(BQ) Part 2 book Pocket protocols for ultrasound presents the following contents: Kidneys, bladder, abdominal aorta, peritoneal free fluid, testicular ultrasound, lower extremity deep venous thrombosis, central venous access, peripheral venous access, arterial access,...
C H A P T E R 20 Kidneys Behzad Hassani K E Y POIN TS • The primary indication to perform a point-of-care ultrasound exam of the kidney is to evaluate for hydronephrosis • Any suspicious mass detected by point-of-care ultrasound warrants further investigation and expert consultation • A bladder ultrasound exam should accompany every renal ultrasound exam A distended bladder due to bladder outlet obstruction is a common cause of hydronephrosis Background Renal ultrasound provides a fast, radiationfree, and cost-effective alternative to computed tomography (CT) for the initial workup of low-risk patients who present with acute symptoms ranging from undifferentiated abdominal pain to painless hematuria.1 Ultrasound is the imaging modality of choice for investigating obstructive uropathy in pregnant and pediatric populations, given the adverse effects of ionizing radiation Focused renal ultrasound can accurately detect and grade hydronephrosis in the context of obstructive uropathy,2 directly visualize large obstructing calculi,3 and characterize renal cysts or solid masses.4 Renal ultrasound can be incorporated into the assessment of any patient with undifferentiated flank or abdominal pain When clinical suspicion for renal calculus is high, the presence of hydronephrosis on the side of pain can be considered as de facto evidence of obstructive uropathy Sensitivity of ultrasound for detecting a calcified stone can be improved with the addition of a single plain film of the abdomen (kidneys, ureter, and bladder, or KUB).5 Severity of hydronephrosis observed may correlate with the duration of obstruction6 and possibly with the size of the obstructing calculus.7 When stone disease is clinically less likely or an alternate pathology such as abdominal aortic aneurysm, gallbladder disease, ovarian torsion, or ectopic pregnancy is being considered, the absence of hydronephrosis effectively guides investigations elsewhere Normal Anatomy Kidneys are retroperitoneal organs that lie in an oblique longitudinal plane with the inferior pole of each kidney more anterior and lateral compared to its superior pole (Figure 20.1) Therefore, the transducer must be positioned obliquely to image the kidney along its long axis The left kidney is located more superiorly and posteriorly compared to the right kidney The left kidney is usually visualized through the acoustic window provided by the spleen due to interference by bowel and stomach gas anteriorly, and the right kidney is often visualized through the acoustic window provided by the liver The right kidney is slightly larger than the left kidney, but both kidneys are normally within 2 cm of each other in any dimension Normal kidneys are 9–12 cm long, 4–6 cm wide, and 2.5–3.5 cm thick Kidneys are divided into two distinct anatomic parts: renal parenchyma and renal sinus (Figure 20.2) Renal parenchyma is further subdivided into renal cortex and medulla The medulla consists of cone-shaped medullary pyramids Renal parenchyma surrounds 153 154 4—ABDOMEN AND PELVIS Liver Spleen Right kidney Left kidney Renal pelvis Aorta Inferior vena cava Ureter Bladder Urethra Figure 20.1 Anatomy of the urinary system the sinus on all sides except at the hilum The hilum is where the renal artery, renal vein, and proximal ureter enter the renal sinus Prominent fatty deposits within the renal sinus give it a hyperechoic appearance and distinguish it from the hypoechoic, grainy renal parenchyma This difference in echogenicity is known as the sonographic double density of the kidney.4,8 Image Acquisition A low-frequency transducer with deep penetration, either phased-array or curvilinear type, is used to image the kidneys The narrower beam width of a phased-array transducer is ideal for imaging between the ribs, but the wider beam width of a curvilinear transducer allows visualization of the entire kidney longitudinally in a single view When scanning a patient with a suspected renal pathology, scan the unaffected side first to obtain a baseline image to compare with the affected side To image the right kidney, place the patient in a supine position with the transducer in a coronal plane in the midaxillary to anterior axillary line at the level of the xiphoid process Center the kidney on the screen and rotate the transducer 15-30 degrees counterclockwise to aim the transducer marker slightly posteriorly to capture a true long-axis view of the right the kidney While holding the transducer in the same location on the skin surface, tilt or fan the transducer