The use of electrolyte-free water clearance, sodium clearance and urine potassium to urine sodium ratio to distinguish between the syndrome of inappropriate antidiuretic hormone release and cerebral salt wasting syndrome in hyponatraemic patients in a neurosurgical intensive care unit
Date
2021
Authors
Magumbeze, Victor
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Abstract
Aim
Deciding the cause of hyponatraemia from physical examination of a patient and laboratory investigations of serum and urine biochemistry is often not possible. The reasons lie in the facts that physical examination cannot accurately define a patient's extracellular fluid volume and laboratory investigations do not provide clear discrimination between different causes of hyponatraemia. The aim of this study is to determine whether or not calculation of the modified electrolyte-free water clearance (MEFWC), sodium clearance (CNa) and the urine potassium to urine sodium ratio (UK/UNA ratio) will provide enough additional evidence to enable a clinician to make an accurate distinction between the syndrome of inappropriate anti-diuretic hormone release (SIADH) and the cerebral salt wasting syndrome (CSWS).
Method
Urine samples, collected over a period of 1 hour from hyponatraemic patients in a neurosurgical intensive care unit, were analyzed for sodium and potassium concentrations. This information was used to calculate values for MEFWC, CNa and UK/UNa ratio. Patients were divided firstly into two primary groups on the basis of whether MEFWC value was positive (Group P) or negative (Group N). Each of the primary groups was then divided into three sub-groups based on whether CNa value was less than normal (sub-groups P1 and N1 ), normal (sub-groups P2 and N2) or greater than normal (sub-groups P3 and N3). Lastly, UK/UNa ratio was calculated for each patient in the study.
Results
A total of 32 patients entered the study. There were 13 patients in Group P and 19 patients in Group N. Further sub-division was such that sub-groups P1, P2 and P3 consisted of 4, 5 and 4 patients, respectively. Sub-groups N 1, N2 and N3 consisted of 0, 9 and 10 patients respectively. The MEFWC for sub-group N2 was -8.87 ± 7.58 millilitres/ hour (ml/ hr) and that for sub-group N3 was -51.48 ± 32.30 ml/ hr. The CNa for these two groups were 58.28 ± 18.47 ml/ hr and 179.88 ± 86.64 ml/ hr, respectively. The UK/UNa ratio was found to be 0.31 ± 0.12 for sub-group N2 and 0.22 ± 0.15 for sub-group N3. Although the differences in MEFWC are not statistically significant he values for CNa are statistically different. By definition, patients with hyponatraemia and negative MEFWC are suffering from the SIADH. UK/UNA ratio of less than unity indicates that neither group has hypovolaemia. The different CNa indicate that patients with SIADH may have different levels of total body sodium (TBNa) and, therefore, different sodium excretion rates.
By definition, patients with positive MEFWC do not have SIADH. Patients in sub-groups P1, P2 and P3 all exhibited positive MEFWC (18.0 ± 9.89 ml/ hr, 49.65 ± 46.08 ml/ hr and 60.28 ± 58.75 ml/ hr, respectively). The differences in MEFWC between the P sub-groups are not statistically significant but do indicate that there is a wide range of total body water (TBH2O) in Group P patients. The CNa values for sub-group P1 (11.69 ± 3.10 ml/ hr), P2 (51.74 ± 19.55 ml/ hr) and P3 (195.17 ± 98.32 ml/ hr) are statistically significantly different. These results indicate that there is a wide variation in TBNa in Group P patients. Only sub-group P1 has UK/UNa ratio value greater than unity (1.95 ± 0.91 ). Thus, sub-group P1 is the only sub-group containing patients that can be classed as having hypovolaemia. It is also the sub-group with the lowest CNa. Sub-groups P2 and P3 do not have hypovolaemia as indicated by UK/UNa ratios less than unity (0.49 ± 0.24 and 0.29 ± 0.11, respectively).
Conclusion
The study demonstrated that calculation of the three indices, MEFWC, CNa and UK/UNa ratio provided enough information to readily identify patients with SIADH - on the basis of negative MEFWC, normal or increased CNa and an absence of hypovolaemia.
The study was able to identify patients with positive MEFWC, normal or increased CNa and absence of hypovolaemia indicating that various combinations of increased TBH2O and increased TBNa were responsible for the hyponatraemic condition in these patients.
A sub-group of patients with positive MEFWC, a low CNa and hypovolaemia was identified -patients who are retaining sodium and water to repair a deficit in TBNa and TBH2O.
The study did not identify any patients with hypovolaemia co-existing with a normal or an increased CNa which could be compatible with a diagnosis of CSWS.
Description
A research report submitted to the Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa in partial fulfilment of the requirement for the degree of Master of Medicine in the Department of Neurosurgery, 2021