Conditions
Diabetes insipidus (DI) is a condition of excessive output (more than three litres a day in adults) of dilute urine. This output cannot be reduced with restricting fluid intake.
DEFINITION
Diabetes insipidus (DI) is a condition of excessive output (more than three litres a day in adults) of dilute urine. This output cannot be reduced with restricting fluid intake, and represents an inability of the kidneys to concentrate urine normally.
CAUSES and AETIOLOGY
There are four known types of DI:
Central DI is due to insufficient antidiuretic hormone (ADH or vasopressin) being made or released by the pituitary gland. The most common causes of central DI are:
- Idiopathic – cause unknown – in 30-50% of cases. It has been proposed that some form of auto-immune process may direct destructive antibodies to the pituitary;
- neurosurgery and trauma to the pituitary gland;
- stroke;
- tumours – primary or secondary, spread from, for example, lung cancer or leukaemia;
- infiltrating diseases like histiocytosis X, eosinophilic granuloma or sarcoidosis; and
- familial – usually autosomal dominant inheritance.
Nephrogenic DI (NDI) is due to the inability of the kidneys to concentrate urine, despite the presence of enough ADH. A mild or transient decline of kidney function like this is often seen in the elderly, in the sick or those with a temporary kidney disorder. ADH resistance causing obvious symptoms can be due to:
- hereditary NDI, found in children, though may not apparent for several years;
- drugs, like lithium, (often used for treating bipolar disorder), amphotericin B, some antivirals, and orlistat. Lithium may cause permanent damage;
- low potassium levels; and
- high calcium levels.
Dipsogenic DI results from a defective thirst mechanism. Normally the hypothalamus regulates the balance between thirst and fluid intake, but when this is defective, the excessive thirst sensation leads to excessive fluid intake. This suppresses ADH and leads to even more urine output, continuing the vicious circle.
Gestational DH: this is DI found during pregnancy due to excess production by the placenta of a hormone which normally destroys ADH.
DIAGNOSIS
A history should determine the extent of polyuria (excessive urine output), the age of onset, and any obvious precipitating causes (like stroke, or neurosurgery). Excessive thirst, or confusion/changes in consciousness may also be present.
The extent of fluid retention can be assessed by measuring plasma sodium concentration, and is confirmed by measuring the patient’s response to the water restriction test, when urine volume and concentration should alter accordingly. The response to administered ADH can also form a diagnostic test, though this is not recommended in children or those in whom NDI is suspected. Measuring plasma ADH can also be done if the results of the water restriction test are not clearcut.
The combination of the above tests will confirm the diagnosis, and also differentiate the different types of DI.
TREATMENT
Treatment of this condition is highly specialised, and undertaken only by experienced endocrinologists.
CENTRAL and GESTATIONAL DI
Hormone replacement is effective in deficiency states, such as gestational and central DI, where desmopressin is used. This needs monitoring, because overdose can result in fluid retention, causing dangerously decreased sodium levels.
In some cases, other drugs are added, such as chlorpropamide, carbamazepine, clofibrate or even diuretics and anti-inflammatories.
NEPHROGENIC DI
The congenital form will require life-long treatment. Variations in the disease occur, so that treatment is only undertaken by specialists.
Discontinuing any known toxic drugs, and treating the underlying causes of electrolyte imbalance are mandatory.
Sodium restriction, mild diuretics and frequent bladder emptying are all used to relieve symptoms and avoid complications (like cystitis due to constantly full bladder, or ureteric dilatation). Water intake must still be encouraged to avoid dehydration, and precautions taken against gastro-oesophageal reflux associated with increased oral fluids.
(Dr AG Hall, Health24, January 2008)