Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

KIDNEYS AND URETERS

In some children with persistent vesicoureteral reflux after ureteral reimplantation, subsequent endoscopic subureteral Teflon injection corrects the reflux in most.

Injection Therapy

Subureteral collagen or polytetrafluoroethylene (Teflon) injection is an alternative therapy to surgical ureteral reimplantation and it corrects reflux in about 80% of patients. In children with neurogenic bladder dysfunction and vesicoureteral reflux, success rates are greater after ureteroneocystostomy than after subureteral Teflon injection, yet the latter procedure is often employed in managing reflux because of relative procedure simplicity and ability, if necessary, to perform secondary reimplantation. The operative cost is considerably less than that for reimplantation, and morbidity is minimal.

Results of endoscopic collagen injection in treating children with reflux into a totally duplicated ureter system were disappointing; eventually many of these children require surgical correction.

Among infants and children with posterior urethral valves and vesicoureteral reflux, reflux resolves in only a minority. Most require correction.

Ultrasonography shows collagen to be hyperechoic compared to the bladder wall shortly after endoscopic subureteral injection, and it gradually becomes isoechoic. On a long-term basis US does not detect some residual periureteral Teflon.

Diverticula

Calyceal

Calyceal diverticula are lined by transitional epithelium and communicate with an adjacent calyx. They can be located anywhere in the collecting system, although upper and lower pole fornices predominate. Their pathogenesis is unknown. Most are incidental findings. An occasional one becomes obstructed or is a site for stone formation and a nidus for infection.

Many of these diverticula have a typical radiographic appearance. They communicate

with an adjacent calyx; thus postcontrast they opacify only after contrast enters the involved calyx, differentiating them from papillary necrosis, which, being in continuity with tubules of Bellini, opacifies before the involved calyx. Some contain debris or stones. Delayed contrast filling is evident with some.

A cyst communicating with a calyx is a nonspecific finding: a pyelogenic cyst or any inflammatory or neoplastic tumor can rupture into a calyx. Radiographically, an abscess that has drained into an adjacent calyx has a similar appearance.

Percutaneous calyceal diverticulectomy (most with stones) consists of stone removal and diverticular neck incision or dilation and diverticular wall fulguration.

Ureteral

The pathogenesis of ureteral diverticula is not clear. Some authors refer to them as pseudodiverticula. They are usually detected during high-quality urography or retrograde pyelography and appear as small ureteral outpouchings. They occur primarily in the upper one third of the ureter and generally are multiple and often bilateral.

Associated urinary tract abnormalities are common, ranging from bladder outlet obstruction, to neoplasms, to renal stones and others.

Calcifications

Urolithiasis

Clinical

Urolithiasis means a calculus within the urinary tract. Renal (nephrolithiasis) and ureteral calculi predominate in developed countries, while bladder calcifications are more common in underdeveloped parts of the world. Stones range from small to large. Most radiologists have encountered a giant staghorn calculus; these have a complex composition. An underlying urinary tract infection is common.

Most patients with nonobstructing stones are asymptomatic, although an occasional stone results in hematuria or infection.

In general, about one third of patients with a first renal stone develop a recurrence within 5 years. These patients are usually investigated for an underlying cause, such as primary hyperparathyroidism, renal tubular acidosis, urinary tract infection, and cystinuria. Urography, lately supplanted by noncontrast CT, is often included in the workup. Stone analysis often guides further studies.

A study based on data from the Swedish Inpatient Register and Swedish Cancer Registry followed over 61,000 patients hospitalized for renal or ureter stones and concluded that although these patients are not at risk for developing a future renal cell cancer, they are at increased risk for renal pelvis, ureter, and bladder cancer (110); chronic irritation and infection appear to play a role because tumors tended to develop on the same side as stones.

Associated Conditions

Epidemiologic studies have established an association between arterial hypertension and renal stone disease. Alterations in calcium metabolism play a role in both entities.

Over 5% of cystic fibrosis patients older than 15 years have had urolithiasis in the past.

Renal stone disease is induced by some drugs. In France, the most common drugs involved are calcium and vitamin D supplements and long-term therapy with carbonic anhydrase inhibitors (111). Furosemide therapy of premature neonates for hyaline membrane disease is associated with urolithiasis.

Occasionally reported is heterotopic ossification within a renal stone. A suture retained in the renal pelvis from prior surgery can act as a nidus for subsequent calcification.

