You respond to a 74-year-old male with a history of small cell lung cancer who is having a generalized seizure. After the seizure stops, he is lethargic. Vital signs are stable and blood glucose is 98 mg/dL. His wife states he has been drinking excessive amounts of water and becoming progressively more confused over the past few days.
Which underlying metabolic abnormality is the most likely cause of this patient's seizure?
- Hyperglycemia due to steroid treatment for his cancer.
- Hyponatremia secondary to Syndrome of Inappropriate Antidiuretic Hormone (SIADH). (correct answer)
- Hypercalcemia of malignancy leading to neurologic irritability.
- Hypokalemia resulting from chemotherapy side effects.
Explanation: Small cell lung cancer is a common cause of SIADH, a condition where the body produces too much antidiuretic hormone. This leads to water retention and a dilutional hyponatremia (low serum sodium). Severe hyponatremia causes cerebral edema, which can manifest as confusion, lethargy, and seizures. The other options are less likely to present with seizures in this context.