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Polyuria and Polydipsia

Polyuria and Polydipsia: Excerpt from The Diagnostic Approach to Symptoms and Signs in Pediatrics

Polyuria (excessive urine volume) is usuallyassociated with polydipsia (excessive thirst).

Principal Causes of Polyuria and Polydipsia

1. Diabetesmellitus
2. Diabetes insipidus
   1. Antidiuretichormone deficiency (central diabetes insipidus)
   2. Antidiuretic hormone resistance (nephrogenicdiabetes insipidus)
3. Primary polydipsia
   1. Compulsivewater drinking
   2. Hypothalamic thirst center defect

Clinical Features and Diagnosis

Diabetes Mellitus

By far,most common cause of polyuria and polydipsia in pediatric populationis diabetes mellitus.

Most common form is insulin-dependentdiabetes mellitus (type 1), in which autoimmune destruction of betacells of pancreas causes diminished insulin secretion and hyperglycemia.Subsequent glucosuria produces osmotic diuresis with polyuria, polydipsia,and polyphagia.

Second form is non–insulin-dependentdiabetes mellitus (type 2), which is characterized by insulin resistanceand usually relative insulin deficiency.

Criteria for diagnosis of diabetesmellitus have been established by American Diabetes Association(1999) and include symptoms of diabetes mellitus plus plasma glucoseconcentration ≥200 mg/dL at any time of day regardlessof time of last meal or fasting plasma glucose concentration ≥126 mg/dL(fasting is defined as no caloric intake for ≥8 hrs).

Diabetes Insipidus

2 formsof diabetes insipidus are antidiuretic hormone (ADH) deficiencyand ADH resistance.

Primary clinical manifestations arepolyuria and polydipsia. Episodes of hypernatremic dehydration withhypotonic urine also may occur.

Antidiuretic Hormone Deficiency (Central Diabetes Insipidus)

Kidney isunable to conserve water with ADH deficiency because of absenceor lack of release of ADH (arginine vasopressin).

Genetic transmission of primary formis autosomal-dominant, and gene locus has been mapped to chromosome20p13.

Secondary form includes lesions thatdamage neurohypophyseal system: head trauma, infection (meningitis,encephalitis), tumors (most commonly craniopharyngioma), Langerhanscell histiocytosis, and metastatic neoplasia (leukemia).

Specific gravity of morning urine specimenshould be ≥1.018 in normal children without any overnight fluidintake. Presence of inappropriately dilute urine with hyperosmolarserum suggests ADH deficiency, and water deprivation test shouldbe performed under controlled supervised conditions.

Inability to concentrate urine by waterdeprivation and resolution with administration of exogenous argininevasopressin confirm diagnosis.

Antidiuretic Hormone Resistance (Nephrogenic Diabetes Insipidus)

The defectin this form of diabetes insipidus is inability of renal tubuleto respond to ADH.

Primary form is usually due to X-linkeddisorder with defect in vasopressin receptor; however, autosomal-dominantand -recessive forms also occur.

Secondary form is due to several disorders(renal dysplasia, medullary cystic disease, cystinosis, sickle celldisease, chronic renal failure, Fanconi syndrome, Bartter syndrome,hypercalcemia) and drugs (lithium, demeclocycline, methoxyflurane,amphotericin B, phenytoin) that produce renal concentrating defects.

Primary Polydipsia

Is the excessiveingestion of water, when it is not needed to maintain fluid balance.

Causes include compulsive water drinkingfor psychological reasons and frequent presentation of fluids toinfants. Another cause of primary polydipsia is hypothalamic damagethat affects thirst center but not ADH release.

When overnight fluids are withheld,concentrated urine can be produced in the morning. Serum vasopressinlevels are low but increase to normal with fluid deprivation.