anteriorly and posteriorly to assess the entire kidney from its most anterior to posterior surface Rotate the transducer 90 degrees counterclockwise from the long-axis view to obtain a transverse cross section of the kidney Tilt or fan the transducer superiorly and inferiorly to assess the superior and inferior poles of the kidney Figure 20.3 illustrates transducer positions to obtain longand short-axis views of the kidney The left kidney is more posterior and superior than the right kidney Placing the patient in a right lateral decubitus position facilitates visualization of the left kidney and reduces interference from the ribs and bowel gas Requesting the patient to hold his breath in maximal inspiration will shift the kidney caudally to further reduce interference from rib shadows Identify the left kidney with the transducer in a coronal plane on the posterior axillary line and then rotate the transducer 15-30 degrees clockwise aiming the transducer marker posteriorly to acquire a true long-axis view (Figure 20.4) Transverse or short-axis views are obtained by rotating the transducer 90 degrees counterclockwise from the long axis (Figure 20.5).4,8 Sagittal and transverse views of the bladder should be obtained for a complete evaluation of the urinary system (see Chapter 21, Bladder) Image Interpretation Perinephric fat and Gerota’s fascia appear as a hyperechoic stripe around the kidneys, and the fibrous capsule gives each kidney a hyperechoic outline Normally, the renal cortex has a homogeneous appearance on ultrasound 155 20—KIDNEYS Superior pole Medial border Renal artery Renal vein Lateral border Renal pelvis Ureter Inferior pole Anterior surface of right kidney Pyramid in renal medulla Renal papilla Major calyx Fibrous capsule Renal cortex Renal sinus Minor calyx Hilum of kidney Renal column Renal pelvis Ureter Internal structure of right kidney Figure 20.2 Cross-sectional anatomy of the kidney that is less echogenic than the adjacent liver or spleen Fluid-filled medullary pyramids are seen as hypoechoic or anechoic triangular prominences in a semicircular arrangement around the sinus As a result, the renal medulla is significantly less echogenic compared to the surrounding cortex The renal sinus normally appears hyperechoic due to fat content, and in the absence of urinary tract obstruction, the sinus appears homogenously hyperechoic with small anechoic pockets of urine The ureter is usually obscured by bowel gas but may be visible with ultrasound when dilated When distended, the ureter appears as a tubular structure extending inferiorly from the renal pelvis.4,8 Pathologic Findings OBSTRUCTIVE UROPATHY AND HYDRONEPHROSIS Progressively enlarging anechoic areas within the hyperechoic sinus develop with distal urinary tract obstruction and indicates hydronephrosis The severity of hydronephrosis is classified as mild, moderate, or severe (Figure 20.6)9 and may correlate with the size of the 156 4—ABDOMEN AND PELVIS B Short axis A B A Long axis A B Figure 20.3 Transducer position for kidney ultrasound in long-axis (A) and short-axis (B) planes distal renal stone Most algorithms incorporate the degree of hydronephrosis into a clinical decision-making pathway.1,7,9 Mild hydronephrosis is defined as enlargement of the calices with preservation of renal papillae The renal sinus is normally hyperechoic but becomes anechoic due to mild central dilation in mild hydronephrosis (Figure 20.7) As mild hydronephrosis progresses, the degree of central dilation of the renal sinus increases, but the structure of the medullary pyramids is preserved (Figure 20.8) It is preservation of medullary pyramidal architecture, not the degree of renal pelvic dilation, that characterizes mild hydronephrosis and differentiates mild from moderate hydronephrosis (Video 20.1 ) Moderate hydronephrosis is characterized by rounding of the calices, obliteration of renal papillae, and blunting of medullary pyramids Progressive dilation of the calyces leads to glovelike splaying of the renal sinus and ballooning of the medullary pyramids that has been called the classic bear-claw appearance 157 20—KIDNEYS Pocket of urine Cortex Medullary pyramid Sinus Figure 20.4 Normal long-axis view of the kidney Note the prominent medullary pyramids and the hyperechoic renal sinus in the absence of hydronephrosis Cortex Sinus Figure 20.5 Normal short-axis view of the kidney Normal Mild Moderate Severe Figure 20.6 Severity of hydronephrosis is graded by the degree of distortion of normal architecture Mild hydronephrosis: enlarged calices with preservation of renal papillae and pyramids Moderate hydronephrosis: dilated calices with obliterated papillae and blunted pyramids Severe hydronephrosis: calyceal ballooning, complete obliteration of papillae and pyramids, cortical thinning 