Medicine – Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)

Syndrome of inappropriate antidiuretic hormone secretion – SIADH is a disorder in which antidiuretic hormone (ADH), also called arginine vasopressin (AVP), continues to act despite low plasma osmolality.

The inappropriate ADH effect causes excessive renal water reabsorption, producing:

Water retention → dilutional hyponatraemia → low plasma osmolality → inappropriately concentrated urine.

Importantly, the major abnormality is water excess rather than primary sodium deficiency.

1. Normal ADH Physiology

ADH is synthesised in the:

Supraoptic and paraventricular nuclei of the hypothalamus.

It is transported to the:

Posterior pituitary,

where it is stored and released.

ADH acts primarily on V₂ receptors in collecting-duct principal cells, increasing insertion of:

Aquaporin-2 water channels.

Therefore:

ADH → V₂ receptors → aquaporin-2 insertion → ↑ renal water reabsorption.

2. What Happens in SIADH?

Normally, falling plasma osmolality should suppress ADH secretion.

In SIADH, ADH secretion or action persists despite:

Hypo-osmolality.

Therefore:

Inappropriate ADH activity

↓

↑ Collecting-duct water reabsorption

↓

Water retained disproportionately to sodium

↓

Dilutional hyponatraemia

↓

↓ Plasma osmolality.

3. Why the Urine Remains Concentrated

When plasma osmolality is low, the normal kidney should produce:

Very dilute urine.

In SIADH, ADH continues to act on the collecting ducts.

Therefore the urine remains:

Inappropriately concentrated.

A typical biochemical pattern is:

↓ Serum Na⁺

↓ Serum osmolality

Urine osmolality >100 mOsm/kg rather than maximally dilute

Urinary sodium often >30 mmol/L when salt intake and renal function are adequate.

4. Volume Status in SIADH

Patients with SIADH are usually clinically:

Euvolaemic.

Although ADH initially causes water retention, the resulting mild extracellular-volume expansion promotes:

Natriuresis – urinary sodium loss.

Consequently, obvious peripheral oedema is generally absent.

Therefore the classic pattern is:

EUVOLEMIC HYPOTONIC HYPONATRAEMIA.

5. Major Causes of SIADH

The causes can be organised into four major groups:

Malignancy.

Pulmonary disease.

Central nervous system disease.

Drugs.

Other causes include postoperative states, pain and nausea.

6. Malignancy

Malignancy is an important cause because certain tumours can produce:

Ectopic ADH.

The classic association is:

Small-cell lung carcinoma – SCLC.

7. Small-Cell Lung Carcinoma

The most important malignancy to remember is:

SMALL-CELL LUNG CARCINOMA.

SCLC is a neuroendocrine tumour capable of ectopic hormone production.

It can produce:

ADH

↓

Water retention

↓

Dilutional hyponatraemia

↓

SIADH.

Therefore:

SMALL-CELL LUNG CANCER + HYPONATRAEMIA → THINK SIADH.

8. Other Malignancies

The original notes also include:

Mesothelioma.

Bladder malignancy.

Prostate malignancy.

Pancreatic malignancy.

Lymphoma.

SIADH has been reported with several malignant diseases, but these associations are considerably less characteristic than:

Small-cell lung carcinoma.

For examination purposes, SCLC should remain the major malignancy association.

9. Pulmonary Disorders

Pulmonary disease can stimulate inappropriate ADH secretion even without ectopic production by a tumour.

Important pulmonary causes include:

Pneumonia.

Pulmonary tuberculosis.

Lung abscess.

Other significant pulmonary illnesses can also precipitate SIADH.

10. Pneumonia

The original notes correctly identify:

Pneumonia

as an important cause.

Pulmonary infection, inflammation, hypoxaemia, physiological stress and associated nausea/pain may stimulate:

ADH release.

Therefore pneumonia can produce:

Hyponatraemia due to SIADH.

11. Pulmonary Tuberculosis

The original notes correctly include:

Tuberculosis – TB.

Pulmonary TB can be associated with inappropriate ADH secretion.

However, when a patient with TB has hyponatraemia, other causes may also need consideration, including:

Adrenal insufficiency

if the adrenal glands are involved.

This matters because adrenal insufficiency can mimic the biochemical appearance of SIADH.

12. Lung Abscess

A:

Lung abscess

may also stimulate inappropriate ADH release as part of severe pulmonary infection.

