Q. In your clinic you see a 3-month-old infant. There was a severe unilateral hydronephrosis detected antenatally. The rest of the renal tract has appeared normal on all the antenatal and postnatal scans. What is the most likely cause of the hydronephrosis?
A. PUJ obstruction.
Q. How does PUJ obstruction present in children?
A.
Most commonly with antenatal hydronephrosis.
In older children, PUJ obstruction may present with loin pain. This pain may be exacerbated by drinking large fluid volumes, where the subsequent diuresis exacerbates the stretching of the upper tract. This is referred to as Deitel’s crisis.
PUJ obstruction may be picked up as an incidental finding during the investigation of other complaints.
More unusually the child may present with a urinary tract infection, a mass or haematuria (a hydronephrotic kidney is more susceptible to trauma).
Q. What is the aetiology of PUJ obstruction in children?
A. Usually the PUJ junction is hypoplastic and aperistaltic in neonates.
A significant proportion of older children, usually after the age of 7 years, who present with PUJ obstruction will have a lower pole anterior accessory vessel crossing the PUJ (these children usually present with intermittent loin pain, which occurs after drinking large amounts; hydronephrosis is only present when patients are symptomatic).
Q. What investigations are useful in children in whom you suspect PUJ obstruction?
A. In children the two most useful tests are ultrasound and dynamic renography.
Ultrasound will give information about the degree of hydronephrosis, the AP renal pelvic diameter, degree of calyceal dilation and whether there is renal cortical thinning. It will usefully exclude ureteric dilation and other renal tract abnormalities, in which case there is no indication for a MCUG. Dynamic renography will give information about differential function of the kidneys, and information as regards how well the kidneys drain. MRI can be used to provide vascular anatomy prior to laparoscopic surgery. Intravenous urography gives very little additional useful information, and is rarely performed in children.
Note: If dynamic diuresis renography is equivocal a Whitaker test can be performed, but this is done very rarely, particularly in children. This investigation requires the placement of a nephrostomy in the affected kidney and a catheter in the bladder. With the patient prone a mixture of contrast with saline is infused via the nephrostomy at a rate of 10 mL/min. The pressure difference between the kidney and bladder is measured: if <15 cm H2O the system is not obstructed, and if >22 cm H2O the kidney is obstructed. Results between 15 and 22 cm H2O are equivocal.
Q. The mother has been told that her baby needs a 99mTc MAG3 scan. What does MAG3 stand for?
A. Mercapto-acetyltriglycine. It is labelled with metastable technetium-99 (99mTc).
Q. How is 99mTc MAG3 handled by the kidney?
A. It undergoes both tubular and glomerular excretion. Dimercapto-succinic acid (DMSA) binds only to the proximal renal tubules. Diethylene-triamine-penta-acetic acid (DTPA) is only filtered by the glomeruli.
Q. She wants to know how it is performed.
A. The study is performed in the nuclear medicine department.
Before the study begins, the mother should ensure that her child has had plenty to drink and is well hydrated. Intravenous access is obtained.
The tracer is injected. Images are acquired, using a gamma camera that records activity over the next 20 minutes or so. Final images are taken after the child has voided, and/or had a change in position. Furosemide may be administered according to a locally agreed protocol. In children, where venous access is often not always easy, the diuretic is often administered at the same time as the tracer.
Q. The mother is very anxious about the dose of radiation. What advice will you give?
A. The radiation dose from a 99mTc MAG3 test is less than 0.4 mSv (milli Sieverts). For a DMSA scan it is approximately 1 mSV. The higher dose is easier to understand when one considers that 99mTc MAG3 is excreted by the kidney, whereas 99mTc DMSA binds to the renal tubule.
In both cases the radiation dose is less than for a plain abdominal x-ray.
Q. Please interpret the MAG-3 renogram shown in Figure 6.4.
A. A series of images are shown taken at the intervals marked. ‘ROI’ indicates the regions of interest that have been drawn around the kidneys; the counts taken from these regions of interest proved the graph shown in the upper right. The differential function is taken by comparing the count from the two kidneys during the early part of the study before the trace has entered the collecting system. ‘Eyeballing’ the 1-2 minute images and the 2-4 minute images it can be seen that the left kidney carries less function than the right. This is borne out by the measured differential of 39% annotated on the bottom right graph. Tracer moves much more slowly through the left system than the right, with significant tracer remaining in the left kidney at the end of the study. This is reflected in the curves of the top right graph. It would have been advantageous to see how much more drains from the left kidney with voiding. There seems to be some bladder emptying between the 15-20 minute and the 25-30 minute images, but the bladder has not completely emptied. It is not clear from the images whether diuretic has been administered.
Figure 6.4 A 99mTc MAG-3 renogram.
Q. What are your indications for pyeloplasty in PUJ obstruction?
A.
Reduced function of the affected kidney. A differential function of less than 40% is significant.
Symptoms such as pain or urinary tract infection. Older children are able to articulate that they have pain. This is more difficult to establish in infants, and when asked, it is unusual for the mothers of babies with PUJ obstruction to report that they think their child is having pain.
Deteriorating renal function (a decrease of differential function of more than 10% is significant) or increasing hydronephrosis on follow-up USS.
Concern that the function of the kidney will decline if left untreated. Based on the ‘Natural History’ studies by Dhillon [4], gross hydronephrosis, with an AP renal pelvis diameter of more than 50 mm, should be surgically treated. Between 20 and 50 mm things are not so clear cut with an increasing proportion requiring surgery with increased AP renal pelvic diameter as shown in Table 6.1.
Table 6.1 Percent of children requiring surgery for increasing hydronephrosis in PUJ obstruction
|
AP renal pelvic diameter in transverse plane in PUJ obstruction (mm)a |
Percent requiring surgery |
|
>50 |
100 |
|
>40 |
80 |
|
>30 |
55 |
|
>20 |
20 |
|
<20 |
1-3 |
a Those infants with a persistent AP renal pelvic diameter of <10 mm are discharged.