Respiratory involvement can vary significantly between different NMDs and within each type of disorder. Reduction of inspiratory muscle strength results initially in restrictive pulmonary impairment with a progressive decrease of forced vital capacity (FVC). Subsequently, ineffective alveolar ventilation may occur, leading to nocturnal hypercapnia and eventually to diurnal hypercapnia. In addition, weakness of expiratory muscles leads to inadequate clearance of airway secretions. Hypoventilation, coupled with an impaired cough, predisposes to atelectasis and respiratory failure. Furthermore, patients with NMDs often experience mild to moderate bulbar dysfunction, affecting their ability to swallow. Children with type 1 spinal muscular atrophy (SMA), myasthenia gravis (MG), and with other rapidly progressive NMDs may develop a more severe bulbar dysfunction with an increased likelihood of aspiration. Finally, respiratory status can be further impaired by sleep apneas, nutritional problems, gastroesophageal reflux, or progressive scoliosis. In patients with compromised respiratory function, anesthetic agents may further decrease respiratory muscle strength, exacerbating hypoventilation, airway secretion retention, aspiration, and obstructive and central apneas. These conditions may lead to nosocomial infections, prolonged intubation, tracheotomy, and eventually death.

Several NMDs are associated with cardiac dysfunctions (cardiomyopathies and/or abnormality of the conduction system) as shown in Table 12.2. However, clinical manifestations of heart failure are often unrecognized until very late stage, owing to musculoskeletal limitations. All children with relevant cardiac dysfunctions have a limited ability to increase cardiac output in response to stress. Consequently, they are at high risk for perioperative cardiac side effects due to negative inotropic effect of volatile and i.v. anesthetic agents, positive pressure ventilation, hypoxemia, and acute anemia. Volatile anesthetics may also induce arrhythmia resulting from sensitization of the heart to catecholamines and from inhibitory effects on voltage-gated K⁺ channels. Finally, children with NMDs with respiratory involvement leading to nocturnal hypoxemia may be affected by right ventricular changes because of pulmonary hypertension.

It is a rare inherited drug-induced disorder of the skeletal muscle characterized by an increased muscle metabolism with excessive heat, carbon dioxide and lactate production, high oxygen consumption, contractures of the muscles, and myofiber breakdown. It is usually triggered when an MH-susceptible individual is exposed to a halogenated agent or succinylcholine and in rare cases to strenuous exercise and/or heat exposure.

It is an uncommon but potentially fatal disorder triggered by succinylcholine or halogenated agents in susceptible patients, characterized by muscle necrosis with release of intracellular muscle constituents (i.e., myoglobin, potassium, and creatine kinase) into the circulation. It can be acute, resulting in hyperkalemic cardiac arrest, or subacute, presenting as dark urine, acute kidney failure, or cardiac arrest in the postanesthetic care unit.

It is a cardiac arrest due to hyperkalemia triggered by succinylcholine in the presence of striated muscle denervation hypersensitivity (upregulation of nicotinic acetylcholine receptors).

Detailed diagnosis is essential to assess the risk during surgery and anesthesia. Thus, preoperative assessment must include a neurological examination to confirm the diagnosis, when feasible, and to identify the level of progression of the disease in each patient. However, diagnostic process may be complex and some patients may lack a definite diagnosis, particularly those manifesting only with isolated elevated creatine kinase levels with or without minor signs. These children are particularly at risk of life-threatening complications related to anesthesia and should be treated as subjects at highest risk level.

In all children with NMDs, preoperative pulmonary evaluation is strongly recommended to assess the risk of respiratory complications and the need for specific perioperative and postoperative management.

Assessment of respiratory function should include an accurate medical history and physical examination, a chest x-ray, an evaluation of sleep-disordered breathing, and the measurements of respiratory function and cough effectiveness. Evaluation of respiratory function and cough effectiveness includes measurement of FVC, peak cough flow (PCF), and diurnal pulse oximetry (SpO₂). SpO₂ less than 95 % in room air has been established as a clinically significant threshold of abnormality, requiring carbon dioxide (PCO₂) level measurement. Preschool or older patients with developmental delay may not be able to perform evaluation tests of respiratory function and cough effectiveness. In these cases, the measurement of the crying vital capacity (i.e., FVC obtained from a tightly fitted mask over the nose and mouth with in-line spirometer) can approximate FVC.

It is crucial to optimize the patient’s respiratory status before surgery. When respiratory function measurements and sleep studies are abnormal, noninvasive ventilation (NIV) and manual or mechanically assisted cough techniques may be indicated. Therefore, planning and coordination with the hospital respiratory therapists is crucial. In particular, mechanical insufflator–exsufflator (MI-E) can increase coughing, promote deep lung inflation, and treat or prevent atelectasis. Consequently, patients with limited respiratory reserve should be trained in these techniques before surgery and assisted with these devices during sedation, regional anesthesia, and in the postoperative period. This strategy is also recommended for patients already using assisted cough techniques and long-term NIV.

Recently, preoperative training in the use of NIV has been recommended for patients with Duchenne muscular dystrophy (DMD) with preoperative FVC <50 % of predicted value and especially for patients at high risk of respiratory failure, defined by an FVC <30 % of predicted value. Moreover, for children over 12 years of age, if PCF is less than 270 L/min, training in assisted cough techniques is advocated before surgery. This strategy has the potential to be applied to adults and children with respiratory involvement resulting from diagnosis other than DMD.

Children with NMDs should undergo a careful assessment of cardiac function as well as optimization of therapy before anesthesia or sedation. In all children with NMDs, an electrocardiogram and echocardiogram should be performed before anesthesia or sedation, if not done in the previous 12 months. Moreover, signs or symptoms of arrhythmias should be promptly investigated with a Holter test. In addition, patients with a high degree of AV blocks may need a cardiac pacemaker before GA.

In all patients with severe cardiac dysfunctions, invasive arterial pressure should be monitored during GA and in the postoperative period.

In children with NMDs without primary myocardial dysfunction (e.g., SMA), preoperative cardiologic evaluation is suggested only if pulmonary hypertension is suspected.

Nutritional status should be optimized before surgery. In case of poor nutritional balance, wound healing can be delayed and the patient could be too weak to adequately clear secretions or maintain ventilation.