[Osmolality and brain]

A modification of serum osmolarity induces always movements of water across cell membranes and therefore variations of cell volume of all tissues, including brain. To avoid a severe cell dehydration or hyperhydration, the organism has several regulation means called osmoregulation. Cerebral osmoregulation is essential as the brain is contained in a unexpandable box. In comparison with other organs, this phenomenon is complex and particular as: 1) cerebral volume consists of 3 sub spaces (intracellular, extracellular and cerebrospinal fluid [CSF]); 2) exists a blood-brain barrier (BBB) which behaves functionally as a semi-permeable membrane, essentially sensitive to osmolar disturbances. This brain volume regulation mechanism is working whatever the nature of the solutes initiating the osmotic deviation (sodium, glucose, mannitol...). Cerebral osmoregulation results from intracerebral osmolar modifications. Thus, every variation of plasma osmolarity elicits a similar variation of intracerebral osmolarity. This phenomenon results from modifications of the brain cell "protective" osmoles content. When the osmolar disturbance occurs quickly (in a few hours), cerebral osmoregulation is not complete. It results essentially from modifications of brain cell inorganic solutes content, i.e. electrolytes (Na, K, Cl) which originate from plasma, CSF and extracellular brain spaces uptake. When the osmolar disturbance is more progressive, cerebral osmoregulation is complete. The brain volume returns then to its initial value, by increasing its brain cell electrolytes, but above all organic "idiogenic" osmoles content. These idiogenic osmoles are identified as amino acids, polyols and trimethylamines. During treatment, the delayed normalization of brain osmolarity compared with plasma osmolarity prescribes a slow correction of the osmolar disturbance, as much as it is a chronic one.