- Neuromuscular disorders (NMDs) in critical care may be divided into those that precipitate admission to the ICU and those that arise during ICU management; the latter are much more common and significantly influence the pace and extent of recovery from critical illness.
- The maximal inspiratory pressure (MIP), maximal expiratory pressure (MEP), vital capacity (VC), and qualitative judgment of oropharyngeal function are the most important parameters to follow in patients with NMDs.
- An effective cough is unlikely with a MEP <40 cm H2O or a VC <30 mL/kg.
- Sleep-related deterioration in hypoventilation and hypoxemia is common in patients with respiratory muscle impairment.
- Many patients with Guillain-Barré syndrome or myasthenia gravis of sufficient severity to precipitate ICU admission will benefit from treatment with plasmapheresis or intravenous immunoglobulin.
- Muscle biopsy is useful in the diagnosis of polymyositis, mitochondrial disease, and other myopathies, and should be considered when electrophysiologic and other testing does not offer a clear diagnosis of peripheral neuropathy or myoneural junction diseases.
- Critical illness myopathy and neuropathy should be considered in all patients with protracted critical illness, particularly when multisystem organ failure has been present; avoidance of paralytics, appropriate correction of the metabolic milieu, early physical therapy, and measures to shorten duration of mechanical ventilation may help ameliorate the impact of these acquired NMDs.
Neuromuscular weakness may result from disorders involving the peripheral nerves, neuromuscular transmission, or skeletal muscles.1–3 Neuromuscular disorders encountered in the critical care setting may be divided into those that result in ICU admission and those that are acquired during treatment of critical illness. In the first part of this chapter we will address neuromuscular disorders that lead to ICU admission, focusing on Guillain-Barré syndrome (GBS), myasthenia gravis (MG), and dermatomyositis/polymyositis (DM/PM), and concluding with a brief discussion of mitochondrial disease, a recently recognized metabolic myopathy. Subsequently, neuromuscular weakness that is acquired after admission to the ICU will be reviewed. However, before discussing the various causes of neuromuscular weakness, a brief review of respiratory muscle impairment is appropriate, since respiratory failure is the most common reason that patients with a primary neuromuscular disorder are admitted to an ICU.
Although respiratory muscle impairment may occasionally develop rapidly, more often its onset is gradual. Diaphragmatic weakness may be initially manifested by orthopnea or sleep disruption due to nocturnal hypoventilation, without prominent dyspnea.2 Because there may be a paucity of symptoms, objective testing is necessary. Maximal inspiratory pressure (MIP), maximum expiratory pressure (MEP), vital capacity (VC), and an assessment of oropharyngeal function are the most important respiratory muscle parameters to follow,4–6 and should be measured frequently in hospitalized patients who have an evolving neuromuscular disorder, with careful attention to serial changes.7 The most sensitive parameters of respiratory muscle strength are the MIP and MEP. Measurement of MIP and MEP requires a maximal effort at residual volume (MIP) and total lung capacity (MEP), using a bedside manometer fitted with a mouthpiece. Normal values for MIP and MEP are approximately –70 cm H...