Therefore:

PULMONARY INFECTION → ADH STIMULATION → POSSIBLE SIADH.

13. Central Nervous System Disorders

Many CNS disorders can disrupt the normal hypothalamic regulation of:

ADH secretion.

The original notes include:

Encephalitis.

Meningitis.

Trauma.

Subarachnoid haemorrhage.

Guillain–Barré syndrome.

Hydrocephalus.

Acute intermittent porphyria.

14. Meningitis and Encephalitis

Inflammation or infection involving the CNS can disturb hypothalamic ADH regulation.

Therefore:

Meningitis

and

Encephalitis

may cause inappropriate ADH release.

The resulting hyponatraemia can worsen neurological symptoms if severe.

15. Head Trauma

The original notes correctly include:

Trauma.

Head injury can disrupt hypothalamic and pituitary regulation.

Interestingly, depending on the nature of the injury, trauma can produce either:

SIADH

or

Central diabetes insipidus.

Therefore careful monitoring of:

Serum sodium and urine output

is important after significant neurological injury.

16. Subarachnoid Haemorrhage

The original notes correctly include:

Subarachnoid haemorrhage – SAH.

SAH is an important neurological setting in which:

Hyponatraemia

may develop.

SIADH is one possible mechanism.

17. SIADH Versus Cerebral Salt Wasting

Hyponatraemia following neurological injury is not automatically SIADH.

A differential diagnosis is:

Cerebral salt wasting.

The key conceptual distinction is:

SIADH → usually clinically euvolaemic.

Cerebral salt wasting → renal sodium loss with hypovolaemia.

Distinguishing them is important because their management differs.

18. Guillain–Barré Syndrome

The original notes correctly include:

Guillain–Barré syndrome – GBS.

GBS can be associated with SIADH, particularly in more severe disease.

Autonomic and neuroendocrine disturbances may contribute to:

Inappropriate ADH secretion.

Therefore significant hyponatraemia can occur during GBS.

19. Hydrocephalus

Hydrocephalus may disturb hypothalamic function or intracranial pressure relationships and can occasionally be associated with:

SIADH.

This is less common than some of the classic CNS causes but remains recognised.

20. Acute Intermittent Porphyria

The original notes correctly include:

Acute intermittent porphyria – AIP.

AIP can produce:

Severe abdominal pain.

Neurological symptoms.

Autonomic disturbance.

Psychiatric manifestations.

and importantly:

Hyponatraemia.

SIADH is an important mechanism contributing to hyponatraemia during an acute attack.

Therefore:

ABDOMINAL PAIN + NEUROPSYCHIATRIC FEATURES + HYPONATRAEMIA → CONSIDER AIP.

21. Drug-Induced SIADH

Medications are an important cause of SIADH and should always be reviewed in a patient with unexplained:

Euvolaemic hyponatraemia.

The original notes include:

Tricyclic antidepressants.

Carbamazepine.

Chlorpropamide.

Phenothiazines.

Several additional modern drug associations are also important.

22. Antidepressants

The original notes correctly include:

Tricyclic antidepressants – TCAs.

However, an especially important modern association is:

Selective serotonin reuptake inhibitors – SSRIs.

Examples include:

Sertraline.

Fluoxetine.

Citalopram.

SSRIs can cause SIADH and hyponatraemia, particularly in:

Older adults and other susceptible patients.

23. Carbamazepine

The original notes correctly identify:

Carbamazepine.

Carbamazepine can enhance the renal effects of ADH and/or promote inappropriate antidiuretic activity.

Therefore:

CARBAMAZEPINE → WATER RETENTION → HYPONATRAEMIA.

The related antiseizure drug:

Oxcarbazepine

is also strongly associated with hyponatraemia.

24. Chlorpropamide

The original notes include:

Chlorpropamide.

Chlorpropamide is an older sulfonylurea that can:

Potentiate the renal action of ADH.

It was historically well recognised as a cause of:

Hyponatraemia/SIADH-like antidiuresis.

Its importance has decreased because chlorpropamide is now used much less frequently.

25. Phenothiazines

The original notes correctly include:

Phenothiazines.

These drugs can promote inappropriate antidiuretic activity and contribute to:

Hyponatraemia.

Medication history is therefore particularly important in patients taking:

Psychotropic drugs.

26. Other Important Drug Causes

Additional medications associated with SIADH or inappropriate antidiuresis include:

SSRIs.

SNRIs.

