The renal function tests: a key to understanding of osmolal disorders in cerebral injury?

Introduction

Hyponatremia in cerebral injuries often associates with Cerebral Salt Wasting Syndrome (CSWS) or Inappropriate Antidiuretic Hormone Secretion Syndrome (IADHS). Typical cause of hypernatremia is central diabetes insipidus (DI). The group of available calculated renal function parameters is applied in the differential diagnosis of these syndromes.

Methods

The aim of the retrospective study is to clarify whether the changes of the renal function parameters predict the development of classic serum changes in the IADHS, CSWS and DI. Twenty patients with cerebral injury were evaluated during 3 days before they developed hypo- or hypernatremia with the aid of the original computer programe calculating renal function parameters. Urine was collected for 24 hours. Patients with renal insufficiency (Ckr < 0.8 ml/s), haemodynamicaly unstable and those who were treated with dose of furosemide higher than 1 mg/kg/day or with thiazide diuretics were excluded from the study. The courses of electrolyte free water clearence (EWC), electrolyte clearence (Cel) and natremia (SNa) in IADHS, CSWS and in DI were statistically evaluated using paired t-test (NCSS v.4).

Results

CSWS was diagnosed in nine patients. Cel (l/24 h) was gradually increasing above normal levels: 4.67 ± 3.45 vs 6.00 ± 2.70 (P > 0.05) vs 6.35 ± 4.99 (P > 0.05). Nevertheless the changes during the development of hyponatremia (141.2 ± 2.9 vs 136.4 ± 3.0 [P < 0.01] vs 132.0 ± 4.2 [P < 0.01]) were not statistically significant. EWC (l/24 h) was in normal range. Cel was stable at the normal to slightly increased levels in six IADHS patients. There was progressive decrease of SNa (139.3 ± 3.3 vs 136.9 ± 1.8 [P > 0.05] vs 133.1 ± 1.7 [P < 0.05]) accompanied by the inappropriate negative but nonsignificantly decreasing levels of EWC: –0.31 ± 2.12 vs –0.11 ± 1.44 (P > 0.05) vs –0.94 ± 0.69 (P > 0.05). Cel was found at normal levels in five patients who developed DI. EWC was grossly elevated: 1.46 ± 0.63 vs 2.85 ± 3.25 (P > 0.05) vs 2.30 ± 1.03 (P > 0.05) however the increase during the studied period did not reach statistic significance. The same time SNa increased: 141.5 ± 4.7 vs 152.5 ± 7.3 (P < 0.06) vs 157.4 ± 5.1 (P < 0.05).

Conclusions

The changes in selected renal function tests are not predictive for the development of the tonicity shifts in cerebral injury patients. Despite these preliminary results the renal function tests remain a valuable and cheap aid for the early diagnosis of the syndromes. An early correction of the infusion and diuretic therapy can minimize the disorder developing later.