158 4—ABDOMEN AND PELVIS Mild hydronephrosis Cortex Sinus Cortex Renal sinus replaced by severe hydronephrosis Figure 20.10 Severe hydronephrosis Figure 20.7 Mild hydronephrosis Perinephric fluid Cortex Preserved pyramid Mild hydronephrosis Sinus Figure 20.11 Longitudinal view of the right kidney Note the small amount of perinephric fluid, indicative of calyceal rupture and urinary extravasation Figure 20.8 Mild hydronephrosis Cortex Sinus Moderate hydronephrosis Figure 20.9 Moderate hydronephrosis of moderate hydronephrosis (Figure 20.9 and Video 20.2) Preservation of the outer cortex is the distinguishing feature between moderate and severe hydronephrosis Severe hydronephrosis is defined as calyceal ballooning with variable degrees of cortical thinning (Figure 20.10) Fingerlike projections that characterize moderate hydronephrosis coalesce into one large anechoic collection of urine that completely obliterates the sinus and medullary pyramids All that remains of the normal renal architecture is a rim of outer cortex Total distortion of normal renal architecture is the key distinguishing feature of severe hydronephrosis (Video 20.3) A small amount of perinephric fluid due to calyceal rupture and urinary extravasation may be seen with hydronephrosis (Figure 20.11) Perinephric fluid is associated with significant 159 20—KIDNEYS Parapelvic cysts Cortical cyst Cortex Sinus Figure 20.12 Large cortical cyst Figure 20.13 Parapelvic cysts risk of infection or perinephric abscess formation and requires close follow-up.1 Several conditions can mimic the ultrasound appearance of hydronephrosis Two general guiding principles should be followed to distinguish between true hydronephrosis and its mimics: (1) trace the anechoic areas of suspected hydronephrosis to the renal pelvis where the areas should coalesce; and (2) scan the kidney in both the longitudinal and transverse planes to thoroughly delineate the architecture of the area of suspected hydronephrosis Medullary pyramids may appear anechoic and can be mistaken for collections of urine; however, the triangular pyramids are separated from each other by cortical tissue and should be seen as distinct entities from the hyperechoic renal sinus Cortical and parapelvic cysts can also mimic hydronephrosis, but they are distinguished by their smoothwalled, spherical shape that is not contiguous with the renal pelvis (Figures 20-12 and 20-13) Renal hilar vessels are also anechoic and can be mistaken for hydronephrosis Color-flow Doppler can differentiate vasculature from hydronephrosis.8 Renal Calculus Ultrasound has low sensitivity for detecting ureteral calculi.3 Stones may be seen within the renal parenchyma, at the ureteropelvic junction proximally or at the ureterovesicular junction distally In general, ureters are obscured by bowel gas and are difficult to assess as they travel from kidney to bladder Stones are hyperechoic and exhibit acoustic shadowing (Figure 20.14 and Video 20.4) Renal calculus Acoustic shadowing Figure 20.14 Large renal calculus in the right kidney seen in along-axis view Note the associated prominent acoustic shadowing Severity of hydronephrosis may correlate with stone size.7 Patients with renal colic and moderate or severe hydronephrosis are significantly more likely to have stones >5 mm than patients with mild or no hydronephrosis.7 Stones 10 mm are unlikely to pass and will likely require urologic intervention.10 Renal Cyst Renal cysts are common and usually benign, but renal malignancies may present as cystic structures on ultrasound A benign cyst must fulfill all of the following criteria8: Thin-walled and smooth, with no septations, internal echoes, or solid elements 160 4—ABDOMEN AND PELVIS Septations Heterogeneous mass Debris Renal sinus Figure 20.15 Large complex cyst with internal septations seen in a transverse view of the right kidney Figure 20.17 Irregularly shaped renal mass with heterogeneous echogenicity Hypertrophied column of Bertin Multiple cysts Figure 20.16 Polycystic kidney disease 2 Round or oval shape that is well demarcated from the adjacent parenchyma and appears homogeneous in all imaging planes Posterior acoustic enhancement must be evident behind the cyst If the above criteria are not fulfilled, the presence of a complex cyst (Figure 20.15), renal abscess, or malignancy must be considered and warrants further workup Multiple renal cysts are seen in polycystic kidney disease (PCKD) and acquired renal cystic disease (ARCD) PCKD represents an extreme example on the spectrum of renal cystic disease (Figure 20.16 and Video 20.5) PCKD is characterized by an abundance of irregular cysts of varying size that distort the normal renal architecture bilaterally These patients often present to acute care settings with flank pain, hematuria, hypertension, and