Syndrome of inappropriate Antidiuretic hormone (SIADH) at a glance

SIAD refers that the hyponatremia and hypo-osmolality resulting from inappropriate, continued secretion or action of the ADH hormone despite of normal or increased plasma volume, which result in impaired water excretion.

We know ADH is secreted from posterior pituitary gland & responsible for concentrated urine. When the osmolality of the body increase, ADH is secreted and acts on the V₂ receptors of the collecting tubules of the kidneys and helps to reabsorb free water from the renal medulla. But in SIAD, ADH is secreted inappropriately and causes more water reabsorption and the Na⁺  concentration in the ECF become low . The Condition became Hypotonic and Hyponatremic which is the main effect of SIAD.

Types:

1. Acute

2. Chronic – in cancer patients it may be chronic.

Causes of SIAD:  SIAD may be occurs due to –

A. Increased secretion of ADH:

CNS:       Stroke, hemorrhage, Infection, trauma, psychosis, Pituitary tumors,porphyria

Pulmonary conditions:       Pneumonia, Tuberculosis, acute respiratory failure, Asthma, Atelectasis

Drugs:    Cyclophosphamide , vinccristine ,Amiodarone, Ciprofloxcacine ,Antipsycotic drugs ,SSRIs ,TCAs,

                MAOIs,Bromocriptine , Carbamasepine.oral hypoglycemic agents ,

Postoperative state:           Major abdominal operation, Thoracic surgery

B. Ectopic secretion of ADH:

                Lung cancers, tumor of duodenum & pancreas, Thymus tumor, Sarcoma, Malignant

                 histiocytosis ,mesothelioma ,Olfactory neuroblastoma ,

C. Increase sensitivity to ADH:              NSAIDs , Cyclophosphamide , tolbutamide , carbamazepine,

                Chlorpromide.

D. Miscellaneous :               Exogenous administration of vasopressin ,malnutrition , Cachexia ,AIDS

Diagnosis:

                SIAD is a diagnosis of exclusion, it is necessary to rule out the thyroid,renal,liver ,cardiac, adrenal

 Dysfunction through laboratory testing.

Markers are

a. Low plasma sodium concentration (<130 mmol/L)

b . Low plasma osmolality (<270 mmol/kg) {normal 300 mmol/kg }

c. Urine osmolality not minimally low (>150 mmol/kg)

d.Urine sodium concentration not minimally low ( > 30 mmol/L )

e. Low normal plasma urea , creatinine ,uric acid

[a,b,c,d,e taken from Davidson’s Principles and practice of medicine 22nd edition ]

Sign /Symptom

If hyponatremia occurs gradually:

·         Asymptomatic

·         Anorexia

·         Nausea

·         Vomiting

·         Irritability

·         Headache

·         Abdominal cramp

                If Hyponatremia occurs rapidly:

·         Sign of cerebral oedema my developed

·         Headhache

·         Nausea

·         Muscle cramps

·         Generalized weakness

·         Hyporeflexia

·         Confusion,coma

·         Seizures

Treatment:

Before treatment others cause of hyponatremia must be excluded

• If asymptomatic hyponatremia only fluid restriction may improve gradually (600 -1000 ml/day)
• If mild symptomatic hyponatremia loop diuretics may be added except thiazide with fluid restriction.
• If sever hyponatremia infusion must be first choive but correction must not more than 10 mEq/L/day. Because if it corrected more than 10 mEq/L/day demyelination syndrome may develop.
• Vasopressin receptor antagonist may be used such as tolvaptan.

Collected & Concise only for postgraduate student