Disorders of tonicity, hyponatraemia and hypernatraemia, are common in neurosurgical patients. Tonicity is sensed by the circumventricular organs while the volume state is sensed by the kidney and peripheral baroreceptors; these two signals are integrated in the hypothalamus. Volume is maintained through the renin-angiotensin-aldosterone axis, while tonicity is defended by arginine vasopressin (antidiuretic hormone) and the thirst response. Edelman found that plasma sodium is dependent on the exchangeable sodium, potassium and free-water in the body. Thus, changes in tonicity must be due to disproportionate flux of these species in and out of the body. Sodium concentration may be measured by flame photometry and indirect, or direct, ion-sensitive electrodes. Only the latter method is not affected by changes in plasma composition. Classification of hyponatraemia by the volume state is imprecise. We compare the tonicity of the urine, given by the sodium potassium sum, to that of the plasma to determine the renal response to the dysnatraemia. We may then assess the activity of the renin-angiotensin-aldosterone axis using urinary sodium and fractional excretion of sodium, urate or urea. Together, with clinical context, these help us determine the aetiology of the dysnatraemia. Symptomatic individuals and those with intracranial catastrophes require prompt treatment and vigilant monitoring. Otherwise, in the absence of hypovolaemia, free-water restriction and correction of any reversible causes should be the mainstay of treatment for hyponatraemia. Hypernatraemia should be corrected with free-water, and concurrent disorders of volume should be addressed. Monitoring for overcorrection of hyponatraemia is necessary to avoid osmotic demyelination.

张力障碍、低钠血症和高钠血症在神经外科患者中很常见。张力由心室周围器官感知，而容量状态由肾脏和外周压力感受器感知；这两个信号整合在下丘脑中。容量通过肾素-血管紧张素-醛固酮轴维持，而张力则通过精氨酸加压素（抗利尿激素）和口渴反应来保护。爱德曼发现血浆钠依赖于体内可交换的钠、钾和游离水。因此，张力的变化一定是由于这些物种进出身体的不成比例的流量。钠浓度可以通过火焰光度法和间接或直接离子敏感电极来测量。只有后一种方法不受血浆成分变化的影响。根据容量状态对低钠血症进行分类是不准确的。我们将钠钾总和给出的尿液张力与血浆张力进行比较，以确定肾脏对钠血症的反应。然后我们可以使用尿钠和钠、尿酸盐或尿素的排泄分数来评估肾素-血管紧张素-醛固酮轴的活性。结合临床背景，这些有助于我们确定钠血症的病因。有症状的人和有颅内灾难的人需要及时治疗和警惕监测。否则，在没有低血容量的情况下，限制自由饮水和纠正任何可逆的原因应该是治疗低钠血症的主要方法。应使用游离水纠正高钠血症，并应解决并发的容量障碍。