SIADH is an important cause of hyponatremia in hospitalized patients.

1. SIADH
SYNDROME OF INAPPROPRIATE
ANTIDIURETIC HORMONE SECRETION
Speaker- Dr Rahul Arya

2. ANTI DIURETIC HORMONE
 ADH/AVP is
nonapeptide that is
synthesized in
hypothalamus and
transported to posterior
pituitary where it is
stored.
 AVP secretion is
regulated by effective
osmotic pressure of
body fluids.

3. MECHANISM OF ACTION OF AVP
 Stimuli for AVP secretion
 Hyperosmolarity – sensed by osmoreceptors in
the hypothalamus.
 Circulating volume depletion – sensed by
baroreceptors in carotid sinus, aortic arch,
pulmonary veins and left atrium.

5. SIADH
 It is a disorder of impaired water excretion caused
by the inability to suppress the secretion of
antidiuretic hormone (ADH).
 Inappropriate, continued secretion or action of ADH
despite normal or increased plasma volume.
 Results in impaired water excretion, and
subsequently hyponatremia and hypo-osmolality.

6.  The release of ADH is not inhibited by a reduction
in plasma osmolality.
 The non-physiological secretion of AVP results in
enhanced water reabsorption, leading to dilutional
hyponatremia.
 Transient expansion of extracellular fluid volume.

7. NEUROLOGICAL PATHOPHYSIOLOGY
 Hyponatremia and hypo-
osmolality cause acute
cerebral oedema.
 Brain ECF moves into
CSF.
 Rapid adaptation occur
within hours as a result
of loss of electrolytes.
 Slow adaptation occur
over several days with
loss of organic
osmolytes.

8. ETIOLOGY
 Increased secretion of ADH-
 CNS- stroke, hemorrhage, infection, trauma,
psychosis.
 Drugs- cyclophosphamide, vincristine, vinblastine,
amiodarone, ciprofloxacin, theophylline,
antipsychotic drugs (haloperidol, thioridazine,
thiothixene), SSRI, TCAs, MAOIs, bromocriptine,
clofibrate.
 Pulmonary conditions- pneumonia, ARDS, asthma,
atelectasis.
 Post operative states- major abdominal or thoracic
surgeries.

9.  Ectopic secretion of ADH-
Lung cancers, tumors of duodenum and pancreas,
olfactory neuroblastoma, malignant histiocytosis,
mesothelioma, occult tumors.
 Increased sensitivity to ADH-
NSAIDs, cyclophosphamide, tolbutamide,
carbamazepine, mizoribine, chlorpropamide.
 Miscellaneous-
exogenous administration of vasopressin,
desmopressin, cachexia, malnutrition, AIDS.

10. CLINICAL FEATURES
 Signs and symptoms depends on both degree of
hyponatremia and rate at which hyponatremia
develops.
 When sodium conc’n decreased slowly-
asymptomatic or non specific symptoms like
anorexia, nausea, vomiting, irritability, headaches
and abdominal cramps.
 Rapid decline- more severe symptoms.
 Serum sodium conc <120 mEq/L or serum
osmolality <240mOsm/kg is serious irrespective of
rate of decline.

11.  Patient can have cerebral edema manisfesting as
headache, nausea, restlessness, irritability, muscle
cramps, generalized weakness, hyporeflexia,
confusion, coma, seizures, brainstem herniation or
death.

12. EVALUATION AND DIAGNOSIS
 Careful history– comorbities, current medications
and patient’s symptoms.
 No significant findings in physical examination but
signs of dehydration or edema would make
diagnosis unlikely.
 Key points in diagnosing SIADH are-
 Serum sodium concentration.
 Tonicity of plasma and urine
 Urine sodium concentration
 Clinical volume status.

13. DIAGNOSIS OF SIADH
 Essential Features-
 Plasma osmolality <275 mOsm/kg.
 Urinary osmolality >100 mOsm/kg.
 Urinary Na >20 mEq/l with normal dietry salt intake.
 Normal thyroid, adrenal, cardiac, liver and kidney
function.
 Clinical euvolemia i.e no clinical signs of volume
depletion/ excess like tachycardia, decreased skin
turgor, dry mucus membrane, edema, ascites.
 No recent use of diuretic agent.

14. DIAGNOSIS OF SIADH
 Supplemental features-
 Plasma uric acid <4 mg/dl.
 Blood urea nitrogen <10 mg/dl.
 Correction of hyponatremia through fluid restriction.

15. CEREBRAL SALT WASTING SYNDROME
 Rare syndrome seen in patients with cerebral
tumors, subarachnoid hemorrhage, patients who
have undergone trans-sphenoidal pituitary surgery.
 Mimics SIADH i.e hyponatremia, increased urine
osmolality, urine Na > 20 mEq/l and urine osmolality
> serum osmolality.
 It represents appropriate water resorption with salt
wasting and a secondarily hypovolemic state

16. SIADH VS CEREBRAL SALT WASTING SYNDROME
Clinical Features SIADH CSW
Plasma Na Low Low
ECF volume Normal or slightly
increased
Decreased
Postural hypotension Absent Present
ADH Increased Increased
Urine osmolality Inappropriately high Appropriately high
Urine Na excretion Increased >40 mEq/l
because of volume
expansion
Increased >40 mEq/l
because of salt wasting
Plasma uric acid level Low due to volume
expansion
Low due to urinary
losses
Effect of isotonic saline May worsen
hyponatremia
Improves hyponatremia
Treatment Free water restriction,
hypertonic saline
infusion,
diuretics,vaptans
Infusion of isotonic
saline, Salt loading

17. TREATMENT
 Treatment depends on-
 Symptoms
 Serum sodium concentration
 Rapidity of onset of hyponatremia
 Primary etiology

18.  Mild asymptomatic hyponatremia(serum Na
>125mEq/L)-
 Fluid restriction is the 1st line treatment.
 It generally improves with correction of underlying
cause and restriction of free fluid intake to 800-
1000 ml/d.
 If no response, fluid intake can be restricted to 500-
600 ml/d.

