NON-DILATED RENAL OBSTRUCTION

Non-dilated renal obstruction
Obstruction may occasionally be present in the acute stages before renal dilatation is apparent: beware—the finding of a non-dilated PCS on ultrasound does not exclude obstruction in any patient with symptoms of renal colic. Spectral Doppler is useful in diagnosing acute, early renal obstruction, before PCS dilatation develops, because of the associated increase in blood flow resistance in the affected kidney. This causes an increase in the resistance and pulsatility indices (RI and PI) on the obstructed side, due to a reduction in diastolic flow. A raised RI in itself is a non-specific finding, not necessarily indicating obstruction; it is known to be age-related or can be associated with extrinsic compression of the kidney (for example by a fluid collection or mass) or with some chronic renal diseases or vascular disorders. This can be overcome by analysing Doppler spectra from both kidneys and evaluating any difference between the two sides. A marked difference in the RI between the kidneys in a patient with renal colic points towards obstruction of the kidney with the higher resistance. A difference in RI of greater than 6 is highly suspicious of obstruction in a patient with renal colic; a reduction in the RI on the affected side can be observed when the obstruction has been relieved or after the renal PCS has become dilated. This effect often does not persist once the kidney dilates, presumably because the intrarenal pressure is relieved, which emphasizes the use of Doppler in acute cases, before dilatation has become established.14 Because of the vagaries of the stage of obstruction, renal pressure, etc. the interpretation of RI should be made cautiously. IVU will show delayed PCS opacification and is also more useful than ultrasound in assessing the level of obstruction. CT IVU, as mentioned previously, is more commonly fulfilling the role previously held by the IVU.

Vesicoureteric junction
The normal ureters may be identified on ultrasound with high-resolution equipment, as they enter the bladder. Jets of urine emerge into the bladder at these points and can be demonstrated with colour Doppler. An absent or reduced number of jets may indicate obstruction on that side; this finding again should be interpreted cautiously. Ureteric jet analysis is not routinely performed at most hospitals as a diagnostic test of renal obstruction.

Careful scanning at the VUJs can identify significant anomalies:
● Reflux can be seen to dilate the ureter intermittently.
● A ureterocoele may be diagnosed as it dilates with the passage of urine; it may not be obvious until the operator has watched carefully for a few minutes.
● Stones may become lodged at the VUJ, causing proximal dilatation.

Non-obstructive hydronephrosis
Not all renal dilatation is the result of an obstructive process and the kidney may frequently be dilated for other reasons.

Reflux
This is the most common cause of non-obstructive renal dilatation, and is normally diagnosed in children. Reflux is associated with recurrent urinary tract infections and can result in reflux nephropathy, in which the renal parenchyma is irretrievably damaged. Reflux can be distinguished from other causes of renal dilatation by observing the dilatation of the ureters at the bladder base, due to the retrograde passage of urine.

Postobstructive dilatation
Dilatation of a once severely obstructed kidney may persist. The PCS remains baggy and dilated despite the obstruction having been relieved.

Papillary necrosis
The renal papillae, which are situated in the medulla adjacent to the calyces, are susceptible to ischaemia due to relatively low oxygenation in the region of the medullary junction. This is particularly associated with diabetic patients and those on long-term anti-inflammatory or analgesic medication. The papillae tend to necrose and slough off, causing blunting of calyces on IVU. Sloughed-off papillae may lodge in the entrance to the calyces, causing obstruction. Papillary necrosis is difficult to detect on ultrasound unless advanced. It appears as prominent calyces with increased corticomedullary differentiation. IVU is the imaging method of choice.

Congenital megacalyces
This is a congenital condition in which the PCS is dilated due to poor development of the papillae. The calyces are normally markedly enlarged but the cortex is normal and the ureters are of normal calibre and not dilated. Occasionally this is associated with congenital megaureter in which the muscular layer of the ureter is atonic.