In SIADH, there is inappropriate secretion of ADH  despite normal or increased plasma volume. This will cause water retention and dilutional hyponatremia.

Pathophysiology

There are many reasons for inappropriate ADH secretion.  

Respiratory disorders

• Pneumonia    
• Bronchiloitis 
• Tuberculosis
• Lung abscess

Neurological disorders

• Meningitis 
• Encephalitis 
• Head injury
• Subdural / Subarachnoid haematoma
• Cerebral infarction

 Malignancy 

• Lymphoma (non-Hodgkin)

Drug effects 

• Cyclophosphamide
• Carbamazepine

Clinical Presentation and Diagnosis

Depending on the severity and the rate of development, a hyponatremia patient may or may not be symptomatic because slow, progressive hyponatremia is associated with fewer symptoms than a rapid drop of serum sodium to the same value.

Patients could develop 

• Confusion
• Disorientation
• Delirium
• Generalized muscle weakness
• Generalized seizures
• Tremor
• Ataxia
• Dysarthria
• Coma 

Initial Investigations and Management

The following investigations will help to confirm the diagnosis 

• Serum Na+, potassium, chloride, and bicarbonate
• Plasma osmolality and Urine osmolality 
• Serum creatinine and blood Urea 

The Following Criteria  are used to diagnose SIADH

• Decreased serum osmolality (<275 mOsm/kg)
• Increased urine osmolality (>100 mOsm/kg)
• Euvolaemia
• Increased urine sodium (>20 mmol/L)
• No other cause for hyponatraemia (no diuretic use and no suspicion of hypothyroidism, cortisol deficiency, marked hyperproteinaemia, hyperlipidaemia or hyperglycaemia).

Osmolality and Osmolality urine; Sodium and Sodium urine; levels of Creatinine, Urea and Urate tend to be low in SIADH, but high in patients in whom Hyponatraemia is associated with hypovolaemia.

Management

1. Since SIADH occurs secondary to another cause, the aetiology needs to be addressed and treated appropriately.
   
2. Maintenance of strict fluid balance is of utmost importance.

Input and the urine output must be measured. Catheterization is necessary in instances where the output measurement is not possible. In an infant or a young child, checking the diaper weight is advised. Input should be measured for all the liquids, and in a child where fluid intake is difficult to measure, for example, in a breast-fed child, the feeds may need to be expressed so that the intake amount can be certain.

3. Regular monitoring of sodium levels is important as it can aid the management.
   
4. Fluid restriction is the treatment and is successful most of the time. 
   
5. If the sodium levels fall <130 mmol/L or if the child becomes symptomatic, referral to a paediatric endocrinology service is necessary, with the management ideally done in the ICU/HDU setting. 
   
6. Patients with symptomatic hyponatraemia or resistant and severe hyponatraemia will need immediate treatment with 3% saline, but should be cautious to avoid rapid correction of hyponatraemia. The correction of sodium should not exceed more than 8 mEq/L per 24 hours or 0.5 to 1 mEq/L per hour.

Follow-up plan
Strict fluid balance is a must with careful monitoring of the sodium level. 

Indications to refer to an Endocrinologist 
If there’s severe resistant hyponatraemia or symptomatic hyponatraemia, the patient should be discussed with the paediatric endocrinology team regarding further management.  

Prognosis
The prognosis for patients with SIADH depends on the cause, and for benign causes, the prognosis is excellent and reversible once the aetiology is treated.

References:

1. The Royal Children’s hospital Melbourne, The Royal Children’s Hospital Melbourne. Available at: https://www.rch.org.au/clinicalguide/guideline_index/hyponatraemia/ (Accessed: 24 February 2025). 
2. Yasir M, Mechanic OJ. Syndrome of Inappropriate Antidiuretic Hormone Secretion. [Updated 2023 Mar 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing;2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507777/
3. NHS choices. Available at: https://clinicalguidelines.scot.nhs.uk/ggc-paediatric-guidelines/ggc-paediatric-guidelines/biochemistry/hyponatraemia-treatment-algorithm-paediatrics-190/ (Accessed: 24 February 2025). 

Professor Manori Gamage and Dr Dinendra Siriwardena 
Reviewed by Dr Navoda Atapattu