Patients with significant weakness of oropharyngeal muscles may present with intermittent choking, slurred speech, dysphonia, difficulty in clearing secretions, or aspiration pneumonia. Patients may exhibit shortness of breath at rest or on exertion, or they may have nonspecific symptoms such as restlessness, difficulty sleeping, or fatigue. Early morning headache, daytime somnolence, and poor school performance are suggestive of nocturnal hypoventilation, which causes hypoxemia and hypercapnia. Weakness of the diaphragm, to a greater extent than weakness of the intercostals, can result in daytime dependency on accessory muscles and can lead to nighttime hypoventilation, as the intercostal muscles can become hypotonic during rapid eye movement sleep (i.e., REM atonia). Positional symptoms may be described by patients with acute NMDs. Patients with diaphragmatic weakness commonly use accessory muscles of breathing and report becoming distressed when supine. Patients with an intact diaphragm but weak intercostals and abdominal muscles may develop respiratory distress in the upright position.

Abnormalities or progressive deterioration in blood gas values indicate respiratory failure. Arterial blood gas monitoring may not be the optimal method for determining when a patient with NMD has a significant problem, since changes in blood gases and oxygen-hemoglobin saturation tend to occur late, at a time when patients can no longer compensate for increasing weakness.

A simple, single-breath counting test may be used at the bedside in older children and adolescents to assess severity and follow progression of weakness due to acute NMD. If the patient can count up to 10 in one breath, the forced vital capacity is likely to be at least 15-20 mL/kg. If they can count up to 25, the vital capacity is ˜30-40 mL/kg. Monitoring the trend in single-breath counting may help to determine disease progression and the need for mechanical ventilation before the development of respiratory failure.

In children older than 5 years, a spirometric assessment can be used at the bedside to monitor the severity and progression of weakness. A normal vital capacity is measured from maximum inspiration to maximum expiration (normal values range from 55 to 80 mL/kg). Maximum static respiratory pressures are measured: maximum inspiratory pressure (measured at residual volume with normal values of -100 to -120 cm H₂O for adult males and -80 to -100 cm H₂O for adult females) and maximum expiratory pressure (measured at total lung capacity with normal values 150-240 cm H₂O for adult males and 108-160 cm H₂O for adult females). These measurements are considered sensitive indicators of respiratory muscle strength. Both minimal values and changes in values have been associated with clinically significant deterioration, including an inability to cough and clear secretions, increased incidence of pulmonary infections, increased nighttime ventilatory insufficiency, hypercarbia, and need for ventilatory assistance. Exact minimal measurements that require intervention are not available because of variations in respiratory impairment among NMDs or among patients, and the lack of data in children. A “20/30/40 rule” has been suggested for adult patients for minimal values that raise concern for respiratory failure. A vital capacity of ˜20 mL/kg, a maximum inspiratory pressure less negative than -30 cm H₂O, or a maximum expiratory pressure of <40 cm H₂O are minimal values that raise concern for respiratory problems in adults with acute NMD. The threshold for all the three values may also be lower in children. Values that fall by 50% from baseline or >30% in a 24-hour period are of concern. It has been observed that life-threatening respiratory failure occurs when vital capacity drops below 20 mL/kg or 30% of predicted values (3).

Radiographic findings are often late or nonspecific. Helpful findings on chest x-ray may include an elevated hemidiaphragm, in fixed position during inspiration and expiration, suggestive of hemidiaphragmatic weakness or paralysis. A bell-shaped thoracic cage indicates long-standing intercostal muscle weakness. Nonspecific findings include patchy atelectasis or areas of consolidation in patients with aspiration or infection of the respiratory tract.

Ventilatory muscle insufficiency and diaphragmatic weakness may not correlate with general neuromuscular weakness. Respiratory function may be compromised even before clinical signs of ventilatory insufficiency are obvious. Because of concomitant pulmonary infection, progression to respiratory failure may be more rapid than that predicted by the underlying
condition. Respiratory status, therefore, must be closely and carefully monitored.

Airway protection should be assessed. Nasal speech, gurgling sounds, difficulty in swallowing, or protrusion of the tongue indicates significant bulbar muscle involvement and imminent airway obstruction and ventilatory failure. Clinical manifestations of respiratory muscle weakness include increased respiratory rate, decreased tidal volume, paradoxical inward chest movement during inspiration, and frequent change in breathing pattern. Active use of accessory muscles of respiration, acidosis, mechanical airway obstruction, and pneumonitis indicate weakness of major muscles of respiration. Hypercapnia is a late finding. Whenever possible, bedside spirometry should be performed. A forced vital capacity that falls below 15-20 mL/kg indicates increased risk of ventilatory failure. Patients unable to generate -20 to -30 cm H₂O of negative inspiratory force (NIF) on manometer testing are also at higher risk for respiratory insufficiency. These assessments should be conducted every 2-4 hours. Once admitted to the PICU, these patients should be monitored closely and oral fluids and feeds should be discontinued to prevent aspiration. There should be a low threshold for intervening with endotracheal intubation and mechanical ventilation in patients exhibiting declining respiratory function.

Endotracheal intubation is important because (a) the oropharyngeal muscles are weak and there is risk of severe aspiration into the lungs via an unprotected airway; (b) the thoracic pump is weak and there is risk of declining tidal volumes, hypoxemia, and hypercapnia; and (c) the cough and clearance of airway secretions may be poor and there is risk of inadequate pulmonary toilet. An elective endotracheal intubation may prevent sudden respiratory arrest and its consequences (4).

If in doubt, early initiation of ventilator support should be preferred in order to ensure adequate minute ventilation. In severe cases, endotracheal intubation and controlled mechanical ventilation may be required. If the patient is able to generate some respiratory muscle effort, synchronized intermittent mandatory ventilation (SIMV), pressure-support ventilation (PSV), or combination of both is appropriate. With SIMV, unassisted, spontaneous breathing may burden the accessory ventilatory muscles, but in PSV, inability to trigger breaths or unanticipated changes in lung compliance can result in insufficient ventilation. If fatigue, hypoxemia, or hypercapnia is encountered, the support from either mode must be increased. Available data are limited regarding the use of noninvasive ventilation (NIV) for children with acute NMDs. NIV is a safe and effective first-line therapy for hypoxemic acute respiratory failure (ARF

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