Oxcarbazepine.

Some antipsychotic drugs.

Cyclophosphamide.

Vincristine.

Desmopressin and other vasopressin-related therapies, which can cause water retention through direct antidiuretic effects.

Drug-induced hyponatraemia may also involve mechanisms other than classic SIADH, so the clinical context remains important.

27. Pain, Nausea and Surgery

An important addition to the original list is:

Pain.

Nausea.

Surgery/postoperative stress.

These are potent non-osmotic stimuli for:

ADH release.

Therefore hospitalised postoperative patients may develop transient:

Hyponatraemia due to increased ADH activity.

28. Symptoms of SIADH

The symptoms are mainly caused by:

Hyponatraemia

and the resulting movement of water into brain cells.

The severity depends on both:

How low the sodium falls

and

How rapidly it falls.

29. Mild or Moderate Hyponatraemia

Patients may develop:

Nausea.

Headache.

Fatigue.

Dizziness.

Difficulty concentrating.

Confusion.

Some patients with chronic mild hyponatraemia may have relatively subtle symptoms.

30. Severe Acute Hyponatraemia

Rapidly developing severe hyponatraemia can produce:

Cerebral oedema.

This may cause:

Vomiting.

Marked confusion.

Seizures.

Reduced consciousness.

Coma.

Therefore:

SEVERE SYMPTOMATIC HYPONATRAEMIA IS A MEDICAL EMERGENCY.

31. Diagnostic Pattern of SIADH

The characteristic laboratory pattern is:

Hyponatraemia.

↓

Low measured serum osmolality.

↓

Urine remains:

Inappropriately concentrated.

↓

Urinary sodium is generally:

Not appropriately suppressed.

↓

Patient appears:

Clinically euvolaemic.

32. SIADH Is a Diagnosis of Exclusion

This is extremely important.

A patient should not be diagnosed with SIADH simply because they have:

Low sodium + concentrated urine.

Other causes of hypotonic hyponatraemia must be excluded.

Particularly important are:

Adrenal insufficiency.

Hypothyroidism when clinically relevant.

Renal failure.

Diuretic-related hyponatraemia.

Hypovolaemia.

Heart failure/cirrhosis and other oedematous states.

33. Adrenal Insufficiency

Adrenal insufficiency is particularly important because cortisol deficiency increases:

ADH secretion.

Therefore adrenal insufficiency may closely resemble SIADH.

Primary adrenal insufficiency may additionally cause:

Hyperkalaemia

because of aldosterone deficiency.

Therefore:

HYPONATRAEMIA SHOULD NOT AUTOMATICALLY BE LABELLED SIADH WITHOUT CONSIDERING ADRENAL INSUFFICIENCY.

34. Serum Osmolality

True SIADH produces:

Hypotonic hyponatraemia.

Therefore:

Serum osmolality is low.

This helps distinguish it from situations in which serum sodium is low but plasma tonicity is not truly reduced.

35. Urine Osmolality

In appropriate physiological suppression of ADH, hypotonic plasma should cause the kidneys to produce:

Maximally dilute urine.

In SIADH, this does not happen.

Therefore:

Urine osmolality is >100 mOsm/kg in typical diagnostic criteria.

The key concept is:

The urine is too concentrated for the low plasma osmolality.

36. Urinary Sodium

In SIADH, urinary sodium is often:

>30 mmol/L

when dietary sodium intake is adequate and there is no confounding renal dysfunction or diuretic use.

This occurs because the patient is not truly sodium-depleted and mild volume expansion promotes:

Natriuresis.

37. Treatment Principles

Management depends on:

Severity of hyponatraemia.

Presence of neurological symptoms.

How rapidly the sodium fell.

Underlying cause.

Treatment must be cautious because excessively rapid correction of chronic hyponatraemia can cause severe neurological injury.

38. Treat the Underlying Cause

Whenever possible:

Treat pneumonia or other infection.

Treat the underlying malignancy.

Review and stop the causative medication when appropriate.

Treat CNS disease.

Correcting the underlying cause may allow normal ADH regulation to return.

39. Fluid Restriction

For many patients with chronic or mild-to-moderate SIADH, a major initial strategy is:

Fluid restriction.

This reduces further free-water accumulation.

The effectiveness depends on the severity of SIADH and the patient’s urinary electrolyte and osmolality profile.

40. Severe Symptomatic Hyponatraemia

Patients with severe neurological symptoms such as:

Seizures.

