flowchart TD
A["Hyponatremia: Na \< 135 mEq/L"] --> B{"Serum osmolality?"}
B -->|"Low"| C["Hypotonic hyponatremia (true)\nMost common"]
B -->|"Normal"| D["Isotonic hyponatremia\nPseudohyponatremia"]
B -->|"High"| E["Hypertonic hyponatremia\nHyperglycemia, mannitol"]
C --> F{"Volume status?"}
F -->|"Hypovolemic"| G["GI losses\nDiuretics\nAdrenal insufficiency"]
F -->|"Euvolemic"| H["SIADH\nCNS infections\nHypothyroidism\nPsychogenic polydipsia"]
F -->|"Hypervolemic"| I["CHF\nNephrotic syndrome\nLiver failure"]
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Rapid correction of chronic hyponatremia can cause osmotic demyelination syndrome! Do not exceed hourly or daily maximum correction limits.
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Clinically osmotic demyelination syndrome presents in a biphasic pattern. The first phase is usually attributable not to the demyelination but rather to the inciting electrolyte abnormality, with patients being acutely encephalopathic. Following rapid reversal of this abnormality, the patient transiently improves before progressing onto the classic osmotic demyelination syndrome features 2-3 days later. When pontine involvement is prominent, clinical features may include:
The earliest change is seen on DWI with diffusion restriction in the lower pons. This is seen within 24 hours of the onset of quadriplegia 3. This same region demonstrates eventual high T2 signal and later a low T1 signal. The T1 and T2 changes may take up to two weeks to develop. This region has a classic trident-shaped appearance. The overall appearance on T2/FLAIR axial MRI has also been likened to the face of a pig, referred to as piglet sign.
Once osmotic demyelination syndrome has occurred, management is supportive in nature. Thus, prevention of the syndrome developing is vital, in particular, ensuring that if there is hyponatremia, serum sodium correction should be gradual. One article states that serum sodium level correction should be 4 mEq/L to 6 mEq/L within 1 to 2 hours, with no more than 10 mEq/L correction within the initial 24 hours 7.
Some patients show complete recovery however six-month survival rate is only 5-10%.