19.  Mild symptomatic hyponatremia-
 Fluid restriction.
 Loop diuretic- it interfere with the action of ADH in
collecting tubules by inhibiting free water
reabsorption.
 The osmolality of infused saline must exceed the
osmolality of patient’s urine.

20.  Severe symptomatic hyponatremia (serum Na
<125 mEq/L)-
 Fluid restriction
 Hypertonic saline- infused via pump and urine
osmolality can be followed to guide therapy.
 Hypertonic saline can be switched to isotonic saline
when urine osmolality is <300 mOsm/L.
 Aggressive and overly rapid correction may induce
central pontine myelinosis.

21. CENTRAL PONTINE MYELINOSIS
 It is a demyelinating condition affecting pontine and
extrapontine neurons.
 It leads to quadriplegia, pseudobulbar palsy,
seizures, coma or even death.
 High risk patients are-
 Patient with hypokalemia
 Burn patient
 Patient on thiazide diuretics
 Alcoholics
 Elderly

22.  Serum Na level should be raised at rate no faster
then 1-2 mEq/h and rate should not exceed 8-12
mEq/d.
 Once serum Na rises above 125 mEq/l, risk of
seizures and death is reduced and daily correction
is slowed to 5-6 mEq/d.

23.  Chronic SIADH-
 High sodium diet with loop diuretics.
 Treatment of underlying cause.
 Other agents which can be used are-
 Demeclocycline
 Lithium
 Urea
 Vaptans

24. DEMECLOCYCLINE
 Tetracycline antibiotic.
 Interfers with action of ADH on collecting tubules.
 Induces nephrogenic diabetes insipidus in 70% of
cases.
 Onset of action takes over a week.
 Dose is 600-1200 mg daily in divided doses.
 May lead to irreversible renal failure.

25. LITHIUM
 Causes nephrogenic diabetes insipidus in 65% of
cases.
 Reduces aquaporin-2 expression.
 Effect takes 3-4 days to set in.
 Narrow therapeutic index 0.6-1.2 mEq/l.
 It induces tubulo-interstitial nephritis that can lead
to irreversible NDI and end stage renal failure.

26. UREA
 Causes osmotic diuresis and enhanced water
excretion.
 15-30 g of urea in glass of orange juice 2-3 times a
day after meals.
 Very cost effective and correct hyponatremia slowly
by 2-3 mmol/l/d.
 Because of its bitter taste many patient are not able
to tolerate it.

27. OTHER OPTIONS..
 Extracorporeal Treatments-
 Veno-venous hemofiltration.
 SLEDD (Slow Low Efficacy Daily Dialysis).

28. VAPTANS
 Vasopressin receptor antagonist.
 3 types of vasopressin receptors:-
subtype location action
V1a Vascular smooth
muscle
Vasoconstriction
V1b Anterior pituitary ACTH release
V2 Renal cortical and
medullary collecting
duct
Insertion of the water
channel aquaporin-2
in the luminal
membrane of the
collecting duct, thus
making it more
permeable to water.

29.  Types of Vaptans-
1. Non selective (mixed V1a/V2)- Conivaptan.
1. V1a selective- Relcovaptan.
2. V1b selective-Nelivaptan.
3. V2 selective- Lexivaptan
- Mozavaptan
- Satavaptan
- Tolvaptan.

32.  Few studies have been performed to study efficacy
of vaptans in SIADH patients.
 Several studies of short duration reported that
vaptans were efficacious in increasing serum
sodium concentration.
 There is however paucity of long term observations
in SIADH.
 But there is little doubt that these agents will be
effective in treatment of chronic hyponatremia.

33. CONIVAPTAN
 Conivaptan is available as an intravenous
preparation.
 There is initial loading dose of 20 mg over 30 min
followed by continuous infusion at a rate of 20 mg/d
for up to 4 days.
 Side effects- infusion site reactions, postural
hypotension, mild to moderate increase in BUN or
creatinine and significantly increased thirst.
 It is an efficient t/t for hyponatremia of 117-128
mmol/l.

34. TOLVAPTAN
 Tolvaptan is available as a tablet usually taken once
a day in morning.
 The recommended dosage is 15-30 mg/day.
 Patient should discontinue any previous fluid
restriction and drink fluid freely though not
excessively.
 Long term t/t over 1-2 year is effective and no
tachyphylaxis occurred.

35.  Adverse effects- constipation, nausea, dizziness,
weakness, hyperglycemia and UTI.
 The patients treated with vaptans no longer need
fluid restriction, correction of hyponatremia occur
efficiently and quickly and hospitalization is shorter.

36. SUMMARY
 SIADH is most common cause of hponatremia in
hospitalized patients.
 Symptoms of SIADH depends on degree of
hyponatremia and rate at which it develops.
 SIADH is a diagnosis of exclusion, and adrenal,
cardiac, liver, kidney and thyroid dysfunction must
be ruled out.
 Mild symptomatic hyponatremia is treated with fluid
restriction.

37.  Severe symptomatic hyponatremia is treated with
hypertonic saline in addition to fluid restriction.
 To avoid neurological complications the serum
sodium level should be raised no faster than 1 to 2
mEq/h and no faster than 8-12 mEq/day.
 The use of Vaptans has been considered a
breakthrough in treatment of hyponatremia;
however long term reports on the safety profile is
not available yet.