Marked reduced consciousness.

or other manifestations of severe acute hyponatraemia may require carefully controlled:

Hypertonic saline.

This should be performed with close monitoring of serum sodium.

41. Avoid Over-Rapid Sodium Correction

An important complication of excessively rapid correction of chronic hyponatraemia is:

Osmotic demyelination syndrome – ODS.

Therefore:

Serum sodium must be corrected in a controlled manner.

Patients at particularly high risk of ODS include those with profound chronic hyponatraemia, malnutrition, alcohol-related disease, liver disease and hypokalaemia.

42. Other Treatment Options

Selected patients with persistent SIADH may require additional approaches such as:

Increased solute intake or oral urea.

Loop diuretics in selected circumstances.

Vasopressin receptor antagonists – vaptans – in carefully selected patients.

These treatments require clinical judgement and monitoring.

43. Demeclocycline

As discussed with nephrogenic diabetes insipidus:

Demeclocycline

reduces renal responsiveness to ADH.

Therefore it can induce:

Nephrogenic DI

and was historically used to counteract SIADH.

Its use is now limited in many settings because of:

Nephrotoxicity and other disadvantages.

44. Causes of SIADH – Note Form

MALIGNANCY:

Small-cell lung carcinoma – classic and most important.

Mesothelioma and several other malignancies have been reported.

Bladder/prostate/pancreatic malignancies and lymphoma are less classic associations.

PULMONARY:

Pneumonia.

Tuberculosis.

Lung abscess.

Other severe pulmonary disease.

CNS:

Meningitis.

Encephalitis.

Head trauma.

Subarachnoid haemorrhage.

Guillain–Barré syndrome.

Hydrocephalus.

Acute intermittent porphyria.

DRUGS:

SSRIs/SNRIs.

Tricyclic antidepressants.

Carbamazepine.

Oxcarbazepine.

Phenothiazines/other selected antipsychotics.

Chlorpropamide – historical.

Cyclophosphamide.

Vincristine.

OTHER STIMULI:

Pain.

Nausea.

Postoperative stress.

45. Diagnostic Pattern – Note Form

Serum sodium:

↓.

Serum osmolality:

↓.

Urine osmolality:

Inappropriately ↑ relative to plasma.

Usually >100 mOsm/kg rather than maximally dilute.

Urinary sodium:

Often >30 mmol/L when intake and renal function are appropriate.

Clinical volume status:

Usually euvolaemic.

Renal function:

Should not explain the water-handling abnormality.

Adrenal insufficiency:

Must be excluded.

Thyroid disease:

Consider/exclude where appropriate.

46. SIADH Versus Diabetes Insipidus – Copyable Comparison

SIADH

ADH effect:

Too much/inappropriately persistent.

↓

Kidney retains:

Water.

↓

Urine volume:

Reduced or relatively low.

↓

Urine:

Inappropriately concentrated.

↓

Serum sodium:

Low.

↓

Serum osmolality:

Low.

DI

ADH effect:

Too little ADH in central DI

or

Kidney resistant to ADH in nephrogenic DI.

↓

Kidney loses:

Free water.

↓

Urine volume:

High.

↓

Urine:

Very dilute.

↓

Serum sodium:

May become high if water intake is insufficient.

↓

Serum osmolality:

May become high.

47. Important Clarifications to the Original Notes

The most important malignant cause to remember is:

SMALL-CELL LUNG CARCINOMA → ECTOPIC ADH → SIADH.

Other malignancies can be associated, but they are less characteristic.

For pulmonary disease, remember:

PNEUMONIA + TB + LUNG ABSCESS.

For CNS disease, remember:

MENINGITIS/ENCEPHALITIS + SAH + TRAUMA + GBS.

For medications, the original list remains useful, but modern high-yield additions include:

SSRIs and oxcarbazepine.

Chlorpropamide is now largely a:

Historical association because the drug is rarely used.

Most importantly:

SIADH IS A DIAGNOSIS OF EXCLUSION.

Low sodium alone does not diagnose SIADH.

Key Clinical Pattern

For rapid recall:

SIADH = INAPPROPRIATELY HIGH ADH EFFECT → WATER RETENTION → DILUTIONAL HYPONATRAEMIA.

Think:

SCLC → CLASSIC MALIGNANCY.

PNEUMONIA / TB → PULMONARY.

MENINGITIS / ENCEPHALITIS / SAH / TRAUMA → CNS.

