INTRODUCTION

Osmolality is the number of solute particles in a kilogram (kg) of solvent. Serum osmolality is chiefly determined by the sodium concentration as seen in the following equation:

An estimate can be made by doubling the serum Na⁺ and adding 10, if glucose and BUN are normal. Serum osmolality is calculated by the formula above or by freezing point depression. A difference between the two values indicates the presence of other osmotically active particles, e.g. ethylene glycol, mannitol. Decreasing the colloid oncotic pressure has minimal effect on brain water content in the normal brain.

Normal serum osmolality; cat/dog: 280–305 mOsm/kg.

In experimental animals, a reduction in plasma osmolality of as little as 5% under otherwise normal circumstances causes brain oedema and increases intracranial pressure (ICP).

Less than 260 or more than 330 mEq/l are likely to produce cerebral dysfunction but it is the speed of change in osmolality that is most likely to provoke neurological signs.

• Causes of hyper-osmolality: hypernatraemia, severe hyperglycaemia

• Causes of hypo-osmolality: hyponatraemia/water intoxication

• Clinical signs most commonly occur with rapid changes in osmolality.

Normal serum sodium: 140–150 mEq/l (dog), 150–160 mEq/l (cat). Lethargy commences with Na >170 mEq/l, or total osmolalities of ≥350 mOsm/kg. Signs of hyper-osmolality may be intermittent, worsening during periods of fluid loss from vomiting and diarrhoea. Osmotic dehydration and shrinkage of the brain may tear meningeal blood vessels.