Struvite and Staghorn Calculi 

Updated: Jan 02, 2019
Author: Maxwell Meng, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS 

Overview

Practice Essentials

Upper urinary tract stones that involve the renal pelvis and extend into at least 2 calyces are classified as staghorn calculi[1] (see image below). Although all types of urinary stones can potentially form staghorn calculi, approximately 75% are composed of a struvite-carbonate-apatite matrix. Struvite is magnesium ammonium phosphate; a Swedish geologist named Ulex discovered the substance in bat droppings and named it after his friend and mentor, the 19th-century Russian diplomat and naturalist Baron von Struve.[2]

Struvite and staghorn calculi. Plain abdominal rad Struvite and staghorn calculi. Plain abdominal radiograph demonstrating a right staghorn calculus and a smaller left renal pelvic stone. The patient is a 72-year-old woman.

Struvite stones are also known as triple-phosphate (3 cations associated with 1 anion), infection (or infection-induced), phosphatic, and urease stones. Other, less common staghorn calculi can be composed of mixtures of calcium oxalate and calcium phosphate. 

In the past, removal of large complex renal calculi required major open operations, with attendant morbidity and no guarantee of complete stone removal.  The current approach favors minimally invasive modalities such as extracorporeal shockwave lithotripsy (SWL) and percutaneous nephrolithotomy (PNL). See Treatment.

History of the Procedure

The concept that urinary tract infections play a role in lithogenesis is not new. Hippocrates noted the relationship between renal calculi and loin abscesses. In 1817, Marcet recognized the association of phosphate calculi with infection, alkaline urine, and ammoniacal urine. Not until the early 20th century did Brown propose that urea-splitting bacteria were responsible for urinary ammonia, alkalinity, and stone formation.[3] The isolation of urease, the first enzyme ever purified, earned Sumner[4] the Nobel Prize for Chemistry in 1946. Urease-producing organisms are listed in Etiology.

Problem

Struvite stones are invariably associated with urinary tract infections. Specifically, the presence of urease-producing bacteria, including Ureaplasma urealyticum and Proteus species (most common), Staphylococcus species, Klebsiella species, Providencia species, and Pseudomonas species, leads to the hydrolysis of urea into ammonium and hydroxyl ions. Escherichia coli does not produce urease and is not associated with struvite stone formation. Other common bacteria that have not been shown to produce urea include Citrobacter freundii, enterococci, and streptococci.

The resulting increase in ammonium and phosphate concentrations combined with the alkalotic urine (pH >7.2) is necessary for struvite and carbonate apatite crystallization. Magnesium ammonium phosphate crystals (MgNH4 PO4 •6H2 O) are admixed with carbonate apatite (Ca10 (PO4) 6•CO3) in varying proportions along with matrix. The proportion of matrix, typically low molecular weight mucoproteins, is greater than in other types of calcium-based stones and is thought to protect the bacteria from antimicrobials.

Epidemiology

Frequency

Although calcium oxalate stones are most prevalent in the Western world, struvite calculi account for up to 30% of urinary tract stones worldwide. In the United States, 10-15% of all stones are composed of struvite. They are found more frequently in women and in persons older than 50 years, likely reflecting the population at increased risk of recurrent or persistent urinary tract infections. Accordingly, treatment of struvite stones must also address the source of these infections.

The natural history of struvite calculi mandates the complete removal of stones. First, infection stones generally grow rapidly, and any remaining stone material may serve as a nidus for future stone formation. Second, even after complete stone removal, struvite stones recur in approximately 10% of patients; if residual stones or fragments are left after treatment, recurrence rates approach 85%.

Third, struvite stones are a potential source of significant morbidity. Previously, it was believed that asymptomatic struvite stones could be managed expectantly; however, studies have demonstrated that 30% of patients treated conservatively (ie, no surgery to remove stones) died of renal failure or of pyelonephritis and sepsis.

In rare cases, chronic irritation, infection, and inflammation from staghorn calculi can cause squamous metaplasia, leading to squamous cell carcinoma of the renal collecting system. These malignancies carry a very poor prognosis, with a 5-year survival rate of less than 10%.[5]

Priestley and Dunn reported a 41% 5-year survival rate in patients with untreated unilateral struvite stones.[6] These data underscore the importance of approaches, primarily surgical, to completely remove the stone material.

 

Etiology

Gram-positive bacteria that cause struvite stones are as follows:

  • Staphylococcus aureus

  • Staphylococcus epidermidis

  • Corynebacterium species (ie, C ulcerans, C renale, C ovis, C hofmannii, C murium, C equi)

  • Mycobacterium rhodochrous group

  • Micrococcus varians

  • Bacillus species

  • Clostridium tetani

  • Peptococcus asaccharolyticus

Gram-negative bacteria that cause struvite stones are as follows:

  • Bacteroides corrodens

  • Helicobacter pylori

  • Bordetella pertussis

  • Bordetella bronchiseptica

  • Haemophilus influenzae

  • Haemophilus parainfluenzae

  • Proteus species (ie, P mirabilis, P morganii, P rettgeri)

  • Providencia stuartii

  • Klebsiella species (K pneumoniae, K oxytoca)

