There are three broad mechanisms:
■ increased urethral resistance, i.e., bladder outlet obstruction (BOO)
■ low bladder pressure, i.e., impaired bladder contractility
■ interruption of sensory or motor innervation of the bladder
Causes in Men
The commonest cause is benign prostatic enlargement (BPE) due to benign prostatic hyperplasia (BPH) leading to BOO; less common causes include malignant enlargement of the prostate, ure-thral stricture, and, rarely, prostatic abscess.
10 J. REYNARD
Urinary retention in men is either spontaneous or precipitated by an event. Precipitated retention is less likely to recur once the event that caused it has been removed. Spontaneous retention is more likely to recur after a trial of catheter removal, and therefore is more likely to require definitive treatment, e.g., transurethral resection of the prostate (TURP). Precipitating events include anaesthetics and other drugs (anticholinergics, sympathomimetic agents such as ephedrine in nasal deconges-tants); nonprostatic abdominal or perineal surgery; and immobility following surgical procedures, e.g., total hip replacement.
Causes in Women
There are more possible causes in women, but acute urinary retention is less common than it is in men. The causes include pelvic prolapse (cystocoele, rectocoele, uterine), the prolapsing organ directly compressing the urethra; urethral stricture; ure-thral diverticulum; postsurgery for 'stress' incontinence; Fowler's syndrome (impaired relaxation of external sphincter occurring in premenopausal women, often in association with polycystic ovaries); and pelvic masses (e.g., ovarian masses) (Fowler 2003).
Causes in Either Sex
A wide variety of pathologies can cause urinary retention in both men and women: haematuria leading to clot retention; drugs (as above); pain (adrenergic stimulation of the bladder neck); postoperative retention; sacral (S2-S4) nerve compression or damage—so-called cauda equina compression (due to prolapsed L2-L3 disc or L3-L4 intervertebral disc, trauma to the vertebrae, benign or metastatic tumours); radical pelvic surgery damaging the parasympathetic plexus (radical hysterectomy, abdomino-perineal resection); pelvic fracture rupturing the urethra (more likely in men than women); neurotropic viruses involving the sensory dorsal root ganglia of S2-S4 (herpes simplex or zoster); multiple sclerosis; transverse myelitis; diabetic cystopathy; damage to dorsal columns of spinal cord causing loss of bladder sensation (tabes dorsalis, pernicious anaemia).