Causes of nocturia

There are three ways in which nocturia can be caused:

• By problems of fluid balance
• By neurological diseases affecting bladder control
• By disorders of the lower urinary tract (LUT)^[3] 

It is easy to overlook the first two categories while concentrating on the urinary tract.

Fluid balance causes of nocturia^[2]

Polyuria (day and night) - defined as urine volume >40 ml/kg/24 hours

• Excess fluid intake - including alcohol^[4]
• Diabetes mellitus (DM)
• Diabetes insipidus
• Hypercalcaemia
• Renal failure (more likely in chronic kidney disease rather than acute kidney injury)^[5]

Nocturnal polyuria - defined as normal 24-hour urine volume, with nocturnal volume >35% total

• Excessive evening fluid intake, including alcohol^[4]
• Diuretics (may depend on time of day taken)
• Disruption of normal vasopressin (antidiuretic hormone) secretion - more common in the elderly
• Nocturnal redistribution of fluid - cardiac failure; other causes of oedema - eg, venous stasis
• Sleep apnoea (unknown mechanism)

Neurological causes of nocturia

The bladder is controlled via the brain, spinal cord tracts, sacral segments S2-S4 and peripheral nerves. Therefore, many neurological conditions affect bladder function. Nocturia may be a symptom because:

• The neurological problem may cause urinary frequency: this can occur in multiple sclerosis (MS), and has been reported as an early feature of cervical cord compression and tethered spinal cord syndrome (TSCS).^[6]
• The neurology may cause retention of urine, which either results in frequency and true nocturia, or leads to overflow incontinence, which may be misinterpreted as nocturia.

Doctors should be aware that:

• If retention occurs in women or patients aged under 60, who are unlikely to have bladder obstruction, neurological causes need to be considered.
• Important, urgent conditions to diagnose are:
  • Cord compression and cauda equina syndrome (CES); the bladder innervation is easily damaged, and prompt referral/treatment can save bladder function.
• TSCS - although usually less acute, this again needs early referral.^[6]

Other common neurological disorders causing urinary symptoms are:

• Parkinson's disease^[7]
• Diabetic cystopathy

LUT causes of nocturia^[2]

This is a 'low nocturnal bladder capacity', which can be classified as due to:

• Bladder outflow obstruction (where chronic retention in effect lowers any additional bladder capacity):
  • Prostatic disease: benign prostatic hypertrophy, prostate cancer
  • Urethral disease - this occurs both in men and in women^[8]
• Bladder overactivity
• Sensory urgency
• Urinary tract infection
• Inflammation - eg, interstitial cystitis
• Malignancy
• Pregnancy

Assessment of nocturia in primary care

• Nocturia in often ascribed to prostatic disease (in men), without due consideration of other causes.
• Other factors, or a combination, are just as likely to be the cause.
• The cause of nocturia can usually be determined by simple assessment using history, examination and a voiding diary, plus urodynamics if necessary.^[9]

History

• Clarify the patient's symptoms, and ask about other LUT symptoms.
• Establish how these symptoms are affecting life and sleep.
• Consider whether this is a fluid balance problem:
  • Fluid intake pattern (including alcohol).
  • Excessive thirst suggests diabetes (mellitus or insipidus) or hypercalcaemia.
  • Establish whether there are any systemic illnesses which could be contributing - eg, cardiac failure, obstructive sleep apnoea, oedema, chronic kidney failure.
• Explore whether there any neurological or spinal symptoms: neck or back pain, limb weakness or sensory loss. Important symptoms and signs are:
  • Abnormal gait or spasticity, suggesting upper motor neurone lesions.
  • Limb sensory loss or weakness: widespread or bilateral symptoms are worrying and suggestive of cord or cauda equina lesions (nerve root lesions usually cause more localised and unilateral symptoms).
  • Constipation can also occur as part of sphincter disturbance.
  • Saddle area sensory loss and sexual dysfunction ('numb bum') suggest CES.
• Medication: consider any contributing medicines - eg, diuretics, excessive calcium supplementation or antacids,^[10] or lithium.

Examination

• Percuss the abdomen to examine for an enlarged bladder.
• Establish whether there is leg oedema present.
• Urine dipstick will screen for, but not exclude, DM, infection, haematuria and proteinuria.
• Other relevant examination, depending on the suspected cause:
  • Cardiovascular
  • Neurological - especially important if there is urinary retention where obstructive causes are unlikely, ie in women and the under-60s
  • Rectal examination (men) to assess the prostate; pelvic examination (women)

Investigations in primary care^[1]

• Voiding diary by the patient, including the time and volume of fluid intake and urine output.
• Blood tests: renal function, electrolytes, glucose and calcium.
• Midstream urine culture and microscopy.
• Urodynamics: GPs may have direct access to these clinics, which assess urinary flow rate and residual volume. Some clinics perform additional measurements, such as bladder capacity by ultrasound, bladder pressures using a urethral catheter, or fluoroscopic pressure and flow measurement.

Further management

Urgent problems which will need same-day referral are:

• Metabolic problems - eg, DM if ketotic, dehydrated or severely hyperglycaemic; hypercalcaemia; significant renal failure, electrolyte disturbance or lithium toxicity.
• Neurological problems: suspected cord compression or CES.
• Urological problems: acute retention, chronic retention with renal impairment.

Other problems, after initial assessment and investigations, can be classified according to the type of cause above, and then managed accordingly. Note that several conditions may co-exist, all contributing to nocturia - eg, cardiac impairment, DM and prostatic hypertrophy.