This classification includes Becker muscular dystrophy and Duchenne muscular dystrophy. They are the same disease in terms of their genetic origin: the dystrophy gene; however, they differ in terms of the dystrophin deficit which is almost complete in Duchenne's disease, but only partial in Becker's disease. This is why they have a very different clinical presentation.

Duchenne's disease is the most common and severe muscular dystrophy; the mean age of diagnosis is 3 to 5 years for Duchenne's dystrophy, whereas for Becker's dystrophy is adulthood.³ It presents mostly in males; however, some women carriers of the gene may present muscular or cardiac pathology.^4-6

The clinical sign in Duchenne's disease is severe progressive muscular weakness that begins during childhood (approximately around 18 months of age), leading to a definite loss of gait at around 10 years of age, due to a pathological muscular reorganization with fatty infiltration and increased fibrous tissue that replaces the muscular tissue.³

The progressive course of Duchenne's dystrophy compromises the cardiac and respiratory function due to the involvement of the respiratory muscles. The overall survival of these patients is 20 to 30 years, and they die because of ventilatory failure in 90% of the cases and 10% due to heart failure.^5,7,8 After 30 years, over 90% of the patients with Duchenne present cardiac involvement.⁵ The characteristic clinical findings are progressive weakness of the lower extremities, pseudohypertrophy of the gastrocnemious muscles (evident between 2 and 6 years of age), high levels of creatinkinase, chronic pulmonary disease featuring a restrictive pattern due to involvement of the respiratory and laryngeal muscles. During adolescence, there are signs of cardiomyopathy.^5,8,9 These patients need frequent surgical procedures such as correction of neuromuscular scoliosis, orthopedic surgeries, muscular biopsies, and, in advanced stages, gastrostomies and tracheostomies.

Becker's dystrophy is characterized by muscular weakness of the lower extremities and the pelvis; its progression is slower than Duchenne's dystrophy as the dystrophin deficit is partial. Symptoms usually present during adolescence and patients may develop dilated cardiomyopathy at age 30 (never before 16 years old).⁹ The survival is longer, between 30 and 60 years, and although the patient experience less respiratory comorbidities,^5,10 they present alterations in the electrocardiogram (EKG) in up to 75% of the cases, with a higher risk of developing heart failure.^8,11

The muscle contracture test with caffeine halothane may be positive (sensitivity 95%-97% and specificity 80%-90%) for malignant hyperthermia. Nevertheless, this test yields abnormal contractures in patients with Becker and Duchnne muscular dystrophy, with higher incidence of false positives as the compromised myocytes are voltage dependent.⁶ Up to date, there are no other diagnostic methods to establish the genuine susceptibility of these patients to malignant hyperthermia.

This neuromuscular and multisystemic disease is characterized by abnormal sodium and chloride channels, produces distal, axial, pharyngeal, and respiratory muscle weakness associated with cardiac arrhythmias, cardiomyopathy, diabetes mellitus, thyroid dysfunction, and cognitive disorders.¹

Steinert myotonic dystrophy is classified into 2 types: muscular dystrophy type 1 (MD1) or classical muscular dystrophy (Steinert disease) of congenital nature due to alteration of the transcription of the genes encoding for chloride channels, insulin receptors, troponins, and N-methyl

De Aspartate (NMDA) receptors. It has a widely variable onset in life, with a very diverse presentation that may manifest between 10 and 40 years of age¹; and type 2 muscular dystrophy (MD2) or proximal myotonic myopathy, that usually presents in adults and has a lower anesthetic risk than type 1. Cognitive dysfunction, cardiomyopathy, and sudden death are more common in type 1 merosin-deficient.^12-15

Ullrich's congenital muscular dystrophy and limb-girdle muscular dystrophy belong in this group of pathologies. Ullrich's disease is a severe neuromuscular disorder caused by mutations in the genes encoding for collagen VI, which is a fundamental glycoprotein of the extracellular matrix responsible for maintaining tissue integrityand provides a structural network to support cells in all tissues.^16,17 It manifests since the neonatal period with congenital hypotony, and many patients are unable to walk, presenting proximal joint contractures, distal hyper-laxity with rapidly progressing scoliosis and normal intelligence, normal cardiac function, and normal or slightly elevated levels of creatinkinases.^17-19 The usual survival extends to the second decade of life, and death occurs because of respiratory failure.^20-22 There is a more benign form of the disease called Bethlem myopathy or slowly progressive muscular dystrophy.

