Page 1184 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 87: Neuromuscular Diseases Leading to Respiratory Failure 821

CHAPTER Neuromuscular Diseases TABLE 87-1 Causes of Acute and Subacute Bilateral Weakness
87 Leading to Respiratory Syndrome/level of Representative Disorders
Abnormality
Failure Basilar artery occlusion Embolic, thrombotic, vasculitic

William Marinelli Myelopathy Cord compression (eg, abscess, neoplasm,
disc herniation, trauma)
James W. Leatherman
Transverse myelitis
Central nervous Poliomyelitis
KEY POINTS system infections West Nile virus
• Neuromuscular disorders (NMDs) in critical care may be divided Central nervous Neurotoxic fish poisoning
into those that precipitate admission to the ICU and those that system toxins
arise during ICU management. Peripheral nerve Guillain-Barre syndrome
• Many patients who present to the ICU as a result of an underlying disorders
neuromuscular disorder will have a previously defined diagnosis. Phrenic nerve injury (eg, trauma, surgery neoplasm)
However, when a patient presents with recent onset of acute or Infections with phrenic nerve involvement (eg, diphtheria, herpes
subacute bilateral muscle weakness, a broad differential diagnosis zoster, Lyme disease, West Nile
must be considered. Parsonage-Turner Syndrome with phrenic nerve involvement
• A rapidly progressive spinal cord lesion is the most important Heavy metal toxicity
diagnosis to consider and immediately exclude in patients present-
ing with ascending or flaccid paralysis. Vasculitic neuropathy
Disorders of Myasthenia gravis
• The maximal inspiratory pressure (MIP), maximal expiratory
pressure (MEP), vital capacity (VC), and qualitative judgment of neuromuscular Eaton-Lambert syndrome
oropharyngeal function are the most important parameters to fol- transmission Botulism
low in patients with NMDs. Tick paralysis
• An effective cough is unlikely with a MEP <40 cm H O and risk of
2
hypercapnia increases when MIP is less negative than −30 cm H O. Organophosphate poisoning
2
A VC <30 mL/kg impairs secretion clearance and respiratory fail- Penicillamine toxicity
ure is common at values <15 to 20 mL/kg. Myopathic disorders Dermatomyositis/polymyositis
• Sleep-related deterioration in alveolar ventilation resulting in hyper- Metabolic myopathy (eg, mitochondrial disease)
capnia and hypoxia is common in patients with respiratory muscle
impairment. Toxic myopathy (eg, corticosteroid injury, alcohol, cocaine,
rhabdomyolysis)
• Most patients with Guillain-Barré syndrome or myasthenia gravis
of sufficient severity to precipitate ICU admission will benefit from Electrolyte disorders Hypokalemia
treatment with plasma exchange or intravenous immunoglobulin. Periodic paralysis
• Muscle biopsy is useful in the diagnosis of polymyositis, mitochon- Hypophosphatemia
drial disease, and other myopathies, and should be considered when Hyperkalemia
electrophysiologic and other testing does not offer a clear diagnosis Hypermagnesemia
of peripheral neuropathy or myoneural junction diseases.
Hypocalcemia

NEUROMUSCULAR DISORDERS IN CRITICAL CARE: Although lesions involving the upper and lower motor neuron may
GENERAL ASSESSMENT AND MANAGEMENT occasionally be responsible, more often the underlying disorder affects
the peripheral nerves (eg, Guillain-Barré syndrome, GBS), neuromuscu-
Neuromuscular weakness may result from disorders involving the lar junction (eg, myasthenia gravis, MG), or skeletal muscles (eg, derma-
peripheral nerves, neuromuscular transmission, or skeletal muscles. tomyositis and polymyositis, DM/PM). In this chapter, we will address
Neuromuscular disorders encountered in the critical care setting may neuromuscular disorders that may present with acute or subacute
be divided into those that result in ICU admission and those that are declines in respiratory muscle strength leading to acute respiratory
acquired during treatment of critical illness. Most patients who present failure. Our discussion will primarily focus on GBS, MG, and DM/PM.
to the ICU as a result of an underlying neuromuscular disorder will have We will also offer a brief review of several additional disorders that
a previously defined diagnosis. However, when a patient presents with should be considered in the differential diagnosis of patients presenting
recent onset of acute or subacute bilateral muscle weakness, a broad dif- to the ICU with progressive neuromuscular impairment. Before discuss-
ferential diagnosis must be considered (Table 87-1). The initial approach ing individual disorders, we will review the evaluation and management
to differential diagnosis attempts to define the principal level of abnormal- of respiratory muscle weakness.
(Table 87-2). Additional diagnostic tests such as neuroimaging, nerve ■ RESPIRATORY MUSCLE WEAKNESS
ity based on the patient’s history and findings on neurologic examination
conduction, and electromyogram (EMG) studies are often needed to There are three primary mechanisms by which respiratory failure
establish the underlying disorder more reliably. An easy to remember develops as a direct consequence of an underlying neuromuscular dis-
mnemonic, MUSCLES, may be helpful in remembering some of the most order: (1) weakness of inspiratory muscle, particularly the diaphragm,
common causes of generalized weakness in the ICU (Table 87-3). (2) inadequate expiratory muscle function, and (3) impairment in
1
Involvement of respiratory muscles is the most common reason that muscles of the upper airway. The primary clinical consequences of
2-4
patients with primary neuromuscular disorders are admitted to the ICU. impairment in one or more of these muscle groups include inadequate

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