Myopathy

Etiology

Infection

  • Viral – influenza, parainfluenza, Coxsackie, HIV, cytomegalovirus, echovirus, adenovirus, Epstein Barr virus
  • Bacterial – pyomyositis, lyme myositis
  • Fungal
  • Trichinosis
  • Toxoplasmosis

Inflammatory

  • Polymyositis
  • Dermatomyositis
  • Inclusion body myositis
  • Juvenile dermatomyositis
  • Vasculitis
  • Overlap syndromes – lupus, scleroderma, rheumatoid arthritis, Sjögren’s syndrome
  • Rheumatoid arthritis
  • Sjögren’s syndrome

Endocrine

  • Hypothyroidism
  • Cushing’s syndrome (or exogenous steroid administration)

Electrolyte-Related

  • Hypokalemia
  • Hypophosphatemia
  • Hypocalcemia
  • Hypernatremia
  • Hyponatremia

Metabolic

  • Disorders of carbohydrate, lipid, and purine metabolism

Rhabdomyolysis (see Rhabdomyolysis, [[Rhabdomyolysis]])

  • Crush Trauma
  • Alcohol abuse, including hyperkinetic state with delirium tremens
  • Exertion, especially with environmental heat illness
  • Malignant hyperthermia
  • Seizures (see xxxx, [[xxxx]])
  • Vascular surgery

Drug/Toxin

  • Alcohol
  • Cocaine
  • Corticosteroids
  • Heroin
  • colchicine
  • Antimalarial drugs
  • HMG-CoA reductase inhibitors
  • Penicillamine
  • Zidovudine

Other

1) Hypothyroidism (neuropathy and myopathy): inspiratory and expiratory weakness
-Also decreases ventilatory drive and may cause UAO
-Normal lung volumes by PFT’s (unless obese also)
2) Hyperthyroidism (proximal and distal myopathy): inspiratory and expiratory weakness
-Altered type 1 fibers in intercostals and diaphragm
-Restrictive PFT’s
3) SLE (neuropathy and myopathy):
4) Polydermatomyositis: see ILD

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Physiology

  • Myopathy
  • Hypoventilation (leads to hypoxia and acidosis with resultant pulmonary vasoconstriction)

Diagnosis

ABG

  • Hypoxemia/hypercapnia

PFT

  • General Comments: restrictive pattern
  • Decreased FEV1 (with normal-increased ratio): FEV1 curve may be truncated near RV when expiratory muscle weakness is severe
  • Decreased VC (due to muscle weakness, decreased lung and chest wall compliance)
  • Decreased TLC
  • Increased RV (inversely related to MEP)
  • Normal-decreased FRC
  • Slightly increased Raw (with normal-increased specific airway conductance)
  • Normal DLCO (especially if corrected for TLC)
  • Decreased MVV: related to respiratory muscle strength
  • Decreased MIP/ MEP (usually decreased to same degree): most reliable tests (spirometry normal early, but MIP/ MEP usually abnormal)
  • Decreased lung compliance (due to chronically low lung volumes: microatelectasis/ reduced surface tension/ altered interstitial elastic fibers)
  • Transdiaphragmatic pressure (with esophageal and gastric balloons): reveals decreased pressure gradient

CXR

  • Low lung volumes

Diaphragmatic EMG/ NCV

  • Useful to diagnose myopathy

Creatine Kinase (CK)

  • May be elevated

Aldolase

  • May be elevated

Muscle Biopsy

  • Diagnostic

Clinical Manifestations

Neurologic Manifestations

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Symptoms:
-Exertional dyspnea
-Fatigue

Signs:
-Proximal muscle weakness
-Tachypnea (in severe disease)

Complications:
– Acute/Chronic Hypoventilation (see Acute Hypoventilation, [[Acute Hypoventilation]] and Chronic Hypoventilation, [[Chronic Hypoventilation]])
-Aspiration pneumonia
-Sleep-disordered breathing (nocturnal hypoxia/ hypercapnia)
-Pulmonary HTN
-Cor pulmonale


Critical Illness Polymyopathy (aka Necrotizing Myopathy from Steroids + NMJ Blockers)

  • Clinical features of critical illness polymyopathy include a flaccid paralysis of extremities that affects proximal more than distal preserved sensory function (seen in critical illness polyneuropathy), normal or reduced deep tendon reflexes, elevated creatine kinase, usually around day 4, and inability to wean from mechanical ventilator
  • The use of corticosteroids is a strong risk factor
  • Non-decremental response to repetitive nerve stimulation is a major diagnostic feature

References

  • Paresis acquired in the intensive care unit: a prospective multicenter study. JAMA 2002;288:2859.
  • Persistent neuromuscular and neurophysiologic abnormalities in long-term survivors of prolonged critical illness. Crit Care Med 2003;31:1012
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