  • Pasteurella species

  • Pseudomonas aeruginosa

  • Aeromonas hydrophilia

  • Yersinia enterocolitica

  • Brucella species

  • Flavobacterium species

  • Serratia marcescens

  • Ureaplasma urealyticum

  • Mycoplasma T-strain

Yeasts that cause struvite stones are as follows:

  • Cryptococcus species

  • Rhodotorula species

  • Sporobolomyces species

  • Trichosporon cutaneum

  • Candida humicola

Pathophysiology

Two conditions must coexist for the formation of struvite calculi. These are (1) alkaline urine (pH >7.2) and (2) the presence of ammonia in the urine. This leads to magnesium ammonium phosphate and carbonate apatite crystallization. The conversions of urea to ammonia, ammonia to ammonium, and acidification from carbon dioxide are as follows:

H2 NCONH2 + H2 O → 2NH3 + CO2 2NH3 + H2 O → 2NH4+ + 2OH- (increase pH >7.2)CO2 + H2 O → H+ + HCO3 → 2H+ + CO32-

Presentation

The clinical presentation of patients with struvite stones can be variable. Consider struvite stones in patients with risk factors for developing urinary tract infections (eg, prior urinary diversion or urologic surgery, presence of indwelling catheters, neurogenic bladder, vesicoureteral reflux, other anatomic abnormalities).

Infections may result in pyelonephritis, pyonephrosis, or perinephric abscess. Symptoms may include flank pain classic for renal colic, fever, urinary symptoms (eg, frequency, dysuria), and hematuria (either gross or microscopic). However, struvite stones rarely manifest as a solitary ureteral stone with acute renal colic in the absence of prior intervention. Concomitant urinary obstruction and hydronephrosis may be present and can result in nausea or vomiting.

In institutionalized patients susceptible to infection stones, the ability to elicit symptoms may be limited; sepsis may be the only evidence of an underlying struvite staghorn calculus. Note that patients with struvite calculi can be asymptomatic, even when calculi occupy the entire renal collecting system. Even with progression to xanthogranulomatous pyelonephritis, 25% of patients may remain completely free of symptoms. Systemic manifestations of large struvite stones and associated chronic infection include generalized fatigue, malaise, and weight loss.

Indications

Staghorn calculi represent a less-common nephrolithiasis subgroup so named because the significant stone burden that fills the renal pelvis and calyces forms a shape on radiographs that resembles a deer's horns. Most staghorn stones in Western society are composed of struvite and can cause significant morbidity and mortality if left untreated; therefore, large struvite stones must typically be removed.

Interestingly, an article investigating the structural analysis of renal calculi in northern India reported that over 90% of staghorn stones were composed of oxalates.[7] In a study from southern Thailand, the most common component of staghorn calculi was uric acid; struvite was found in only 11.6% of cases.[8] A US study, published in 2011, reported that slightly over half of complete staghorn calculi were metabolic in origin, consisting of calcium phosphate (55%), uric acid (21%), calcium oxalate (14%), or cystine (10%).[9]

Unlike other urinary stones that commonly produce symptoms (eg, renal colic) that necessitate intervention, treatment of struvite stones often occurs in patients without classic signs of nephrolithiasis; this is because large staghorn calculi may not cause acute renal or ureteral dilatation and resultant pain.

Relevant Anatomy

A comprehensive discussion of renal anatomy is beyond the scope of this article; however, several points relevant to endourologic techniques are discussed.

First, the kidneys are retroperitoneal organs enclosed within several layers, including the adjacent adherent renal capsule and the renal Gerota fascia surrounding the perinephric fat. Severe renal infections associated with struvite stones may lead to abscess formation, both within the kidney and within the Gerota fascia (ie, perinephric abscess).

Second, the kidneys are intimately associated with many nearby organs. On the right side, the liver may be posterolateral to the kidney at the level of the superior pole; on the left side, the spleen resides in an analogous position. These organs may be injured during percutaneous renal access. On both sides, the colon has retroperitoneal portions that can be located posterior to the kidneys. Studies have demonstrated that retrorenal colon positions are present in up to 10% of patients.

A single kidney contains 5-14 calyces, each of which drains a renal papilla. These minor calyces may coalesce to form major calyces, all of which subsequently drain into an infundibulum.

The placement of percutaneous tubes into the kidney should be guided by the following three principles:

  1. Access should not be placed through an infundibulum, because of greater risks of vascular injury
  2. In all areas of the kidney (both superior and inferior), access should be gained near the fornix of the calyx
  3. Entry into a posterior calyx allows the greatest ability to examine and remove stones in the renal pelvis and in additional infundibula and calyces

Contraindications

The presence of an active, untreated urinary tract infection is a contraindication to stone removal. Patients with struvite stones have chronic bacteriuria, and their urine is never sterilized by antibiotics alone; however, appropriate antibiotics should be administered prior to surgical intervention in an attempt to minimize the potential for sepsis during treatment. Similarly, if concomitant urinary obstruction and purulent infection exist (ie, pyonephrosis), percutaneous drainage and antibiotics are necessary before further manipulation of the stone and urinary tract.

