48: Urolithiasis—Ureteral Surgical Management

Este capítulo durará aproximadamente 10 minutos para leer.

Por Sheila Mallenahalli, BS 1  & Pankaj P. Dangle, MD, MCh, FAAP 2, 3
  1. University of Alabama at Birmingham School of Medicine, Birmingham, AL, USA
  2. Riley Hospital for Children, Indianapolis, IN,USA
  3. Department of Urology, Indiana University School of Medicine, Indianapolis, IN,USA

Introduction

Nephrolithiasis is the formation of stones inside kidney due to deposition of various salts or minerals. These stones can remain asymptomatic for long periods of time until they become large enough to become symptomatic.1 This may present as a combination of flank or abdominal pain, urinary tract infection, or hematuria.2 However in infants and younger children, the presentation may be less specific with symptoms such as constipation, vomiting, dysuria, and generalized abdominal tenderness.1

Embryology

The kidney develops from three primary layers of the mesoderm—the pronephros, mesonephros, and metanephros. The pronephros is the most primitive stage of kidney development. It will eventually regress and is no longer part of the adult kidney. The mesonephros eventually forms part of the male genitalia ductal system and regresses in females. The ureteric bud branches off from the mesonephros and will induce the mesoderm to start formation of the ductal system and the ureters. The metanephros develops into the nephron system.3

Epidemiology

Pediatric nephrolithiasis is much less common than in the adult population.1 Historically, the prevalence in children is likely between 2–3%, but this number has been steadily on the rise.2 Studies over the last 20 years have estimated this increase to be between 4–10% per year, with 12- to 17-year-old adolescents having the most rapid rates of increase.4 It is also thought that female patients may be at a higher risk for stone disease than male patients, presumably due to the higher risk of urinary tract infections in females as compared to males. No identifiable cause is found in most patients, however 9–24% of patients will have a potential origin like metabolic abnormalities , neurologic disease, or a urinary tract malformation.2 There may also be a geographical component to stone formation within the United States, with higher rates of incidence of nephrolithiasis being present in the Midwest and Southeast regions.4 Of children who present with stone disease, between 75–84% will present with low serum citrate and high calcium levels and 4–20% will have a simultaneous urinary tract infection.4,2 There may further be an environmental aspect to stone formation as well with greater rates being associated in hot and dry climates due to a higher risk of dehydration.1 Calcium phosphate stones were previously the most commonly seen in children, but this has now shifted to calcium oxalate stones instead.4

Pathogenesis

Many different metabolic, anatomic, genetic, and environmental factors may play a role in stone formation. Children presenting with nephrolithiasis have a higher likelihood of having a genetic cause, with the prevalence being around 15–17% of all children who present with a stone burden.5,6 Anatomic anomalies such as a single kidney or a uretero-pelvic obstruction can also cause stone formation due to urinary stasis.7

Struvite stones are traditionally associated with an infectious nidus, particularly from urease producing bacteria.8 This increases the ammonia content and raises the pH, which causes phosphate and magnesium to precipitate and results in struvite stones.7 Over time, however, the predominant etiology for nephrolithiasis have changed from infectious causes to now primarily metabolic causes.7

Hypercalciuria is one of the more common causes of stone formation in children. This is traditionally due to changes in calcium absorption in the gastrointestinal or renal systems or disturbances in bone formation. Genetic disorders that can also cause hypercalciuria include Fanconi syndrome, Bartter syndrome, Liddle’s syndrome.7 Children tend to be less likely to form uric acid stones due to their higher urine pH.9 However, inborn errors of purine and pyrimidine metabolism can present with uric acid urolithiasis due to a high uric acid level.9 This includes hypoxanthine-guanine phosphoribosyl transferase deficiency which can lead to Lesch-Nyhan syndrome or phosphoribosyl pyrophosphate synthetase super activity, both of which commonly present with uric acid stones.9 Lesch-Nyhan patients who take allopurinol are also at risk of xanthine stone formation.9 Other causes of hyperuricemia can include probenecid, mannitol, losartan, or high-dose salicylates.9

