ADH is secreted by supraoptic and paraventricular nuclei in the hypothalamus and transmitted via the neuronal axons to the posterior pituitary where it is secreted.
It is released when a decrease in the effective circulatory volume is sensed by vascular baroreceptors primarily located in the large arterial vessels.
ADH continues to be secreted regardless of the plasma osmolality.
Usually ADH is secreted when plasma osmolality increase.
As a result urine continues to concentrate while plasma osmolality remain low.
Urine concentration and urinary Na inappropriately high in a patient with hyponatraemia and hypo osmolar serum.
Clinical features:
Inappropriate secretion of ADH leads to retention of water and hyponatraemia.
The presentation is usually vague, with confusion, nausea, irritability and, later, fits and coma. There is no oedema.
Mild symptoms usually occur with plasma sodium levels below 125 mmol/L and serious manifestations are likely below 115 mmol/L. The elderly may show symptoms with milder abnormalities.
Common causes:
Tumours :
1. Small-cell carcinoma of lung
2. Prostate
3. Thymus
4. Pancreas
5. Lymphomas
Pulmonary lesions:
1. Pneumonia
2. Tuberculosis
3. Lung abscess
CNS causes:
1. Meningitis
2. Tumours
3. Head injury
4. Subdural haematoma
5. Cerebral abscess
6. SLE vasculitis
Metabolic causes :
1. Alcohol withdrawal
2. Porphyria
Drugs :
1. Chlorpropamide
2. Carbamazepine
3. Cyclophosphamide
4. Vincristine
5. Phenothiazines
Diagnosis:
Urine osmolality above plasma osmolality is diagnostic
Dil plasma : plasma osmo <260 , plasma Na <120
Conc urine : urine osmo > 280 , urine Na > 20
The usual features are:
• dilutional hyponatraemia due to excessive water retention
• low plasma osmolality with 'inappropriate' urine osmolality which is higher than plasma osmolality
• continued urinary sodium excretion > 30 mmol/L
• absence of hypokalaemia (or hypotension)
• Normal renal and adrenal and thyroid function.
Treatment:
The underlying cause should be corrected where possible. Symptomatic relief can be obtained by the following measures:
1. Fluid intake should be restricted to 500-1000 mL daily. If tolerated, and complied with, this will correct the biochemical abnormalities in almost every case.
2. Plasma osmolality and sodium and bodyweight should be measured frequently.
3. If water restriction is poorly tolerated or ineffective, demeclocycline (600-1200 mg daily) may be given; this inhibits the action of vasopressin on the kidney, causing a reversible form of nephrogenic diabetes insipidus. It often, however, causes photosensitive rashes.
4. When the syndrome is very severe, rarely hypertonic saline (300 mmol/L slowly i.v.) is given and furosemide may be used. These treatments are potentially dangerous and should only be used with extreme caution.
furhter reading
A 65-year-old female is admitted with an intracranial bleed under the care of the neurosurgeons. Following Magnetic resonance angiography she undergoes clipping of an cerebral arterial aneurysm and was well the following morning.
ReplyDeleteThe surgical team document the following blood chemistry results on successive postoperative days:
Day 1:Plasma Sodium 130 mmol/L, Potassium 3.5 mmol/L, Urea 4.2 mmol/L, Creatinine 95 µmol/L
Day 2:Plasma Sodium 127 mmol/L, Potassium 3.4 mmol/L, Urea 4.2 mmol/L, Creatinine 90 µmol/L
Day 3:Plasma Sodium 124 mmol/L, Potassium 3.4 mmol/L, Urea 4.4 mmol/L, Creatinine 76 µmol/L
Day 4:Plasma sodium 120 mmol/L, Potassium 3.5 mmol/L, Urea 5 mmol/L, Creatinine 70 µmol/L
Normal Range:Plasma sodium 137-144 mmol/L, Potassium 3.5-4.9 mmol/L, Urea 2.5-7.5 mmol/L, Creatinine 60-110 µmol/L
On day 4 she was commenced on a fluid restriction of 1 litre per day. Investigations at that time show:
Plasma osmolality:262 mOsmol/L (278-305)
Urine osmolality:700 mOsmol/L (350-1000)
Urine sodium:70 mmol/L
Select the most likely diagnosis to explain these findings?
1-Cranial diabetes insipidus
2-Hypoadrenalism
3-Fluid overload
4-Syndrome of inappropriate ADH (SIADH)
5-Sick cell syndrome