Can The Degree of Hydronephrosis on Ultrasound Predict Kidney Stone Size?

Background

Symptomatic renal colic is a common complaint presenting to the emergency department (ED), with a rate of 126 to 226 per 100,000 ED visits [1]. In the ED, CT is frequently used to make the definitive diagnosis as it allows for determination of stone size and location, degree of hydronephrosis, and evaluation of other pathology that may mimic renal colic. However this is a particularly worrisome approach in patients with recurrent ureteral stones who have been exposed to numerous previous CT imaging studies. Previous data has shown that emergency physician performed ultrasound is accurate at identifying hydronephrosis, which in combination with hematuria, is sufficient for the diagnosis of renal colic [2,3]. Furthermore,  an ultrasound first approach has been shown to be safe and reasonable as an initial evaluation for suspected renal colic [4]. What ultrasound does not tell us about renal colic is the size of the ureteral stone, which can be useful in determining the need for immediate intervention versus medical management. The following study seeks to determine if the degree of hydronephrosis seen on ultrasound performed by emergency physicians, can be predictive of ureteral stone size. 

Can the degree of hydronephrosis on ultrasound predict kidney stone size?

Clinical Question

Can the degree of hydronephrosis on ultrasound predict kidney stone size?

Methods & Study Design

  • Design
    • Retrospective chart review of emergency department (ED) patients at a single academic medical center
  • Population + Inclusion Criteria 
    • Adult patient presenting to the emergency department who had confirmed ureterolithiasis on noncontrast CT and a focused emergency renal ultrasound performed
  • Exclusion criteria
    • No specific criteria
  • Intervention
    • A focused renal ultrasound was performed in the ED by an emergency medicine resident or attending to evaluate for the presence of hydronephrosis as an indicator of obstructive ureterolithiasis
    • All ultrasound examinations were subsequently reviewed for quality assurance by an emergency ultrasound fellowship trained emergency physician
  • Outcomes
    • Each focused renal ultrasound classified the degree of hydronephrosis as none, mild, moderate, or severe and this was compared to the ureteral stone size on noncontrast CT
      • Definitions:
        • Mild hydronephrosis was defined as enlargement of the calices withpreservation of the renal papillae
        • Moderate hydronephrosis was defined as rounding of the calices with obliteration of therenal papillae
        • Severe hydronephrosis was defined as caliceal ballooning with cortical thinning
    • Ureteral stone size was stratified into 2 groups, those 5mm or smaller and those larger than 5 mm, based on the likelihood of successfully spontaneous stone passage

Results

Increasing degree of hydronephrosis seen on focused ultrasound was associated with an increasing proportion of ureteral calculi larger than 5 mm. 113 (87.6%) patients with less severe hydronephrosis  (none or mild) had ureteral calculi 5 mm or smaller. Of the remaining 16 (12.4%) patients with less severe hydronephrosis, none of these patients had ureteral stones larger than 10 mm. There was good interobserver agreement between the degree of hydronephrosis as determined by the performing emergency physician and the quality assurance review (k = 0.847).

Strengths & Limitations

  • Strengths
    • Majority of ultrasound examinations performed by ED physicians making this applicable to point-of-care ultrasound
    • Gold standard was size of ureteral stone on noncontrast CT
    • Good interobserver agreement between ED ultrasound operator and quality assurance review
  • Limitations
    • Retrospective chart review
    • This study only enrolled patients who both a focused renal ultrasound and confirmed ureterolithiasis on noncontrast CT; this would have missed patients who only had either a focused renal ultrasound or noncontrast CT alone (selection bias)
    • No patient centered outcomes data

Authors Conclusion

"In conclusion, our results demonstrate a relationship between the degree of hydronephrosis as determined by emergency physicians on focused emergency ultrasound and ureteral calculi size; patients with less severe hydronephrosis were less likely to have larger ureteral calculi. This suggests that ultrasound can help identify many, but not all, patients who are at lower risk for having larger ureteral calculi.

