How does contrast/dye given during a CT scan harm your kidneys? What can you do to prevent and minimize the damage?
Certain types of CT scans will often require that the patient get intravenous (iv) "dye" or contrast to make the organs stand out and delineate them better. This usually would lead to better images with greater sensitivity that help the radiologist in picking up features that would otherwise get missed. The downside however is that this very contrast could sometimes cause damage to the kidneys. The medical term for this is "contrast induced nephropathy" (CIN). What do you then do in a situation where iv contrast is necessarily required? Lets try and understand a few nuances about contrast-mediated kidney damage, and what you can do about it?
IV CONTRAST CAN TEMPORARILY REDUCE BLOOD SUPPLY TO THE KIDNEYS
Radiological contrast can lead to a drop in the kidney's blood supply, and this is one of the ways that contrast hurts the kidneys. This is not fully understood, but could be related to certain vasoactive mediators (chemicals that make the blood vessels contract). These chemicals include something called "adenosine", and another called "endothelin".
CONTRAST COULD BE DIRECTLY TOXIC TO THE KIDNEYS
In addition to reducing the blood supply to the kidneys, contrast could also be directly toxic to the kidneys. This could be related to the tendency of contrast to increase the level of certain destructive substances called "free oxygen radicals". This is the second major way in which contrast hurts the kidneys.
NOT EVERYONE IS AT THE SAME RISK OF CONTRAST NEPHROPATHY
In spite of the above mechanisms in which contrast could possibly hurt the kidneys of a person receiving it, not everyone has the same risk of getting CIN. The rule of thumb is that the worse your baseline kidney function, the more likely are you to develop contrast nephropathy. Other factors that increase your risk are diabetes, heart failure, and certain types of cancers like myeloma. In fact, we now have a numerical risk calculator that incorporates a few of these risk factors and helps you come up with an objective number to predict the chances of your kidneys taking a hit from the contrast. Here is the link for that.
THE AMOUNT AND TYPE OF CONTRAST MATTERS
One major, yet easily modifiable risk factor that can reduce the chances of contrast nephropathy is the amount and type of contrast that you receive. It should be made sure that redundant amount of contrast over and above what is necessary should not be given to the patient. Furthermore, older types of contrast (yes, they come in different brands) which have higher osmolality are more likely to hurt the kidneys and should be avoided.
CONTRAST NEPHROPATHY IS USUALLY MILD AND REVERSIBLE
In an overwhelming number of cases, contrast nephropathy (which will be diagnosed by abnormalities in your kidney function number, creatinine within the first three days of receiving contrast) is mild. Even if it does develop, most patients will see their kidney function bounce back and normalize by about a week. However, if you do have preexisting kidney disease (CKD) or other risk factors mentioned above, this might not necessarily happen. Worsening of kidney disease to the point where you need dialysis is rare.
CONTRAST NEPHROPATHY RISK CAN BE MINIMIZED BY TAKING PREVENTIVE STEPS
If the CT scan requiring the contrast is non-emergent, certain preventive steps can be taken. These can often minimize the risk of the kidneys getting hurt. Usually your kidney doctor would prescribe a regimen of intravenous fluids, and make sure that you are not dehydrated. Medications that could hurt your kidneys, like non-steroidal anti inflammatory drugs (NSAIDS) for instance, should be held. A drug called "acetylcysteine" interferes with production of the above mentioned free-oxygen radicals. However, it is now falling out of favor since most medical studies have failed to prove that it works. A newer antioxidant called trimetazidine has also shown some promise in contrast nephropathy prevention. Ask your nephrologist about what strategy might work best for you.
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