List the causes, and features of rhabdomyolysis, and outline the principles of management
Aetiology: consider trauma and muscle compression (including immobility), exertional rhabdomyolysis (eg. heat stroke, grand mal seizures), drugs and toxins (via coma/immobility, agitation/hyperthermia, myotoxins eg. HMG-CoA reductase inhibitors, myonecrosis secondary to non-depolarising neuromuscular blockers), infections, inflammatory myopathies (eg. polymyositis), electrolyte abnormalities (esp. hypokalaemia and hypophosphataemia), hyperthermia (eg. malignant hyperpyrexia and neuroleptic malignant syndrome), metabolic myopathies.
Presentation: Consider history of exertion, fitting, drug exposure (including illicit), immobility, family history, and previous episodes. Patient may complain of painful or weak muscles, and pigmented urine. Investigations reveal markedly elevated muscle enzymes (especially CK), acute oliguric renal failure, and electrolyte abnormalities (hyperkalaemia, hyperphosphataemia, hypocalcaemia, hyperuricaemia and metabolic acidosis).
Principles of management: consider general supportive care, adequate fluid resuscitation, forced alkaline diuresis (including mannitol), specific treatment of underlying cause (eg. dantrolene, phosphate replacement, cooling, removal of precipitants, treatment of infection, fasciotomies etc), and correction of electrolyte abnormalities.
The causes of rhabdomyolysis are discussed in Question 26.3 from the second paper of 2013.
A more extensive discussion of rhabdomyolysis can be found among the Required Reading summaries.
To simplify revision, I reproduce the list of differentials below:
Causes of rhabdomyolysis
- Vascular - muscle ischaemia, eg. ischaemic limb, or myocardial infarction
- Infectious - eg. necrotising fasciitis
- Neoplastic, eg. sarcoma
- Drug-related, eg. due to MDMA or statins, or due to neuroleptic-malignant syndrome
- Congenital, eg. some sort of congenital myopathy
- Autoimmune, eg polymyositis or dermatomyositis
- Traumatic, eg. crush injury, blast injury, compartment syndrome, immobilityetc
- Environmental, eg. hyperthermic injury, "heat stroke"
- Endocrine, eg. hyperthyroidism or phaeochromocytoma
Manifestations of rhabdomyolysis
- Historical features: Trauma, seizures, immobility, drug exposure
- Symptoms:- muscle pain, decreased mobility, weakness, tea-coloured urine
- Signs: Muscle compartment swelling, tenderness, weakness, fever
- Biochemistry: Elevated CK, AST, LDH, urinary myoglobin; renal dysfunction and electrolyte abnormalities (particularly hyperkalemia, hypocalcemia, hyperphosphataemia, hyperuricemia, lactic acidosis)
- Imaging: CT or MRI evidence of muscle oedema; ultrasound evidence of decreased compartment perfusion
Management of rhabdomyolysis
A recent meta-analysis of management strategies for rhabdomyolysis has presented the following conclusions:
- Commence IV fluids within 6 hours - as early as possible
- Aim for a urine output greater than 300ml/hr
- Use of sodium bicarbonate is only indicated to correct systemic acidosis
- Use of mannitol is only indicated if urine output >300ml/hr cannot be maintained
Dialysis may be commenced to improve the removal of myoglobin, if a high-permeability membrane filter is available. Even if it is not, standard CVVHDF seems to decrease the risk of renal injury.
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