Matt Flemer presented an excellent case of hyponatremia at TMC Afternoon Delight today. Hyponatremia is very common in hospitalized patients (15-40% of patients in terms of prevalence) and has a high mortality rate. While symptomatic patients should quickly receive hypertonic saline, a key part of the management is to identify and treat the underlying cause. While some approaches use volume status as a guide to find the etiology, this can be difficult to evaluate. Differentiation of hypovolemia from euvolemia on physical exam has a sensitivity of 50-80% and specificity of 30-50%. We prefer an approach that focuses on objective data that can be obtained on presentation (serum osm, urine osm, urine Na):
The patient was a 55 year old man who presented with LLE swelling, redness, and pain for 3 days and was found to have a sodium level of 123. His only past medical history was possible schizophrenia, and he was not taking any medications. He smoked 1 ppd since childhood, had a significant remote drinking history, and denied illicits. He lived at the Mission, and his family history was unknown. On exam he was noted to be hypertensive with diffuse wheezing. His LLE was swollen, warm, and erythematous below the knee. His labs were significant for Na 123 with serum osm 265, urine osm 829, and urine Na 93. AM Cortisol and TSH were within normal limits. Creatinine was 0.8, and bicarb was 25. LE US was negative for DVT. CXR revealed a R infrahilar airspace opacity, and CT chest revealed a total collapse of the right middle lobe and mediastinal lymphadenopathy suggestive of possible malignancy. He underwent bronchoscopy with biopsy of his mediastinal lymph nodes. Path was positive for small cell lung cancer, which is a known trigger for SIADH. Additional potential triggers in this patient include pain due to LLE cellulitis and post-obstructive pneumonia.
Pearls:
- Volume status is difficult to assess on physical exam, and objective tests are better for evaluation of hyponatremia.
- Post-obstructive pneumonia can be an initial presentation of lung cancer. Paraneoplastic syndromes are common including SIADH with small cell lung cancer.
- SIADH can be triggered by a wide range of etiologies such as CNS pathology, malignancy (esp small cell lung cancer), medications (opiates included), ecstasy, pulmonary conditions, pain, nausea, and surgery.
Summary of Guidelines: https://jasn.asnjournals.org/content/28/5/1340
Great EMCrit article (includes the excellent point that in an unstable/seizing patient 2 amps of bicarb from the crash cart = hypertonic saline):