A 42-year-old man is brought to the emergency department by his family after suffering a generalized seizure that lasted for 10 seconds. His past medical history includes depression, schizophrenia, 20 pack-year smoking history, heavy alcohol use, but no documented cirrhosis. His pulse is 94 beats/min, blood pressure 108/62 mm Hg, respiration rate 14 breaths/min, and is afebrile. Physical examination demonstrates normal skin turgor, clear lungs, normal cardiac examination, trace edema, and nonfocal neurologic examination. Head computed tomography (CT) is negative and chest X-ray shows a possible hilar/perihilar mass with mediastinal widening. His labs are:
Alcohol level is 0 and toxicology screen is negative.
What is the most likely etiology of this patient’s symptoms?A. Syndrome of inappropriate antidiuretic hormone (ADH)
Correct Answer: A
Hyponatremia, defined as serum sodium <135 mEq/L, is the most common electrolyte abnormality in hospitalized patients and presents in 12% to 17% patients admitted to the ICU. Most patients fall in the mild range of 130 to 135 mEq/L, some are moderate 125 to 129 mEq/L, and few are severe <125 mEq/L, as in this patient. Symptoms of hyponatremia include lethargy, dysarthria, disorientation, and seizures. This patient is presenting with severe symptomatic hypotonic hyponatremia, likely due to subsequently diagnosed small-cell lung cancer as suggested by findings on the chest imaging.
This patient has hypotonic hyponatremia. The next step is to assess the patient’s volume status. This patient has euvolemic hypotonic hyponatremia as suggested by normal skin turgor and lack of significant edema. If the patient was hypovolemic (with decreased skin turgor), this patient’s process would be more consistent with cerebral salt wasting. On the other hand, cirrhosis causes hypervolemic hypotonic hyponatremia.
In a patient with euvolemic hypotonic hyponatremia, the next step is to assess the urine osmolality. If urinary osmolality is low (<100 mOsm/kg), it suggests primary psychogenic polydipsia such as in schizophrenia or other psychoses. Other causes include low solute intake from beer potomania syndrome. The high urine osmolality seen in this patient indicates an abnormal concentrating effect, also indicated by urinary sodium >20 mEq/L. Syndrome of inappropriate antidiuretic hormone (SIADH) is the most likely cause of hyponatremia in this patient with hypotonic hyponatremia, euvolemia, natriuresis, and inappropriately concentrated urine. SIADH is a diagnosis of exclusion made in the correct clinical context. SIADH can be caused by small-cell lung cancer, pneumonia, lung abscess, cystic fibrosis, SAH, stroke, brain tumors, meningitis, or brain abscess, and common medications, such as nonsteroidal agents, tricyclic antidepressants, selective serotonin reuptake inhibitors, chemotherapy agents, opiates, and haloperidol.