. The diaphragm attaches posteriorly just inferior to T12, and the posterior aspect of the diaphragm provides most of the excursion distally to expand the lungs during inspiration. The additional expansion of the lungs provided by the rib cage is an extremely complicated and poorly understood process. It is generally accepted that the most proximal ribs are relatively fixed in position, but the ribs distal to them in the middle of the chest rotate outward anteriorly through the costovertebral joints; this motion is mediated by the contraction of the intercostal muscles. The more distal ribs rotate outward anterior laterally. This rotatory motion is made possible by the flexibility of the chondral ribs centrally. The average man by the age of 60 years has lost 700 mL of vital capacity as a consequence of normal aging, 3 most likely as the result of loss of muscle tone and stiffening of the chondral cartilages from calcification.
In summary, normal respiration depends on a complex sequence of biomechanical events in the thorax; these range from the low energy demands of quiet breathing, producing tidal volume, up to the maximum capacity efforts associated with maximum breathing, producing FVC. Optimal thoracic volume and normal alignment of the ribs, coupled with normal excursion of the costovertebral joints and contraction of the diaphragm, produce respiration that is clinically normal. However, any pathologic process, such as abnormal development of the spine, can introduce complex deformity that degrades the biomechanical efficiency of the thoracic engine of respiration and produces significant restrictive lung disease.
4.2 Assessing the Effects of Abnormal Spinal Development on Respiration
4.2.1 Clinical Presentation
In the child with early onset scoliosis, the first step is to determine the time of onset of the deformity and also define the baseline respiratory status. Can the child keep up with the activities of his or her peer group? Does the child participate in all activities at recess, or are sedentary activities preferred? Does the child participate in active sports? On shopping trips, is the child able to walk unlimited distances without fatigue or breathlessness? It is also important to determine the frequency and type of respiratory illnesses that the child is experiencing, including upper respiratory infections, episodes of respiratory syncytial virus (RSV) bronchiolitis, and frank bacterial or viral pneumonia, and also to note if hospitalization was necessary and whether respiratory support such as oxygen or even intubation was needed. The pattern of frequency of illness should be noted, with increasing bouts of respiratory illness a negative factor. Any need for chronic dependency on respiratory aids, such as continuous positive airway pressure (CPAP), bi-level positive airway pressure (BiPAP), nasal oxygen, or ventilator dependency, should be noted, along with a time line of use. The goal of the respiratory history in children with early onset scoliosis is to establish either that there are no clinical sequelae of the spinal deformity or that there are early signs of respiratory dysfunction, and efforts should be made to establish whether there is progression of the respiratory dysfunction.
4.2.2 Physical Examination of the Patient with Early Onset Scoliosis
The child is evaluated for deformity. The examiner notes the apex of the curve, curve flexibility, unequal shoulder heights, and truncal shift, as well as head balance over the pelvis. In addition, the respiratory status needs to be determined. Check for clubbing of the digits, flaring of the nostrils with breathing, and cyanosis of the lips and nail beds. The resting respiratory rate should be recorded because this reflects the ability of the thoracic engine of respiration to oxygenate the body. Elevation of the respiratory rate above normal means that thoracic function is inadequate to maintain oxygenation with normal rates of respiration, so the mechanism has to be put into “overdrive” with a higher respiratory rate per minute. This raises caloric expenditure, making weight gain more difficult for the growing child, and very rapid rates of respiration interfere with the acts of eating and speaking. A normal respiratory rate is 30 to 80 per minute for a newborn, 20 to 40 per minute for ages 2 to 5 years, and 15 to 25 per minute after the age of 6 years. 4 A brief “challenge test” is helpful in borderline cases. The child is asked to run back and forth a short distance in the halls of the clinic, and the normal child usually does this with ease, but the child on the edge of clinical respiratory insufficiency will become uncomfortable after a short while, complain about breathing hard, and stop running.
4.2.3 Physical Examination of the Patient with Thoracic Insufficiency Syndrome in Early Onset Scoliosis
It is important to determine whether the child with early onset scoliosis has thoracic insufficiency syndrome. 4 This is defined as an inability of the thorax to support normal respiration or lung growth. Significant, progressive thoracic insufficiency syndrome can lead to early mortality from restrictive lung disease. Respiration is abnormal when on physical examination anomalies of the thorax are seen during respiration, such as areas of paradoxical respiration (the chest wall segment collapses inward during inspiration) or an absence of chest wall motion over areas of fused ribs associated with congenital anomalies of the spine. There should also be a general evaluation of the symmetry and size of the thorax. The chest circumference can be measured at the nipple line and compared with normative values to derive a percent predicted value. Low values suggest hypoplastic thorax and probable restrictive lung disease due to reduced volume of the lungs. One important test during the physical examination of these children is an assessment for the marionette sign (Fig. 4.1), in which the patient’s head bobs with respiration. When the base of the thorax is too close to the pelvis, in conditions such as lumbar gibbus in patients with myelomeningocele, diaphragm excursion is blocked distally by the abdominal organs, increasing the work of breathing and limiting lung expansion by the diaphragm. This is termed secondary thoracic insufficiency syndrome. The same phenomenon is seen unilaterally in pelvic obliquity in neuromuscular scoliosis. As the patient inhales, the diaphragm contracts and encounters resistance from the abdominal organs, which are pushed proximally by the spine deformity, so the diaphragm, in essence, is doing a push-up against the body weight, raising the torso a few centimeters and straining to expand the lungs. This is not sustainable, and any condition that further increases the abdominal volume in these children, such as constipation, can tip them over the edge with the development of respiratory failure.
Both the rib cage expansion and diaphragmatic excursion of thoracic function can be assessed clinically to some degree. Rotation of the thoracic spine commonly results in a rib hump on the convex side in early onset scoliosis, and rib cage expansion is commonly hampered in this situation. This can be assessed with the “thumb excursion” test (Fig. 4.2). The examiner places his or her hands at the base of the thorax with the thumbs extended medially to be equidistant from the spine; the hands are held lightly on the surface of the chest, with the metacarpal phalangeal joints of the hands in the midaxillary line. The child is instructed to take a deep breath, and there should be symmetric motion of the thumbs greater than 1 cm away from the midline of the spine resulting from the outward motion of hemithorax expansion. This is graded +3. If hemithorax motion is limited by deformity, the movement of the chest outward with inspiration is limited, and if the distance is only 0.5 to 1 cm, the grade is +2. With further stiffness of the chest wall, the distance the thumb moves with respiration may be slight, less than 0.5 cm. Complete absence of motion is graded as +0. In segments of the chest that are affected by fused ribs in congenital scoliosis, and on the side of the rib hump in infantile scoliosis, the thumb excursion test grade is usually +0. Absent careful evaluation, these children may be assumed to have “normal respiration,” but the presence of such abnormalities on physical examination clearly defines their respiration as abnormal. They may have normal activity levels, but commonly the respiratory rate is elevated at rest. This can be described as occult respiratory insufficiency, in which the respiratory mechanism compensates by increasing the rate of breathing, with more energy expended to maintain normal body oxygenation.
Once the physical examination points out specific deformities of the spine and rib cage in early onset scoliosis, imaging techniques are the next step in further defining the abnormalities.
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