Our Conclusion

This paper identifies a correlation between the degree of hydronephrosis on ultrasound and ureteral stone size seen on noncontrast CT. Essentially, patients with minimal or no hydronephrosis are very unlikely to have have a large (>5 mm) ureteral stone. Unfortunately, focused ultrasound is not perfect, and in this study  ~12.4% of patients with minimal or no hydronephrosis still had a large ureteral stone. What I found reassuring was that in this group, none of the patients had a ureteral stone > 10 mm, which at most institutions is the cut off for allowing a trial of passage. Even dissecting the data further, of the patients with moderate hydronephrosis, only 2 out of 43 (4.6%) patients had a stone > 10 mm.

This study suggests that focused renal ultrasound can be used to screen patients with suspected renal colic and potentially avoid an unnecessary CT scan. As with any focused ultrasound, the decision to obtain a CT should not be based solely the degree of hydronephrosis but also in conjunction with the clinical history, physical exam and other pertinent factors (previous ureteral stone, previous need for stone intervention, other concerning diagnoses on differential, pain control, institutional culture, urinalysis, etc). 

The Bottom Line

Ultrasound can be used to identify many, but not all, patients who are at lower risk for having larger ureteral calculi. 

Authors

This post was written by Michael Macias, MD, Ultrasound Fellow at UCSD.

References

    1. Teichman JMH. Clinical practice. Acute renal colic from ureteral calculus. N Engl J Med 2004;350:684-93.

    2. Rosen CL, Brown DFM, Sagarin MJ, et al. Ultrasonography by emergency physicians in patients with suspected ureteral colic. J Emerg Med 1998;16:865-70.

    3. Gaspari RJ, Horst K. Emergency ultrasound and urinalysis in the evaluation of flank pain. Acad Emerg Med 2005;12:1180-4.

    4. Smith-Bindman R, e. (2018). Ultrasonography versus computed tomography for suspected nephrolithiasis. - PubMed - NCBI . Ncbi.nlm.nih.gov. Retrieved 3 March 2018, from https://www.ncbi.nlm.nih.gov/pubmed/25229916
    5. S, G. (2018). Can the degree of hydronephrosis on ultrasound predict kidney stone size? - PubMed - NCBI . Ncbi.nlm.nih.gov. Retrieved 3 March 2018, from https://www.ncbi.nlm.nih.gov/pubmed/20837260

Case # 9: A Transplant Dilemma

A 52 year old male with a h/o kidney transplant presents to the emergency department with pain over his transplanted kidney site (right pelvic region). He also notes increased weakness, nausea and a significant decrease in urine output. He denies any fever. He states he is compliant with his anti-rejection medications.

Vitals: T 99.0 HR 105 BP 165/91  RR 18 O2 98% on RA

A bedside ultrasound is performed, what is the next best step in management?

tx_severe hydro

Answer and Learning Points

Answer

Insertion of foley catheter. The clip above demonstrates severe hydronephrosis of the patient's transplanted kidney. A foley was inserted in the emergency department with immediate output of 1.5 L of clear urine. The patient was found to be in renal failure secondary to his urinary outlet obstruction. He was admitted to transplant surgery and his renal function improved over the next day; he was discharged home with a leg bag and urology follow up. Below is a repeat ultrasound of his transplanted kidney after drainage of his bladder: 

Learning Points

    • Urinary obstruction in a transplanted kidney can be missed initially as pain over the patient's graft site and decreased urine output is easily contributed to possible rejection or infection.
    • The differential diagnosis of acute renal failure in the transplanted kidney is broad (see table below) and emergency department management should include a thorough evaluation for prerenal, intrinsic and post renal causes, in consultation with a transplant service.
    • All renal transplant patients presenting with acute renal failure should have a formal renal ultrasound with doppler to evaluate the graft however often this is not available immediately and a bedside ultrasound can assist with rapid identification of acute urinary obstruction.

Author

This post was written by Michael Macias, MD, Ultrasound Fellow at UCSD.

References

    1. Kadambi PV., Brennan DC., Chon J. (2017). Evaluation and diagnosis of the patient with renal allograft dysfunction. In T.W. Post, B. Murphy, & A. Lam (Eds.), UptoDate. Available from https://www.uptodate.com/contents/evaluation-and-diagnosis-of-the-patient-with-renal-allograft-dysfunction