If pyonephrosis is seen at the time of percutaneous renal access, percutaneous drainage should be left in place to maximally drain the infected collecting system, and nephrolithotomy should be deferred. The patient should be left on antibiotics and admitted for monitoring for urosepsis. Stone removal should be deferred to a later time to minimize infectious complications.

 

Workup

Laboratory Studies

Prior to treatment of struvite stones, complete serum laboratory studies are required, including the following:

  • Complete blood cell count
  • Prothrombin and activated partial thromboplastin times
  • Serum electrolyte evaluations
  • Creatinine measurements

Chronic anemia may necessitate preoperative blood typing and screening for potential blood transfusion, especially if open or percutaneous surgery is planned.

More importantly, urinalysis and urine culture should be performed several days before surgery, and specific antibiotic therapy should be initiated at least 24 hours prior to treatment. Findings from cultures of voided urine may not accurately reflect renal microbiology, with a negative culture finding or discrepant organisms.

Additional aspects of the metabolic evaluation of urinary stones should be pursued because up to 50% of patients with infection-related stones have concomitant metabolic abnormalities. Thus, a 24-hour urinary collection (for calcium, oxalate, uric acid, citrate, phosphate, uric acid, magnesium, sodium, total volume, and pH) and simultaneous serum tests for calcium, uric acid, electrolytes, and phosphate are indicated. If the serum calcium level is elevated, it should be rechecked along with serum parathyroid hormone levels.

If the patient has undergone prior stone removal surgery, information regarding the chemical composition of any previous stones is extremely important.

Imaging Studies

Plain abdominal radiography usually documents the extent of struvite staghorn calculi; however, additional imaging tests that reveal the anatomy of the renal collecting system can be helpful. In addition, guidelines from the American Urological Association/Endourological Society advise that in some patients with staghorn calculi and compromised renal function, adequate assessment of function requires a nuclear renal scan or a contrast-enhanced imaging study (eg, computed tomography [CT], magnetic resonance [MR], or intravenous [IV] urography.[10, 11]

Intravenous urography can clearly delineate the pelvic calyceal anatomy. Currently, however, noncontrast CT scanning followed by intravenous contrast CT scanning is obtained most often in the evaluation of urinary stones. CT scans also display the adjacent structures and may aid in selecting the safest percutaneous tract to access the renal collecting system.

Narrow, scarred infundibula indicate the need for percutaneous nephrostomy (PCN), while wide, large renal infundibula suggest that extracorporeal shockwave lithotripsy (SWL) might be adequate. If the passageway between the calyces and renal pelvis is open and unrestricted, stone fragments produced during SWL are much more likely to pass.

Traditionally, staghorn calculi were defined as partial if the renal pelvic stone extended into at least 2 calyceal groups or complete if at least 80% of the collecting system was filled. Some experts argue that, to compare published stone-free rates, especially in the era of minimally invasive modalities, an improved classification system based on stone size should be implemented.

CT scanning with 3-dimensional reconstruction offers accurate stone volumes, but the added radiographic analysis is costly, time consuming, and neither practical nor readily available. Lam and associates reported a simple 2-dimensional electronic computerized tracing technique that calculated stone surface area, which correlated well with stone volume.[12]

Performing nuclear renography is not necessary, but the findings are helpful for determining the relative function of the affected kidney. If the kidney has minimal function, nephrectomy may be needed. However, overall and relative renal function must be considered prior to removal of the kidney.

Dual energy CT with advanced postacquisition processing has shown some promise in preoperative determination of stone composition. In a small sample of patients, Zilberman et al demonstrated that all struvite stones have a measured attenuation of 900 Hounsfield Units (HU) or less.[13] This type of work was reproduced in both human models and another small population of test subjects.[14] Accurate preoperative prediction of stone composition may assist with planning for perioperative antibiotic and surgical management.

Ultrasonography alone is insufficient, but images show coexisting hydronephrosis. MRI does not help visualize urinary calculi; therefore, this modality has no role in preoperative and postoperative imaging of struvite stones.

 

Treatment

Medical Therapy

Staghorn calculi resulting from urease-producing bacteria are best managed with complete surgical removal of the stones. Medical therapy alone cannot rid the patient of struvite calculi and is typically adjunctive in nature. Nevertheless, nonsurgical measures may control life-threatening sequelae of untreated stones and may represent the best option in patients with significant comorbidities. A guideline from the American Urological Association recommends that patients with residual or recurrent struvite stones may be offered treatment with acetohydroxamic acid (AHA), but only after surgical options have been exhausted.[15]

Urease inhibitors

The most successful method of oral chemolysis is with urease inhibitors. AHA is the most widely used irreversible inhibitor of bacterial urease. AHA has a high renal clearance, can penetrate the bacterial cell wall, and acts synergistically with several antibiotics.

Although studies have demonstrated that AHA inhibition of bacterial urease decreases urinary alkalinity and ammonia levels even in the presence of infection, 20% of patients experience associated adverse effects. These include phlebitis, deep venous thrombosis, and hemolytic anemia. In addition, in patients with impaired renal function (serum creatinine level >2.5 mg/dL) has limited effectiveness and increased toxicity.

Other measures

Suppressive antibiotic therapy may prevent pyelonephritis and associated systemic infection and may help inhibit stone growth.