Cystinuria is another genetic disease that can cause cystine stone formation due to defects in cystine and other dibasic amino acid reabsorption.10 This disease also has the predilection to cause recurrent stones due to the poor solubility of cystine.7 Other disease-stone associations include calcium oxalate stones in Chron disease and cystic fibrosis, and struvite, calcium phosphate, and calcium oxalate stones in myelomeningocele.10

Evaluation and Diagnosis

As previously mentioned, nephrolithiasis may present with a wide range of symptoms including flank pain, hematuria, and dysuria. A history of recurrent UTIs or a urinalysis with isolated leukocytes may be also further indication of renal stone formation.1 If nephrolithiasis is suspected, a renal and bladder ultrasound is the appropriate first diagnostic step in the pediatric population to confirm a stone burden due to its low cost, speed, and the lack of radiation and sedation needed.4 If the ultrasound is inconclusive or further imaging is required due to a high clinical suspicion, only then should ultra-low dose non-contrast CT be used. It is important to note however that adding a CT scan post-ultrasound very infrequently changes management.8 Stones that were not seen on ultrasound were also shown to not be significant clinically.11 In the past 20 years, there has been a shift toward using CT more frequently, but the advantages in image sensitivity that it provides must be weighed with the cons it poses specifically in children.8 On x-ray and CT, there may also be differences identifiable for the different types of stones. For example, cystine stones are radiopaque while struvite stones can be seen on x-ray due to their calcium content.9

Initial laboratory workups for children presenting with nephrolithiasis symptoms traditionally also includes a 24-hour urine collection and serum studies to evaluate various ion and solute levels.9 In children who are presenting with a first nephrolithiasis event, it is important to also evaluate for underlying metabolic or anatomic causes of stone formation.1 A common initial laboratory workup may include a urine analysis with evaluation of crystal formation. This may provide some indication of either genetic or metabolic causes of stone formation such as hyperuricosuria, hypercalciuria, or cystinuria.1 Uric acid stones will also need urine and serum uric acid levels, while xanthine stones will need serum uric acid levels as well as 24-hour xanthine, hypoxanthine, and uric acid excretion. If the stone is passed spontaneously, it is also prudent to further analyze the stone itself, traditionally through x-ray diffraction or infrared spectroscopy. This is even possible for stone fragments < 1 mg that are passed even after lithotripsy.1

Treatment Options and Outcomes

Initial treatment for nephrolithiasis should always start with increasing the patient’s fluid status in order to decrease urinary solute concentration, usually totaling to 70–100 mL/kg/day.1 Acute interventions should also focus on urinary decompression if necessary, usually in the setting of UTIs, pyelonephritis, or other acute systemic symtpoms.11 This can be done via percutaneous nephrostomy (PCN) tube or stent placement with cystoscopy.11 If infectious processes are suspected, the appropriate antibiotics should also be initiated. Uric acid stone burdens may benefit from urine alkalinization or allopurinol if significant hyperuricemia is present.9 The patient’s pain status must also be evaluated and treated with appropriate analgesics.10

If the stone is <10 mm and non-obstructive, a combination of alpha-blocker or calcium channel blockers may be used to prevent contraction of the smooth muscle of the ureter to allow spontaneous stone passage with this medical expulsion therapy.1 Surgical interventions may be required if a stone is not likely to pass spontaneously and now primarily occur in a minimally invasive fashion. This may be required in up to 30% of children presenting with nephrolithiasis, while stones <4 mm are more likely to pass spontaneously.8,10 Open procedures are still performed, but primarily in the setting of anatomical abnormalities or an abnormally high stone burden. The type of surgical approach appropriate, however, is specific to the stone size and location.

