Pediatric orthopedic anesthesia: Review

Pediatric Orthopedic Anesthesia: Review Mashallah Goodorzi

HERE have been enormous changes in pediatric orthopedic surgery, such as the shift from rehabilitation of paralyzed patients, treatment of chronic osteomyelitis, and simplistic management of fractures and trauma to major reconstructive surgery in skeletally immature patients of all ages. The range of patients presenting for pediatric orthopedic surgery has accordingly widened to include both the very young to teenagers with multiple congenital abnormalities and residuals of postnatal acquired deformities. Greater demands are concomitantly being made on the anesthesiologist. Children with congenital anomalies and the residuals of infection and trauma who require anesthesia for orthopedic surgical procedures include all stages of development from birth through adolescence. A pediatric anesthesiologist faces several recurring concerns regarding orthopedic procedures: positioning, blood loss, fluid replacement, airway management, malignant hyperthermia, maintenance of body temperature, and postoperative pain management. Due to the pathology of the diseases and deformities, these patients may need extensive rehabilitation, prolonged and repeated hospitalization, and long-term immobilization that may lead to emotional or psychiatric problems. Mentally impaired patients may become difficult to handle. Efforts must be made to make their hospitalization as tolerable as possible. The requirements for each child are different. They may benefit from preanesthetic psychological as well as pharmacologic preparation. The anesthesiologist must be aware of unusual associated syndromes that may have clinical anesthetic or orthopedic implications. This review presents complicated

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From the Department of Anesthesia, Childrens Hospital Los Angeles, University of Southern California School of Medicine, Los Angeles, CA. Address reprint requests to Mashallah GoodarzL MD, Department of Anesthesia, Childrens Hospital Los Angeles, 4650 Sunset Blvd, Los Angeles, CA 90027. Copyright 9 1997 by W.B. Saunders Company 0277-0326/97/1604-000855.00/0

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issues that are relatively common and that have led to different complications in their management. There are too many syndromes to be discussed in detail. However, the anesthesiologist should review the physiology and pathology of any defined syndrome before undertaking anesthetic procedures. There are several excellent syndrome references. 1,2Further anatomic factors, especially those affecting the stability of the cervical spine, must be carefully studied. The following material will address some of the more frequent "problem patients" requiring anesthetic techniques for orthopedic procedures. OSTEOGENESIS IMPERFECTA (BRITTLE BONES) Osteogenesis imperfecta (OI) is an inherited condition in which the bones are variably, abnormally brittle. The three main signs are bone fragility, blue sclera, and deafness (osteosclerotic type). The disease usually begins during fetal life, but is rarely noticed before infancy or early childhood unless there is significant involvement. The basic defect is mesenchymal hypoplasia that primarily affects the bones. The incidence ranges from 1:20,000 to 60,000 deliveries, with a higher incidence in females. Osteogenesis imperfecta is manifested in two clinical forms: congenita, with severe manifestations evident at birth, and tarda, in which skeletal deformities and fractures occur later in life. Affected children have small bowed limbs, megalocephaly, short neck, dwarfism, and blue sclera. Spinal osteoporosis and laxity of joints secondary to a lack of connective tissues are present. The teeth are easily broken. Otosclerosis occurs in 60% of cases. Capillary fragility causes subcutaneous hemorrhages. The incidence of inguinal and umbilical hernia is high. While the serum calcium and phosphate levels are normal, the overall mineral content of the bones is diminished. Abnormal platelet function may lead to a bleeding diathesis. Platelet aggregation is abnormal, although platelet count is normal. Anesthetic M a n a g e m e n t

Numerous problems concerning anatomic and physiologic abnormalities associated with OI exist

Seminars in Anesthesia, Vol 16, No 4 (December),1997: pp 328-339

PEDIATRIC ORTHOPEDIC ANESTHESIA for the anesthesiologist. The following concerns are the most important: easily fractured teeth and bones; airway abnormality that may involve a large head, tongue, short neck, or thoracic deformities; and a tendency to develop hyperthermia due to abnormal metabolism. Other anomalies include congenital heart disease (patent ductus arteriosus, atrial septal defect, ventricular septal defec0, emphysema, and premature arteriosclerosis. The most important is the susceptibility to develop additional fractures during treatment. Extreme care must be taken when handling these patients. Adequate padding of the operating table is essential. Airway control must be managed with great care since abrupt extension or flexion of the head could fracture the cervical spine, and intubation could easily chip the teeth and may cause fracture of mandible. In addition, patients with OI may have a disproportionately large tongue that may cause obstruction before endotracheal intubation. Presence of megalocephaly and the short neck could also make intubation difficult. Marked thoracic deformities may cause poor respiratory function, even though the ventilation-perfusion relationships are relatively normal. Another important anesthetic consideration is the tendency to develop hyperthermia along with excessive diaphoresis. 3'4Although this hyperthermia does not appear to be the same or as serious as the malignant type, patients with OI indeed share some similarities with malignant hyperthermia individuals, such as autosomal dominant inheritance and connective tissue abnormalities. Hyperthermia may be related to elevated serum thyroxin levels, which are found in at least half of patients with OI. These individuals have elevated oxygen consumption and heat production relative to their body weight. The hyperthermia may be induced with any anesthetic technique, as well as atropine (which should be deleted from the therapeutic regimen of these patients). There has not been a positive scientific relationship between malignant hyperthermia (MH) and OI patients, s Hyperthermia may be controlled with a cooling blanket and avoidance of excessive coverage of the patient with heavy dressings. It is also desirable to minimize the fasciculation associated with succinylcholine chloride administration, since severe fasciculation may produce fracture and muscle damage. The use of regional or local anesthetic agents may avoid airway problems and decrease the incidence of hyperthermia.

