Decreased urine output: oliguria and anuria
Quantitative indicators of daily urine output form the basis for classifying conditions accompanied by fluid retention in the body. If the urine volume drops below 500 mL per day, it is referred to as oliguria. When this indicator drops below 100 mL per day, anuria is diagnosed.
The key task in the differential diagnosis of complete absence of urination is to distinguish between disorders of urine formation and obstructions in its excretion path.
1. True anuria In this condition, the kidneys completely stop producing urine, as a result of which it does not enter the bladder. True anuria is a classic and frequent clinical sign of acute kidney injury (AKI). The following factors can lead to the cessation of kidney function:
- A sharp impairment of renal blood flow (occurs in shock states of any etiology, including cardiogenic shock).
- Acute toxic damage to nephrons by poisons and intoxications.
- Acute inflammatory processes in the organ parenchyma (e.g., acute nephritis).
2. Acute urinary retention (ishuria) Unlike true anuria, in ishuria the kidneys function completely normally and synthesize urine in the required volume. The problem lies in the inability to empty the bladder. The most important diagnostic sign of acute retention is the percussion finding of a significantly enlarged, fluid-overfilled bladder. Causes of ishuria development include:
- Pathological enlargement of the prostate gland (adenoma, malignant neoplasms).
- Central nervous system disorders disrupting sphincter innervation.
- Paraproctitis.
- Iatrogenic interventions and side effects of pharmacotherapy (use of atropine, narcotic analgesics, or ganglionic blockers).
Increased urine output: polyuria
The diagnosis of polyuria is made with a sustained increase in daily urine volume exceeding three liters. This symptom is very often accompanied by polydipsia — pathologically increased, insatiable thirst.
In clinical practice, it is extremely important to distinguish polyuria from pollakiuria (frequent urination). In polyuria, the total volume of excreted fluid increases significantly, which naturally can lead to more frequent trips to the bathroom. True pollakiuria occurs without an increase in daily urine output, and the portions of excreted urine remain small.
Etiological factors of polyuria can be divided into several large groups depending on pathogenesis:
- Alimentary: associated with simple excessive intake of a large amount of fluid.
- Osmotic: develop against the background of hyperosmolality, a classic example of which is hyperglycemia in diabetes mellitus.
- Central hormonal: caused by a deficiency of antidiuretic hormone (ADH), which is characteristic of diabetes insipidus.
- Renal (tubular): occur with tubular dysfunction (nephrogenic diabetes insipidus), interstitial kidney disease, and genetic mutations affecting aquaporins or ADH receptors.
- Drug-induced and metabolic: excessive urine excretion is triggered by hypercalcemia, lithium intake, or diuretics.
Diurnal rhythm disorders: nocturia
Nocturia is a pathological increase in urination during nighttime hours. The criterion for the presence of this symptom is the patient's need to wake up to empty the bladder more than once per night. When taking the medical history, the physician must clarify in detail the exact number of nighttime awakenings associated with the need to urinate.
It is important to note that in nocturia, the volume of individual urine portions can vary greatly — they can be both significant and very small.
Of particular diagnostic significance is the combined manifestation of polyuria and nocturia (when nighttime output prevails over daytime output). This symptom complex is highly specific and is a characteristic sign of the development of chronic kidney disease (CKD). The insidious nature of the situation lies in the fact that such a disturbance in the rhythm and volume of urine output can remain absolutely the sole clinical manifestation of progressive chronic kidney disease for a long time.
Briefly: answers to topical questions
Which hormones, besides antidiuretic hormone (ADH), are involved in the physiological regulation of urine output?
Several groups of hormones are involved in the physiological regulation of urine output.
- Aldosterone — decreases urine output and increases facultative reabsorption, and also increases the tone of the sympathetic nervous system.
- Natriuretic peptide (ANP) — increases urine output by blocking the reabsorption of ions and water; it is an antagonist of aldosterone.
- Thyroid hormones — increase urine output by enhancing protein metabolism and the accumulation of metabolic products (osmotic component).
- Growth hormone (somatotropin) — acts via metabolism, increasing urine output.
- Sex hormones — increase the concentration of protein metabolic products, which increases urine output.
True anuria — the kidneys are 'idle', the bladder is empty. Ishuria — the kidneys are working, the bladder is overfilled.
Class and exam questions
What is the main difference between polyuria and pollakiuria?
In polyuria, the total daily urine volume is always increased (more than 3 liters). In true pollakiuria, daily diuresis is normal, but urination is frequent, and fluid is excreted in very small portions.
How to quickly distinguish acute urinary retention from true anuria during an examination?
By means of suprapubic percussion. In acute retention (ishuria), a dull sound will be detected over the enlarged, overfilled bladder. In true anuria, the bladder is empty.
What is the clinical significance of the combination of polyuria and nocturia?
This combination, especially with the predominance of nighttime urine output over daytime output, is the most important and often the first clinical marker of chronic kidney disease (CKD).
What else to study on this topic
- Mechanisms of acute kidney injury (AKI) development
- Pathogenesis of central and nephrogenic diabetes insipidus
- The role of antidiuretic hormone (ADH) and aquaporins in water balance regulation
- Differential diagnosis of the causes of acute urinary retention (ishuria)
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