Patients with primary hyperparathyroidism have an increased prevalence of renal stones,but only a small percentage of patients with idiopathic stones are hyperparathyroid.A metabolic defect is detected in about half of patients who develop calcium stones.

Calcium containing stones consist of calcium oxalate and calcium phosphate. Less common are magnesium ammonium phosphate (struvite) stones. Calcium oxalate stones are associated with certain blood groups. The prevalence of these stones in patients with blood group O is several times greater than in patients with blood group A.

ADVANCED IMAGING OF THE ABDOMEN

Patients with gout, Crohn’s disease involving the small bowel, and some myeloproliferative disorders have an increased prevalence of uric acid stones. Pure uric acid stones are lucent.

Cystine stones develop in a setting of cystinuria, a rare autosomal-recessive disorder. These stones form in the first and second decades of life, while uric acid stones form in older patients. Cystine stones are only slightly radiopaque and range from small to large. Recurrent stones are common, and lifetime surveillance is necessary in affected individuals.

Patients with struvite stones are prone to persistent infections.

Imaging

Imaging of suspected urinary obstruction is discussed in the previous Dilated Urinary Tract section.

Unenhanced CT readily identifies ureteral stones; keep in mind that with the use of wide collimation some smaller stones, especially uric acid stones, are insufficiently dense to be detected. Narrow collimation is thus necessary. At 1-mm collimation, stones can be grouped by attenuation: uric acid (least dense), cystine and struvite, calcium oxalate monohydrate, and brushite and hydroxyapatite (most dense) (112). In vitro CT using HU measurements of chemically pure stones identified the chemical composition of uric acid, struvite, and calcium oxalate stones, but could not differentiate calcium oxalate from brushite stones and struvite from cystine stones (113); the use of dual CT kilovoltage aids differentiation.

The sonographic appearance of a calculus reflects its size rather than its internal composition. Calculi are hyperechoic and, aside from small ones, have posterior acoustic shadowing (Fig. 10.31). Ultrasonography is insensitive in measuring stone size. Ultrasonography is better in detecting renal calculi than ureteral calculi. In women, transvaginal US can detect stones in the distal ureter, and this modality should be considered in the pregnant patient.

An interesting stone finding consists of a “twinkling”artifact detected by color and power Doppler US (114); this phenomenon appears as a rapidly changing color complex located behind a stone, similar to a comet’s tail. It aids in stone detection. Also, a twinkling artifact depends, in part, on stone composition. Thus it

KIDNEYS AND URETERS

Figure 10.31. Right renal nephrocalcinosis in a child. Longitudinal US scan identifies numerous discrete hyperechoic foci in the kidney. (Courtesy of Luann Teschmacher, M.D., University of Rochester.)

is present with calcium oxalate dihydrate and calcium phosphate stones, but it is absent with calcium oxalate monohydrate and urate stones (115).

Magnetic resonance reveals a stone as a signal void, regardless of stone composition. T2weighted MRI (MR urography) identifies hyperintense urine, at times in a dilated ureter, and a hypointense or signal void stone at the site of obstruction. A blood clot also results in a hypointense signal.

Conventional radiographs are often employed in follow-up of known stones; stones detected only with helical CT should be followed with that imaging modality. Helical CT allows 3D reconstruction of stone shape, number, and size. The role of helical CT in lithotripsy planning is still evolving.

Therapy

The ideal goal of therapy is to eliminate all fragments. One proposed stone management strategy consists of surveillance for asymptomatic caliceal stones <5mm in diameter; symptomatic stones <20mm in diameter are treated with ESWL, while a combination of percutaneous nephrolithotomy and ESWL is used for stones >20mm in diameter (116); stones resistant to ESWL, such as cystine stones, often require percutaneous nephrolithotomy or ureterorenoscopic endocorporeal lithotripsy.

Extracorporeal Shock-Wave Lithotripsy

Extracorporeal shock-wave lithotripsy (ESWL) is now the treatment of choice for most larger renal or ureteral calculi. Both US and fluoroscopic guidance are employed. Occasionally IV contrast is necessary to define a stone. Large stones are often approached by both ESWL and percutaneous nephrolithotomy, while patients with an underlying congenital anomaly tend to undergo surgical reconstruction and stone clearance.