329 ARTHROGRYPOSIS (AMYOPLASIACONGENITA) Arthrogryposis is derived from two Greek words meaning curved stiff joint. This anomaly manifests itself as persistent joint contractures that are frequently present at birth and that may worsen over time. The disease has numerous synonyms. The most common is arthrogryposis multiplex congenita. Another frequently used term is amyloplasia congenita (AMC), which Sheldon first suggested, believing that this condition was primarily a disease of muscle. 7 Less common terms are congenital myodystrophy, multiple congenital contractures with muscle defects, multiple congenital contructures of joints dystrophia muscularis congenita, and multiple congenital articular rigidities. Numerous causes of this condition have been proposed, with one of the more widely accepted being that some mechanical abnormality resulted in increased intrauterine pressure to the fetus. However, recent studies suggest that arthrogryposis is due to either defective formation or degeneration of the anterior horn cells of spinal cord. The mechanical abnormalities include malposition of the fetus, increased intrauterine pressure, oligohydramnios, and inactivity of the fetus. The muscle abnormality is variable embryonic failure of myoblast formation. Some have described the disease as the intrauterine analogue of polio.6 Associated anomalies are either orthopedic or nonorthopedic. Among the orthopedic anomalies are thoracolumbar scoliosis, rib cage anomalies (asymmetry or pectus excavatum), Klippel-Feil syndrome and torticollis, cleft palate, hypoplasia of the mandible, and defective nasal septum. Dislocated knees and patellae, polydactyly, posteromedial bowing of the tibia and multiple joint, and bone deformities also have been reported. The most frequent nonorthopedic anomalies are in the genitourinary system: hydrocele, cryptorchidism, and absence of labia majora uterus and vagina. Cardiac anomalies such as ventricular septal defect, pulmonary stenosis, and transposition of great vessels also have been reported.

Anesthetic Management Problems arising during anesthetic management include difficulties in endotracheal intubation due to the short neck, micrognathia, and stiff tymporomandibular joints. Potential respiratory problems include hypoventilation, microatelec-

330 tasis, restrictive respiratory patterns, a decreased ability to cough, and an increased incidence of aspiration. The presence of chest deformity and scoliosis increase the work of breathing. An abnormal ventilation to perfusion ratio should be considered very carefully. These patients may be prone to hypotension, respiratory depression, and slow recovery from inhalation or intravenous anesthesia. The possibility of malignant hyperthermia (MH) has been reported in the literature, even though there has not been convincing proof that patients with arthrogryposis are particularly susceptible to the development of malignant hyperthermia. In one report, a 6-month-old child with AMC deVeloped respiratory acidosis and hyperthermia on exposure to halothane, which resolved only with the administration of dantrolene. The association of MH and AMC is most likely coincidental. Baines and Barrett 8 reviewed their institution's experience with AMC over a 32-year period. Sixty-seven patients received general anesthesia 398 times, 51 patients received halothane at least once, and 23 patients were exposed to halothane on at least five occasions each. There were no perioperative complications or evidence of MH. In the last 6 years in our hospital we have had over 50 procedures with AMC patients, all of whom have had exposure to halothane without any evidence of MH. The use of a nondepolarizing muscle relaxant is a safer choice in patients with arthrogryposis congenita.

MUSCULAR DYSTROPHIES The general term muscular dystrophies describes a group of genetic disorders that cause progressive degenerative changes in muscles. The most common and severe form of muscular dystrophy (Duchenne) is an inherited, X-linked recessive disorder with an incidence of approximately 30 per 100,000 live births. 9 While it is transmitted by females, for the most part it is clinically evident in males. The location of the dystrophic genetic defect is on the short arm of the X-chromosome at the Xp21 ban& Deficiency of the dystrophin gene has been found in muscle biopsies of Duchenne patients. Although there is histologic and biochemical evidence that the disease exists in children at birth, clinical manifestations do not usually become manifest until the second to third year of life. Motor develop-

MASHALLAH GOODARZI mental delay is characterized by a late development in walking and frequent falling. A broadbased waddling gait with an exaggerated lumbar lordosis may be noted. The children develop a peculiar manner in which they rise from a supine position, climbing up their own legs (Gower's sign). The distribution of muscle weakness is proximal, with the muscles of the leg being the earliest involved, followed by the upper extremities. In the late stage fat, and connective tissue interdigitate with and replace the muscle fibers, giving the appearance of bulky muscles. The progression of the disease may cause joint contractures by the age of 6 years. By the age of 12 years, severe deterioration in functional abilities confines patients to a wheelchair. This often accelerates the development of contractures and kyphoscoliosis, along with increasing weakness of respiratory muscles. The results are thoracic mechanical problems, reduced lung compliance, a weak cough, retained secretions, repeated infection, ventilation/perfusion mismatch, and, ultimately, hypoxemia. Death is usually due to the respiratory insufficiency. Cardiac involvement is common, with 70% to 90% of patients demonstrating electrocardiopathic abnormalities, l: Most patients have labile sinus tachycardia, abnormalities in the conduction system such as atrial dysarythmias, and atrioventricular or intraventricular conduction disturbances. The heart may be enlarged. Involvement of gastrointestinal smooth muscles causes gastric dilatation, vomiting, abdominal pain, and gastric hypomobility. Prolonged supine position and immobility produce electrolyte imbalance, potassium loss, and hypoproteinemia.

