Anesthesia for Muscular Dystrophies

MUSCULAR DYSTROPHIES

Preoperative Considerations

Muscular dystrophies are a heterogeneous group of hereditary disorders characterized by muscle fiber necrosis and regeneration, leading to muscle degeneration and progressive weakness. Anticipated anesthetic risk is increased by the patient’s overall debilitated status, which may impede clearance of secretions and postoperative ambulation, as well as by increased risk of respiratory failure and pulmo-nary aspiration. Duchenne’s muscular dystrophy is the most common and most severe form of muscu-lar dystrophy. Other muscular dystrophy variants include Becker’s, myotonic, facioscapulohumeral, and limb-girdle dystrophies.

Duchenne’s Muscular Dystrophy

An X-linked recessive disorder, Duchenne’s muscu-lar dystrophy affects males almost exclusively. It has an incidence of approximately one to three cases per 10,000 live male births and most commonly presents between 3 and 5 years of age. Affected individuals produce abnormal dystrophin, a protein found on the sarcolemma of muscle fibers. Patients character-istically develop symmetric proximal muscle weak-ness that is manifested as a gait disturbance. Fatty infiltration typically causes enlargement (pseudohy-pertrophy) of muscles, particularly the calves. Progressive weakness and contractures eventually result in kyphoscoliosis. Many patients are confined to wheelchairs by age 12. Disease progression may be delayed by up to 2–3 years with glucocorticoid therapy in some patients. Intellectual impairment is common but generally nonprogressive. Plasma cre-atine kinase (CK) levels are 10–100 times normal even early in the disease and may reflect an abnor-mal increase in the permeability of muscle cell membranes. Female genetic carriers often also have high plasma CK levels, variable degrees of muscle weakness, and, rarely, cardiac involvement. Plasma myoglobin concentration may also be elevated. The diagnosis is confirmed by muscle biopsy.

Respiratory muscle degeneration in patients with muscular dystrophy interferes with aneffective cough mechanism and leads to retention of secretions and frequent pulmonary infections. The combination of marked kyphoscoliosis and muscle wasting may produce a severe restrictive ventilatory defect. Pulmonary hypertension is common withdisease progression. Degeneration of cardiac muscle in patients with muscular dystrophy I also common, but results in dilated or hypertrophic cardiomyopathy in only 10% of patients. Mitral regurgitation secondary to papillary muscle dys-function is also found in up to 25% of patients. Electrocardiogram abnormalities include P–R inter-val prolongation, QRS and ST-segment abnormali-ties, and prominent R waves over the right precordium with deep Q waves over the left precor-dium. Atrial arrhythmias are common. Death at a relatively young age is usually due to recurrent pul-monary infections, respiratory failure, or cardiac failure.

Becker’s Muscular Dystrophy

Becker’s muscular dystrophy is, like Duchenne’s, an X-linked recessive disorder but is less common (1:30,000 male births). Manifestations are nearly identical to those of Duchenne’s muscular dystrophy except that they usually present later in life (adoles-cence) and progress more slowly. Mental retardation is less common. Patients often reach the fourth or fifth decade, although some may survive into their 80s. Death is usually from respiratory complica-tions. Cardiomyopathy may occur in some cases and may precede severe skeletal weakness.

Myotonic Dystrophy

Myotonic dystrophy is a multisystem disorder that is the most common cause of myotonia, a slowing of relaxation after muscle contraction in response to electrical or percussive stimuli. The disease is auto-somal dominant, with an incidence of 1:8000, and usually becomes clinically apparent in the second to third decade of life, but it has also been reported as a paraneoplastic disorder in association with thy-moma. Myotonia is the principal early manifesta-tion; muscle weakness and atrophy become more prominent as the disease progresses. This weakness and atrophy usually affect cranial muscles (orbicu-laris oculi and oris, masseter, and sternocleidomas-toid), and in contrast to most myopathies, distal muscles more than proximal muscles. Plasma CK levels are normal or slightly elevated.

Multiple organ systems are involved in myo-tonic dystrophy, as evidenced by presenile cataracts, premature frontal baldness, hypersomnolence with sleep apnea, and endocrine dysfunction leading to pancreatic, adrenal, thyroid, and gonadal insuffi-ciency. Respiratory involvement leads to decreased vital capacity, and chronic hypoxemia may cause cor pulmonale. Gastrointestinal hypomotility may pre-dispose patients to pulmonary aspiration. Uterine atony can prolong labor and increase the incidence of retained placenta. Cardiac manifestations, which are often present before other clinical symptoms appear, may include cardiomyopathy, atrial arrhyth-mias, and varying degrees of heart block.

The myotonia is usually described by patients as a “stiffness” that may lessen with continued activity—the so-called “warm-up” phenomenon. Patients often report that cold temperatures worsen stiffness. Antimyotonic treatment may include mexiletine, phenytoin, baclofen, dantrolene, or car-bamazepine. A cardiac pacemaker may be placed in patients with significant conduction defect, even if they are asymptomatic.