Extracorporeal shock-wave lithotripsy is used to treat patients with urolithiasis in a solitary kidney and patients with calculi in anomalous kidneys. An increased risk exists with a solitary kidney, because a subcapsular hematoma after ESWL in a solitary kidney can result in acute renal failure. Extracorporeal shock-wave lithotripsy is performed in children; although the procedure is similar to that in adults, some issues, such as sedation, are more pertinent to children.

The overall success rate for lithotripsy is about 80%,although the rate varies among institutions and with stone size and composition.An ESWL study found that 95% of calculi <15mm in maximum length were successfully disintegrated, while only 8% of those >25mm in length were disintegrated (117); for calculi between 15 and 25mm, however, instead of maximum length, volume was a better indicator, with 90% of calculi <6cm3 successfully disintegrated, while none >6cm3 were disintegrated. Spiculated, low-density stones are cleared more readily than smooth, dense stones. Peak shock wave pressure generated by many ESWL units at the focus is between 80 and 120MPa, and, in theory, the peak pressure required to disintegrate a 6-cm3 calculus was calculated to be approximately 80MPa (117).

Technetium-99m-DMSA scintigraphy performed the day before and at least 6 months after ESWL in children did not identify any significant parenchymal lesions (118); stone fragmentation or elimination was achieved in 90% of these children. Likewise, no blood pressure or renal function changes were evident.

In patients who become stone free after ESWL, the ipsilateral typical recurrence-free rates are about 95% and 65% after 1 and 5 years, respectively, and vary depending on the therapy for any underlying lithogenesis.

Morbid obesity makes ESWL difficult. Patients with a bleeding diathesis require adequate perilithotripsy therapy. The literature suggests that lithotripsy is feasible in patients with calcified aneurysms and cardiac pacemakers, but it requires close monitoring. Pregnancy is generally considered a contraindication to ESWL.

Patients with a pheochromocytoma have had severe reactions while undergoing ESWL.

Less than 1% of patients develop a hematoma; preexisting hypertension is a risk factor for perinephric or subcapsular hematoma. These hematomas can be diagnosed by US and most are managed conservatively. An occasional patient with a perirenal hematoma manifests hypertension, generally transient; a rare patient develops permanent hypertension as a late finding; a temporal relationship to previous lithotripsy is difficult to establish; some of these patients have had decreased renal function prior to lithotripsy.

Fever suggesting postprocedure obstruction is approached with urinary tract drainage and possible ureteroscopy. Drainage catheters are inserted to bypass a heavy stone load. Few complications are associated with these ureteral catheters, although encrustation becomes a problem in catheters retained longer than about 6 weeks.

A resultant fibrous scar, detected by imaging, is presumptive evidence for renal parenchymal damage during lithotripsy. Posttherapy Tc-99m- DMSA scintigraphy should detect any acquired parenchymal scars after ESWL. Anecdotal complications include renal laceration, small bowel perforation, splenic rupture, and even abdominal aortic rupture.

Ureteroscopy

Using an ureteroscopic approach, therapy options for ureteral stones include laser or electrohydraulic lithotripsy or simply the use of endoscopic baskets and similar grasping tools. Even intrarenal calculi are amenable to such therapy.

Ureteroscopy is more successful than ESWL for larger calculi. One option is ureteroscopic laser lithotripsy. Lithotripsy stone fragmentation is more successful in lower ureteral stones than in upper ureteral stones. Stone fragmentation by holmium:yttrium-aluminum-garnet (Ho:YAG) lithotripsy is primarily due to photo-

ADVANCED IMAGING OF THE ABDOMEN

thermal chemical decomposition rather than to photomechanical or photoacoustical fragmentation (119); thus calcium carbonate was found in samples composed of calcium oxalate calculi, calcium pyrophosphate in calcium hydrogen phosphate stones, free sulfur, and cysteine in cystine stones and cyanide in uric acid calculi.

Percutaneous Nephrolithotomy

In a retrospective study of lower pole stones 10 to 20mm in size, a stone-free status was achieved in 44% of patients with ESWL and 72% with percutaneous nephrolithotomy (120); the difference in success rate was less significant for smaller stones. On the other hand, morbidity was higher with the nephrolithotomy technique.

Severe hemorrhage developed in 2% of patients after percutaneous nephrolithotomy (121); renal arteriography revealed a mix of arteriovenous fistulas, false aneurysms, and arteriolar injuries, all successfully treated by embolization.

