What is the cause of palpitations and muscle weakness in a patient after chemotherapy?

Hypocalcemia is the Most Likely Cause

In a patient with tumor lysis syndrome (TLS) after chemotherapy presenting with palpitations and muscle weakness, hypocalcemia (Option A) is the correct answer. This clinical scenario is classic for TLS, where rapid tumor cell breakdown leads to a constellation of metabolic derangements including hypocalcemia, hyperphosphatemia, hyperkalemia, and hyperuricemia 1.

Why Hypocalcemia is the Answer

Tumor Lysis Syndrome Pathophysiology

• TLS occurs when chemotherapy causes massive tumor cell death, releasing intracellular contents into the bloodstream 1
• The syndrome specifically includes hypocalcemia, hyperphosphatemia, hyperuricemia, and hyperkalemia as its defining features 1
• Hypocalcemia in TLS results from calcium-phosphate precipitation when released phosphate binds serum calcium 1

Clinical Manifestations Match the Question

• Hypocalcemia directly causes both palpitations (cardiac arrhythmias) and muscle weakness 2
• Cardiovascular manifestations of hypocalcemia include QT prolongation and arrhythmias that present as palpitations 2
• Neuromuscular symptoms include muscle weakness, tetany, and paresthesias 2

Why Not Hyperkalemia (Option B)

While hyperkalemia is indeed part of TLS 1, the clinical presentation doesn't fit as well:

• Hyperkalemia primarily causes cardiac arrhythmias but is less characteristically associated with muscle weakness as a presenting symptom 2
• Hyperkalemia typically presents with peaked T waves, widened QRS, and potentially life-threatening arrhythmias, but muscle weakness is not its hallmark feature 2
• The combination of palpitations AND muscle weakness points more specifically to hypocalcemia 2

Critical Management Considerations

Immediate Assessment Required

• Check serum calcium, phosphate, potassium, uric acid, and renal function immediately 1, 2
• Obtain ECG to assess for QT prolongation (hypocalcemia) versus peaked T waves/widened QRS (hyperkalemia) 2
• Monitor for progression to more severe manifestations including seizures or life-threatening arrhythmias 2

Treatment Priorities

• Correct hypocalcemia cautiously, especially in the presence of hyperphosphatemia, as aggressive calcium replacement can worsen calcium-phosphate precipitation 1
• Address all components of TLS simultaneously: aggressive hydration, allopurinol or rasburicase for hyperuricemia, and phosphate binders 1
• The rate of correction matters significantly—inappropriate correction speed can cause more harm than the disorder itself 2

Common Pitfall to Avoid

Do not assume hyperkalemia is the answer simply because it's part of TLS and can cause cardiac symptoms. The specific combination of palpitations with prominent muscle weakness is the key clinical clue pointing to hypocalcemia rather than hyperkalemia 2. Both electrolyte abnormalities occur in TLS, but the symptom profile described matches hypocalcemia more precisely 1, 2.