SSRI / CARBAMAZEPINE / OXCARBAZEPINE → DRUGS.

The characteristic biochemical pattern is:

↓ SERUM Na⁺

• ●

↓ SERUM OSMOLALITY

• ●

INAPPROPRIATELY CONCENTRATED URINE

• ●

URINARY Na⁺ NOT SUPPRESSED

• ●

CLINICAL EUVOLAEMIA.

And remember the opposite patterns:

SIADH → TOO MUCH ADH EFFECT → WATER RETAINED → LOW Na⁺.

DI → TOO LITTLE ADH EFFECT → WATER LOST → HIGH-VOLUME DILUTE URINE ± HIGH Na⁺.

1. Normal ADH Physiology ADH is synthesised in the: Supraoptic and paraventricular nuclei of the hypothalamus. It is transported to the: Posterior pituitary, where it is stored and released. ADH acts primarily on V₂ receptors in collecting-duct principal cells, increasing insertion of: Aquaporin-2 water channels. Therefore: ADH → V₂ receptors → aquaporin-2 insertion → ↑ renal water reabsorption.

2. What Happens in SIADH? Normally, falling plasma osmolality should suppress ADH secretion. In SIADH, ADH secretion or action persists despite: Hypo-osmolality. Therefore: Inappropriate ADH activity ↓ ↑ Collecting-duct water reabsorption ↓ Water retained disproportionately to sodium ↓ Dilutional hyponatraemia ↓ ↓ Plasma osmolality.

3. Why the Urine Remains Concentrated When plasma osmolality is low, the normal kidney should produce: Very dilute urine. In SIADH, ADH continues to act on the collecting ducts. Therefore the urine remains: Inappropriately concentrated. A typical biochemical pattern is: ↓ Serum Na⁺ ↓ Serum osmolality Urine osmolality >100 mOsm/kg rather than maximally dilute Urinary sodium often >30 mmol/L when salt intake and renal function are adequate.

4. Volume Status in SIADH Patients with SIADH are usually clinically: Euvolaemic. Although ADH initially causes water retention, the resulting mild extracellular-volume expansion promotes: Natriuresis – urinary sodium loss. Consequently, obvious peripheral oedema is generally absent. Therefore the classic pattern is: EUVOLEMIC HYPOTONIC HYPONATRAEMIA.

5. Major Causes of SIADH The causes can be organised into four major groups: Malignancy. Pulmonary disease. Central nervous system disease. Drugs. Other causes include postoperative states, pain and nausea.

6. Malignancy Malignancy is an important cause because certain tumours can produce: Ectopic ADH. The classic association is: Small-cell lung carcinoma – SCLC.

7. Small-Cell Lung Carcinoma The most important malignancy to remember is: SMALL-CELL LUNG CARCINOMA. SCLC is a neuroendocrine tumour capable of ectopic hormone production. It can produce: ADH ↓ Water retention ↓ Dilutional hyponatraemia ↓ SIADH. Therefore: SMALL-CELL LUNG CANCER + HYPONATRAEMIA → THINK SIADH.

8. Other Malignancies The original notes also include: Mesothelioma. Bladder malignancy. Prostate malignancy. Pancreatic malignancy. Lymphoma. SIADH has been reported with several malignant diseases, but these associations are considerably less characteristic than: Small-cell lung carcinoma. For examination purposes, SCLC should remain the major malignancy association.

9. Pulmonary Disorders Pulmonary disease can stimulate inappropriate ADH secretion even without ectopic production by a tumour. Important pulmonary causes include: Pneumonia. Pulmonary tuberculosis. Lung abscess. Other significant pulmonary illnesses can also precipitate SIADH.

10. Pneumonia The original notes correctly identify: Pneumonia as an important cause. Pulmonary infection, inflammation, hypoxaemia, physiological stress and associated nausea/pain may stimulate: ADH release. Therefore pneumonia can produce: Hyponatraemia due to SIADH.

11. Pulmonary Tuberculosis The original notes correctly include: Tuberculosis – TB. Pulmonary TB can be associated with inappropriate ADH secretion. However, when a patient with TB has hyponatraemia, other causes may also need consideration, including: Adrenal insufficiency if the adrenal glands are involved. This matters because adrenal insufficiency can mimic the biochemical appearance of SIADH.