Staghorn Calculi

Staghorn calculi are usually treated by either ESWL or surgery, but the optimal therapy is still controversial. Lithotripsy alone is effective in about 50% of the patients.

Ultrasonography guidance is an option for percutaneous lithotomy, but the entire stone burden is removed only in a minority of patients; others require either lithotripsy or surgery.

Fiberoptic transurethral nephrolithotripsy has been combined with ESWL to treat staghorn calculi. The stones are initially disintegrated by nephrolithotripsy, and ESWL is then performed for residual fragments.

Nephrocalcinosis

Nephrocalcinosis designates renal parenchymal calcifications. These calcifications range from discrete to diffuse and tend to be bilateral. Nephrocalcinosis can be further subdivided by location—cortex or medulla. In either site calcifications can be dystrophic or metastatic. Dystrophic calcifications (those in abnormal tissue) develop at sites of ischemic, necrotic, or infected tissue, while metastatic nephrocalcinosis develop in normal tissue.

KIDNEYS AND URETERS

Medullary nephrocalcinosis tends to be bilateral and symmetrical. It has developed in a setting of vesicoureteral reflux, presumably related to urine stasis in collecting tubules.

Precontrast CT in medullary nephrocalcinosis reveals multiple medullary calcifications. Some mimic nephrolithiasis.

Ultrasonography appears to be reliable in grading nephrocalcinosis, although reliability in assessing change between examinations is only fair. Ultrasonography in primarily medullary nephrocalcinosis reveals hyperechoic medullary pyramids. Posterior shadowing develops only when the condition is advanced. Ultrasonography findings may precede the development of visible calcifications seen with conventional radiography.

Sarcoidosis

Sarcoidosis typically involves lungs, skin, and, at times, eyes. In a setting of sarcoidosis about 25% of patients develop hypercalciuria and 15% urinary lithiasis. A rare manifestation of renal sarcoidosis is focal granulomatous interstitial nephritis (122), with secondary ectopic calcitriol secretion by the granulomas and resultant nephrocalcinosis.

Rarery, ureteral infiltration by sarcoidosis leads to obstruction and hydronephrosis.

Calcifications in Cysts

An occasional benign cyst contains thin, curvilinear calcifications, probably induced by prior hemorrhage or infection. Any amorphous calcifications should raise suspicion for a neoplasm.

Hyperoxaluria

One of the conditions resulting in diffuse cortical calcifications is oxalosis. When extensive, these calcifications can be identified with conventional radiography.

Gas in the Parenchyma or Collecting System

Gas in the renal parenchyma or collecting system can be secondary to a fistula, abscess, or emphysematous pyelonephritis.

A spontaneous nephrocutaneous or nephroenteric fistula is rare and is usually associated with a staghorn calculus and obstruction. A fistula can be secondary to an extrinsic infection or, less likely, a neoplasm.

Computed tomography shows gas-fluid collections within the kidney, perinephric inflammatory changes, and, at times, extension of renal contrast outside the collecting system. Neither an antegrade nor a retrograde pyelogram may define a renal fistula.

Antegrade stent insertion is the primary therapy of choice for an enteroureteral fistula. If the antegrade approach fails, a cystoscopic retrograde approach will generally also fail. In some patients combining an antegrade with a retrograde approach succeeds in stent placement.

Renal Failure/Insufficiency

Clinical

Differentiation of renal failure from renal insufficiency depends on the degree of loss in renal function, with the final end point of renal failure being end-stage renal disease. The etiologies of renal failure are complex, varied, and often interrelated; some are listed in Table 10.10. Differentiation into acute and chronic renal failure aids in clinical management. A subdivision into prerenal, renal, and postrenal etiologies is helpful. Diabetes mellitus is the most common cause of chronic renal failure in adults. Vesicoureteral reflux is probably the commonest cause of end-stage renal failure in children. Most patients with glomerular kidney disorders have a progressive loss of renal function, with eventual histologic evidence of glomerulosclerosis and tubulointerstitial fibrosis. Presumably such fibrosis is not the initiating factor in renal failure but represents a final pathway.

Urinary tract obstruction is an uncommon cause of renal failure, except in individuals with a single kidney, where it can lead to either acute or chronic renal failure. Ultrasonography readily detects hydronephrosis and can steer the further workup, but keep in mind that an occasional obstructed urinary system does not lead to hydronephrosis. Intravenous urography is not indicated in acute renal failure even with a suspected urinary obstruction.