Oliguria and Anuria vs Nocturia Meaning and Causes

What is oliguria or anuria?

Oliguria is the output of less than 400ml of urine in a 24 hour period.

Although the normal urine output in a day is approximately 1 to 2 liters (30 to 70 fluid ounces), the body only needs to pass out between 300ml to 500ml per day to rid the system of excess solutes and waste products. Another method for identifying oliguria is urine output that is less than 0.5ml/kg/h (millilitres per kilogram of body weight per hour) in adults or less than 1ml/kg/h in infants.

Anuria is a daily output less than 100ml. Rarely is there a total absence of urine output.

Causes of Oliguria and Anuria

In both oliguria and anuria, it has to established whether there is reduced urine production or if the output of urine is being obstructed, either partially or completely. This can be classified according to the cause in relation to the kidney.

  • Pre-renal means that the cause of the diminished urine output lies before the kidneys can produce urine.
  • Renal causes indicates that the cause is due to the kidney itself.
  • Post-renal refers to causes within the urinary tract that is preventing or reducing the output of urine.

Drugs or toxins may contribute to pre-renal and renal causes of oliguria or anuria.

Oliguria is sometimes seen immediately after recovering from any condition where fluid loss was a prominent feature – vomiting, diarrhea or polyuria. This is only temporary. Anuria is a medical emergency and immediate investigation and treatment is necessary.

Pre-renal

  • Low cardiac outputheart failure, myocardial infarction (heart attack), pulmonary embolism.
  • Low blood or fluid volume – dehydration, bleeding.
  • Reduced vascular resistance – sepsis, renal artery occlusion/stenosis and other diseases affecting the arterioles.

Renal

  • Acute tubular necrosis
  • Glomerular disease – primary or secondary to other systemic diseases.
  • Interstitial renal disease

Post-Renal

What is nocturia?

Nocturia is the term for urinary frequency resulting in waking up at night (awaking from sleep) in order to pass urine. The volume of urine is not significantly large as is the case with polyuria (passing >3 litres of urine in a 24 hour period). With nocturia, the total daily urine output is normal or even less than the normal volume. In the event that there is a large urine volume output in a day (>3 liters per day), then the causes of polyuria should first be considered.

Usually there is constant urging which is not alleviated by passing urine. A patient will often report that the bladder does not feel like it is empty even after urination. This feeling, which is known as tenesmus, can be quite distressing and despite the best efforts to force out as much urine as possible, the sensation persists. Nocturia does not usually occur in isolation – often the frequent urination is also evident during the waking hours (i.e. during the day).

Many patients suffering with nocturia may also report urinary incontinence – either the solitary incident or recurrent episodes.

Causes of Nocturia

Nocturia may be due to the consumption of :

  • Drugs (iatrogenic) like diuretics.
  • Foods and drinks with a diuretic action (example : tea, coffee, alcohol).
  • Excessive intake of fluids throughout the day or prior to falling asleep.

The above causes should be excluded before considering any pathological causes.

Psychogenic causes of nocturia may be related to psychogenic polydipsia (perception of unquenchable thirst), anxiety, depression or insomnia and other sleep disturbances. Pregnancy is another common non-pathological cause of nocturia, especially in the latter stages.

Most pathological causes of nocturia involve some degree of obstruction of the lower genotourinary system or are related to kidney disorders.

  • Urinary tract infections (UTI’s)
  • Urinary stones
  • Urethral stricture (narrowing of the urethra)
  • Causes of urinary incontinence
  • Causes of polyuria – diabetes mellitus must always be considered in frequent urination due to the incidence of this endocrine disorder.
  • Benign prostatic hyperplasia (BPH)
  • Prostatitis
  • Prostate cancer
  • Cystitis
  • Cystocele
  • Spinal cord injury/lesion
  • Chronic kidney diseases
  • Age-related changes of the bladder and urethra