Polydipsia and Polyuria Flashcards

Polydipsia

• Defined as excessive thirst that persists over long periods, leading to constant drinking.
• A symptom commonly associated with various medical conditions affecting water homeostasis.

Polyuria

• Characterized by the passage of a large volume of urine over a specific time frame, typically with decreased specific gravity (SG).
• Often results in abnormal excessive urination.

Polydipsia and Polyuria Relationship

• Both symptoms consistently appear in several polysystemic disorders, indicating disturbances in water regulation.

Primary Causes

• Polydipsia generally results from primary polydipsia conditions, which may lead to excessive thirst and urination.

Basic Principles

• The balance of water intake and output is crucial (water in = water out).
• Overhydration and dehydration need to be managed, affecting intracellular fluid levels.

Thirst Mechanism

• Thirst is a physiological desire for water, controlled by the hypothalamus, serving as a stimulus to drink.

Antidiuretic Hormone (ADH) Control

• The control center for ADH, responsible for managing fluid balance, is adjacent to the thirst center and responds to extracellular fluid osmolality.

Primary Thirst (PD)

• Well-understood, often triggered by intracellular dehydration and changes in blood volume, influencing thirst and ADH secretion.

Secondary Thirst (PD)

• Poorly understood and anticipates water needs before deficiencies arise; more common than primary thirst causes.

Dipsenogenic Compounds

• Renin and Angiotensin play a role in stimulating thirst through direct actions on neurons in the thirst center, influenced by certain diseases.

Pathologic Thirst

• Defined as primary polydipsia resulting from conditions like neuronal irritation, compulsive drinking, increased plasma renin, and hypercalcemia.

Neuronal Irritation Causes

• Hypothalamic tumors, trauma, or inflammation can lead to disturbances that trigger thirst.

Compulsive Water Drinking

• Often associated with psychological factors like anxiety or boredom, leading to non-physiological thirst responses.

Hypercalcemia Causes

• Conditions like cancer or kidney disease, resulting in mineral imbalances, can contribute to pathologic thirst.

Renal Concentrating Mechanism

• In a typical 25 lb dog, approximately 68 liters of fluid are filtered daily by the glomeruli, with less than 500 ml excreted as urine.

ADH Osmoreceptor Control System

• ADH is produced by specialized neurons and stored in nerve endings, responding to increased extracellular fluid osmolality by enhancing water reabsorption.

Countercurrent System

• Loops of Henle facilitate water reabsorption along concentration gradients, effectively reducing osmolality.

Isosthenuric Urine

• Refers to urine with a specific gravity similar to plasma (1008 to 1012), indicating renal abnormalities if more concentrated urine is expected.

Iatrogenic Factors

• Conditions or symptoms induced by human actions, generally drug-related.

Disorders Inducing Polyuria and Polydipsia

• A variety of disorders can disrupt tubular function, leading to these symptoms, including diabetes insipidus and mellitus.

Diseases Affecting Renal Concentrating Mechanisms

• Conditions that stimulate primary polydipsia and lead to polyuria include diabetes insipidus, diabetes mellitus, hyperadrenocorticism, and several others.

Diabetes Insipidus

• Two types exist: neurogenic (due to neural injury causing ADH deficiency) and nephrogenic (due to unresponsiveness of tubules to ADH).

Diabetes Mellitus

• characterized by insulin deficiency leading to hyperglycemia and glucosuria, which increases absorbable solute levels and may be tested with dipsticks.

Hyperadrenocorticism

• Involves increased glucocorticoid production, inhibiting ADH in collecting ducts, often associated with Cushing’s disease symptoms.