Becker-Type Muscular Dystrophy Becker-type muscular dystrophy is X-linked recessive. The incidence is estimated at 3 per 100,000, This is a milder and slowly progressive form of dystrophy compared with the Duchenne type. The onset is late, and the majority of patients survive into the fourth and fifth decades of life. 13 Cardiac involvement occurs, along with congestive heart failure. TM

Fascioscapulohumeral Dystrophy Fascioscapulohumeral dystrophy is an autosomal dominant type with an incidence of 3 to 10 per million. The disease appears in adolescence,

PEDIATRIC ORTHOPEDIC ANESTHESIA with a slow progression of weakness of the muscles of the face, shoulder, and upper arm. The disease does not change the life span. Although rare, left axis deviation and atrial paralysis have been reported. Anesthetic Considerations

These patients present for surgical procedures ranging from muscle biopsy to correction of scoliosis. The severity of muscle weakness, particularly with respect to the respiratory system, as well as cardiac and gastrointestinal involvement, should be carefully assessed. Pulmonary function should include measurement of vital capacity, arterial blood gases, maximal expiratory pressure, and maximum inspiratory pressure to detect abnormalities of an early stage of acute and chronic respiratory muscle weakness. Vital capacity measured at supine and then sitting position is one way to detect diaphragmatic weakness. A greater than 25% change in vital capacity correlates well with orthopnea and guides the required postoperative need for extended ventilatory support and intensive chest physiotherapy. A vital capacity of less than 30% of predicted suggests the need for postoperative ventilatory support. The vital capacity appears to be the most important single measurement that can be obtained and a forced vital capacity (FVC) of less than 20 mL/kg indicates increased risk and the need for postoperative ventilation. A 12-lead electrocardiogram with rhythm strip may show conductive defects or other evidence of diffuse myocardial damage. Monitoring should include arterial blood pressure, electrocardiogram, oximetry, endtidal CO2, temperature, and a means of assessing neuromuscular function. Hypoproteinemia, anemia, and fluid and electrolyte imbalance should be corrected preoperatively. Atropine should be avoided to prevent tachycardia and the onset of cardiac failure. There have been reports of tachycardia, hyperthermia, abdominal pain, and dark-colored urine under general anesthesia in patients with Duchenne muscular dystrophy. A number of case reports describe intraoperative succinylcholine- and halothane-induced cardiac arrest associated with acidosis, myoglobinuria, increased serum creatine kinase, and hyperthermia.~~ One explanation of these arrests is hyperkalemia caused by muscle injury induced by succinylcholine and aggravated by

331 halothane. It also has been suggested that these complications are consistent with malignant hyperthermia, since muscle biopsies in these patients have shown susceptibility to malignant hyperthermia, ~5 suggesting that only nontriggering anesthetic agents should be used for patients with a known history of muscular dystrophy. Duchenne muscular dystrophy patients who develop suspicious signs under anesthesia should be tested for MH to identify patients and families at risk. However, Duchenne muscular dystrophy patients who are not MH susceptible may still be at risk of developing rhabdomyolisis or cardiac arrest on exposure to succinylcholine or volatile anesthetic agents. To avoid some of the problems associated with general anesthesia, regional anesthesia should be considered when appropriate. Postoperative respiratory insufficiency as well as cardiac failure are of concern in patients who are already compromised in terms of their disease, along with the added factors presented by the anesthetic agents. An intensive care setting for close observation is required for these patients postoperatively. There are other forms of muscular dystrophy, such as limb-girdle, scapuloperoneal dystrophy, oculopharyngeal, and humeroperoneal, that are slowly progressive, with clinical manifestations appearing at later stages of life. Anesthetic risk should be considered with all kinds of muscular dystrophy, and a thorough preoperative evaluation must be conducted before anesthetic administration.

MYOTONIC SYNDROMES Myotonic syndromes involve the inability of muscles to relax after voluntary contraction because of persistent muscular electrical activity in the muscles. Myotonia may be provoked by other mechanical stimulation or simply by voluntary movement. Electromyographically, myotonia appears as a high-frequency repetitive discharge with a waxing and waning of amplitude and frequency that may be heard as a characteristic "dive-bomber" sound. Instability of muscle membranes leads to repetitive discharges from a single stimulus. This pathophysiology cannot be abolished with nerve or spinal blockades or muscle relaxants. The most important kinds of myotonia are dystrophica congenita and paramyotonia congenita.

