On rhabdo and myopathies – 10/9/18

Thanks to Cameron and Adam for presenting the case of a middle aged man with no significant PMH who presented with diffuse myalgias and chronic progressive proximal muscle weakness, found to have a CK >12k and EMG findings concerning for an inflammatory myopathy, awaiting muscle bx for diagnosis.


Clinical Pearls

  • Rhabdomyolysis literally means dissolution of skeletal muscle and has a broad differential outside of the typical traumatic or exertional processes associated with it see below).
  • The four main inflammatory myopathies are dermatomyositis, polymyositis, inclusion body myositis, and necrotizing autoimmune myositis.
  • Polymyositis is rare and a diagnosis of exclusion after the other three main inflammatory myopathies have been investigated.
  • Overall, the prognosis of inflammatory myopathies is good with appropriate treatment.  The exception is inclusion body myositis which is a progressive disorder without any effective therapy.
  • Pigment nephropathy can occur with rhabdo regardless of the underlying etiology especially in patients with CK >5000.  Aggressive IV hydration to lower CK levels is important to reduce the risk of kidney injury.

Rhabdomyolysis:

DDx:

  • Traumatic
    • Crush injuries, surgery, prolonged compression from immobility or coma
  • Non-traumatic
    • Exertional:
      • Normal muscle: strenuous exercise, heat stroke, seizures, hyperkinetic states
      • Abnormal muscle: metabolic myopathies, mitochondrial myopathies, malignant hyperthermia, NMS
    • Non-exertional
      • Alcoholism
      • Drugs and toxins: lipid-lowering drugs (fibrates, statins), alcohol, heroin, cocaine, meth, colchicine
      • Infections: influenza, coxsackie, EBV, HIV, legionella
      • Electrolyte abnormalities: hypokalemia, hypophosphatemia, hypocalcemia
      • Endocrinopathies: DKA, HHS, hypothyroidism, vitamin D deficiency
      • Inflammatory myopathies (rare)
      • Paraneoplastic
      • Miscellaneous

Inflammatory myopathies

Largest group of potentially treatable myopathies in children and adults.

  • Four subtypes: distinguishing which process is important because each subtype has a different prognosis and response to therapy
    • DM
      • Anti-Mi-2, anti-MDA-5, anti-TIF-1, anti-NXP-2
    • PM
      • Rare, often misdiagnosed
      • Dx of exclusion
    • Necrotizing autoimmune myositis
      • More common than PM
      • Occurs alone or after viral infections or in association with cancer, CTD, or post-statin
      • Anti-SRP or anti-HMGCR
      • Highest CK level
    • Inclusion body myositis
      • Most common in people >50
      • 7.9 cases/million in the US
      • Distal muscles impacted first
      • Facial muscles impacted
      • Muscle atrophy occurs earlier than in others
      • Extramuscular manifestations are uncommon
      • Dysphagia occurs in >50%
      • Muscle atrophy is common
      • Lowest CK level
  • Up to 30% of patients with DM or PM have a constellation of clinical findings termed “antisynthetase syndrome”
    • Acute disease onset
    • Constitutional symptoms (fever, weight loss)
    • Myositis
    • Raynaud’s
    • Mechanic’s hands
    • Non-erosive arthritis
    • ILD
    • Labs show antibodies to tRNA synthetase enzymes (anti-Jo-1)
  • Extramuscular manifestations
    • systemic symptoms
    • cardiac arrhythmias or ventricular dysfunction
    • pulmonary complications (ILD)

Capture

Table above adapted from this and this review article by NEJM.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s