Basic science

There are four mechanisms of AUR3.

  1. Increased resistance to flow: this may be at the bladder neck smooth muscle, which results in a dynamic outflow obstruction or a mechanical obstruction such as urethral stricture or prostatic enlargement
  2. Inappropriate detrusor muscle innervation resulting from neurological causes such as stroke, spinal cord lesions or diabetic neuropathy
  3. Bladder over-distension such as occurs with postoperative pain and alcohol; inability to void leads to bladder distension
  4. Drugs: anti-muscarinic and alpha adrenergic medications

The prostate is rich in adrenergic receptors. In AUR, the maximal pressure rise is in the prostatic urethra, the bladder neck is not tightly closed and the tone of the external sphincter is inhibited. 5a Reductase drugs such as finasteride decrease symptoms of benign prostatic hypertrophy (BPH) and thus the incidence of AUR when taken long term10. However, significant side effects such as impotence and decreased libido may occur.

There are many causes of urinary retention. These do not need to be learnt by rote, but what is important is to always consider neurological causes.

Acute urinary retention aetiology


  • Urethral stricture
  • BPH
  • Acute prostatic haematoma
  • Prostate cancer
  • Bladder neck contracture
  • Urethral stone
  • Foreign body
  • Iatrogenic e.g. urinary stent occlusion


  • Neurogenic: neurogenic bladder
  • Neurological disease: MS, Parkinson’s, DESD (Detrusor-external sphincter dyssynergia), Tabes Dorsalis, Alzheimer’s
  • Spinal cord injury including Cauda Equina Syndrome
  • CVA
  • Tumour
  • Abdominoperineal resection
  • Spinal anaesthesia
  • Lower tract instrumentation
  • Traumatic pain
  • Alcohol toxicity
  • UTI
  • Acute prostatitis, E.coli, proteus

Drugs that cause AUR

  • Decongestants: phenylephrine, pseudoephedrine
  • Antihistamines: diphenhydramine, phenergan
  • Amphetamines
  • Morphine
  • Beta agonists: atropine, isoprenaline, isoproterenol
  • Hyoscine
  • Tricyclic antidepressants
  • Nifedipine

Female specific causes

  • Organ prolapse
  • Pelvic mass
  • Gravid uterus
  • Vulvovaginitis
  • Herpes simplex
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