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Myotonia Dystrophica (Steinert's disease) Myotonia dystrophica, an inherited autosomal dominant disease, is the most common and prevalent form of myotonia, with an incidence of 13 per 100,000 live births. The responsible genes for this myotonia have been localized to the long arm of chromosome 19. The onset of symptoms usually occurs in the second to third decades of life. Weakness, wasting, and atrophy particularly affect the facial sternocleidomastoid, trapezii, and limb musculature. Weak bulbar muscles may result in dysarthria, accumulation of saliva, and dysphagia. Myotonia is enhanced by hypothermia, most evident in the limb and mastication muscles. Premature frontal balding, cataracts, and testicular atrophy are common. Pulmonary and cardiac complications contribute greatly to high morbidity and mortality in this disorder. Cardiac involvement may precede the diagnosis of myotonic dystrophy by years. Atrioventricular and intraventricular conduction disturbances, left axis deviation, and nonspecific ST-segment changes are the most common cardiac abnormalities. Autopsy of the heart muscles has shown degeneration analogous to the skeletal kind. Respiratory insufficiency due to the weakness of the diaphragmatic and thoracic musculature is common in myotonic dystrophy. In the early stages of the disease, the maximum respiratory pressure appears to be a more sensitive indicator of diminished respiratory reserve than either spirometry or mixed venous Pc02. A restrictive pattern develops with progression of the disease, with decreases in expiratory reserve volume, vital capacity, minute ventilation, and maximal breathing capacity. A number of endocrine abnormalities have been reported, including abnormal glucose tolerance with an increased incidence of diabetes mellitus, and decreased androgenic function of gonads and adrenal cortex, resulting in a diminished negative feedback on pituitary growth hormone that leads to acromegalic-like changes. A neonatal or congenital myotonic dystrophy inherited from a myotonic mother has been reported. The pregnancy is often complicated with hydramnios and poor fetal movement prenatally and uterine atony postnatally. The infant is hypotonic at birth and exhibits fascial weakness and severe respiratory distress, or presents shortly after with difficulty in suckling or swallowing.

MASHALLAH GOODARZI The cause of weakness at birth involves maternal intrauterine factors that affect those individuals who have the dominant gene for myotonic dystrophy.

Myotonia Congenita (Thompson's Disease) Myotonia congenita is an inherited autosomal dominant, relatively benign, and rare disorder. It becomes less severe with age. There is no atrophy of muscles. The main complaint of patients is muscle stiffness, which could be induced by initiation of movement or exposure to cold and may be relieved by continued exercise. Muscle hypertrophy may be noted in later life. The pathophysiology underlying this myotonia appears to be decreased chloride conductance across the muscle membrane.

Paramyotonia Congenita Paramyotonia congenita is a rare disorder with autosomal dominant inheritance. It is commonly induced by exposure to cold, and worsens with repetitive activity. The membrane defect appears to be the result of increased sodium permeability, which is further increased by cooling.

Anesthetic Management The degree of muscle weakness and the effects on respiratory and cardiovascular systems and other organ involvement are considered the most important in the management of these patients. The overall mortality in myotonia dystrophica is higher than normal. Recognizing the disorder preoperatively and being aware of problems associated with the anesthetic management of these patients may help to avoid some of these complications. Patients with respiratory weakness are extremely sensitive to respiratory depressant drugs. Small doses of sodium thiopental may cause prolonged apnea, especially after premedication with narcotic or benzodiazopine. Agents that cause shivering should be avoided. ~6Warming intravenous solutions and inhaled gases to a controlled ambient temperature is important. The response to succinylcholine in myotonia is unpredictable. It may induce generalized myotonia, which may impede intubation and adequate ventilation. The response to a nondepolarizing muscle relaxant is normal, but reversal of these drugs with an anticholinesterase may aggravate myotonia or produce longlasting muscle weakness. The

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shorter-acting, nondepolarizing muscle relaxants atracurium, vecuronium, and mivacronium may obviate the need for reversal with an anticholinesterase. 17'~8 While there have been reports of malignant hyperthermia in patients with myotonic dystrophy, the relationship between MH and myotonia congenita is unclear.19 Close monitoring of these patients is required. Epidural, spinal, or local infiltration of anesthesia are the alternative choices to general anesthesia for these patients. JUVENILE RHEUMATOID ARTHRITIS

Juvenile rheumatoid arthritis (JRA) is a chronic, systemic inflammatory disorder of unknown etiology that involves the joints. Susceptibility to JRA seems to be determined by the genes of the major histocompatibility complex. It is likely that multiple stimuli may trigger disease in the immunogeneticaUy susceptible host. Whatever the inciting event, the immune system is activated and a pathologic perpetuation of the inflammatory response occurs, both at the joints and systematically. The incidence is 1% to 3% and involves all racial and ethnic groups. The disease is two to three times more common in women. Synovitis of the cricoarytnoid joints occurs in 26% to 53% of JRA patients. The patients may have a feeling of fullness or tension in their throat, dysphagia, and pain with talking. Caused by chronic ankylosis in the joints, patients develop a husky voice and stridor during sleep secondary to the reduced cord mobility. Vasculitis with involvement of vasovasorum of the neurologic supply to the larynx may also produce vocal cord paralysis. Keenan et al2~ have described a triplane deviation of the larynx in patients with vertical atlantoaxial subluxation. Synovitis of the temporomandibular joint occurs in 66% of JRA patients; joint dysfunction increases with disease duration. Although mouth opening may be impaired because of temporomandibular joint dysfunction, this is rarely of clinical significance with respect to intubation. Approximately 10% of the JRA patients will have the sicca complex, which increases the risk of epistaxis and requires careful vasoconstriction and copious lubrication. Dehydration may lead to a dry and cracked tongue. These patients may not be able to tolerate fasting for long periods. Involvement of the cervical spine is common in JRA, either manifested