Patients with muscular dystrophy usually require orthopedic surgeries early in life, scoliosis correction, muscle biopsies, tendon release, or tendon transfers.^3,41 In case of dystrophies affecting the heart, such as Steinert myotonic dystrophy, Emery-Dreifus, and limb-girdle, the patients will require anesthesia for implanting cardio deibrillators and pacemakers^{1,30,31,36,42} because of the high risk of sudden death caused by ventricular fibrillation, ventricular tachycardia, or complete heart block²⁶; in advanced stages, patients require tracheostomies, gastrostomies, and therapeutic or diagnostic procedures such as computarized tomography-scans and magnetic resonance image.²

The recommendation is a comprehensive evaluation of the patient's functional condition, with emphasis on the most affected organs based on the type of dystrophy. The difficult airway criteria should be assessed as these are frequently present in this type of patients, and any pressure ulcers due to chronic positions should be ruled out.

Table 4 describes the pre-surgical laboratory examinations and complementary tests that should be ordered to patients with muscular dystrophy undergoing surgery. The most frequent recommended tests are creatinkinase levels, Complete Blood Count (CBC) coagulation tests, echocardiogram, EKG, and lung function tests.

Table 4

Source: Authors.

The forced vital capacity (FVC) and the forced expiratory volume in both supine decubitus and seating position should be evaluated. If FVC is below 50% of the expected level, the patient is at high risk of requiring postoperative non-invasive ventilation.^{17,19,20,38,39} Occasionally, there may be a need to do additional tests to assess the diaphragmatic function; a dysfunctional diaphragmatic function requires non-invasive mechanical ventilation support before surgery.

The severity and high perioperative risk criteria are as follows:

PEP: peak expiratory pressure, LVEF: left ventricular ejection fraction.³⁹

Due to the incidence of malignant arrhythmia and sudden death in these patients, it is recommended to conduct electrophysiological studies, in particular for limb-girdle dystrophies, Steinert disease and Emery Dreifuss.¹²

The anesthetic management of patients with muscular dystrophies is particularly difficult because of the risk of rhabdomyolysis leading to hyperpotassemia and cardiac arrest, malignant arrhythmias, increased muscle weakness, airway management problems, and exacerbation of the respiratory failure. It is important to stress that the anesthetic risk depends on the type of surgery, the clinical condition of the individual patient, and the progression of the disease; the following recommendations are general but sound clinical judgment should prevail for each particular patient.

Complete monitoring of these patients will require EKG, non-invasive blood pressure, capnography, pulse oxime-try, temperature, diuresis, ventilation parameters, and neuromuscular relaxation.

The administration of glucose solutions is recommended to lower the risk of rhabdomyolysis and acute hyperpotassemia⁴³; extended fasting shall be avoided and consideration of bronchoaspiration prophylaxis with H2 antagonists, metoclopramide, and sodium citrate shall be considered.^44-46

TIVA is beneficial for these patients.³ Propofol is the most commonly used hypnotic for the induction and maintenance of anesthesia in patients with Duchenne muscular dystrophy.⁴⁷ If the heart is affected, other options may be used, including etomidate and thiopental which have been successfully used in patients with myotonic dystrophies.⁴⁶ Volatile anesthetic agents are not used in most case reports, as these may trigger myotony and crisis of rhabdomyolysis; nitrous oxide is the only inhaled agent used in around 34% to 78% of these reports.^3,48 However, the use of sevofluorane has been reported for anesthetic induction in patients with muscular dystrophy with no associated complications. The reported events of rhabdomyolysis associated with exposure to inhaled agents have 2 situations in common: first, the onset of the adverse event presents in the recovery area when patients are moved, and second, it is more frequent in children less than 8 years old, in whom the disease is less severe, but the muscular cell membrane is more instable and vulnerable to the development of rhabdomyolysis, even with normal muscular activity.^45,48,49 For this reason, some authors conclude that the evidence is not strong enough to associate inhaled agents with adverse events such as rhabdomyolysis and that association is multifactorial, but there are safer anesthetic techniques.^48,49 In myotonic dystrophy, halogenated agents are not contraindicated, but may increase the muscular weakness.^1,12,50 Based on the author's experience, the recommendation is to use inhaled induction to facilitate catheterization of an intravenous access and immediately change to TIVA or regional anesthesia-associated sedation. This strategy shortens the time of exposure to anesthetics which theoretically lowers the probability of muscular contractures and rhabdomyolysis.

