Is "cerebral hyponatraemia" iatrogenic?

[Treatment of hypovolemia in brain injured patients]

The appropriate administration of intravenous fluids in neurosurgical patients remains an area of disagreement between neurosurgeons and anaesthetists. Fluid restriction has long been advocated by the former and is widely believed to reduce or prevent the formation of cerebral oedema. However, such restriction can lead to hypovolaemia which in turn can result in haemodynamic instability. Thus, brain homeostasis should be aimed for through adequate fluid administration and normal or slightly elevated mean arterial pressure. The properties of the endothelium differ between the brain and the remainder of the body. In most non CNS tissues the size of the junctions between endothelial cells averages 65 A. Proteins do not cross these gaps while sodium does. In the brain, the junction size is only 7 A, which is too small to allow crossing by sodium. Investigations with changes in osmotic and oncotic pressure have demonstrated that: 1) reducing osmolality results in oedema formation in all tissues including normal brain; 2) a decrease in oncotic pressure is only associated with peripheral oedema but not in the brain; 3) in case of brain injury, a decrease in osmolality elicits oedema in the part of brain which remained normal; 4) similarly, a decrease in oncotic pressure does not cause an increase in brain oedema in the injured part of the brain. Thus, a major reduction in oncotic pressure is unimportant for the brain, whereas changes in total osmolality are the dominant driving force at this level. To conclude, in a hypovolaemic patient with severe head injury, the crystalloid of choice is NaCl 0.9% and the colloid of choice is hydroxyethylstarch, both with an osmolality > 300 mosm.kg-1. Ringer-lactate is hypoosmotic (255 mosm.kg-1) and may cause or increase cerebral oedema. Mean arterial pressure should be maintained above 80 mmHg.

The effects of hyponatraemia and subarachnoid haemorrhage on the cerebral vasomotor responses of the rabbit

Impairment of cerebral autoregulation and development of hyponatraemia are both implicated in the pathogenesis of delayed cerebral ischaemia and infarction following subarachnoid haemorrhage (SAH) but the pathophysiology and interactions involved are not fully understood. We have studied the effects of hyponatraemia and SAH on the cerebral vasomotor responses of the rabbit. Cerebrovascular reactivity to hypercapnia and cerebral autoregulation to trimetaphan-induced hypotension were determined in normal and hyponatraemic rabbits before and 6 days after experimental SAH produced by two intracisternal injections of autologous blood. Hyponatraemia (mean plasma sodium of 119 mM) was induced gradually over 48 h by administration of Desmopressin and intraperitoneal 5% dextrose. Sham animals received normal saline. The cerebrovascular reactivity (% change +/- SD in cortical CBF/mm Hg PaCO2, measured by hydrogen clearance) of hyponatraemic (4.8 +/- 3.0%) and SAH (1.3 +/- 2.0%) animals was significantly less (p less than 0.05) than control (11.6 +/- 4.0%) and sham (8 +/- 2.0%) animals, whereas the reactivity of hyponatraemic-SAH animals was preserved (9.8 +/- 6.0%). Hyponatraemia and SAH alone each significantly impaired CBF autoregulation but their combined effects were not additive. Systemic hyponatraemia impairs normal cerebral vasomotor responses but does not augment the effects of experimental SAH in the rabbit.

Hyponatremia hypo-osmolarity in neurosurgical patients. "Appropriate secretion of ADH" and "cerebral salt wasting syndrome"

This prospective study is based on 256 patients with severe brain injury. Six patients (2.3%) developed the clinical picture of inappropriate secretion of antidiuretic hormone (SIADH): 3 in the first 3 days following the injury, 3 after more than a week. Their ADH plasmatic level were measured by radio-immunoassay. In the former, many factors, largely iatrogenic, can explain the increased secretion of ADH we found and which is then definitely "appropriate". It should be prevented by fluid restriction. In the latter, we found adequately low ADH levels, when the hypo-osmolarity is taken into account. Here, the aetiology seems to be a renal salt loss, eventually in relation to a natriuric factor (e.g. atrial natriuretic factor), justifying the term: "Cerebral salt wasting syndrome". With the resistance to fluid restriction, the treatment still remains a problem.

Pseudohyponatremia

Pseudohyponatremia should be distinguished from true hyponatremia lest injudicious therapy be instituted. Pseudohyponatremia is caused by a displacement of serum water by elevated concentrations of serum lipids or proteins. Only two (flame photometry and indirect potentiometry) of the three current methods available for measuring serum sodium involve sample dilution and may consequently produce spuriously low sodium values. The third method (direct potentiometry) involves no sample dilution, and sodium measurements are unaffected by hyperlipidemia and hyperproteinemia. As all three methods for sodium measurements may coexist in a clinical laboratory, it is important for the physician to be aware not only of the serum value but also the method employed.

Electrolyte disorders in the elderly

Normal human ageing impairs homeostatic mechanisms in such a way as to exaggerate and prolong the effects of stress. Thus, an event--pathological or traumatic--which produces a trivial change in plasma electrolytes of young people may produce major oscillations of plasma levels in the elderly, which take much longer to return to 'normal levels'. This is especially apparent with perturbations in the plasma levels of sodium and potassium, mainly due to changes in renal function and neurohumeral mechanisms which occur with increasing age. Paradoxically this does not mean that the clinician should be over-enthusiastic in attempting to correct electrolyte imbalance because, for the same reasons, the danger of over-treatment producing the opposite and equally dangerous electrolyte imbalance is ever-present. Indeed, in clinical practice most electrolyte disturbances in old age are iatrogenic in origin. Cautious patience and vigilance should be the clinical approach with elderly patients. A high index of suspicion should lead to a careful appraisal of the drug (diuretic, intravenous fluid) and environmental (dehydration) aetiology of most electrolyte disturbances in old age.

Partial reversal of carbamazepine-induced water intolerance by demeclocycline

The anti-diuretic action of carbamazepine, before and after concurrent treatment with demeclocycline, has been studied in a single epileptic subject, in whom two episodes of status epilepticus had been associated with excessive fluid intake and hyponatraemia. After addition of demeclocyline, free water clearance, plasma arginine vasopressin concentration and serum osmolality (all appreciably reduced after carbamazepine alone) increased but did not revert to normal. The findings are consistent with direct antagonism by demeclocycline of the enhancing effect of carbamazepine on endogenous ADH activity.