Hyponatremia, hyperkalemia, and other fluid and electrolyte disturbance recognition and mechanisms tested on USMLE Step 1.
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- What serum sodium level defines hyponatremia?
- Less than 135 mEq/L
- What are the three volume status categories used to classify hyponatremia?
- Hypovolemic, euvolemic, and hypervolemic
- What hormone is inappropriately elevated in SIADH?
- Antidiuretic hormone (ADH/vasopressin)
- In SIADH, urine osmolality is ____ despite a low serum osmolality.
- inappropriately concentrated
- Name a classic paraneoplastic cause of SIADH.
- Small cell lung cancer
- What is the first-line treatment for chronic SIADH?
- Fluid restriction
- What serious neurologic complication can result from correcting hyponatremia too quickly?
- Osmotic demyelination syndrome (central pontine myelinolysis)
- What is the maximum safe rate of sodium correction in chronic hyponatremia?
- About 8 mEq/L per 24 hours
- What clinical picture is characteristic of osmotic demyelination syndrome?
- Spastic quadriparesis, dysarthria, and dysphagia, resembling locked-in syndrome
- In euvolemic hyponatremia, how does urine osmolality distinguish primary polydipsia from SIADH?
- Primary polydipsia has dilute urine (low urine osmolality); SIADH has concentrated urine
- How does cerebral salt wasting differ from SIADH in volume status?
- Cerebral salt wasting causes hypovolemia; SIADH causes euvolemia
- What are the predominant symptoms of severe hyponatremia?
- Neurologic symptoms such as confusion, seizures, and cerebral edema
- What lab abnormality can cause spurious pseudohyponatremia?
- Severe hyperlipidemia or hyperproteinemia
- What causes translocational hyponatremia?
- Severe hyperglycemia pulling water into the extracellular space
- By how much does serum sodium fall for every 100 mg/dL rise in glucose above normal?
- About 1.6 mEq/L