A CASE OF HYPONATREMIA, RESEMBLING CSWS, RESPONSIVE TO FLUDROCORTISONE, YET CONSIDERED TO BE SIADH

Cerebral or renal salt wasting syndrome (CSWS) presents hyponatremia, difficult to be distinguished from syndrome of inappropriate antidiuretic hormone secretion (SIADH), having a lot in common. The marked difference is volume status, which is so hard to estimate accurately in clinical settings that no differentiation methods have established. 

Fludrocortisone (FC) is considered as effective for CSWS, whereas generally not used for SIADH. Here, we report a case of hyponatremia, in which main physiology was thought to be SIADH, but some clinical findings suggested CSWS, and successfully treated by FC.

Case Presentation: A 54 y-o man with frontal sinus squamous cell carcinoma and brain metastasis, refractory to radiochemotherapy, admitted with complaints of headache, nausea, and anorexia, then hyponatremia (122 mEq/L) was revealed. Thirst and decreased urine output were reported, but no clear signs of hypovolemia were observed. He showed low serum osmolality (245 mOsm/Kg/ H2O), hypouricemia (1.4 mg/dl), and normal renal and thyroid function. Although cortisol level was slightly low (9.2 μg/dL), no adrenal insufficiency sign was observed. Urine was hypertonic with high Na and UA excretion (434 mOsm/Kg・H2O, urine Na/K/Cl 144/21/115 mEq/L, FEUA 25%). Continuous 3% hypertonic NaCl corrected serum Na to 135 mEq/L, yet hypertonic urine (Na+K 260-290 mEq/L) was excreted as much as 3-4L/day and even minor tapering of hypertonic NaCl led to dropping of Na level; these findings evoked CSWS. Highly kept FEUA after correction of serum Na also suggested it. However, raised blood pressure and slight dilation of IVC on echocardiography after initiation of hypertonic NaCl implied SIADH. To quit hypertonic NaCl, V2 receptor antagonist was considered but not used due to concern for causing dehydration. We started oral administration of FC 0.2 μg daily and successfully tapered and stopped hypertonic NaCl.

CSWS features pathological volume loss from kidney and the proposed mechanism causing hyponatremia is that volume depletion stimulates physiological AVP secretion, different from SIADH or adrenal insufficiency. In our case, dilation of IVC and raised blood pressure while administrating NaCl could not disprove tendency toward fluid loss, but could prove absent of volume depletion causing AVP secretion. Thus, hypertonic urine suggested the presence of ongoing AVP effect from other stimuli (e.g. brain tumor, stress), in turn contradicting CSWS. His sinuses cancer inhibited nasal breathing and provoked thirst, leading to insufficient water restriction. It is presumed that to treat SIADH (or adrenal insufficiency) inadequate fluid restriction demanded lots of hypertonic NaCl, inducing significant diuresis, may resembling CSWS.

Although FC was thought to improve hyponatremia in CSWS via restoration of volume, it should act through different mechanism in our case. Even though adrenal insufficiency was not disproved in our case, taking into account the previous report of efficacy of FC for SIADH at least temporarily and the recent research about effect of mineralocorticoid on AVP secretion, it is necessary to reassess how FC improves hyponatremia.

Our case implies the importance of reassessing hyponatremia cases in terms of what triggers and resolves AVP secretion.