Differential Diagnosis

Common Diagnoses

  • Age Related (in Part Caused by A Reduction in Bladder Capacity)
  • Excess Fluid Intake at Bed Time (Especially Alcohol)
  • Any Cause of Swollen Ankles (the Recumbent Posture Redistributes the Fluid Load At Night) – See ‘Swollen Ankles’
  • Cystitis
  • Luts in Men, Secondary to Benign Prostatic Enlargement

Occasional Diagnoses

  • Overactive Bladder Syndrome
  • Lower Urinary Tract Obstruction (other Than Prostate Problems)
  • Any other Cause of Urinary Frequency (See ‘Frequency’)
  • Diabetes Mellitus
  • Any other Cause of Polyuria (See ‘Excessive Urination’)
  • Sleep Apnoea (Causes Overproduction of Urine)

Rare Diagnoses

  • Anxiety
  • Drug Side Effect (Rare because Drugs Likely to Cause Diuresis Are Usually Taken in the Morning)
  • Diabetes Insipidus

Ready Reckoner

Key distinguishing features of the most common diagnoses

Age RelatedExcess Fluid IntakeSwollen AnklesCystitisLUTS
Daytime Ankle OedemaPossibleNoYesNoNo
Daytime FrequencyPossibleNoNoYesPossible
Poor Urinary FlowNoNoNoNoYes
Long-Term ProblemYesPossibleYesNoYes

Possible Investigations

Likely:Urinalysis, MSU, urinary frequency volume chart.

Possible:Blood sugar/HbA1c, PSA.

Small Print:Cystoscopy, urodynamic studies, ultrasound, water deprivation test.

  • Urinalysis: Protein, nitrites, leucocytes and possible haematuria in infection; glucose in diabetes; specific gravity very low in diabetes insipidus.
  • MSU: To confirm infection and identify pathogen.
  • Urinary frequency volume chart: To help distinguish nocturnal polyuria (increased urine production at night) from reduced bladder storage capacity.
  • Blood sugar/HbA1c: To confirm diabetes mellitus.
  • PSA: Pros and cons of this test may be discussed if assessment raises the possibility of prostate cancer.
  • Specialist tests include: Cystoscopy and ultrasound (for lower urinary tract obstruction), urodynamic studies (for unstable bladder) and water deprivation test (for diabetes insipidus).

Top Tips

  • In the elderly, the cause is often multifactorial.
  • The effects: Such as disturbed sleep, a disrupted household, exhaustion and occasional incontinence – may be more important to the patient than the specific diagnosis.
  • Swollen ankles: Of any aetiology – are frequently overlooked as an underlying cause
  • Nocturia may just be a manifestation (albeit the most distressing) of polyuria or urinary frequency. Focus your approach on the underlying problem.

Red Flags

  • Exclude diabetes – but remember that it is not the only cause of polyuria, nocturia and thirst.
  • A habitual ‘nightcap’ may be the cause of nocturia – and may be a pointer to an underlying alcohol problem, especially in solitary elderly males.
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