Ultrasound, a Non-ionizing Evaluation of Stones
In the age of ALARA, US is an appealing nonionizing imaging modality that is reasonable for initial diagnostic evaluation of urolithiasis. US detection of hydroureteronephrosis and stones employs grayscale brightness (B)-mode by directing sound waves through tissue, detecting the time between reflections of waves, and calculating this difference to generate a two-dimensional image. Stones appear as echogenic foci with a posterior acoustic shadow.
Point of Care Ultrasound and Hydronephrosis
US is noninvasive, portable and widely accessible. It is ideal in the initial evaluation of suspected renal colic in the ED, particularly for patients with atypical presentation. In 2019 a multispecialty collaboration of the American College of Emergency Physicians (ACEP), ACR, and the American Urological Association (AUA) published a review of optimal imaging in the diagnosis of suspected renal colic. The consensus recommendations included the following: in young patients (<35yo) with uncomplicated presentation typical for kidney stone, avoid CT; in middle-aged patients (~55yo), avoid CT if there is a prior history of kidney stone; in older patients (>75yo), obtain CT. Point of care US (POCUS) should be utilized particularly in patients with less typical presentation; radiologic performed US should be the initial imaging modality in pregnant and pediatric patients.
POCUS is used to identify and quantify hydronephrosis, which may suggest the presence of ureterolithiasis.[34–37] A 2014 multicenter, pragmatic, comparative effectiveness trial impacting the ACEP, ACR, AUA recommendations (above)[24,38] randomized 2759 patients to POCUS, radiology performed US, or abdominal CT. For US, stone presence was confirmed by patient report of observed stone passage or surgery. There were no significant differences between imaging modalities for diagnostic accuracy of nephrolithiasis with sensitivity of 84–86% and specificity 50–53%. There were no differences in length of stay, mean total costs, high-risk complication, return to ED, hospital admission, or self-reported pain. In this as well as other studies, patients who underwent US at index presentation had lower 90–180 days radiation and subsequent receipt of a CT (20–27% for radiologist US, 41% for POCUS).[38–40]
A systematic review and meta-analysis published in 2018 evaluated five studies with 1,773 subjects. The detection of hydronephrosis on POCUS compared to CT scan was associated with a pooled sensitivity of 70.2%, specificity 75.4%, positive likelihood ratio of 2.85, and negative likelihood ratio of 0.39. Two recent studies have compared performance between ED physicians and radiologists in detection of hydronephrosis using POCUS and reported inter-rater reliability between 77% and 87.5%.[34,37] Efficacy is improved for POCUS with larger stones and by qualifying hydronephrosis on a binary outcome (e.g 'zero to mild' vs 'moderate to severe,' or 'present' vs 'absent').[34–36] In more recent studies, POCUS has been associated with shorter ED stay[41,42] and lower medical costs.[39,42] It should be noted that a more recent study did report lower sensitivity (81%) and specificity (59%) of POCUS than those reported in the above meta-analysis.
Finally, abdominal X-ray may be used as an adjunct to US for stone identification. Abdominal X-ray alone has poor sensitivity and may overestimate stone size.[44,45] However, one study reported that addition of X-ray to POCUS identification of hydronephrosis increased sensitivity (73.5% to 88%), though did not largely improve specificity (92.7–93%). This was reiterated in a study that demonstrated an improvement in sensitivity from 60% to 92%, explicitly for stones that would require intervention. Clinicians should keep in mind that, whereas an abdominal radiograph should be lower radiation exposure than CT imaging, a 2018 study reported a higher than published radiation exposure of 2.15mSv; only a quarter of abdominal X-rays had an effective dose < 1mSv.
Curr Opin Urol. 2022;32(4):397-404. © 2022 Wolters Kluwer Health, Inc.