SIADH

Question 1

69-year-old man presents to his physician with complaint of malaise for the past 2 months. He also has a unrelenting cough that is bothersome to him and keeps him from sleeping well at night. In addition, he realized that he had unintentionally lost 18 pounds in the past months after weighing himself when his wife
remarked that he looked thinner than usual. The patient has a 40 pack-year smoking history. Labs are drawn
and he is found to have a serum sodium level of 120 mEq/L. A chest radiograph is obtained and shown in the
image

What is the Cause?

What is Paraneoplastic Syndrome?

What is SIADH? Causes
Signs and symptoms
Labs

What is ADH Half Life?

Answer 1

Hyponatremia (SIADH) is associated with small cell lung cancer in the context of Paraneoplastic syndrome

paraneoplastic syndromes occur separately from the tumor itself, but result from substances produced by the tumor, symptoms may affect various body systems (i.e., endocrine, hematologic, and neuromuscular)

syndrome of inappropriate antidiuretic hormone (SIADH) is characterized by excessive free water retention and impaired water excretion, leading to dilutional hyponatremia

causes of SIADH?
Ectopic production
Such as in sclc

CNS disorders
Infection such meningitis encephalitis
Malignancy stroke

Drugs cyclophosphamide carbamazepine nsaids ssri

Symptoms & Signs of SIADH:

• Symptoms Depends on level of hyponatremia
o Mild symptoms (sodium usually 125-130
nausea and malaise
o moderate symptoms (115-125 mEq/L)
headaches lethargy weakness
o severe symptoms (< 120 mEq/L)
seizures coma respiratory arrest

• Physical exam
o euvolemic fluid status ▪ absence of edema ▪ normal skin turgor

Serum labs
↓ serum osmolality (< 275 mOsm/kg)
↓ serum sodium (< 135 mEq/L)
↓ serum uric acid (< 4 mg/dL)
adrenal and thyroid function are normal

Urine studies
urine osmolality > serum osmolality
↑ urine osmolality (> 100 mOsm/kg)
↑ urine sodium (40 mEq/L)

circulates in a free (non-protein bound) peptide, diffuses readily into the extracellular fluid and is present at very low concentrations (<40 pmol/L) and has a very short half-life (15–20 minutes), making routine diagnostic measurement impractical.

Question 2

Action of ADH

Synthesis and Storage of ADH

Management of SIADH?

Answer 2

The main action of ADH in the kidney is to regulate the volume and osmolarity of the urine. Specifically, it acts in
the distal convoluted tubule (DCT) and collecting ducts (CD).
When plasma osmolality increases, ADH secretion will increased.
ADH acts through a G-protein coupled receptor to increase the transcription and insertion of Aquaporin–2 channels to the apical membrane of the DCT and CD cells. Increasing the water, the permeability of the DCT and CD cells to water.

Its synthesis occurs in the supraoptic and paraventricular nuclei in the hypothalamus, then transported to the posterior pituitary gland via the neurohypophysial capillaries. In the posterior pituitary gland, its synthesis is completed, and it is stored here until it is ready to be secreted into the circulation

“Management depends on symptom severity and duration of hyponatremia.”
1. Treat the underlying cause — for example, remove the offending drug or treat a lung or CNS disorder.
2. Sodium correction:
• In acute or severe symptomatic hyponatremia, use hypertonic 3% saline to prevent cerebral edema.
• In chronic cases, correct slowly to avoid osmotic demyelination syndrome.
3. Medical management:
• Fluid restriction — first-line for mild or asymptomatic cases.
• Hypertonic (3%) saline — for severe symptoms or acute hyponatremia.
• Salt tablets — may be used as adjuvant therapy in asymptomatic or chronic cases.
• ADH receptor antagonists (demeclocycline, tolvaptan, conivaptan) — not used routinely; reserved for resistant or chronic SIADH.

Question 3

Correction of hyponatremia

Answer 3

Treatment depends on acuity and severity of symptoms.”

For acute or severely symptomatic hyponatremia:
• Correct aggressively with hypertonic saline (3% NaCl) to prevent cerebral edema.
• No strict limit in the first few hours — the goal is to relieve symptoms, not reach normal sodium immediately.
• Once stable, identify and treat the underlying cause.

For chronic hyponatremia:
• Correct slowly and carefully to avoid osmotic demyelination syndrome (ODS).
• Recommended rate of correction is not more than 6–8 mEq/L in the first 24 hours (some experts even limit to ≤6).
• Patients with sodium <120 mEq/L usually need ICU monitoring and specialist input.

Question 4

What is the recommended rate of correction of hyponatremia?

Answer 4

For chronic hyponatremia, correction must be slow to avoid osmotic demyelination syndrome.

In patients at normal risk, the goal is to raise serum sodium by 4–8 mEq/L in 24 hours, with a maximum correction of 10–12 mEq/L in 24 hours, and not more than 18 mEq/L in 48 hours.

In high-risk patients — such as alcoholics, malnourished, liver disease, or very low sodium — the correction should be even slower: 4–6 mEq/L in 24 hours, with a maximum of 8 mEq/L in 24 hours.”