333 Table 1. Six "Alarm Signals"

1. Severeneck pain, often radiating to the occiput 2. Disturbed bladder function varying from incontinence to retention 3. Diminished motor power in the arm or legs 4. "Jumping" legs 5. Numbnessor tingling in the fingers or feet 6. A "marble sensation" in the limbs or trunk

as clinical symptoms (45% to 70%) or as discrete radiologic abnormalities. Synovitis may occur at any of the joints or bursa of the cervical spine. Pathology of the cervical spine may be divided into two distinct anatomic regions: the occipitoatlanto-axial complex and the subaxial cervical spine. Atlanto-axial subluxation is the more common pathology in J R A . 19 The subluxation is maximal in flexion, and in most cases luxation will reduce with extension. There is destruction of the C1-C2 facet joints and articular masses, and the dens may invaginate through the foramen magnum. Ten percent to 50% of patients are reported to develop neurologic symptoms related to brain stem compression. Preoperative goals in the evaluation of the cervical spine are to determine stability and whether there are neurologic symptoms. Meijers et al2~ described six "alarm signals" (Table 1) that appear to indicate spinal cord damage. The heart and vasculature are frequently involved in JRA. The incidence of symptomatic pericarditis is approximately 1% in JRA patients, but echocardiography demonstrates involvement of 33% to 40% of patients with pericarditis. There is a 10-fold male predominance and most patients have pleural effusions. Lesions similar to rheumatoid nodules may involve the myocardium and the valves. Mortality related to cardiac disease in patients with JRA is twice that of the general population. The respiratory system, like the cardiovascular, is frequently involved in JRA, but commonly this involvement is asymptomatic. Pleural effusion may occur at any time, and adhesions are common. The incidence of pulmonary fibrosis in JRA patients is greater than 10%, with rare clear radiologic evidence. Fibrosis alveolitis usually occurs early in the course of the disease, within 2 to 3 years of the onset of joint symptoms. Multiple drugs are used for JRA, each one

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of them having toxicities that are important to evaluate. Aspirin compounds interfere with platelet function for 4 to 7 days after discontinuation of these medications. There is much debate regarding the safety of regional anesthesia for patients exposed to these drugs. Bleeding time is rarely a useful test in these patients, unless the medication has been stopped. The goal is to demonstrate a normal platelet function. Increasing numbers of JRA patients are being treated with methotrexate, which can cause methotraxate-induced lung disease. Finally, we recommend a full coverage of hydrocortisone, or the equivalent, in the 24 hours beginning 6 to 8 hours preoperatively, with a gradual tapering dose. This is for all patients who have required prednisone for a period longer than 1 week within the preceding 6 months.

Anesthetic Management Management begins with a preoperative interview designed to establish duration, systemic manifestation, and drug therapy. The physical examination covers the airway, cervical spine, and cardiovascular and respiratory systems. Routine laboratory work plus cervical spine radiography, especially in chronic JRA cases, are necessary. Airway management is a great concern in administering general anesthesia. When cervical spine abnormalities are present, these patients should be intubated while awake. 22 Different methods of dealing with difficult airways have been described. No method has proven to be superior, and success with low morbidity is usually a function of operator experience. Fiberoptic laryngoscopy is the most useful, and every attempt should be made to make the patient comfortable with intravenous sedation and topicalization; if a nasal route is chosen, vasoconstriction of nasal mucosa should be considered. Maintenance of anesthesia should be tailored to the needs of the patient. Positioning is critically important; copious padding should be applied to all surfaces. Extubation should be delayed until there is clear evidence that the patient can maintain the airway. Regional anesthesia is an acceptable technique for JRA patients. The evidence of adverse reaction to regional anesthesia is very low. Supraclavicular block is a good approach for upper extremity surgery. Intravenous regional anesthesia is an excellent technique for hand and forearm

MASHALLAH GOODARZI Table 2. Classification of Scoliosis

1. Idiopathic 2. Neuromuscular (cerebral palsy, polio, or muscular dystrophy) 3. Congenital 4. Neurofibromatosis 5. Mesenchymal disorder (Marfan's syndrome) 6. Rheumatoid disease 7. Extraspinal contractures (post-burn) 8. Osteochondrodystrophies (dwarfism) 9. Infection of the spine 10. Metabolic disease (rickets) 11. Trauma 12. Tumors (eg, osteoid osteoma, bone cyst)

surgery. Lower extremity anesthesia may be accomplished with epidural or spinal technique. Since the lumbar and thoracic spines are less frequently involved, access is not usually a problem. SCOLIOSIS (SPINAL FUSION)

Scoliosis, a lateral curvature of the spine, has been divided into structural and nonstructural patterns, depending on the presence or lack of flexibility. Nonstructural scoliosis, such as those related to leg length discrepancies or sciatica, may be resolved completely with attention to the underlying causes, In the structural type, there is a failure to have equal flexibility on either the left or right bending films. This is often associated with rotational deformities of the vertebral bodies and the surrounding structures, such as the rib cage. The magnitude is measured in degrees and is determined by the Cobb technique. The upper and lower end vertebrae of the curve determined at the angle formed by the intersec. tion of perpendiculars to the lines drawn through the end plates of the vertebral bodies (Fig 1). The direction of the curve is determined by the convexity, which would be either right or left. The overall incidence is 1.8 per 1,000, with a higher rate in females. The progression is most likely to occur in the rapid adolescent growth phase. Table 2 shows a classification of scoliosis.

