Page 1190 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 87: Neuromuscular Diseases Leading to Respiratory Failure 827

and muscle tenderness are common but are usually not severe. Creatine be considered in the presence of serious conduction abnormalities, ven-
phosphokinase (CK) elevation is the most consistent indicator of muscle tricular dysrhythmias, or severe cardiomyopathy.
inflammation. However, other muscle enzymes (aldolase, aspartate Alternative immunosuppressive agents, including methotrexate,
aminotransferase, alanine aminotransferase, and lactate dehydrogenase) azathioprine, or cyclophosphamide, should be considered in patients
may also be elevated. Electromyography typically reveals features of a with poor prognostic markers or a limited response to corticosteroids.
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generalized myopathic disorder, but findings may be normal in 10% to These immunosuppressive agents, particularly methotrexate or aza-
15% of patients. Biopsy of a muscle that has been demonstrated to be thioprine, may also be used if there is corticosteroid intolerance and
weak is the most definitive test, revealing variable degrees of type I and as corticosteroid sparing agents for long-term maintenance therapy.
II fiber necrosis and inflammation. MR imaging has been demonstrated Although only limited studies are available for guidance, therapy with
to identify muscle inflammation and edema, which can facilitate local- IVIg has been associated with significant increases in muscle strength
ization of diagnostic biopsies in selected patients. Skin biopsy may be in patients with DM/PM, and should be considered in patients unre-
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diagnostically helpful in patients who present with predominant features sponsive to corticosteroids. In one study of 35 patients with PM who
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of DM. The clinical presentation of DM/PM commonly shares features were refractory to therapy with prednisone and at least one additional
with several connective tissue disorders, and rheumatologic consulta- immunosuppressive agent, significant improvement in strength was
tion is advisable. noted in 71% of patients after treatment with IVIg, and all patients
Respiratory and cardiovascular complications of DM/PM are the had a significant reduction in CK values. IVIg has also been reported
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main concerns in the ICU. 71,74,75 Respiratory complications of DM/PM to be beneficial in the management of life-threatening esophageal
include respiratory muscle weakness; interstitial lung disease; pneumo- involvement in DM/PM. Alternative therapy with cyclosporine,
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nia resulting from aspiration or immunosuppression; and drug-induced tacrolimus, alkylating agents, tumor necrosis factor inhibitors, and
lung disease. The presence of dysphagia suggests pharyngeal muscle rituximab has been used for patients whose symptoms are unrespon-
dysfunction, which greatly increases the risk for aspiration, the most sive to standard treatment. 83,84
commonly reported pulmonary complication of DM/PM. In addition, The clinical course of DM/PM is quite variable, ranging from a
striated muscle weakness involving the proximal esophagus may further relatively indolent course to a relentlessly progressive process that is
increase the risk for aspiration. Respiratory muscle dysfunction with unresponsive to therapy. Symptom duration of greater than 6 months
inspiratory and expiratory muscle involvement has been reported in up before diagnosis, severe symptoms, and the presence of dysphagia are
to one-third of patients. Interstitial lung disease has also been identified clinical predictors of poor outcome in DM/PM. 80,85 Serial neuromuscu-
in approximately one-third of patients with DM/PM, with nonspecific lar examinations have been the principal method of following patients
interstitial pneumonitis being the most common underlying histopatho- and determining their response to therapeutic interventions. However,
logic lesion. 76-78 Typical findings on high-resolution chest CT include MR imaging may offer additional benefits in assessing the response to
reticular and ground-glass opacities in the lower lung fields, variable therapy. In the future, use of myositis-specific autoantibody testing
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presence of consolidation in the lung periphery, and the absence of a will likely be an important parameter in deciding on optimal treatment
fibrotic honeycomb-like appearance. An analysis of 70 patients with regimens and estimating prognosis in patients with DM/PM.