Sterilization of the urine with antimicrobial treatment alone can partially dissolve some struvite stones. However, only a single case report exists in the literature of prolonged oral antibiotic (dicloxacillin) therapy alone resulting in complete resolution of an infection stone.[16] Therefore, suppressive antibiotics should be viewed primarily as a means of inhibiting stone growth and as infection prophylaxis.

Dietary manipulation with a low-phosphorus, low-calcium diet and aluminum hydroxide gel (in an attempt to limit substrate [ie, phosphate] for struvite stones) has had only modest success and carries a significant risk of calcium abnormalities (hypercalciuria) and possible aluminum toxicity.

Other medical interventions, such as urinary acidification with oral ammonium chloride, have had limited long-term clinical utility.

Surgical Therapy

In the past, removal of large complex renal calculi required either anatrophic nephrolithotomy (bivalving the kidney on the lateral aspect) or pyelolithotomy (opening the renal pelvis). These are both major open operations with attendant morbidity. Moreover, these procedures did not always ensure complete stone removal, with the incidence rate of residual fragments ranging from 12-36%.

The advent of minimally invasive modalities to treat renal stones revolutionized the approach to staghorn calculi. These include extracorporeal shockwave lithotripsy (SWL) and percutaneous nephrolithotomy (PNL).

Extracorporeal shockwave lithotripsy

SWL was introduced in 1982 and is used to fragment urinary stones in a variety of locations without requiring an incision or instrumentation of the urinary tract.

Struvite calculi are effectively broken by SWL because of the multiple laminations within the stones. However, even with excellent stone fragmentation by primary SWL, repeat therapy via SWL must be performed in 50% of patients because of a large stone burden. If multiple sessions are anticipated, the renal pelvis should be treated first. Real-time monitoring of stone fragmentation using fluoroscopy is important to target and shock all areas of the stone.

In addition, the potential for urinary obstruction during spontaneous stone passage usually requires the placement of an indwelling ureteral stent. In up to 40% of patients, a percutaneous nephrostomy (PCN) tube is subsequently required to allow adequate renal drainage.[17]

If the renal infundibula are narrow, stone fragments that stem from SWL are unlikely to pass and remain in the calyces. Percutaneous nephrolithotomy (PNL) is the preferred surgical therapy in these cases. Large, wide infundibula that permit easy passage of fragments increase the stone-free rate and overall success of SWL treatment for staghorn calculi.

Percutaneous nephrolithotomy

PNL refers to the creation of a tract from the skin to the renal collecting system, thus permitting use of a nephroscope and instruments via this tract to fragment and remove stones. More recently, flexible ureteroscopes combined with small holmium laser fibers have allowed retrograde access to the kidney for stone destruction.

PNL was developed and popularized in the 1980s with the proliferation of miniaturized instruments. Access to the kidney is obtained under ultrasound or fluoroscopic guidance, and the tract is typically dilated to 24-30F. Through this tract, a rigid nephroscope is introduced to visualize the stone and collecting system and to guide fragmentation. Energy sources for this purpose include ultrasonic, electrohydraulic, and pneumatic lithotrites and laser.

The use of a flexible nephroscope allows examination of the entire kidney and helps ensure complete stone removal. Multiple PCN tracts may be used in cases of branched, complex staghorn calculi.

After PNL, a PCN tube is placed to optimize urinary and fragment drainage. The first photo below illustrates the results of the patient shown in the second image. Right PNL was performed via a single lower-pole access during a single session, rendering the patient stone free. The left stone was later treated using SWL.

Struvite and staghorn calculi. Plain abdominal rad Struvite and staghorn calculi. Plain abdominal radiograph of a 72-year-old woman. She underwent right percutaneous nephrolithotomy, with the path of renal access demonstrated by the remaining nephrostomy tube. She was rendered stone free in the single-session procedure.
Struvite and staghorn calculi. Plain abdominal rad Struvite and staghorn calculi. Plain abdominal radiograph demonstrating a right staghorn calculus and a smaller left renal pelvic stone. The patient is a 72-year-old woman.

Some practices have advocated placing the patient in a flexed prone or supine position in preference to a flat prone position, to more easily access the stone percutaneously and clear a larger stone burden. In addition to possibly improving stone clearance, these positions may provide better comfort for the surgeon during the procedure.[18, 19]  

Other advances

Technologic advances have resulted in instruments that can reach the kidney from the urethral meatus. Both laser and electrohydraulic ureteroscopic lithotripsy are possible and can significantly fragment staghorn calculi.

Case reports describe a synchronous bidirectional technique that combines percutaneous nephroscopy and retrograde intrarenal surgery to successfully treat complex, branched staghorn calculi, lessening the need for multiple flank punctures. As with SWL, the passage of a large stone burden requires an indwelling ureteral stent, and multiple treatments may be required.

For both monotherapy SWL and retrograde ureteroscopic lithotripsy, rendering the patient stone free is difficult, especially in the setting of dilated collecting systems and dependent lower pole calyces.

Surgical principles

Although multiple surgical approaches to staghorn calculi are available, several principles must be kept in mind. First, complete removal of all stone material is the goal of any procedure. Simple debulking does not prevent future infections, stone formation, or impairment of renal function.