Operative and Nonoperative Therapy Various techniques, such as orthosis, exercise, and electrostimulation, have been investigated. The aim of these devices is to prevent progression of the curves. All may have problems, de-

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pending on patient compliance. Realistically, only orthotic treatment achieves any control of progression of the curvature(s). The main indication for surgery is the progression of the curve despite adequate nonsurgical therapy. The goal of surgery is fusion of the spinal curve to prevent further deterioration of the curve. A magnitude less than 40 degrees generally is not an indication for surgery, unless there is severe pain, neurologic deficits, or documented recent progression related to scoliosis (especially certain congenital curves with unbalanced hemivertebra). Anterior spinal surgery is used in more severe curvatures, which are less flexible, or in congenital deformities, which require combined anterior and posterior osteotomy and fusion.

Preoperative Evaluation Preoperative clinical evaluation for scoliosis patients should focus on the patient's respiratory and cardiovascular systems. The respiratory system usually shows the greatest impact of the disease process. 23 A history of recent or recurrent respiratory infections, smoking, or asthmatic conditions should all be carefully evaluated. Routine chest inspection and auscultation are indicated, and a chest radiograph should be obtained. Arterial blood gas analysis and simple spirometry (vital capacity and forced expired volume in 1 second [FEVl] in the supine and seated positions commonly show a reduced arterial oxygen tension and a restrictive defect. In pediatric patients, a large group with scoliosis have other congenital anomalies. Elliot and Eugene 24 have shown that 35% of their patients were diagnosed as having cerebral palsy. Often these children are developmentally delayed and malnourished in addition to their neurologic and musculoskeletal problems. As a group, they had the largest number of complications preoperatively. Spina bifida patients occupied 18% of their patient population. These patients had the highest level of anxiety due to previous operative procedures, a considerable amount of blood loss, and the longest duration of surgery. The patients with Duchenne muscular dystrophy had a very high amount of blood loss (179.7%) of their estimated blood volume, and the duration of surgery was as long as that for spina bifida patients (9 to 9.5 hours). Neurofibromatosis, Marfan's syndrome, and Scheuermann's syndrome comprised 20% of

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their patient population. Pulmonary function testing often shows impairment, specifically of the vital capacity and presence of ventilation/perfusion abnormalities. Defective lung function will be further impaired postoperatively, necessitating ventilatory suport in the immediate postoperative period. Those patients who are uncooperative or difficult to test preoperatively frequently also have incoordination of the swallowing and cough reflexes, with a history of repeated pulmonary infections, putting them at high risk for anesthesia induction and maintenance. Any patients with a complicated history of pulmonary infection should be admitted before surgery for chest physiotherapy to optimize their respiratory status. CARDIOVASCULAR AND NEUROLOGIC STATUS

Children with dystrophic neuromuscular disorders and those who have pulmonary hypertension due to chronic hypoxia ought to have echocardiography, in addition to an electrocardiogram and chest x-ray. If the cardiovascular system is compromised, then surgery is deferred until the patient is treated and in optimal condition. Children with cerebral palsy may have multiple neurologic involvement. The degree of spasticity, athetoid movements, and limb contracture are often significant with respect to the positioning of the patient. Incoordination of the pharyngeal muscles causes poor swallowing and varying degrees of upper airway obstruction. Diminished cough reflexes increase the potential for repeated aspiration, all of which should be evaluated preoperatively. Many of these children have convulsive disorders, which are controlled with anticonvulsive medications. The blood level of anticonvulsive drugs should be known and every effort should be made to bring them to the optimal level of therapy preoperatively. Spina bifida patients with hydrocephalus usually have a ventricular shunt to control the intracranial hypertension. The site of the shunt and its functional adequacy are important factors in the management of anesthesia. Impairment of bladder function is often present in children with neurologic disease. Chronic bladder infection should be treated preoperatively. Psychological preparation of these patients is very important. Two factors must be considered before pharmacologic premedication is used. The

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MASHALLAH GOODARZI

neurologic status, especially the protective reflexes, should not be compromised. It is essential to explain to the patient the possibility of recall if a wake-up test is performed. Other procedures, such as evoked potential monitoring, and blood loss should be discussed in detail. Blood loss is an invariable fact in these procedures. In our institution, blood loss ranged from 35% of estimated blood volume in idiopathic scoliosis to 200% of estimateb blood volume in a complicated scoliosis. If possible, the patient may donate up to 4 U of autologous blood during the 4 weeks before surgery. In addition to the autologous blood, one should ensure that packed cells are also available. The patient should be familiar with routine postoperative procedures, such as intensive care unit admission, pain control, the necessity of movement while in the bed, and chest physiotherapy if indicated.