12. Lung Abscess A: Lung abscess may also stimulate inappropriate ADH release as part of severe pulmonary infection. Therefore: PULMONARY INFECTION → ADH STIMULATION → POSSIBLE SIADH.

13. Central Nervous System Disorders Many CNS disorders can disrupt the normal hypothalamic regulation of: ADH secretion. The original notes include: Encephalitis. Meningitis. Trauma. Subarachnoid haemorrhage. Guillain–Barré syndrome. Hydrocephalus. Acute intermittent porphyria.

14. Meningitis and Encephalitis Inflammation or infection involving the CNS can disturb hypothalamic ADH regulation. Therefore: Meningitis and Encephalitis may cause inappropriate ADH release. The resulting hyponatraemia can worsen neurological symptoms if severe.

15. Head Trauma The original notes correctly include: Trauma. Head injury can disrupt hypothalamic and pituitary regulation. Interestingly, depending on the nature of the injury, trauma can produce either: SIADH or Central diabetes insipidus. Therefore careful monitoring of: Serum sodium and urine output is important after significant neurological injury.

16. Subarachnoid Haemorrhage The original notes correctly include: Subarachnoid haemorrhage – SAH. SAH is an important neurological setting in which: Hyponatraemia may develop. SIADH is one possible mechanism.

17. SIADH Versus Cerebral Salt Wasting Hyponatraemia following neurological injury is not automatically SIADH. A differential diagnosis is: Cerebral salt wasting. The key conceptual distinction is: SIADH → usually clinically euvolaemic. Cerebral salt wasting → renal sodium loss with hypovolaemia. Distinguishing them is important because their management differs.

18. Guillain–Barré Syndrome The original notes correctly include: Guillain–Barré syndrome – GBS. GBS can be associated with SIADH, particularly in more severe disease. Autonomic and neuroendocrine disturbances may contribute to: Inappropriate ADH secretion. Therefore significant hyponatraemia can occur during GBS.

19. Hydrocephalus Hydrocephalus may disturb hypothalamic function or intracranial pressure relationships and can occasionally be associated with: SIADH. This is less common than some of the classic CNS causes but remains recognised.

20. Acute Intermittent Porphyria The original notes correctly include: Acute intermittent porphyria – AIP. AIP can produce: Severe abdominal pain. Neurological symptoms. Autonomic disturbance. Psychiatric manifestations. and importantly: Hyponatraemia. SIADH is an important mechanism contributing to hyponatraemia during an acute attack. Therefore: ABDOMINAL PAIN + NEUROPSYCHIATRIC FEATURES + HYPONATRAEMIA → CONSIDER AIP.

21. Drug-Induced SIADH Medications are an important cause of SIADH and should always be reviewed in a patient with unexplained: Euvolaemic hyponatraemia. The original notes include: Tricyclic antidepressants. Carbamazepine. Chlorpropamide. Phenothiazines. Several additional modern drug associations are also important.

22. Antidepressants The original notes correctly include: Tricyclic antidepressants – TCAs. However, an especially important modern association is: Selective serotonin reuptake inhibitors – SSRIs. Examples include: Sertraline. Fluoxetine. Citalopram. SSRIs can cause SIADH and hyponatraemia, particularly in: Older adults and other susceptible patients.

23. Carbamazepine The original notes correctly identify: Carbamazepine. Carbamazepine can enhance the renal effects of ADH and/or promote inappropriate antidiuretic activity. Therefore: CARBAMAZEPINE → WATER RETENTION → HYPONATRAEMIA. The related antiseizure drug: Oxcarbazepine is also strongly associated with hyponatraemia.

24. Chlorpropamide The original notes include: Chlorpropamide. Chlorpropamide is an older sulfonylurea that can: Potentiate the renal action of ADH. It was historically well recognised as a cause of: Hyponatraemia/SIADH-like antidiuresis. Its importance has decreased because chlorpropamide is now used much less frequently.

25. Phenothiazines The original notes correctly include: Phenothiazines. These drugs can promote inappropriate antidiuretic activity and contribute to: Hyponatraemia. Medication history is therefore particularly important in patients taking: Psychotropic drugs.

26. Other Important Drug Causes Additional medications associated with SIADH or inappropriate antidiuresis include: SSRIs. SNRIs. Oxcarbazepine. Some antipsychotic drugs. Cyclophosphamide. Vincristine. Desmopressin and other vasopressin-related therapies, which can cause water retention through direct antidiuretic effects. Drug-induced hyponatraemia may also involve mechanisms other than classic SIADH, so the clinical context remains important.