Second, the patient should be counseled that multiple interventions may be required. If ureteroscopy or SWL is the primary treatment modality, the need for subsequent SWL, ureteroscopy, or PNL must be discussed. Use of combination therapy is a reasonable approach to ensure removal of all residual fragments. An example of this is the so-called sandwich technique, with initial PNL followed by SWL and then second-look PNL. Patient outcomes have been demonstrated to improve with increasing PNL experience.

Third, the immediate use of adjunctive measures can be considered. Postoperative oral acetohydroxamic acid (AHA) and antibiotics may delay the regrowth of struvite stones. In addition, direct irrigation of the collecting system is possible through the nephrostomy tube after PNL.

Lavage chemolysis for residual fragments consists of acidification of the urine with solutions such as Suby G or hemiacidrin (Renacidin). Although in vitro and in vivo data support some efficacy of direct acidification and ion exchange of stone calcium for magnesium, caution must be used when performing irrigation to ensure sterile urine, low intrarenal pressure, and normal serum magnesium levels. Hypermagnesemia (and associated toxicity) is more common in patients with compromised renal function.

In patients who have undergone single-tract PNL for complex renal calculi, Chen et al reported successful removal of residual stones using flexible ureteroscopy and holmium laser lithotripsy. Of their 27 patients, nine had full staghorn calculi and seven had partial staghorn calculi. After the flexible ureteroscopy and laser lithotripsy, the overall stone-free rate was 88.9%.[20]

Recommendations

A guideline on the surgical treatment of stones from the American Urological Association and the Endourological Society, issued in 2016, recommends PNL as first treatment for most patients with staghorn calculi. Compared with open surgery for staghorn calculi, PNL provides comparable stone-free rates, decreased intraoperative and postoperative complications, decreased length of hospital stay, earlier return to work, and a much smaller surgical incision. Complete stone removal should be the ultimate goal.[10, 11]

The guideline suggests that patients who are not considered candidates for PNL may be offered staged ureteroscopy (URS). SWL or URS should be offered to patients with symptomatic lower pole renal stones that are ≤10 mm.[10, 11]

Although open surgery was once the criterion standard, currently its use as the initial procedure should be limited to the following situations:

  • The kidney requires complete removal
  • The intrarenal anatomy is not amenable to other approaches
  • The stone cannot be expected to be removed by a reasonable number of minimally invasive procedures

 

 

Preoperative Details

Patients rarely require additional procedures prior to removal of the stones; however, if the patient is uroseptic and has evidence of urinary obstruction (eg, hydronephrosis) or pyonephrosis, placement of a PCN tube or ureteral stent may be necessary. Only when the patient is clinically stable after adequate urinary drainage and administration of intravenous antibiotics should definitive surgical intervention for stone removal be performed.

Intraoperative Details

In performing PNL and open stone removal, be aware of the potential for an underlying anatomic abnormality such as ureteropelvic junction obstruction or ureteral stricture. Their role in staghorn stone formation should be considered, but the abnormality may not be best addressed at the same setting. After the stone has been removed, subsequent treatment can correct the defect.

A meta-analysis by Astroza et al of 1311 PNLs performed on patients with staghorn calculi found that higher stone-free rates were achieved in a shorter surgical time when patients are positioned in the prone position versus supine positioning. Complication rates were comparable for both positions.[21]

For PCNL, either an upper-pole or a lower-pole approach can be used to gain access into the collecting system. In a study that included 76 patients with complete staghorn calculi, Blum et al reported no significant differences between the two approaches in terms of percentage of procedures completed using a single tract, stone-free rates, and overall median operative time; however, complication rates were significantly lower with prone lower-pole versus upper-pole access (3.4% vs 23.5%, P = 0.02).[22]

Postoperative Details

In analyzing outcomes of the treatment modalities, important considerations are (1) the probability of being stone free and (2) the probability of undergoing secondary unplanned procedures. Thus, careful assessment of the postoperative stone status is important.

Plain abdominal radiography may help visualize large stone fragments after open surgery, PNL, and SWL. An advantage of PNL includes the ability to fill the collecting system with contrast and to perform antegrade nephrostography, typically 1-2 days after surgery if the patient remains afebrile. Evidence of significant residual stones or urinary obstruction may lead to second-look nephroscopy or maintenance of the nephrostomy tube, respectively.

Most patients are discharged in 3 days after PNL, without the need for percutaneous renal drainage. Secondary SWL is usually indicated if a stone (or multiple stones >1 cm) remains in peripheral calyces. This can be performed during the same hospitalization 1-2 days after the initial PNL.

In a review of 31 patients who underwent repeat PNL after prior failed attempts at percutaneous stone removal, Borofsky et al reported uniform success with salvage PNL. Most of the cases involved staghorn calculi, and in most cases, initial PNL failed because of unsuitable access to the stone. These authors concluded that in the hands of experienced surgeons, salvage PNL can have outcomes no different from those of primary PNL.[23]

Follow-up

After all treatment modalities, the patient should be closely monitored for signs of infection, renal dysfunction, and bleeding.