ANESTHESIA AND MONITORING The anesthetic choice is predicated by the patient's preoperative condition and severity of the disease. Inhalation or intravenous anesthetic techniques are both acceptable for induction. A nondepolarizing muscle relaxant is a good choice for intubation, and the patient should be kept paralyzed during the entire procedure to prevent an increase in intra-abdominal pressure and pressure on the inferior vena cava (IVC). Maintenance of anesthesia may be achieved with either narcotic or inhalation techniques. In our institution a combination of inhalation and narcotic anesthesia (balanced technique) is used because the need for high concentrations of inhaled agents may be kept at a minimum to prevent interference with somatosensory evoked potential monitoring. Routine monitoring (electrocardiography, blood pressure, pulse oximeter, endtidal Co2, nerve stimulator, esophageal stethoscope, temperature, urine output), plus an arterial line with two large intravenous cannulas are indicated for scoliosis surgery. There recently have been reports of venous air emboli during both anterior and posterior spinal instrumentations, which is an alarm for routine monitoring of these surgical procedures by Doppler ultrasound or echocardiography and insertion of a central venous catheter, the position of which should be evaluated by chest x-ray film. In our institution, among the 250 posterior and anterior spinal fusion cases seen in the past 5 years, we had one patient with severe scoliosis and mylomeningo-

cele and ventriculoperitoneal shunt who developed massive fatal air emboli during anterior spinal instrtunentation. The percentage of air emboli in these patients is not clear because most of them are not monitored for detection of air emboli. Studies have shown that the use of multifenestrated catheter has a higher chance of aspirating air in the event of massive air emboli. Proper positioning on the frame to avoid pressure areas and to prevent nerve injury is very important. To simplify the movement of the patient from the supine to the prone position, the various monitors are disconnected, with the pulse oximeter being the last. This is the first monitor to be connected in the prone position, and the cardiovascular and respiratory status of the patient should be assessed to ensure stability in the prone position. The face should be checked to prevent any pressure on the eyes. Decompression of the stomach before turning may prevent increased intragastric pressure. Placement of a pulmonary artery catheter is not indicated in the vast majority of cases. It should, however, be seriously considered in those patients in whom left ventricular function is compromised or in whom fluid replacement demands extreme vigilance because of concomitant pathology. Various methods have been suggested to reduce the blood loss. Positioning the patient on a well-padded frame with enough support for the anterolateral aspect of the upper thoracic and pelvic girdle is very important. The patient's abdomen should be free of any pressure to minimize the pressure on the IVC. Minimizing pressure on the abdomen maintains functional residual capacity at the near-normal level. Complete relaxation of the abdominal and diaphragmatic muscles decreases the intra-abdominal pressure and the pressure on the IVC. Every effort should be made to prevent coughing, bucking, airway obstruction, gastric inflation, and straining to decrease the pressure on the IVC. Infiltration of epinephrine (1/400,00 concentration) before incision should decrease the blood loss from subcutaneous tissues. The dose of epinephrine in 1:100,000 solution should not exceed 0.15 mL/kg per 10-minute period and 0.45 mL/ kg per hour if halothane anesthesia is used. However, a larger dose may be used if other inhalation agents are used. The use of hypotensive anesthesia for reducing the amount of blood loss has been suggested, a6'27 The recommendation is to keep systolic blood

PEDIATRIC ORTHOPEDIC ANESTHESIA pressure at 55 mm Hg to prevent any detrimental effect to the blood supply of the spinal cord. Different techniques have been advocated to induce the hypotensive anesthesia. The most appropriate one with the fewest side effects for the patient should be used. Hypotensive technique does not change the length of the surgery and can interfere with somatosensory-evoked potential monitoring in some occasions with deep hypotension. Even though there can be less blood loss intraoperatively, overall the total amount of blood loss intraoperatively and postoperatively is not any different from those procedures without hypotensive technique. A major concern about hypotensive anesthesia during scoliosis surgery has been the potentially increased risk of spinal cord injury. It is feared that reducing spinal cord blood flow by hypotensive techniques could increase the risk of spinal cord injury. The results of different studies 2s have suggested that there is little, if any, increased risk of neurologic injury with hypotensive anesthetic techniques. Somatosensory evoked potential has shown no significant change with moderate hypotension.2s The distraction of the spine under moderate hypotension is no more hazardous than during normotensive stages. Important factors govern the success and safety of induced hypotension during scoliosis surgery. The benefit gained from the use of this technique depends on the skill of the surgeon. Good surgical technique is enough to match the high standards of hypotensive anesthetic technique. Intraoperative recovery of lost blood entails recovery of the drainage from the suction bottles. The red blood cells are filtered, washed, concentrated, and then reinfused into the patient. Intraoperative blood recovery systems may recover approximately 50% of the lost red blood cells. 29 Hemodilution decreases the loss of red blood cell mass by reducing the hematocrit of the blood lost intraoperatively. The patient is phlebotomized in the operating room, and several units of blood are removed and preserved. Circulating volume is maintained by crystalloid replacement. The operation is performed at normal blood pressure, and at the end of the operation the patient is diuresed of the excess fluid and the patient's own blood is transfused.3~ Hemodilution decreases the oxygen-carrying capacity of the blood, so the patient must be monitored carefully.

