Hyponatremia Signs and Symptoms by Severity Level
Mild Hyponatremia (130-135 mEq/L)
Mild hyponatremia is not benign and requires clinical attention despite often being asymptomatic. 1, 2
- Nausea and vomiting 2, 3
- Muscle cramps 2
- Gait instability and balance disturbances leading to increased fall risk (21% vs 5% in normonatremic patients) 2, 4
- Lethargy and generalized weakness 2, 3
- Headaches 2, 3
- Dizziness 2
- Cognitive impairment with altered memory and complex information processing 2, 4
- Mild neurocognitive deficits 3
Critical pitfall: Even at this level, mortality risk increases 60-fold (11.2% vs 0.19% in normonatremic patients) when sodium drops below 130 mEq/L, and fracture rates increase significantly (23.3% vs 17.3% over 7.4 years). 2, 4
Moderate Hyponatremia (125-129 mEq/L or 120-125 mEq/L)
At this level, symptoms become more pronounced and require active management. 3
- More prominent nausea and vomiting 3
- Increased weakness 3
- Worsening headache 3
- Confusion beginning to develop 5
- Clouding of consciousness (present in 76% of patients with sodium ≤120 mEq/L) 6
Severe Hyponatremia (<120-125 mEq/L)
Severe hyponatremia constitutes a medical emergency requiring immediate intervention. 2, 3
Neurological Manifestations:
- Confusion and delirium 2, 3
- Altered consciousness progressing to obtundation 2, 4
- Seizures (3.3% incidence) 2, 6, 3
- Coma (11% of patients with sodium ≤120 mEq/L) 6, 3
- Ataxia 3
- Long tract signs including hemiparesis (6.0% incidence) 6
- Hallucinations (0.5% incidence) 6
- Tremor (1.0% incidence) 6
- Acute psychosis (0.5% incidence) 6
Life-Threatening Complications:
- Respiratory distress 4
- Cardiorespiratory distress 4
- Brain herniation (rare) 3
- Death - mortality rate of 20.2% in hospitalized patients with sodium <115 mEq/L 7, 3
Key Clinical Determinants of Symptom Severity
The severity of symptoms depends on three critical factors: 4, 5
- Rapidity of development: Acute hyponatremia (<48 hours) causes more severe symptoms than chronic hyponatremia at the same sodium level 8, 4, 5
- Duration: Chronic hyponatremia allows for cerebral adaptation, reducing acute symptoms but causing long-term complications 4, 5
- Absolute sodium level: Lower sodium concentrations correlate with more severe manifestations 4
Special Considerations
In neurosurgical patients, even mild hyponatremia (131-135 mEq/L) requires closer monitoring as it may indicate cerebral salt wasting or SIADH, which can progress rapidly. 1
Chronic mild hyponatremia (130-135 mEq/L) is associated with secondary osteoporosis, increased fracture risk, and persistent cognitive impairment that improves with correction. 2, 4
Predictors of poor outcome in severe hyponatremia include: hypoxia/respiratory failure, presence of neurologic symptoms, sepsis, and paradoxically, slow correction rates in severely symptomatic patients. 7