Postoperative serum electrolyte evaluations, CBC counts, and creatinine studies should be performed. The patient should continue on the appropriate parenterally administered antibiotic. If an indwelling ureteral stent was placed prior to SWL, the patient should return for stent removal in 3-4 weeks.

Patients should be evaluated in an outpatient setting in 3 weeks after any surgery for staghorn stones. Renal ultrasonography, abdominal radiography, and serum creatinine studies are usually performed. In addition, if a preoperative metabolic evaluation of stone disease (24-h urine collection) was not performed, conduct one postoperatively once the patient is no longer hospitalized and on a controlled diet. Patients should be exhibiting no symptoms, eating their regular diet, and following their usual lifestyle activities when such a study is performed.

In a 2014 expert opinion guideline on medical management of kidney stones, the American Urological Association recommended that after removal of struvite stones, patients should be closely monitored for persistent or recurrent urinary tract infections with urease-producing organisms. Monitoring should include periodic urine culture. In some cases, long-term prophylactic antibiotic therapy may be appropriate for helping prevent recurrences.[15]

For patient education information, see Kidney Stones.

Complications

Infectious complications, including pyelonephritis and sepsis, may occur after either open or noninvasive (ie, SWL) treatment. Typically, infections occur early after PNL, but they may be delayed after SWL, when stone passage and obstruction occur. The incidence of significant blood loss and the need for transfusion after SWL is low, probably well below 0.1%.

The authors' recent experience demonstrates a PNL-associated transfusion rate of less than 5%. Early recognition of bleeding after PNL is paramount. Venous bleeding can usually be managed by balloon tamponade or clamping of the PCN tube; arterial bleeding may necessitate renal arteriography and embolization (or rarely nephrectomy). However, kidney loss after percutaneous stone removal, although more common than after SWL, is rare (0.1%).

Overall, the mortality rate associated with staghorn stone treatment is extremely low (SWL, 0.06%; PNL, 0.1%; PNL and SWL, 0.2%) and is typically related to cardiac dysfunction. Although significant morbidity is encountered more often with open surgery and PNL, SWL is associated with both planned and unplanned secondary interventions.

Injury to adjacent organs, including the spleen, liver, colon, and lungs, has been reported with PNL. Knowledge of perirenal anatomy, prompt recognition, and appropriate intraoperative and perioperative management usually minimize associated morbidity. For example, placement of a chest tube effectively drains air and/or fluid from the pleural cavity. Supracostal renal access during PNL is associated with a greater likelihood of pulmonary complications.

Perforation of the renal pelvis may occur during PNL. Adequate visualization, careful intracorporeal lithotripsy, and sufficient irrigation drainage help reduce this complication. Typically, it is identified immediately, with evidence of urothelial disruption or perinephric fat. Recognition of this complication is important so that the surgery can be terminated, minimizing extravasation of irrigation fluid and the potential for significant fluid absorption and spread of infection. Placement of the PCN tube is sufficient to ensure urinary drainage and healing of the urothelium. The nephrostomy tube is kept in place for 3-4 weeks to ensure healing, at which time nephrostography is performed and repeat nephroscopy can be considered for residual stone fragments.

During PNL, irrigating fluid may also be absorbed through open venous sinuses; dilutional hyponatremia and hypothermia may result. Long-term complications of both open surgery and PNL include intrarenal stricture or stenosis of the collecting system. Infundibular stenosis was recently reported as a rare complication, typically within the first year after surgery, and was associated with more complex stones and PNL.

Treatment complications for staghorn stones

Complications include the following:

  • Perforation of the renal pelvis

  • Hydrothorax/pneumothorax

  • Perirenal hematoma

  • Significant blood loss

  • Vascular injury

  • Transfusion

  • Urinoma

  • Sepsis/pyelonephritis

  • Stent/nephrostomy tube migration

  • Renal impairment

  • Wound infection

  • Loss of kidney

  • Injury of adjacent organ (eg, spleen, liver, colon)

  • Deep vein thrombosis

  • Death

Outcome and Prognosis

Multiple measures of treatment outcomes are available for evaluation. At best, monotherapy SWL for struvite staghorn stones yields stone-free rates of 60%. Residual fragments remain in 22-70% of patients, and re-treatment is necessary in 32-88% of patients.

In patients with a smaller stone burden (>500 mm2 surface area), stone-free rates may approach 90%. After monotherapy PNL, reported stone-free status is achieved in approximately 80% of patients. These outcomes are further improved in correlation to surgeon experience. Combining PNL with subsequent SWL yields stone-free rates comparable to those of PNL alone; this likely reflects the aggressiveness of the initial PNL and attempts to remove residual stones via flexible nephroscopy.

While the goal of the physician is to ensure stone-free status, patients are interested in direct outcomes. Prevention of patient symptoms and associated stone-related morbidity, such as infection, are important means of assessing treatment success. Studies have demonstrated that, even in the presence of small stone fragments after SWL monotherapy and perioperative antibiotics for 2 weeks, 86% of patients were cured of persistent infection. Conversely, achieving stone-free status does not ensure resolution of persistent urinary infections. Important considerations in these patients include anatomic abnormalities, neurogenic bladder, indwelling catheters, or urinary diversion. Long-term freedom from bacteriuria is probably not possible in these situations.