337 When conditions such as marked reduction in oxygen delivery to the tissues (anemia, low fixed cardiac output, low oxygen saturation); presence of cerebral, renal, or cardiac disease (right to left shunt); sickle cell disease; and uncorrected polycythemia exist, hypotensive anesthesia and hemodilution should be considered contraindicated. We have used intrathecal morphine in a series of anterior and posterior spinal instrumentations after the induction of anesthesia and have found this technique to be very helpful in reducing the anesthetic requirement and decreasing the amount of blood loss. This could be due to the blockade of pain receptors or blockade of autonomic nervous system due to local anesthetic activity of opioid drugs. The other advantages of intrathecal morphine is coverage for postoperative pain control. The average duration of pain control postoperatively is approximately 15 to 20 hours, when it is given at the dose of 20 #g/kg. Assessing and adequately replacing the blood loss is a major concern. Sponges should be weighed as soon as they are removed from the surgical field. Volumes of irrigation must be measured accurately, and blood on the floor, gowns, and drapes should be taken into consideration. Avoidance of hypothermia is important and can be achieved by heating and humidifying the inspired gases and using the circle system. The technique of anesthesia should be modified for anterior spinal fusion because of intra-abdominal or thoracic approaches. Bleeding usually is not as excessive. Selective one-lung ventilation of the dependent lung may be done in older children. Patients should be well monitored and serial blood gases should be checked during singlelung ventilation. It is advisable to expand the lung intermittently during the procedures. Postoperative care is equal to that of a thoracotomy. Control of secretion, atelectasis, hypoventilation, and pain are important. INTRAOPERATIVE SPINAL CORD MONITORING Two methods are routinely used: wake-up test during operation and somatosensory-evoked potential monitoring. The intraoperative awakening test is a gross test of spinal motor function.31 It is unreliable in patients with mental retardation, cerebral palsy, or lower limb palsy, and in very

338 young patients. The use of the awakening test needs prior preparation of the patient and special technique of anesthesia, with close cooperation between anesthesiologist and surgical teams. In our institution, the wake,up test is reserved only for those patients who develop a prolonged somatosensory-evoked potential abnormality that is not reversible after a short period of time. MacEwen et a132reported that the incidence of neurologic injury was 1,17%, with 114 occurrences in 9,680 patients. Thirteen of these patients had injury to cranial or peripheral nerves, most of which completely resolved. The incidence of permanent spinal cord injury in their study was approximately 0.6%. Other investigators reporting on the use of the wake-up test for monitoring have noted an apparent spinal cord injury rate of 2% to 5%. In this group, the deficits were usually resolved intraoperatively by modification of the amount of spinal column distraction. The use of sublaminar wiring has a higher incidence of neurologic injury. The causes for this neurological injury usually arise from stretching and distraction of blood vessels, which decreases the blood supply and produces ischemia of neural elements. Cerebral neurons can survive a period of ischemia, although there will be no outward sign of neuronal activity during the ischemia, but when the blood supply returns to normal the neuron activities return to normal. Somatosensory-evoked potentials assess the function of sensory pathways responsible for vibration and position senses. Application of electrical stimuli to the peripheral nerve produce signals, termed evoked potentials, that can be recorded at various points along the sensory pathway. Evoked potential monitoring is safe and permits serial measurements over long periods of time. The major disadvantage is sensitivity to physiologic changes (temperature, hypotension, anesthetic agents). The anesthesiologist must provide a stable pharmacologic and physiologic situation to prevent alteration of recorded evoked potentials. Volatile anesthesia produces more disturbing signals than intravenous anesthesia. In our institution, the use of low doses of volatile agents with narcotic or intrathecal morphine has produced stable and successful monitoring. Despite all careful screening, false-positive changes have occurred with evoked potential monitoring at a rate of 40%. The injury to the ventral portion

MASHALLAH GOODARZI of the spinal cord in which the major motor pathways reside might go undetected with the present monitoring modality.33 Motor-evoked potential monitoring that applies stimuli to the motor pathway cephalad or proximal to the surgical site and that transmits either transcranially or to the spinal cord is available in some centers. There are concerns of injury to the central nervous system due to the intense stimuli required during such monitoring.

POSTOPERATIVE MANAGEMENT Blood loss may continue to be significant. The patient must be monitored with urine output, electrocardiogram, pulse-oximeter, blood pressure, and, if needed, central venous pressure. The amounts and type of fluid replacement required are indicated by all these parameters as well as regular evaluation of hemoglobin, hematocrit, electrolyte, albumin, and serum protein content. Urine output should be kept at no less than 1 mL/kg/hr. Hematocrit should be at the level of 30%. The translocation of fluid to the extracellular space is at a maximum for the initial 12 to 24 hours postoperatively. Diuresis starts after this initial period. Ringer lactate is the ideal choice to keep the urine output at the normal level. Blood glucose should be checked periodically to prevent hypoglycemia. Packed blood cells and other blood products are given only when necessary. Those patients who have borderline respiratory function or neurologic deficit involving reflex protection of airways or cardiovascular instability may need postoperative ventilatory supports, which can best be performed by nasotracheal intubation. Postoperative pain management of these patients has changed from the mainstay of intravenous narcotics to epidural narcotics (morphine 0.5 #g/kg/hr, fentanyl 1 to 2 #g/kg/hr, sufentanyl 0.2 #g/kg/hr). The injection of 20 #g/kg morphine intrathecally after the induction of anesthesia or a continuous intrathecal catheter with morphine may control the postoperative pain very safely. Regardless of the route of administration, the continuous measurement of oxygen saturation is necessary to ensure that there is no inadvertent hypoxemia or hypoventilation. Paralytic ileus is frequently present for the initial 48 to 72 hours postoperatively, and can be treated by insertion of a nasogastric tube to prevent disten-

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