Facioscapulohumeral Dystrophy

Facioscapulohumeral dystrophy, an autosomal dominant disorder with an incidence of approxi-mately 1–3:100,000, affects both sexes, although more females than males are asymptomatic. Patients usually present in the second or third decade of life with weakness that is confined primarily to the muscles of the face and the shoulder girdle. Muscles in the lower extremities are less commonly affected, and respiratory muscles are usually spared. The dis-ease is slowly progressive with a variable course. Plasma CK levels are usually normal or only slightly elevated. Cardiac involvement is rare, but loss of all atrial electrical activity with an inability to atrially pace the heart has been reported; ventricular pacing is still possible in these patients. Longevity is mini-mally affected.

Limb-Girdle Dystrophy

Limb-girdle muscular dystrophy is a heteroge-neous group of genetic neuromuscular diseases. Limb-girdle syndromes include severe childhood autosomal recessive muscular dystrophy and other incompletely defined autosomal recessive syn-dromes such as Erb’s (scapulohumeral type) and Leyden–Mobius (pelvifemoral type) dystrophies. Most patients present in childhood to the second or third decade of life with slowly progressive muscle weakness that may involve the shoulder girdle, the hip girdle, or both. Plasma CK levels are usually ele-vated. Cardiac involvement is relatively uncommon but may present as frequent arrhythmias or conges-tive heart failure. Respiratory complications, such as hypoventilation and recurrent respiratory infec-tions, may occur.

Anesthetic Considerations

A. Duchenne’s and Becker’s Muscular Dystrophies

The anesthetic management of these patients is com-plicated not only by muscle weakness but also by cardiac and pulmonary manifestations. An associa-tion with malignant hyperthermia has been sug-gested but is unproven. Preoperative premedication with sedatives or opioids should be avoided because of increased aspiration risk due to respiratory mus-cle weakness, gastric hypomotility, or both. Intraoperative positioning may be complicated by kyphoscoliosis or by flexion contractures of theextremities or neck. Succinylcholine should be avoided in patients with Duchenne’s or

Becker’s muscular dystrophies because of unpredict-able response and the risk of inducing severe hyperkalemia or triggering malignant hyperther-mia. Although some patients exhibit a normal response to nondepolarizing NMBs, others may be very sensitive. Marked respiratory and circulatory depression may be seen with volatile anesthetics in patients with advanced disease, and regional or local anesthesia may be preferable in these patients. Perioperative morbidity is usually due to respiratory complications. Patients with vital capacities less than 30% of predicted appear to be at greatest risk and often require temporary postoperative mechanical ventilation.

B. Myotonic Dystrophy

Patients with myotonic dystrophy are at increased risk for perioperative respiratory and cardiac com-plications. Most perioperative problems arise in patients with severe weakness and in those cases in which surgeons and anesthesiologists are unaware of the diagnosis. The diagnosis of myotonic dystro-phy has been made in some patients in the course of investigating prolonged apnea following general anesthesia.

Patients with myotonic dystrophy have altered responses to a number of anesthetic medicines. They are often very sensitive to even small doses of opioids, sedatives, and inhalation and intrave-nous anesthetic agents, all of which may cause sud-den and prolonged apnea. Premedication should therefore be avoided. Succinylcholine is relatively contraindicated because it may precipitate intense myotonic contractions, complicating orotracheal intubation. Myotonic contraction of respiratory, chest wall, or laryngeal muscles may make ventila-tion difficult or impossible. Other drugs that act on the motor end plate, such as decamethonium, neo-stigmine, and physostigmine, can aggravate myo-tonia. Regional anesthesia may be preferentially employed, but does not always prevent myotonic contractions.

The response to nondepolarizing NMBs is reported to be normal; however, they do not con-sistently prevent or relieve myotonic contractions. As reversal of nondepolarizing NMBs can induce myotonic contractions, the use of short-acting non-depolarizing agents is recommended. Postoperative shivering commonly associated with volatile agents, particularly when associated with decreased body temperature, can induce myotonic contractions in the recovery room. Small doses of meperidine can often prevent such shivering and may preempt myo-tonic contractions.

Induction of anesthesia without complications has been reported with a number of agents includinginhalation agents and propofol. Neuromuscular blockade, if needed, should employ short-acting NMBs. An association between myotonic dystrophy and malignant hyperthermia has been suggested but not established. Nitrous oxide and inhalation agents can be used as maintenance anesthesia. Reversal with anticholinesterases should be avoided, if possible.

The principal postoperative complications of myotonic dystrophy are prolonged hypoventilation, atelectasis, aspiration, and pneumonia. Close post-operative monitoring should be accompanied by aggressive pulmonary hygiene with physical therapy and incentive spirometry. Aspiration prophylaxis is indicated. Patients undergoing upper abdominal surgery or those with severe proximal weakness are more likely to experience pulmonary complications. Perioperative cardiac conduction abnormalities are less likely to occur but still warrant close cardiovas-cular monitoring.

C. Other Forms of Muscular Dystrophy

Patients with facioscapulohumeral and limb-girdle muscular dystrophy generally have normal responses to anesthetic agents. Nevertheless, because of the great variability and overlap among the various forms of muscular dystrophy, sedative-hypnotics, opioids, and nondepolarizing NMBs should be used cautiously, and succinylcholine should be avoided.