84. Hypotonicity. Pathogenesis and consequences

Hypoosmolarity may develop due to:

• Water poisoning
• Moderate water intake in renal failure
• Syndrome of inappropriate ADH (SIADH)
• Severe oedema
• Salt deprivation
• Sweating and drinking just water
• Addison’s disease
• Renal tubular acidosis IV
• Cystic fibrosis

Again are many of these causes seen before because they cause hypovolaemia or hypervolaemia. Those that are new are bolded, for more details on the others can you check topic 81 and 82.

In salt deprivation will there by hypoosmolarity because the level of salt in the ECS decreases.

When sweating, you lose hypoosmolar fluid containing a lot of water and a little salt, which makes you hypertonic. However, if you sweat a lot and rehydrate yourself with just water and not salt will you be normovolaemic but hypotonic.

Compensation and consequences

Compensation occurs with suppression of ADH release, increased aldosterone and increased salt craving.

Any deviation of intracellular osmotic pressure (either hyperosmolarity or hypoosmolarity) causes non-specific inhibition of enzymes, as the enzymes are dependant on a certain osmolarity to function properly. This causes abnormalities in cell metabolism and function.

Hypoosmolarity causes cells to swell, which increases the intracranial pressure. Dizziness, headache, vomiting and potentially coma may occur. The intracranial pressure may occur so much that cerebral herniation occurs.