Diagnostic Approach

Polyuriamust be distinguished from small volume urinary frequency, whichis common in pediatric practice. Children with polyuria often havenocturia and are unable to sleep through the night without wakingup to urinate. Most children with urinary frequency do not havepolyuria or a defect in urinary concentrating ability. Common causesof isolated urinary frequency are habit, attention-seeking behavior,and urinary tract infection.

Random sample of urine with specificgravity of >1.028 and absence of polyuria rules out a concentrationdefect. Even urinary specific gravity of >1.020 on randomor early morning sample indicates sufficient urinary concentrationsuch that symptomatic diabetes insipidus is unlikely. Children whohave urine with a somewhat lower than normal specific gravity butwho can sleep through the night without passing urine do not needfurther evaluation.

Presence of polyuria, dehydration,and high urinary specific gravity is evidence for osmotic diuresis,which is most commonly caused by diabetes mellitus. Dilute urineassociated with polyuria suggests diabetes insipidus or psychogenicpolydipsia. If blood glucose and urea nitrogen are normal, high serumosmolality with hyposmolar urine suggests ADH deficiency or resistance.Low serum osmolality with hyposmolar urine suggests primary polydipsia.

With either ADH deficiency or resistance,urine specific gravity rarely exceeds 1.005 and urinary osmolalityrarely exceeds 200 mOsm/kg. Water deprivation test thatdemonstrates inability to concentrate urine indicates diabetes insipidusand distinguishes it from primary polydipsia. If urine remains hypotonicwith dehydration, next step is to determine response to exogenousvasopressin, which distinguishes ADH deficiency from resistance.With ADH deficiency, administration of vasopressin causes diminishingof symptoms and increase in urine specific gravity, whereas no responseoccurs with ADH resistance.

With suspected renal disease, certaintests should be performed: CBC and differential; UA; urine culture;serum electrolytes, calcium, phosphorus, and creatinine; blood ureanitrogen; hemoglobin electrophoresis; and renal U/S. Othertests (e.g., determination of serum and urinary amino acids, voiding cystourethrography,and renal biopsy) may be necessary to define specific renal abnormality.

Psychosocial history of emotional disturbance,including episodes of compulsive water drinking and formation ofconcentrated urine with fluid deprivation, are evidence for psychogenicpolydipsia. This disorder may sometimes be difficult to distinguishfrom hypothalamic thirst defect, and consultation with a pediatricendocrinologist is recommended.

References

1. Barratt TM, et al., ed. Pediatric nephrology,4th ed. Baltimore: Lippincott Williams & Wilkins, 1999.
2. Expert Committee on the Diagnosis and Classificationof Diabetes Mellitus. Report of the expert committee on the diagnosisand classification of diabetes mellitus. Diabetes Care 1999;22(suppl1):5–19.
3. Horev Z, Cohen AH. Compulsive water drinking in infantsand young children. Clin Pediatr 1994;11:209–213.
4. Leung AKC, et al. Polyuria in childhood. Clin Pediatr1991;30:634–640.
5. Online Mendelian Inheritance in Man (OMIM). McKusick-NathansInstitute for Genetic Medicine, Johns Hopkins University (Baltimore,MD) and National Center for Biotechnology Information, NationalLibrary of Medicine (Bethesda, MD), 2000. World Wide Web URL: http://www.ncbi.nlm.nih.gov/omim.
6. Rudolph AM, ed. Rudolph's pediatrics, 20thed. Stamford, CT: Appleton & Lange, 1996.

Book Source Details

• Book Title: The Diagnostic Approach to Symptoms and Signs in Pediatrics
• Author(s): Paul S. Bellet
• Year of Publication: 2006
• Copyright Details: The Diagnostic Approach to Symptoms and Signs in Pediatrics, Copyright © 2006 Lippincott Williams & Wilkins.

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More About This Book: Title: The Diagnostic Approach to Symptoms and Signs in Pediatrics Authors: Paul S. Bellet Publisher: Lippincott Williams & Wilkins Copyright: 2006 ISBN: 0-78172-899-1

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