Chronic Hypertonic Hyponatremia in Lung Cancer: The Utility of the Osmolar Gap

Hyponatremia is a frequent electrolyte disorder encountered in clinical practice. The differential diagnosis is vast and requires a methodical approach to determining the cause. Hypotonic hyponatremia is the commonest form and syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a typical cause, especially in patients who have cancer. On the other hand, hyponatremia with elevated serum osmolality (hypertonic hyponatremia) is not common, and as such, clinicians are less familiar with this clinical scenario. Clinicians are even less familiar with the use of calculated osmolality and the osmolar gap in such scenarios. Therefore, it is easy to misdiagnose a patient with SIADH in the presence of cancer if a thorough biochemical analysis is not performed. We describe the case of a 56-year-old man who presented with episodes of dizziness and was demonstrated to have chronic hypertonic hyponatremia. Because of a previous diagnosis of mild SIADH several months prior, the presence of severe hyponatremia on a background of cancer, and relatively elevated urine osmolality, the elevated serum osmolality was not appreciated and the serum osmolar gap was not calculated. This resulted in a misdiagnosis of chronic SIADH. It was only after the osmolar gap was calculated and found to be significantly elevated that the markedly elevated serum ethanol level was discovered, and a diagnosis of alcohol-related hypertonic hyponatremia was made. On reassessment, the patient admitted to drinking large quantities of high alcohol-containing drinks for over the preceding three months. On gradual alcohol cessation, his blood results returned to normal. This case report highlights the importance of thorough assessment and the use of osmolar gap when navigating through the differential diagnosis of hyponatremia.