renal failure ARCD is another condition associated with multiple renal cysts that is present in patients with end-stage renal disease on hemodialysis and is associated with higher risk of renal malignancy While most patients with chronic kidney disease have Figure 20.18 Hypertrophied column of Bertin bilaterally shrunken and hyperechoic kidneys (Video 20.6), those with ARCD have numerous cysts Renal Mass Renal malignancies detected incidentally during abdominal imaging are associated with lower morbidity and mortality rates.11 Any suspicious mass detected by point-of-care ultrasound warrants further investigation and expert consultation (Figure 20.17).11 Normal variants that can mimic renal malignancies include prominent columns of Bertin, which are hypertrophied cortical tissue that distort the calyces in the renal sinus (Figure 20.18) Renal cell carcinoma is the most common type of renal malignancy in adults These tumors are 20—KIDNEYS notoriously heterogeneous in their sonographic appearance They can be isoechoic, hypoechoic, or hyperechoic relative to adjacent parenchyma, and they may have a partial cystic appearance that can be mistaken for benign cysts.4 Angiomyolipoma is the most common type of benign tumor of the kidney The tumors are well-demarcated hyperechoic masses located within the renal cortex There is significant overlap in the appearance of angiomyolipomas and echogenic renal cell carcinoma.4 Therefore, providers using point-ofcare ultrasound should obtain additional radiographic imaging and seek expert consultation when incidental masses are detected PEARLS AND PITFALLS • Hydronephrosis may be underrecognized in hypovolemic patients due to transient collapse of calyces Sensitivity of ultrasound in detecting hydronephrosis is increased after fluid resuscitation in patients with volume depletion • A bladder ultrasound exam should accompany a renal ultrasound exam A distended bladder due to bladder outlet obstruction may cause bilateral hydronephrosis, and 161 a repeat renal ultrasound exam should be performed after bladder decompression • Varying degrees of hydronephrosis, most often on the right side, is common in pregnancy and may not be pathologic • Renal calculi are common and detection of a nonobstructing parenchymal stone may be unrelated to the patient’s clinical presentation • Unilateral hydronephrosis may be due to external ureteral compression by a mass lesion or retroperitoneal lymphadenopathy, and these pathologies should be considered • Absence of hydronephrosis does not rule out ureterolithiasis because small calculi may not create significant obstruction • High-risk patients (age >50) who present with flank pain and hydronephrosis on bedside ultrasound exam may have a ruptured abdominal aortic aneurysm, and any abnormal ultrasound findings in these patients warrants additional workup • All renal masses are malignant until proven otherwise Detection of a renal mass by point-of-care ultrasound requires further workup with additional radiographic imaging and expert consultation Case Studies Case Case Presentation A 20-year-old man presents to the emergency department after hours of severe left lower quadrant abdominal pain The pain began on his left flank, migrated to the left lower quadrant, and is associated with nausea He denies prior history of similar pain His vital signs are normal but he appears uncomfortable Physical exam reveals moderate tenderness to deep palpation of the left lower quadrant without peritoneal signs, and mild left costovertebral angle tenderness White blood cell count is 23,000 with a left shift Your primary differential diagnosis includes pyelonephritis, diverticulitis, and kidney stone Ultrasound Findings A focused bedside ultrasound exam of the abdomen is performed No free fluid is seen, but mild hydronephrosis of the left kidney is detected (Video 20.7), suggesting renal colic as a possible etiology A comprehensive abdominal ultrasound confirms the presence of left-sided hydronephrosis (Figure 20.19) and reveals a mm partially obstructing calculus in the distal left ureter (Figure 20.20) Bilateral ureteral jets are seen on the posterior bladder wall using color flow Doppler ultrasound (Figure 20.21) Case Resolution The patient is discharged with oral analgesics and a trial of medical expulsion therapy He is given outpatient urology follow-up He spontaneously passes the stone after days and recovers without complications Providers can accurately detect and grade hydronephrosis using bedside ultrasound When clinical suspicion of renal calculus is high, unilateral hydronephrosis serves as indirect evidence of obstructive uropathy due to a stone Moderate to severe hydronephrosis correlates with a stone size >5 mm, even though the actual stone may not be visualized Stones