The syndrome of inappropriate secretion of antidiuretic hormone and fluid restriction in meningitis--how strong is the evidence?

In patients with meningitis, fluid restriction is recommended to counter the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) and to reduce cerebral oedema. However, any effects of an increased plasma level of ADH upon cerebral oedema would be due not to fluid retention but to hypoosmolality. In a literature review of fluid and electrolyte disturbances and the effect of fluid therapy in bacterial/tuberculous meningitis, the prevalence of hyponatraemia, hypoosmolality and SIADH varied considerably; apparently, non-osmotic stimuli for the secretion of ADH, e.g. intracranial hypertension and hypovolaemia, were present in most patients. Neither clinical nor experimental studies have confirmed that fluid restriction reduces the cerebral oedema in meningitis. Furthermore, compared with maintenance therapy, fluid restriction did not improve outcome in a randomized controlled study. Thus, we find no evidence to support the use of fluid restriction in patients with meningitis. Fluid therapy in acute bacterial meningitis should aim at avoiding hypovolaemia and hypoosmolality based on the assumptions that (i) ADH is increased by non-osmotic stimuli; (ii) elevated ADH is less important for cerebral oedema than severe hypoosmolality, which may in itself induce or aggravate oedema; (iii) maintenance fluid therapy aiming at isoosmolality will not worsen neurological outcome; and (iv) hypovolaemia is difficult to detect, and detrimental for cerebral perfusion, in these patients.