DISEASES OF THE LOWER URINARY TRACT

There are five main groups of disorders in the lower urinary tract: infection, which is often secondary to stasis of urine, following obstruction to flow; obstruction by intrinsic occlusion or extrinsic pressure; stone formation, which is often secondary to stasis of urine combined with infection; tumour formation, i.e. neoplasia of transitional-cell epithelium; developmental abnormalities.

Infection

Infections in the lower urinary tract are predisposed by obstruction and stasis. Lower urinary tract infection is usually due to Gram-negative coliform

bacilli, e.g. E. coli and Proteus, which are normally commensals in the large bowel; because they have a short urethra, women are particularly prone to developing ascending infections. In men, lower urinary tract infection is usually associated with structural abnormalities of the lower urinary tract and stasis due to obstruction. Diabetes mellitus also predisposes to infection.

Morphology. The pelvicalyceal system is dark reddish brown as a result of acute inflammation of the usually smooth creamy mucosal lining due to bacterial infection. The kidney is also congested and some small scattered abscesses are present in the cortex and medulla (acute pyelonephritis). Obstruction of the drainage of urine from the kidney causes hydronephrosis. Obstruction, one of the most important consequences of disease of the lower urinary tract, may occur at any place in the tract: renal pelvis (calculi, tumours), periureteric junction (stricture, calculi, extrinsic compression), ureter (calculi, extrinsic compression-pregnancy, tumour, fibrosis), bladder (tumour, calculi), urethra (prostatic hyperplasia or carcinoma, urethral valves, urethral stricture).

If obstruction occurs in the urethra, the bladder develops dilatation and secondary hypertrophy of muscle in its wall. This predisposes to development of outpouching of the bladder mucosa (diverticulae).

If obstruction occurs in a ureter, there is dilatation of the ureter (megaureter), with progressive dilatation

of the renal pelvicalyceal system, termed hydronephrosis. Fluid entering the collecting ducts cannot empty into the renal pelvis and there is intrarenal resorption of fluid. At this stage, if the obstruction is relieved, renal function returns to normal. However, if obstruction persists, there is atrophy of renal tubules, glomerular hyalinization, and fibrosis. As an end-stage, the renal parenchyma becomes severely atrophic and renal function is permanently impaired.

Urinary tract obstruction also predisposes to infection and stone formation.

Urinary calculi

Urinary calculi may form anywhere in the lower urinary tract (urolithiasis), the most common sites being the pelvicalyceal system and bladder. The two main predisposing factors for stone formation are increased concentration of solute in urine (low fluid throughput or primary increase in metabolite), and reduced solubility of solute in urine (due to persistently abnormal urinary pH). Conditions that cause these factors to operate are low fluid intake, urine stasis, persistent urinary tract infection, and primary metabolic disturbances.

The most common urinary stones, accounting for 80% of cases, are composed of calcium oxalate or phosphate. Half of these cases are associated with idiopathic hypercalciuria, with only about 10% being

caused by hypercalcemia. Other cases may be caused by hyperoxaluria, which has several associations, e.g. inflammatory bowel disease. The second most common type of calculi, accounting for 15% of cases, are those composed of magnesium, ammonium, and calcium phosphates (struvite). They are associated with infection in the lower urinary tract as a result of urea-splitting organisms, which make urine permanently alkaline

Uric acid stones account for about 5% of cases and are predisposed by conditions causing hyperuricaemia, e.g. gout. However, 50% of patients with uric acid stones do not have hyperuricemia, and it is suggested that production of persistently acid urine is the predisposing factor. Cystine stones are rare, accounting for under 1% of cases. They are seen in heritable tubular transport defects causing cystinuria.

The renal pelvis is filled with a large calculus that is shaped to its contours, resembling the horn of a stag. The calyceal system at the lower pole contains separate rounded calculi.






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