[Nephrology] 부적절 항이뇨 증후군, 진단, Syndrome of Inappropriate Antidiuresis (SIAD), diagnosis

SIAD는 euvolemic hypoosmolality의 가장 흔한 원인이고, 모든 hypoosmolar 환자들 중에 20 ~40%를 차지하는 임상에서 접하는 모든 원인의 hypoosmolality 중에 단일 원인으로 가장 흔합니다. ​ SIAD 진단에 필요한 임상 기준은 기본적으로 1967년 Schwartz and Bartter가 1967년 제시한대로 유지되고 있습니다. 핑크색으로 되어 있는 5가지 기준은 꼭 알고 있어야 합니다 . 임상에서 SIAD를 평가할 때 적용하는 기준들입니다. ​ 1. Decreased effective osmolality of the extracellular fluid (P osm <275 mOsm/kg H 2 O). Pseudohyponatremia or hyperglycemia alone must be excluded. ​ 2. Inappropriate urinary concentration at some level of hypoosmolality. This does not mean that urine osmolality is greater than plasma osmolality, only less than maximally dilute (i.e., urine osmolality >100 mOsm/kg H 2 O). Also, urine osmolality need not be elevated inappropriately at all levels of plasma osmolality, because in the reset osmostat variant form of SIAD, vasopressin secretion can be suppressed with resultant maximal urinary dilution if plasma osmolality is decreased to sufficiently low levels. ​ 3. Clinical euvolemia, as defined by the absence of signs of hypovolemia (orthostasis, tachycardia, decreased skin turgor, dry mucous membranes) or hypervolemia (subcutaneous edema, ascites). Hypovolemia or hypervolemia strongly suggests different causes of hypoosmolality. Patients with SIAD can become hypovolemic or hypervolemic for other reasons, but in such cases it is impossible to diagnose the underlying inappropriate antidiuresis until the patient is rendered euvolemic and is found to have persistent hypoosmolality. ​ 4. Elevated urinary sodium excretion while on a normal salt and water intake. This criterion is included because of its utility in differentiating between hypoosmolality caused by a decreased EABV, in which case renal Na + conservation occurs, and distal dilution-induced disorders, in which urine Na + excretion is normal or increased secondary to ECF volume expansion. Patients with SIAD can have low urine Na + excretion if they subsequently become hypovolemic or solute depleted, conditions that sometimes follow severe salt and water restriction. Consequently, a high urine Na + excretion is the rule in most patients with SIAD; its presence does not guarantee this diagnosis, and its absence does not rule out the diagnosis. ​ 5. Absence of other potential causes of euvolemic hypoosmolality. Most notably are hypothyroidism, hypocortisolism (Addison disease or pituitary ACTH insufficiency), and diuretic use. ​ REF. Williams Textbook of Endocrinology, 14th edition 이와 관련된 내용은 세균맨 The쉬운 내과 [문제·해설]에도 언급되어 있습니다. ​ 해리슨 20판에 언급되어 있는 SIAD에 대한 임상 특징입니다. ​ A practical approach for determining the cause of hyponatremia IVIG: intravenous immune globulin; TURP: transurethral resection of the prostate; BP: blood pressure; ACTH: adrenocorticotropic hormone; ADH: antidiuretic hormone; TSH: thyroid-stimulating hormone; SIADH: syndrome of inappropriate antidiuretic hormone secretion. * A simple and convenient correction of the serum sodium for hyperglycemia is as follows: Add 2 mEq/L to the serum sodium for every 100 mg/dL of serum glucose above the normal value. ¶ Impaired water excretion in renal failure occurs if there is severe impairment in glomerular filtration rate. Patients with mild-to-moderate impairment in glomerular filtration rate are typically able to excrete water loads. The measured plasma osmolality may be high in patients with renal failure because of high urea concentrations. However, urea is an ineffective osmole, and such patients have hypotonic hyponatremia even if the plasma osmolality is normal. Δ Thiazide-induced hyponatremia may be protracted. An extensive evaluation for other etiologies can be delayed for several weeks in mildly hyponatremic patients. ◊ If the serum sodium is 125 mEq/L or less, we do not give isotonic saline. In such patients, the evaluation can be delayed until the sodium is slowly raised to higher levels. § Although patients with hyponatremia due to heart failure or cirrhosis will usually have edema that is clinically apparent, hypovolemia may not always be apparent by clinical exam. Thus, in a patient who appears to be euvolemic but whose urine chemistries are consistent with hypovolemia, infusion of isotonic saline (eg, 1 liter over one hour) can be helpful. ​ ​ REF. Williams Textbook of Endocrinology, 14th edition Harrison's Internal Medicine, 20th edition UpToDate 2020.08.05