A 45-year-old hospitalized patient is being evaluated for derangements in sodium and water… — USMLE Step 2 CK

A $45$ -year-old hospitalized patient is being evaluated for derangements in sodium and water balance. Evaluate the following four clinical laboratory profiles. Match each laboratory profile and dynamic test result with the most likely underlying pathophysiologic diagnosis.

Serum $Na^+$ : $122$ mEq/L, Serum Osmolality: $255$ mOsm/kg, Urine Osmolality: $550$ mOsm/kg, Urine $Na^+$ : $50$ mEq/L in a clinically euvolemic patient.Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
Serum $Na^+$ : $148$ mEq/L, Serum Osmolality: $305$ mOsm/kg, Urine Osmolality: $150$ mOsm/kg. Exogenous desmopressin administration increases Urine Osmolality to $600$ mOsm/kg.Central diabetes insipidus
Serum $Na^+$ : $149$ mEq/L, Serum Osmolality: $308$ mOsm/kg, Urine Osmolality: $140$ mOsm/kg. Exogenous desmopressin administration results in Urine Osmolality of $145$ mOsm/kg.Nephrogenic diabetes insipidus
Serum $Na^+$ : $130$ mEq/L, Serum Osmolality: $270$ mOsm/kg, Urine Osmolality: $80$ mOsm/kg in a patient with a history of schizophrenia.Primary polydipsia

Cevap

Profile 1 maps to SIADH, Profile 2 maps to Central diabetes insipidus, Profile 3 maps to Nephrogenic diabetes insipidus, and Profile 4 maps to Primary polydipsia.

Each laboratory profile distinctly maps to a water balance disorder. SIADH and primary polydipsia present with hyponatremia and low serum osmolality, but are differentiated by urine osmolality (inappropriately high in SIADH, appropriately low in primary polydipsia). Central and nephrogenic diabetes insipidus present with hypernatremia and dilute urine, and are differentiated by their ability to concentrate urine in response to exogenous desmopressin.

Adım Adım Çözüm

Evaluate the serum sodium and serum osmolality for each laboratory profile.

Profiles 1 and 4 indicate hypotonic hyponatremia (water excess); Profiles 2 and 3 indicate hypertonic hypernatremia (water deficit).

This fundamentally divides the differential diagnosis into states of water retention versus water loss.

For the hyponatremic profiles, evaluate the urine osmolality.

Profile 1 has inappropriately concentrated urine (

>100

mOsm/kg) pointing to SIADH. Profile 4 has appropriately maximally dilute urine (

<100

mOsm/kg) pointing to primary polydipsia.

To determine if the kidneys are appropriately excreting the excess free water in response to the hyponatremia.

For the hypernatremic profiles, evaluate the response to exogenous desmopressin (ADH).

Profile 2 shows a massive increase in urine osmolality, indicating central diabetes insipidus. Profile 3 shows no significant change, indicating nephrogenic diabetes insipidus.

To differentiate between central lack of ADH production and peripheral renal resistance to ADH.

Anahtar Kavram

Diagnosis of water balance disorders (diabetes insipidus, SIADH, primary polydipsia) through interpretation of serum and urine osmolality, and response to desmopressin.

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