27. Pain, Nausea and Surgery An important addition to the original list is: Pain. Nausea. Surgery/postoperative stress. These are potent non-osmotic stimuli for: ADH release. Therefore hospitalised postoperative patients may develop transient: Hyponatraemia due to increased ADH activity.

28. Symptoms of SIADH The symptoms are mainly caused by: Hyponatraemia and the resulting movement of water into brain cells. The severity depends on both: How low the sodium falls and How rapidly it falls.

29. Mild or Moderate Hyponatraemia Patients may develop: Nausea. Headache. Fatigue. Dizziness. Difficulty concentrating. Confusion. Some patients with chronic mild hyponatraemia may have relatively subtle symptoms.

30. Severe Acute Hyponatraemia Rapidly developing severe hyponatraemia can produce: Cerebral oedema. This may cause: Vomiting. Marked confusion. Seizures. Reduced consciousness. Coma. Therefore: SEVERE SYMPTOMATIC HYPONATRAEMIA IS A MEDICAL EMERGENCY.

31. Diagnostic Pattern of SIADH The characteristic laboratory pattern is: Hyponatraemia. ↓ Low measured serum osmolality. ↓ Urine remains: Inappropriately concentrated. ↓ Urinary sodium is generally: Not appropriately suppressed. ↓ Patient appears: Clinically euvolaemic.

32. SIADH Is a Diagnosis of Exclusion This is extremely important. A patient should not be diagnosed with SIADH simply because they have: Low sodium + concentrated urine. Other causes of hypotonic hyponatraemia must be excluded. Particularly important are: Adrenal insufficiency. Hypothyroidism when clinically relevant. Renal failure. Diuretic-related hyponatraemia. Hypovolaemia. Heart failure/cirrhosis and other oedematous states.

33. Adrenal Insufficiency Adrenal insufficiency is particularly important because cortisol deficiency increases: ADH secretion. Therefore adrenal insufficiency may closely resemble SIADH. Primary adrenal insufficiency may additionally cause: Hyperkalaemia because of aldosterone deficiency. Therefore: HYPONATRAEMIA SHOULD NOT AUTOMATICALLY BE LABELLED SIADH WITHOUT CONSIDERING ADRENAL INSUFFICIENCY.

34. Serum Osmolality True SIADH produces: Hypotonic hyponatraemia. Therefore: Serum osmolality is low. This helps distinguish it from situations in which serum sodium is low but plasma tonicity is not truly reduced.

35. Urine Osmolality In appropriate physiological suppression of ADH, hypotonic plasma should cause the kidneys to produce: Maximally dilute urine. In SIADH, this does not happen. Therefore: Urine osmolality is >100 mOsm/kg in typical diagnostic criteria. The key concept is: The urine is too concentrated for the low plasma osmolality.

36. Urinary Sodium In SIADH, urinary sodium is often: >30 mmol/L when dietary sodium intake is adequate and there is no confounding renal dysfunction or diuretic use. This occurs because the patient is not truly sodium-depleted and mild volume expansion promotes: Natriuresis.

37. Treatment Principles Management depends on: Severity of hyponatraemia. Presence of neurological symptoms. How rapidly the sodium fell. Underlying cause. Treatment must be cautious because excessively rapid correction of chronic hyponatraemia can cause severe neurological injury.

38. Treat the Underlying Cause Whenever possible: Treat pneumonia or other infection. Treat the underlying malignancy. Review and stop the causative medication when appropriate. Treat CNS disease. Correcting the underlying cause may allow normal ADH regulation to return.

39. Fluid Restriction For many patients with chronic or mild-to-moderate SIADH, a major initial strategy is: Fluid restriction. This reduces further free-water accumulation. The effectiveness depends on the severity of SIADH and the patient’s urinary electrolyte and osmolality profile.

40. Severe Symptomatic Hyponatraemia Patients with severe neurological symptoms such as: Seizures. Marked reduced consciousness. or other manifestations of severe acute hyponatraemia may require carefully controlled: Hypertonic saline. This should be performed with close monitoring of serum sodium.

41. Avoid Over-Rapid Sodium Correction An important complication of excessively rapid correction of chronic hyponatraemia is: Osmotic demyelination syndrome – ODS. Therefore: Serum sodium must be corrected in a controlled manner. Patients at particularly high risk of ODS include those with profound chronic hyponatraemia, malnutrition, alcohol-related disease, liver disease and hypokalaemia.

