From the Guidelines
Hyperosmolality has severe implications, including dehydration, increased risk of mortality, and disability.
Key Implications
- Dehydration: Hyperosmolality is a key indicator of dehydration, particularly in older adults, where renal function is often poor and clinical judgment may be fallible 1.
- Mortality and Disability: Older adults with raised serum osmolality (>300 mOsm/kg) have an increased risk of mortality and disability, with one study showing a doubling in risk of 4-year disability 1.
Diagnostic Threshold
- A serum or plasma osmolality >300 mOsm/kg is considered the threshold for dehydration, as established by Cheuvront et al. and supported by multiple cohort studies 1.
Physiological Effects
- Hyperosmolality triggers protection mechanisms, such as thirst and increased concentration of urine by the kidney, to maintain fluid balance 1.
- In older adults, poor renal function and clinical judgment limitations make serum osmolality a crucial indicator of hydration status 1.
From the Research
Implications of Hyperosmolality
The implications of hyperosmolality are multifaceted and can have significant effects on various bodily functions. Some of the key implications include:
- Cellular dehydration: Hyperosmolality can cause water to be drawn out of cells, leading to cellular dehydration 2, 3.
- Disruption of homeostatic mechanisms: Hyperosmolality can occur when there are defects in the two major homeostatic mechanisms required for water balance, namely thirst and arginine vasopressin (AVP) release 2.
- Neurologic symptoms: Hyperosmolality can cause serious neurologic symptoms, including seizures, coma, and death 2, 4.
- Respiratory depression: Hyperosmolality can lead to respiratory depression, which can be life-threatening 5.
- Cardiac and immune system disorders: Hyperosmolality can impair cardiac and immune function, particularly at levels above 310 mOsm/kg H2O 6.
- Kidney function impairment: Hyperosmolality can also impair kidney function, leading to a range of complications 6.
- Blood-brain barrier permeability: Hyperosmolality can increase blood-brain barrier permeability, leading to further complications in patients with traumatic brain injury 6.
Organ-Specific Effects
The effects of hyperosmolality can vary depending on the organ or system affected. For example:
- Renal inner medulla: Cells in the renal inner medulla are normally exposed to high levels of NaCl and urea, which can cause numerous perturbations, including elevation of reactive oxygen species, cytoskeletal rearrangement, and inhibition of DNA replication and transcription 3.
- Brain: Hyperosmolality can cause serious neurologic symptoms, including seizures, coma, and death, particularly in patients with traumatic brain injury 2, 4, 6.
- Heart: Hyperosmolality can impair cardiac function, particularly at levels above 310 mOsm/kg H2O 6.
- Immune system: Hyperosmolality can impair immune function, particularly at levels above 310 mOsm/kg H2O 6.