Enlargement of the third ventricle and hyponatraemia in aneurysmal subarachnoid haemorrhage.

Hyponatraemia following aneurysmal subarachnoid haemorrhage is associated with an increased risk of cerebral infarction. Whether the development of hyponatraemia was related to enlargement of the third ventricle on the admission CT scan was investigated in a consecutive series of 133 patients who were seen within 72 hours of aneurysmal haemorrhage. Hyponatraemia occurred significantly more often in patients with enlargement of the third ventricle (with or without dilatation of the lateral ventricles) than in patients with a normal ventricular system (20/41 versus 24/92, p = 0.016). After ventricular drainage, the sodium levels returned to normal in two patients in whom the size of the third ventricle decreased and not in four patients with persistent enlargement of the third ventricle. The significant relationship between enlargement of the third ventricle and hyponatraemia remained after adjustment for the amount of cisternal blood, but not after adjustment for the amount of intraventricular blood. These results suggest that the size of the third ventricle is an important but not the only factor in the relationship between acute hydrocephalus and hyponatraemia in patients with aneurysmal subarachnoid haemorrhage.