Potential deleterious effects of staghorn calculi and treatments for the stone have been a source of concern. However, studies have demonstrated the general safety of both SWL and PNL in the management of large stones, even with a solitary kidney and chronic renal insufficiency. Effects of SWL and PNL are minimal, with only slight decreases in renal function after intervention. Patients who progress to severe renal insufficiency associated with staghorn stones usually present initially with compromised renal function (serum creatinine level >3 mg/dL).

Future and Controversies

Many aspects of struvite staghorn calculi require further study.

Standardized classification of renal anatomy and staghorn calculi may improve staging of the stones. This will allow more accurate comparison of treatment modalities. Also, uniform methods of reporting treatment outcomes are needed.

Determining which endpoints (eg, stone free, clearance of infection, preservation of renal function, resolution of symptoms) are most important is necessary. Elucidating some of these factors will help in selecting the appropriate surgical approach and goals of intervention. Continued technological advancements in minimally invasive instruments and increasing worldwide surgical PNL experience will continue to lessen the morbidity associated with staghorn calculi therapy.

A better understanding of the etiology of infection staghorn stones may direct rational treatment. The potential role of microorganisms, such as nanobacteria, must be defined. In addition, development of more effective medical treatments may significantly alter management strategies. Urease inhibitors with less toxicity may have increased general utility. Also, drugs effective in acidification of the urine could halt stone formation and growth even in the presence of persistent infection.

 

Guidelines

Guidelines Summary

The American Urological Association/Endourological Society have published a guideline on the surgical management of stones.[10, 11] Recommendations relevant to adult patients with struvite and staghorn calculi include the following:

  • Clinicians should obtain a non-contrast computed tomography (CT) scan on patients before performing percutaneous nephrolithotomy (PNL) (strong recommendation; evidence level grade C)
  • Clinicians may obtain a non-contrast CT scan to help select the best candidate for shockwave lithotripsy (SWL) versus ureteroscopy (conditional recommendation; grade C)
  • Clinicians may obtain a functional imaging study (DTPA or MAG‐3) if clinically significant loss of renal function in the involved kidney or kidneys is suspected (conditional recommendation; grade C)
  • Clinicians must obtain a urinalysis prior to intervention; a urine culture should be obtained in patients with clinical or laboratory signs of infection (strong recommendation; grade B)
  • Clinicians should obtain a complete blood cell count and platelet count in patients undergoing procedures where there is a significant risk of hemorrhage or for patients with symptoms suggesting anemia, thrombocytopenia, or infection; serum electrolytes and creatinine should be obtained if reduced renal function is suspected (expert opinion)
  • In patients with complex stones or anatomy, clinicians may obtain additional contrast imaging if further definition of the collecting system and the ureteral anatomy is needed (conditional recommendation; grade C)
  • In symptomatic patients with a total renal stone burden >20 mm, clinicians should offer PNL as first-line therapy (strong recommendation; grade B)
  • In patients with total renal stone burden >20 mm, clinicians should not offer SWL as first-line therapy (moderate recommendation; grade C)
  • Nephrectomy may be performed when the involved kidney has negligible function in patients requiring treatment (conditional recommendation; grade C)
  • For patients with symptomatic (flank pain), non-obstructing, caliceal stones without another obvious etiology for pain, clinicians may offer stone treatment (moderate recommendation; grade C)
  • SWL should not be offered as first-line therapy to patients with >10 mm lower pole stones (strong recommendation; grade B)
  • Patients with lower pole stones >10 mm in size should be informed that PNL has a higher stone-free rate but greater morbidity (strong recommendation; grade B)
  • Flexible nephroscopy should be a routine part of standard PNL (strong recommendation; grade B)
  • Normal saline irrigation must be used for PNL and ureteroscopy (strong recommendation; grade B)
  • Patients not considered candidates for PNL may be offered staged ureteroscopy (moderate recommendation; grade C)
  • Alpha-blockers may be prescribed to facilitate passage of stone fragments after SWL (moderate recommendation; grade B)
  • SWL should not be used in patients with anatomic or functional obstruction of the collecting system or ureter distal to the stone (strong recommendation; grade C)
  • Staghorn stones should be removed if attendant comorbidities do not preclude treatment (clinical principle)
  • When residual fragments are present, endoscopic procedures should be offered to render the patient stone free, especially if infection stones are suspected (moderate recommendation; grade C)
  • Stone material should be sent for analysis (clinical principle)
  • Open/ laparoscopic /robotic surgery should not be offered as first-line therapy to most patients with stones; exceptions include rare cases of anatomic abnormalities, with large or complex stones, or those requiring concomitant reconstruction (strong recommendation; grade C)
  • A safety guide wire should be used for most endoscopic procedures (expert opinion)
  • Antimicrobial prophylaxis should be administered prior to stone intervention and is based primarily on prior urine culture results, the local antibiogram, and in consultation with the current Best Practice Policy Statement on Urologic Surgery Antibiotic Prophylaxis (clinical principle)
  • If purulent urine is encountered during endoscopic intervention, the clinician should abort stone removal procedures, establish appropriate drainage, continue antibiotic therapy, and obtain a urine culture (strong recommendation; grade C)
  • If initial SWL fails, endoscopic therapy should be offered as the next treatment option (moderate recommendation; grade C)
  • Ureteroscopy should be used as first-line therapy in most patients who require stone intervention in the setting of uncorrected bleeding diatheses or who require continuous anticoagulation/antiplatelet therapy (strong recommendation; grade C)
 

Medication

Medication Summary

When pharmacologic therapy is warranted, the most successful method of oral chemolysis is to administer a urease inhibitor. Urinary irrigants may also be used to solubilize and dissolve certain types of calculi.

