Not all PCS dilatation, i.e. hydronephrosis, is pathological, or indeed obstructive, that is, there can be dilatation without physiological obstruction. Conversely, not all obstructive uropathy necessarily results in PCS dilatation. Ultrasound images Physiological dilatation Mild dilatation of the renal collecting system is a common finding, most commonly secondary to an over-distended bladder. Following micturition, the collecting system decompresses and returns to normal. An external renal pelvis (see above) is a nonobstructive ‘baggy’ dilatation of the pelvis and can be regarded as a normal variant. The intrarenal collecting system is normal in this situation (Fig. Below). Pregnancy is another common cause of mild PCS dilatation, more frequently on the right, particularly in the second and third trimester. This is thought to be due partly to pressure on the ureters from the advancing pregnancy and partly hormonal. It is however wrong to assume that the kidney is not obstructed just because the patient is pregnant. If symptomatic, the suspicion of obstruction in a dilated system is increased, particularly if echoes are present in the PCS.
TS through the RK demonstrating a baggy extrarenal pelvis. The PCS remains undilated, and this should not be confused with hydronephrosis
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