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Clinical Summary

• Patient: 63-year-old female
• Key findings: CPK 13,000 U/L (>10× ULN), urine myoglobin >1,200 ng/mL, lower limb weakness
• Core syndrome: Rhabdomyolysis with myoglobinuria + proximal myopathy

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Differential Diagnosis

1. Inflammatory Myopathies (Top Priority at This Age)

IMNM is a particularly strong differential here given CPK of 13,000 and the severity of weakness — it can produce rhabdomyolysis-range CPK and occurs in older females, often statin-associated.

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2. Drug/Toxin-Induced Myopathy & Rhabdomyolysis

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3. Metabolic & Endocrine Myopathies

Hypothyroidism should be excluded early — it is a reversible, common cause of CPK elevation up to 10× ULN with proximal weakness in older women, and can precipitate true rhabdomyolysis.

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4. Inherited Metabolic Myopathies (Late-Onset)

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5. Infectious Myositis

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6. Neuromuscular Diseases Causing Weakness (Lower CPK expected)

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7. Paraneoplastic Myopathy

• In a 63-year-old female, occult malignancy (lung, ovarian, breast, GI) can present with inflammatory myositis or necrotizing myopathy.
• Dermatomyositis in patients >40 years has a ~15–30% association with underlying malignancy.

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8. Trauma / Immobilization / Compression

• Prolonged immobilization, falls, seizures, or compartment syndrome can cause pressure-induced rhabdomyolysis with secondary lower limb weakness.

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Priority Workup (Based on CPK 13,000 + Myoglobinuria)

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Immediate Concern: Rhabdomyolysis Complications

• Acute kidney injury (myoglobin-induced tubular toxicity) — the most urgent complication
• Hyperkalemia, hyperphosphatemia, hypocalcemia
• Disseminated intravascular coagulation (DIC)
• Cardiac arrhythmia from electrolyte disturbances

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Statin-Induced Myopathy: Management

Classification of Statin-Associated Muscle Symptoms (SAMS)

Per ACC/AHA guidelines, withhold statin therapy for CK >5× ULN. The NLA threshold is >3× ULN (Assessment and Management of SAMS, p. 33).

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Step 1 — Immediate Actions

1. Discontinue the offending statin immediately — mandatory in any patient with CK >5× ULN or myoglobinuria
2. Identify and treat precipitating factors:
   • Hypothyroidism (check TSH)
   • CKD, dehydration
   • Drug interactions (fibrates, cyclosporine, azole antifungals, macrolides — all inhibit CYP3A4/OATP1B1 and raise statin levels)
   • Recent strenuous exercise, trauma, infection
3. Exclude IMNM — anti-HMGCR antibodies; IMNM does NOT resolve with statin discontinuation and requires immunosuppression

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Step 2 — Rhabdomyolysis Management (CPK 13,000 + Myoglobinuria)

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Step 3 — Cardiovascular Risk Must Still Be Addressed

A. Statin Rechallenge Strategy

• Try at least 2–3 different statins metabolized by different pathways before declaring statin intolerance
• Prefer hydrophilic statins first (lower muscle penetration):
  • Rosuvastatin (hydrophilic, long half-life)
  • Pravastatin (hydrophilic, not CYP3A4-metabolized)
• Lipophilic statins (higher myopathy risk): simvastatin > lovastatin > atorvastatin > fluvastatin/pitavastatin
• Alternative dosing regimens (not FDA-approved but widely used):
  • Alternate-day dosing (rosuvastatin or atorvastatin — long half-lives)
  • Dose de-escalation
  • Low-dose daily (e.g., rosuvastatin 5 mg daily)

The majority of patients with SAMS can tolerate rechallenge with an alternative statin or lower dose (Role of Nonstatin Therapies, p. 41).

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B. Non-Statin Lipid-Lowering Alternatives

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Step 4 — Monitoring After Rechallenge or Dose Reduction

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Special Considerations for This Patient (63F)

• Rule out IMNM (anti-HMGCR antibodies) — statin-associated IMNM does not resolve with discontinuation; requires steroids ± IVIG ± azathioprine/mycophenolate
• Check TSH — coexisting hypothyroidism dramatically amplifies statin myotoxicity
• Review all interacting drugs — fibrates, antifungals, macrolides, amlodipine, amiodarone
• Renal monitoring is paramount given CPK 13,000 and active myoglobinuria

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Summary Algorithm

CPK 13,000 + Myoglobinuria + Statin use
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  STOP statin immediately
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  Aggressive IV fluids, electrolyte correction, serial CK
           ↓
  Exclude IMNM (anti-HMGCR), hypothyroidism, drug interactions
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  CK normalizes? → Rechallenge with hydrophilic statin (rosuvastatin/pravastatin)
  at low dose / alternate-day dosing
           ↓
  If intolerant to ≥2 statins → Bempedoic acid + Ezetimibe ± PCSK9 inhibitor