42. Other Treatment Options Selected patients with persistent SIADH may require additional approaches such as: Increased solute intake or oral urea. Loop diuretics in selected circumstances. Vasopressin receptor antagonists – vaptans – in carefully selected patients. These treatments require clinical judgement and monitoring.

43. Demeclocycline As discussed with nephrogenic diabetes insipidus: Demeclocycline reduces renal responsiveness to ADH. Therefore it can induce: Nephrogenic DI and was historically used to counteract SIADH. Its use is now limited in many settings because of: Nephrotoxicity and other disadvantages.

44. Causes of SIADH – Note Form MALIGNANCY: Small-cell lung carcinoma – classic and most important. Mesothelioma and several other malignancies have been reported. Bladder/prostate/pancreatic malignancies and lymphoma are less classic associations.

PULMONARY: Pneumonia. Tuberculosis. Lung abscess. Other severe pulmonary disease.

CNS: Meningitis. Encephalitis. Head trauma. Subarachnoid haemorrhage. Guillain–Barré syndrome. Hydrocephalus. Acute intermittent porphyria.

DRUGS: SSRIs/SNRIs. Tricyclic antidepressants. Carbamazepine. Oxcarbazepine. Phenothiazines/other selected antipsychotics. Chlorpropamide – historical. Cyclophosphamide. Vincristine.

OTHER STIMULI: Pain. Nausea. Postoperative stress.

45. Diagnostic Pattern – Note Form Serum sodium: ↓.

Serum osmolality: ↓.

Urine osmolality: Inappropriately ↑ relative to plasma. Usually >100 mOsm/kg rather than maximally dilute.

Urinary sodium: Often >30 mmol/L when intake and renal function are appropriate.

Clinical volume status: Usually euvolaemic.

Renal function: Should not explain the water-handling abnormality.

Adrenal insufficiency: Must be excluded.

Thyroid disease: Consider/exclude where appropriate.

46. SIADH Versus Diabetes Insipidus – Copyable Comparison SIADH ADH effect: Too much/inappropriately persistent. ↓ Kidney retains: Water. ↓ Urine volume: Reduced or relatively low. ↓ Urine: Inappropriately concentrated. ↓ Serum sodium: Low. ↓ Serum osmolality: Low.

DI ADH effect: Too little ADH in central DI or Kidney resistant to ADH in nephrogenic DI. ↓ Kidney loses: Free water. ↓ Urine volume: High. ↓ Urine: Very dilute. ↓ Serum sodium: May become high if water intake is insufficient. ↓ Serum osmolality: May become high.

47. Important Clarifications to the Original Notes The most important malignant cause to remember is: SMALL-CELL LUNG CARCINOMA → ECTOPIC ADH → SIADH. Other malignancies can be associated, but they are less characteristic.

For pulmonary disease, remember: PNEUMONIA + TB + LUNG ABSCESS.

For CNS disease, remember: MENINGITIS/ENCEPHALITIS + SAH + TRAUMA + GBS.

For medications, the original list remains useful, but modern high-yield additions include: SSRIs and oxcarbazepine. Chlorpropamide is now largely a: Historical association because the drug is rarely used.

Most importantly: SIADH IS A DIAGNOSIS OF EXCLUSION. Low sodium alone does not diagnose SIADH.

Key Clinical Pattern For rapid recall: SIADH = INAPPROPRIATELY HIGH ADH EFFECT → WATER RETENTION → DILUTIONAL HYPONATRAEMIA. Think: SCLC → CLASSIC MALIGNANCY. PNEUMONIA / TB → PULMONARY. MENINGITIS / ENCEPHALITIS / SAH / TRAUMA → CNS. SSRI / CARBAMAZEPINE / OXCARBAZEPINE → DRUGS. The characteristic biochemical pattern is: ↓ SERUM Na⁺  ●  ↓ SERUM OSMOLALITY  ●  INAPPROPRIATELY CONCENTRATED URINE  ●  URINARY Na⁺ NOT SUPPRESSED  ●  CLINICAL EUVOLAEMIA. And remember the opposite patterns: SIADH → TOO MUCH ADH EFFECT → WATER RETAINED → LOW Na⁺. DI → TOO LITTLE ADH EFFECT → WATER LOST → HIGH-VOLUME DILUTE URINE ± HIGH Na⁺.