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)

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