Diabetes insipidus
A disorder of water metabolism, diabetes insipidus results from a deficiency of circulating vasopressin (also called antidiuretic hormone) or from renal resistance to this hormone. Pituitary diabetes insipidus is caused by deficiency of vasopressin, whereas nephrogenic diabetes insipidus is caused by renal tubular resistance to the action of vasopressin. Diabetes insipidus is characterized by excessive fluid intake and hypotonic polyuria.
The disorder may start in childhood or early adulthood (median age of onset is 21) and is more common in men than in women. Incidence is slightly higher today than in the past.
In uncomplicated diabetes insipidus, the prognosis is good with adequate water replacement, and patients usually lead normal lives.
Causes
Pituitary diabetes insipidus results from intracranial neoplastic or metastatic lesions, hypophysectomy or other neurosurgery, a skull fracture, or head trauma that damages the neurohypophyseal structures. It can also result from infection, granulomatous disease, and vascular lesions; it may be idiopathic and, rarely, familial.
The hypothalamus synthesizes vasopressin. The posterior pituitary gland (or neurohypophysis) stores vasopressin and releases it into the general circulation, where it causes the kidneys to reabsorb water by making the distal tubules and collecting duct cells water-permeable.
In pituitary diabetes insipidus, the absence of vasopressin allows the filtered water to be excreted in the urine instead of being reabsorbed. In renal diabetes insipidus, the kidney doesn’t respond to vasopressin, which is usually present in high concentrations.
Signs and symptoms
The patient’s history typically shows an abrupt onset of extreme polyuria (usually 4 to 16 L/day of dilute urine, but sometimes as much as 30 L/day). As a result, the patient is extremely thirsty and drinks great quantities of water to compensate for the body’s water loss. This disorder may also result in hourly nocturia.
If the patient is unable to obtain adequate quantities of water, features of diabetes insipidus include signs and symptoms of dehydration (poor tissue turgor, dry mucous membranes, constipation, muscle weakness, dizziness, and hypotension). Polyuria usually begins abruptly, commonly appearing within 1 to 2 days after a basal skull fracture, a stroke, or surgery.
Relieving cerebral edema or increased intracranial pressure may cause all of these symptoms to subside just as rapidly as they began.
Diagnosis
Urinalysis reveals almost colorless urine of low osmolality (50 to 200 mOsm/kg, less than that of plasma) and low specific gravity (less than 1.005). However, a diagnosis requires the water deprivation test to provide evidence of vasopressin deficiency, resulting in the kidneys’inability to concentrate urine.
Water deprivation test
In this test, after baseline vital signs, weight, and urine and plasma osmolalities are obtained, the patient is deprived of fluids and observed to make sure he doesn’t drink anything surreptitiously. Hourly measurements then record the total volume of urine output, body weight, urine osmolality or specific gravity, and plasma osmolality. Throughout the test, blood pressure and pulse rate must be monitored for signs of orthostatic hypotension.
Fluid deprivation continues until the patient loses 3% of his body weight (indicating severe dehydration). When urine osmolality stops increasing in three consecutive hourly specimens, the patient receives 5 units of aqueous vasopressin subcutaneously (S.C.).
Hourly measurements of urine volume and specific gravity continue after S.C. injection of aqueous vasopressin. Patients with pituitary diabetes insipidus respond to exogenous vasopressin with decreased urine output and increased specific gravity. Patients with nephrogenic diabetes insipidus show no response to vasopressin.
Treatment
Until the cause of diabetes insipidus can be identified and eliminated, administration of various forms of vasopressin can control fluid balance and prevent dehydration.
Vasopressin injection
This aqueous preparation is administered S.C. or I.M. several times a day because it’s effective for only 2 to 6 hours. This form of the drug is used as a diagnostic agent and, rarely, in acute disease.
Desmopressin acetate
This drug can be given orally, by nasal spray that’s absorbed through the mucous membranes or by S.C. or I.V. injection. Desmopressin acetate is effective for 8 to 20 hours, depending on the dosage.
Special considerations
❑ Record fluid intake and output carefully. Maintain fluid intake that’s adequate to prevent severe dehydration.
❑ Watch for signs of hypovolemic shock, and monitor blood pressure and heart and respiratory rates regularly, especially during the water deprivation test. Check the patient’s weight daily.
❑ If the patient is dizzy or has muscle weakness, institute safety precautions, including keeping the side rails up. Assist him with walking.
❑ Monitor urine specific gravity between doses. Watch for a decrease in specific gravity accompanied by increasing urine output, indicating the recurrence of polyuria and necessitating administration of the next dose of medication or a dosage increase.
❑ Provide meticulous skin and mouth care; apply petroleum jelly, as needed, to cracked or sore lips.
❑ Before discharge, teach the patient how to monitor intake and output.
Clinical tip Teach the patient the signs and symptoms of water intoxication, an adverse effect of excessive doses of desmopressin.
❑ Teach the patient that recurrence of polyuria, as reflected on the intake and output sheet, indicates that the dosage is too low.
❑ Advise the patient to wear a medical identification bracelet and to carry his medication with him at all times.
❑ Teach the parents of a child with diabetes insipidus about normal growth and development.
❑ Refer patients with diabetes insipidus and their families for counseling and psychosocial adjustment or coping and support groups, if necessary.
Book Source Details
- Book Title: Handbook of Diseases
- Author(s): Springhouse
- Year of Publication: 2003
- Copyright Details: Handbook of Diseases, Copyright © 2003 Lippincott Williams & Wilkins.
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Copyright Details: Handbook of Diseases, Copyright © 2008 Williams & Wilkins.
More About Causes of Diabetes-like symptoms
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More About This Book:
Title: Handbook of Diseases
Authors: Springhouse
Publisher: Lippincott Williams & Wilkins
Copyright: 2003
ISBN: 1-58255-266-5
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» Next page: Polydipsia (Signs & Symptoms: A 2-in-1 Reference for Nurses)
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