Case Author(s): Lisa Oakley, M.D. and Jerold Wallis, M.D. , 2/20/98 . Rating: #D3, #Q3

Diagnosis: Dehydration

Brief history:

Elderly paralyzed man with intermittent urinary tract infections.

Images:

Dynamic posterior renal images (one minute per image for 20 minutes)

View main image(rs) in a separate image viewer

View second image(rs). Posterior renal flow (4 seconds per image)

View third image(rs). Renogram curve and split function

View fourth image(rs). Static posterior image at 25 minutes

Full history/Diagnosis is available below

Diagnosis: Dehydration

Full history:

72 year old man, quadraplegic, with a history of intermittent urinary tract infections, but with history of normal uptake and excretion on prior renal scintigraphy. This study was requested for routine evaluation of renal function.

Radiopharmaceutical:

Tc99m-MAG3

Findings:

Posterior radionuclide angiogram demonstrates prompt, symmetric perfusion to both kidneys. Sequential images show the kidneys to be of normal size and morphology. There is progressive uptake of tracer by both kidneys with increasing accumulation of tracer in the renal parenchyma over the ensuing twenty minutes of image acquisition. There is some degree of excretion into the renal calyces, but overall, the excretion is markedly diminished. These findings are confirmed on the renogram curve.

The split renal function is within normal limits. The right kidney contributes 54% and the left kidney 46% of total renal function.

Discussion:

Progressively rising renal function curves can be seen in both obstruction and decreased renal function. Given the normal appearance of the renal collecting systems and the persistent parenchymal activity, the problem in this patient is that of decreased excretion.

Although the scintigraphic findings are consistent with symmetric renal injury (e.g. acute tubular necrosis), the clinical history did not suggest any reason for substantial decline in renal function. Since the prior exam, the patient had no episodes of hypotension, nephrotoxic agents, etc.

Upon further questioning, the patient had been NPO for over 12 hours and no pre-test hydration had been given.

It is important to hydrate all patients undergoing renal scintigraphy. Often times, the patient is dehydrated because they assume or have been (inappropriately) told that they should be NPO past midnight. At our institution, the procedure manual states that the patient should receive 16 ounces (or 500 ml) of oral liquids (ideally water) before beginning the study. However, this requires extra time and vigilence on the part of the technologist, which is not always a simple matter, especially in a busy Nuclear Medicine Department.

As seen in this case, a dehydrated patient with normal renal function can have a very abnormal appearing renal scan. The normal response of the kidney in the pre-renal state is to conserve fluids and thus decrease renal excretion.

Followup:

The patient was hydrated with 250 ml oral and 400 ml intravenous fluids. A repeat posterior image of the kidneys was obtained about 20 minutes later and compared with the pre-hydration image.

Compared with the pre-hydration image, the post-hydration image clearly shows significant wash-out of tracer. Therefore, this study was interpreted as normal in the setting of patient dehydration.

View followup image(rs). Posterior static images pre- and post-hydration, scaled to show true change in renal activity.

Major teaching point(s):

1) Hydration is important in renal examinations.

2) Care must be taken in comparison of static images to assess interval change. It is best if the images are acquired for the same amount of time and into the same size digital matrix. Upon display, the images must be scaled to a common maximum. If the two images were filmed such that each image was scaled to its own maximal pixel count (commonly done by some nuclear medicine systems), the kidneys would appear similar in intensity, erroneously suggesting that no washout had occurred.

In this case, both the pre- and post-hydration images were acquired for two minutes. The maximum pixel count is 414 on the pre-hydration image and 177 on the post-hydration image. When displaying the two images, both were scaled the same maximum pixel count (414 in this case) in order to visually appreciate the absolute decrease in renal counts on the post-hydration image.

Differential Diagnosis List

Causes of poor excretion of tracer include:

1) Medical renal disease (Need to correlate with the patient's creatinine. Also, one would not expect such improved tracer excretion with hydration.)

2) Acute tubular necrosis

3) Obstruction (One would expect to see associated hydronephrosis, and likely have better parenchymal clearance of tracer unless obstruction was quite high grade.)

4) Dehydration

ACR Codes and Keywords:

References and General Discussion of Renal Scintigraphy (Anatomic field:Genitourinary System, Category:Normal, Technique, Congenital Anomaly)

Search for similar cases.

Edit this case

Add comments about this case

Return to the Teaching File home page.

Case number: rs015

Copyright by Wash U MO