Kidney and Bladder Ultrasound – Optimal Approach for Evaluation of Kidney Stones
Ultrasound or CT in Diagnosis of Kidney Stones
Two tests are available for diagnosing kidney stones, CT scan or renal ultrasound. With wide availability of CT scans the pendulum has swung towards using CT scans for diagnosis of kidney stones.
CT scan is easy to perform. While a single CT scan in adults is fairly safe and is associated with fairly small amount of radiation, many patients with kidney stones will have multiple recurrent episodes of kidney stones requiring multiple CT scans during their life. I have seen patients who had 5-10 CAT scans in a period of 1-2 years for evaluation of kidney stones. These patients have been exposed to significant amount of radiation in a very short period of time which is very concerning.
In addition they are likely to have a need for CT scan for evaluation of other symptoms at some point in their life. There is significant evidence that accumulated radiation from exposure to multiple CT scans can have detrimental effect on life and may be associated with a small risk of developing certain cancers. Radiation induced cancers typically do not develop immediately but take many years to occur.
While sometimes CT scan provides valuable information that cannot be obtained with other methods, in my practice over the past 5 years the use of CT scans for evaluation and management of patients with kidney stones has decreased while the use of ultrasound has increased.
Ultrasound is Effective in Diagnosing Patients with Kidney Stones
For patients with large kidney stones renal ultrasound is essentially equivalent to CT scan in diagnosis of kidney stones. CT scan is certainly useful for surgical planning in patients with large stones to define anatomy and decide which approach would be ideal especially when the question is whether the patient needs PCNL.
Renal Ultrasound in the Emergency Room
In the emergency room situation, in a patient with renal colic, the finding of kidney stone on ultrasound as well as hydronephrosis is diagnostic. A new randomized control study published in New England Journal of Medicine compared CT scan to ultrasound in evaluation of patients with symptoms of renal colic. This study found no significant difference in terms of diagnostic efficacy and more importantly ultrasound did not miss significantly more dangerous diagnosis then did CT scan. These patients had a very low risk of having another diagnosis that was dangerous.
Renal Ultrasound and Ureteral Stones
A counter-argument for using ultrasound in diagnosing of kidney stones is that ureteral stones are usually difficult to visualize with ultrasound and unless they are located at UPJ or in the distal ureter will not be seen with ultrasound. This is correct but practically this does not make a difference. There is 3 possibilities:
1. Patient has hydronephrosis and the stone is not visible on ultrasound. In this situation the most likely explanation is a ureteral stone.
There are 3 possibilities:
(A) that the stone is in the ureter or that it has passed. If the stone has passed the patient’s pain will resolve spontaneously over the next 24-48 hours. A repeat ultrasound in a few weeks documenting resolution of hydronephrosis is all that is needed.
(B) If the stone was in the ureter, a KUB can be at obtained with minimal radiation exposure. A large stone greater than 7-9 mm will most likely be visible on KUB. Smaller stones that are also more likely to pass will not be visible on KUB. If the patient is comfortable, has no fever and has normal renal function, an optimal approach is for patient to try to pass the stone. The exact location of the stone does not matter all that much as long as the patient is willing to try to pass the stone.
(C) If a patient with hydronephrosis has fever, severe pain that cannot be controlled with medications, or abnormal renal function, the size or location of the stone do not matter as the patient needs to be taken to the operating room for surgical intervention. In this situation, CT scan will not make any immediate difference.
Other diagnoses that may cause ureteral obstruction such as ureteral tumor or ureteral stricture cannot be diagnosed by ultrasound. However more often than not a CT scan performed for evaluation of kidney stones without intravenous contrast will miss these diagnoses as well.
2. Patient has no hydronephrosis and the stone is not visible on ultrasound. This is a challenging situation. There are 2 possibilities. First the patient has a renal colic due to a ureteral stone that has not produced hydronephrosis yet. The other possibility is that the pain is not caused by a stone but has a different etiology. The 2 most common dangerous diagnosis that could be mistaken for renal colic are appendicitis and cholecystitis. Both of these can be evaluated and diagnosed with ultrasound with fairly good confidence. The most common diagnosis other than kidney stone is musculoskeletal pain or sciatica. As long as the patient is stable and comfortable they can be managed expectantly without exposing them to unnecessary radiation. Should they fail expectant management, CT scan can then be performed. This approach is likely to decrease radiation exposure from CT scans by 70-80%.
3. Patient has a stone visible on renal or bladder ultrasound: in this situation, ultrasound is diagnostic. CT scan will not provide any additional information unless there is a suspicion of a different diagnosis in which case a different CT scan protocol is most likely needed (with IV and PO contrast).