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DM/PM and diffuse interstitial lung disease found a musculoskeletal
presentation in 36%, pulmonary presentation in 30%, and a combina- Additional Disorders to Consider in the Differential Diagnosis of Acute to
tion of musculoskeletal and pulmonary symptoms in 21%. When Subacute Neuromuscular Weakness Presenting the ICU: The assessment
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patients initially present with isolated pulmonary disease, treatment of patients presenting to the ICU with acute respiratory failure in the
with corticosteroids may suppress or obscure musculoskeletal symp- setting of progressive neuromuscular weakness requires a careful and
toms, thereby delaying the diagnosis of DM/PM for weeks to years. thorough consideration of a broad differential of heterogeneous dis-
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Defining the primary cause of dyspnea is important as both interstitial orders (Tables 87-1 and 87-2). The development of acute respiratory
lung disease and respiratory muscle weakness may result in respiratory failure may prompt admission to the ICU in patients with known chronic
impairment, and these conditions may coexist. 77 progressive disorders, including amyotrophic lateral sclerosis, postpolio
Cardiac features include tachyarrhythmias; conduction abnormalities; syndrome, and the muscular dystrophies. The acute respiratory distress
heart failure with a dilated cardiomyopathy resulting from myocarditis; in these patients is commonly triggered by a complication of their pro-
and chronic pulmonary hypertension. 71,75 Chronic pulmonary hyperten- gressive disease, such as pneumonia or pulmonary embolism. As previ-
sion may result from several mechanisms, including cardiomyopathy ously emphasized, a rapidly progressive spinal cord lesion is the most
with elevation in left-sided filling pressures, chronic hypoxia due to important diagnosis to consider and immediately exclude in patients
interstitial lung disease, or the development of a primary pulmonary presenting with ascending or flaccid paralysis. This constellation of
hypertension–like syndrome. Thromboembolic disease, resulting from symptoms may be confused with rapidly progressive GBS. In addition,
prolonged neuromuscular weakness and immobility, may also result in phrenic nerve injury and diaphragmatic weakness may result from
dyspnea and pulmonary hypertension. trauma, surgery, neoplasm, inflammatory disorders (eg, systemic vascu-
Corticosteroids are often beneficial in DM/PM. Most patients respond litis), and infections. Infections with herpes zoster, Lyme disease, West
3
to corticosteroids with normalization in muscle enzymes by 4 to 6 weeks Nile virus, and diphtheria have also been associated with phrenic nerve
and improvement in muscle strength within 2 to 3 months. Therapy injury. Parsonage-Turner syndrome, an acute brachial plexus neuropa-
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2
with corticosteroids is usually initiated at a dosage of 0.5 to 1.5 mg/kg thy, which commonly presents with relatively severe shoulder pain, may
prednisone, with gradual tapering after a complete response is demon- also be associated with diaphragmatic weakness from involvement of
strated. High-dose corticosteroids have also been used in patients with the phrenic nerve. Finally, disorders of neuromuscular transmission
3
severe myositis. Corticosteroids have also been considered the primary that may mimic MG include Eaton-Lambert syndrome, botulism, tick
therapeutic agents for interstitial lung disease associated with DM/PM paralysis, organophosphate toxicity, snake venom toxicity, and a myas-
since their initial clinical and pathologic association in the mid-1970s. thenic- like syndrome induced by penicillamine. The following discus-
47
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Pharyngeal muscle involvement in DM/PM usually responds to cortico- sion will offer a brief review of several disorders to be considered in the
steroid therapy. However, surgical division of the cricopharyngeal muscle differential diagnosis of neuromuscular weakness presenting to the ICU.
may be necessary for severe cricopharyngeal achalasia that is refractory Botulism results in a toxin-mediated irreversible inhibition of neu-
to immunosuppressive therapy. Myocarditis in DM/PM is an indication romuscular transmission, resulting in an acute symmetric descending
for corticosteroids and other cardiac complications (congestive heart paralysis, which typically begins with bulbar and eye muscle
failure, pulmonary arterial hypertension) are managed with appropriate impairment. 4,86,87 Bilateral and symmetrical cranial nerve involvement
pharmacologic therapy. Insertion of a pacemaker-defibrillator should is characteristic. The early bulbar involvement of botulism initially

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