8 Metabolic encephalopathy
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 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.
Sudden hypotonic water load, decreased renal capacity to excrete water, excessive/acute inappropriate ADH activity, psychogenic polydipsia or acute CNS disease (cerebral salt-wasting syndrome). It is uncommon in veterinary practice.