Rhabdomyolysis

Question 1

What is rhabdomyolysis?

Answer 1

Muscle disintegration with release of muscle proteins into the systemic circulation, notably creatinine kinase and myoglobin. It can lead to AKI and renal failure as well as compartment syndrome and muscle ischaemia and necrosis.

Question 2

What are the causes of rhabdomyolysis?

Answer 2

Traumatic causes:

• fractures, crush injuries
• exercise, seizures, tetanus
• electrical current/shock

Atraumatic causes:

• congenital myopathies
• endocrinopathies
• drugs (MDMA, cocaine, amphetamines, carbon monoxide, cyanide)
• hyperthermia (exercise, NMS, MH)
• vascular occlusion
• infection (Nec fasc)
• autoimmune diseases (dermatomyositis, polymyositis)

Question 3

What is the pathophysiology of rhabdomyolysis?

Answer 3

• Muscle damage
• increased muscle activity or impaired O2 supply
• ATP depletion
• Disruption of cellular transport systems (ATPase)
• Active transport of [Ca]i leads to hyperactivity of proteases, leading to myofilament and cell membrane damage.
• Leakage of cell contents into plasma (PO4, K, CK, irate, myoglobin)

Question 4

What are the complications of rhabdomyolysis?

Answer 4

1. Electrolyte dysfunction:
   - hyperphosphataemia
   - hyperkalaemia
   - hypocalcaemia
2. Renal failure/AKI
3. Hyperuricaemia
4. Compartment syndrome
5. DIC

Question 5

What is the mechanism of renal failure/AKI in rhabdomyolysis?

Answer 5

1. Myoglobin combines with Tamm-Horsfall protein resulting in tubular obstruction from insoluble casts.
2. Hyperuricaemia leads to uric acid obstruction of tubules
3. Haem moiety is nephrotoxic via lipid peroxidation
4. Renal vasoconstriction from myoglobin scavenging of NO.

Question 6

How is rhabdomyolysis diagnosed?

Answer 6

1. History and examination: Myalgia, weakness, dark urine.
2. Blood tests: CK, phosphate, potassium, calcium, U&Es, urate, LFTs, ABG
3. Urine myoglobin and microscopy
4. ECG
5. Radiology +/- compartment pressures.

Question 7

How is rhabdomyolysis managed?

Answer 7

1. ABCDE approach, correcting abnormalities as they are found.
2. IV access and crystalloid infusion - targeting UO 2-3ml/kg/hr
3. Bicarbonate infusion - urinary alkalinisation increases solubility of myoglobin (aim for urinary pH >6.5)
4. Treat hyperkalaemia
5. Fasciotomies and debridement
6. RRT

Question 8

Any drugs you should avoid in rhabdomyolysis?

Answer 8

1. Suxamethonium

2. Statins

Question 9

What is acute limb compartment syndrome?

Answer 9

Increased pressure within a closed compartment that compromise the perfusion and function of the organs within that compartment.

Question 10

What can cause acute limb compartment syndrome?

Answer 10

May be by swelling of intracompartmental contents or by external restriction.

Common causes include:

1. Trauma:
   - Fractures, particularly in forearm or lower leg.
   - crush injuries
   - circumferential burns
2. Vascular injury
   - dissection
   - ischaemia-reperfusion injury
3. Iatrogenic:
   - large bore femoral cannulas (intra-aortic balloon pump)
   - surgical positioning/long lie
   - drug extravasation

Question 11

What are the compartments of the lower leg?

Answer 11

Lateral, anterior, deep posterior and superficial posterior.

Question 12

How is compartment syndrome diagnosed?

Answer 12

History and examination:

• precipitating injury or insult
• pain disproportionate or refractory to analgesia
• pain on passive stretching
• paraesthesia in nerves associated with suspected compartment
• pain remote to the site of injury/surgical site.

Compartment pressures:

• compartment pressure >30mmHg (normal is <10-12mmHg)
• compartment perfusion pressure (diastolic BP - compartment pressure) <30mmHg