dilation have been defined differently by different sources, with anteroposterior diameters ranging between 10 and 20 mm. About 13% of normal healthy adults have a transverse pelvic diameter of over 10 mm. Grade 1 (mild) – Mild renal pelvis dilation (anteroposterior diameter less than 10 mm in fetuses) without dilation of the calyces nor parenchymal atrophy Grade 2 (mild) – Moderate renal pelvis dilation (between 10 and 15 mm in fetuses), including a few calyces Grade 3 (moderate) – Renal pelvis dilation with all calyces uniformly dilated. Normal renal parenchyma Grade 4 (severe) – As grade 3 but with thinning of the renal parenchyma Treatment Treatment of hydronephrosis focuses upon the removal of the obstruction and drainage of the urine that has accumulated behind the obstruction. Therefore, the specific treatment depends upon where the obstruction lies. Acute obstruction of the upper urinary tract is usually treated by the insertion of a nephrostomy tube. Chronic upper urinary tract obstruction is treated by the insertion of a ureteric stent or a pyeloplasty. Lower urinary tract obstruction (such as that caused by bladder outflow obstruction secondary to prostatic hypertrophy) is usually treated by insertion of a urinary catheter or a suprapubic catheter. Surgery is not required in all prenatally detected cases. Prognosis The prognosis of hydronephrosis is extremely variable and depends on the condition leading to hydronephrosis, whether one (unilateral) or both (bilateral) kidneys are affected, the pre-existing kidney function, the duration of hydronephrosis (acute or chronic), and whether hydronephrosis occurred in developing or mature kidneys. Permanent kidney damage can occur from prolonged hydronephrosis secondary to compression of kidney tissue and ischemia. For example, unilateral hydronephrosis caused by an obstructing stone will likely resolve when the stone passes, and the likelihood of recovery is excellent. Alternately, severe bilateral prenatal hydronephrosis (such as occurs with posterior urethral valves) will likely carry a poor long-term prognosis, because obstruction while the kidneys are developing causes permanent kidney damage even if the obstruction is relieved postnatal. Hydronephrosis can be a cause of pyonephrosis, which is a urological emergency. References Kidney diseases Nephrology