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Hypoparathyroidism
5%
1/19
Liver failure
21%
4/19
Rhabdomyolysis
Salicylates
Saponification
32%
6/19
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This patient is presenting with a notable respiratory alkalosis without metabolic compensation suggesting an overdose of aspirin (salicylates), which early in the intoxication causes hyperstimulation of the medullary respiratory system and hyperventilation. When a patient has an acute respiratory alkalosis this can lower the free calcium, causing symptoms of hypocalcemia (paresthesias and muscle spasms). Salicylate overdose classically occurs as a suicide attempt or in pediatric patients as an accidental ingestion. Early in the ingestion, salicylates stimulate the medullary respiratory center causing a respiratory alkalosis. When a respiratory alkalosis occurs, the elevated pH is compensated by albumin's release of hydrogen ions in an attempt to bring the pH back to normal. To maintain electrical neutrality, albumin binds free calcium causing the free calcium level to decrease while the total calcium level remains the same. Since free calcium is the active fraction of total calcium, patients will experience symptoms of hypocalcemia including paresthesias (with perioral numbness being common), spasticity, and possible QT segment changes (QT prolongation being common in hypocalcemia). Later in the presentation of salicylate overdose, an overwhelming metabolic acidosis occurs secondary to renal wasting of bicarbonate, inhibition of oxidative phosphorylation, and increased fatty acid metabolism. The treatment of salicylate overdose is sodium bicarbonate. Incorrect Answers: Answer 1: Hypoparathyroidism can cause hypocalcemia and would occur after a thyroidectomy. The parathyroid glands sit on the thyroid; thus, patients may present with hypocalcemia and hyperphosphatemia. It would be unlikely to occur spontaneously. Answer 2: Liver failure can cause a low serum albumin which would lower the total calcium but not the free calcium. For this reason, the patient would not have symptoms of hypocalcemia since it is the free calcium which is the active portion. However, in end-stage cirrhosis, the liver also loses 25-hydroxylase capabilities which would decrease the level of active vitamin D thus causing hypocalcemia. Answer 3: Rhabdomyolysis can occur in patients who are seizing or those who use cocaine or amphetamines when there is the destruction of skeletal muscles. Sequestration of calcium by the sarcoplasmic reticulum, precipitation with phosphorus, and deposition in necrotic cells can cause hypocalcemia; however, an isolated respiratory alkalosis would not be seen in this condition. Answer 5: Saponification can occur in chronic pancreatitis where the inflamed fat surrounding the pancreas binds to free calcium causing hypocalcemia. The management of pancreatitis is generally IV fluids, analgesia, and supplementation with pancreatic enzymes. The calcium level should only be repleted in severe cases and symptomatic patients. Bullet Summary: Salicylate overdose causes medullary hyperstimulation leading to hyperventilation, a respiratory alkalosis, and a decrease in the serum free calcium, which can cause paresthesias and muscle spasms.
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