Urease Inhibitors

Class Summary

These agents inhibit the hydrolysis of urea and the production of ammonia.

Acetohydroxamic acid (Lithostat)

Reversibly inhibits bacterial enzyme urease, thereby inhibiting hydrolysis of urea and production of ammonia in urine infected with urea-splitting organisms. Reduced ammonia levels and decreased pH enhance effectiveness of antimicrobial agents and increase cure rate of these infections. Does not acidify urine directly, nor does it have direct antibacterial effect. In patients with urea-splitting urinary infections (often accompanied by struvite stone disease) that are recalcitrant to other management, reduces pathologically elevated urinary ammonia and pH levels.

Genitourinary Irrigants

Class Summary

Magnesium from the irrigating solution is exchanged for calcium within the stone. Additionally, the acidic pH of the solution is able to solubilize and dissolve magnesium stones.

Citric acid/glucono-delta-lactone/magnesium carbonate (Renacidin)

Action on susceptible apatite calculi results from exchange of magnesium from irrigating solution for insoluble calcium contained in stone matrix or calcification. Magnesium salts thereby formed are soluble in gluconocitrate irrigating solution, resulting in dissolution of calculus. Struvite calculi are composed mainly of magnesium ammonium phosphates, which are solubilized by hemiacidrin due to acidic pH. Essential that patients be free from urinary tract infections prior to initiating chemolytic therapy. Used for local irrigation dissolution of renal calculi composed of apatite (a calcium carbonate-phosphate compound) or struvite (magnesium ammonium phosphates) in patients who are not candidates for surgical removal of calculi. Also used as adjunctive therapy to dissolve residual apatite or struvite calculi and fragments after surgery or to achieve partial dissolution of renal calculi to facilitate surgical removal.

 

Questions & Answers

Overview

What are struvite and staghorn calculi?

When were struvite and staghorn calculi first identified?

What is the role of urinary tract infection in the etiology of struvite and staghorn calculi?

What is the prevalence of struvite and staghorn calculi?

Which gram-positive bacteria cause struvite and staghorn calculi?

Which gram-negative bacteria cause struvite and staghorn calculi?

Which yeasts cause struvite and staghorn calculi?

What is the pathophysiology of struvite and staghorn calculi?

Which clinical history findings are characteristic of struvite and staghorn calculi?

When is surgery indicated for the treatment of struvite and staghorn calculi?

What is the renal anatomy relevant to struvite and staghorn calculi?

What are the anatomic principles of percutaneous tube placement for removal of struvite and staghorn calculi?

What are contraindications for surgical removal of struvite and staghorn calculi?

Workup

What is the role of lab tests in the preoperative workup of struvite and staghorn calculi?

What is the role of imaging studies in the preoperative workup of struvite and staghorn calculi?

Treatment

How are struvite and staghorn calculi treated?

What is the role of urease inhibitors in the treatment of struvite and staghorn calculi?

What is the role of antibiotic therapy in the treatment of struvite and staghorn calculi?

Which dietary modifications are used in the treatment of struvite and staghorn calculi?

What is the role of surgery in the treatment of struvite and staghorn calculi?

What is the role of extracorporeal shockwave lithotripsy in the treatment of struvite and staghorn calculi?

What is the role of percutaneous nephrolithotomy in the treatment of struvite and staghorn calculi?

What is the role of combined percutaneous nephroscopy and retrograde intrarenal surgery in the treatment of struvite and staghorn calculi?

What are the general surgical principles for struvite and staghorn calculi removal?

What are the AUA/ES guidelines on use of PNL for struvite and staghorn calculi treatment?

When is an open procedure indicated for removal of struvite and staghorn calculi?

When is placement of a PCN tube or ureteral stent indicated in the treatment of struvite and staghorn calculi?

How is surgery performed for the removal of struvite and staghorn calculi?

What is included in the postoperative monitoring following struvite and staghorn calculi removal?

What is included in long-term monitoring following struvite and staghorn calculi removal?

What are the possible complications of struvite and staghorn calculi treatment?

What is the efficacy of struvite and staghorn calculi surgery?

Which aspects of struvite and staghorn calculi treatment require further study?

Guidelines

What are the AUA/ES guidelines on the surgical management of struvite and staghorn calculi?

Medications

What is the role of medications in the treatment of struvite and staghorn calculi?

Which medications in the drug class Genitourinary Irrigants are used in the treatment of Struvite and Staghorn Calculi?

Which medications in the drug class Urease Inhibitors are used in the treatment of Struvite and Staghorn Calculi?