Opioids such as remifentanil and fentanyl have proven to be safe in these patients.¹

Low-dose ketamine and midazolam have been reported as safe options for sedation-analgesia associated with regional techniques (spinal or epidural) in these patients.^47,51,52 It should be highlighted that the onset of action of non-depolarizing muscle relaxants is faster, their behavior is erratic, with longer duration and a high risk of residual paralysis; hence, monitoring of the neuromuscular function is critical.^9,53

The most frequently used non-depolarizing muscle relaxant is rocuronium,^47,48 which is able to neutralize its pharmacological action using a specific reversal agent. Although the use of sugamadex in patients with neuro-muscular disorders has not been fully established and its role in patients with dystrophies and dilated cardiomyopathy is not totally clear; sugamadex use has been reported in patients with dystrophies and dilated cardiomyopathy,¹⁰ at a dose of 4mg/kg (dose described for adults) with 100% recovery of the neuromuscular function, and minimal adverse cardiovascular events.⁵⁴ In contrast, neostigmine may trigger acute myotony, rhabdomyolysis, malignant arrhythmias, and heart failure.^10,54 The risk of malignant hyperthermia is similar to that of the general population.⁵⁵

Regional caudal or epidural spinal anesthesia may be reliably and successfully used in these patients, although cases of myotonic contractures have been described in incomplete neuraxial block, and technically, it can be more challenging because of the spinal abnormalities.^3,42,55-58

Patients programed for major surgery should receive anti-fibrinolytic therapy with tranexamic acid and blood-sparing techniques, as well as warming strategies with hot fluids and electric blankets to prevent hypothermia that may lead to myoclonic contractures and increased bleeding.^47,58 These patients require complete monitoring, including non-invasive blood pressure monitoring, capnography, cardioscope, pulse oximeter, airway pressure, temperature, urinary catheter, central venous catheter, and arterial line.^3,59 It should be highlighted that fluid therapy in these patients should be done with potassium-free crystalloids, and there are no absolute contraindications to the use of colloids.⁶⁰

Patients with muscular dystrophy have an absolute contraindication to receive depolarizing muscle relaxants such as succinylcholine and care should be exercised with the use of inhaled anesthetics that may trigger a rhabdomyolysis crisis.^8,47,55 The Food and Drug Administration in 1992 issued an alert to avoid succinylcholine in patients with a history of myopathies or unclear congenital hypotony. Inhaled anesthetics may be used with care in children with dystrophies and the pre-surgical potassium and creatine kinase levels must be monitored, in addition to cardiac function.⁹

Patients with muscular dystrophies must have an anesthetic recovery in intensive monitoring units, particularly in symptomatic patients following major abdominal surgery or when the muscle dysfunction is severe.⁵⁷ Good analgesia is a must as it reduces any respiratory complications. However, keep in mind that these patients are more sensitive to the effects of opiates (systemic and neuraxial) and therefore present a higher risk of respiratory depression, exacerbated gastrointestinal paresis, and increased risk of reflux, aspiration, and ventilation dysfunction. In case of respiratory depression following reversal of the neuromuscular relaxation, consider deferring the extubation 24 to 48 hours, or consider the use of non-invasive mechanical ventilation.⁵⁵

Non-steroidal anti-inflammatory drugs should be used with care as these may also trigger crises of rhabdomyolysis.⁴³ The use of intravenous or rectal paracetamol has been reported,^61,62 regional epidural anesthesia, and ultrasound-guided plexus nerve block, as well as para-vertebral blocks for thoracic procedures.^61,63,64

Patients with muscular dystrophy have a high risk of apnea and death following extubation, over the next 24 hours after surgery.⁵⁵

In the initial stages of the disease with mild involvement and preserved gait, sedation and analgesia with low-dose benzodiazepines and opioids may be used; however, in patients in advanced stages and with compromised respiratory function, the use of opioids and sedatives during the postoperative period should be avoided.^3,45 If required, use them with care and keep the patient under strict surveillance in the intensive care unit.^11,64

For extubation, the recommendation is to do it with the patient awake, and if non-depolarizing muscle relaxants need to be used, complete reversal must be ensured to avoid residual relaxation.^4,10,44,64

Regardless of the anesthetic agent used, patients with dystrophies have a high risk of complications including respiratory failure, rhabdomyolysis, arrhythmias, cardiac arrest, and reactions similar to malignant hyperthermia that need acute symptomatic treatment, but fail to resolve with dantrolene.^{44,46,55,63,64} Patients with dystrophies may be classified based on the perioperative risk, in accordance with MIRS (muscular impairment rating scale); the risk is intermediate in grade MIRS 3, and very high in grades 4 and 5 (Table 5).⁶⁵ The surgical risk also increases in major surgery and in patients who were administered muscle relaxant. Table 6 describes the most frequent perioperative complications for each muscular dystrophy.

Table 5

Source: Authors.

Table 6

Source: Authors.