For example, extracorporeal shockwave lithotripsy (ESWL) is first line for proximal ureteric stones while ureteroscopy (URS) is preferred for distal ureteric stones.1 ESWL involves shock pulses directed at the stone itself.12 Starting from low energy levels and working up, the shearing forces produced are intended to disrupt the stone and break it up into fragments which can then be spontaneously passed. URS involves using a guidewire to lead the ureteroscope to the location of the stone. A laser is then placed in direct contact with the stone to fragment it.13 Stones in the lower calyxes of the kidneys are usually removed with either retrograde intrarenal surgery (RIRS) or micro-perc technique.1 RIRS involves access the stone location by ascending the urinary tract through the ureter and upwards.14 Micro-perc is a version of percutaneous nephrolithotomy (PCNL)—these techniques involve making an incision in the flank and inserting a needle to access the stone for extraction or fragmentation via laser.14 Micro-perc involves the use of a smaller needle. Once inside the renal pelvis, the approach largely depends on the size of the stone. If greater than 15 mm, RIRS, micro-perc, or PCNL is preferred while ESWL is used for stones <15 mm. PCNL is also used for staghorn caliculi due to complex nature of their shape.1,10

Complications

Each of the techniques described above have been shown to have significant benefits, but also can have various complications as well. Stones larger than 15 mm may need multiple approaches because they provide a larger technical difficulty. The micro-perc technique has a high stone-free rate post-procedure, but has a decreased effectivity in stones larger than 20 mm.1

While ESWL is a commonly used, it requires great focusing so as not to disturb surrounding structures. There are also concerns about an increased risk for hypertension in the pediatric population as well as renal colic due to potential scarring and subsequent lower renal function.4,11 Furthermore, patients will still need to spontaneously pass the stone fragments that result from this procedure which could cause further pain and discomfort.

Although it has a very high stone-free rate, PCNL is more invasive than other techniques and thus poses more risks such as the need for blood products after hemorrhage.11 URS also has been proven to be more difficult in children due to the smaller ureter size than must be maneuvered.4 Stent placement prior to URS have been shown to improve ureteral access and lower complication rates, however non-stented URS should be trialed prior to stent placement.11 Furthermore, URS also poses risks for subsequent UTI post-operatively as well.8

The amount of radiation present during these different procedures is also an important factor when treating children—studies have shown URS to have significantly lower radiation exposure than other techniques like PCNL.4 This is especially important to consider when repeat interventions are performed for multiple instances of stone formation.11

Suggested Follow Up

After the initial stone incident, a subsequent urinalysis should be performed in 3–6 months.4 This however varies significantly with regards to institutional guidelines and provider preference.15 As children have a high recurrence rate of stone formation, important risk factors or causes should be investigated to prevent subsequent incidences. Some follow up can be more specific to the stone type. For example, patients with hypercalciuria can be placed on thiazide diuretics and followed up to measure subsequent urine calcium levels and uric acid stones may require follow up of serum and urine uric acid levels especially if treated with allopurinol.10 More generally, children should also be advised to increase their fluid intake to prevent dehydration stimulated stone formation.

Conclusions

Nephrolithiasis in children can be due to many causes such as an infectious nidus or metabolic disturbances. Initial imaging involves ultrasound to confirm stone burden, and initial management should focus on pain stabilization and urinary decompression. Further management is highly dependent on stone size and location. If unable to pass spontaneously or through MET, surgical interventions like ESWL, URS, RIRS, micro-perc, or PCNL should be considered. Follow up should include subsequent urinalysis and serum and urine studies.

Key Points

  • Initial imaging should be renal and bladder ultrasound, with CT being used only if absolutely necessary.
  • If unable to pass spontaneously, surgical management is highly dependent on stone size and location. Possible interventions include ESWL, URS, RIRS, micro-perc, or PCNL.
  • Follow up for nephrolithiasis should include urinalysis 3–6 months post-incident and any necessary serum and urine studies to monitor metabolite levels.

Summary

Pediatric nephrolithiasis has historically occurred in 2–3% of children, but this number has been steadily on the rise. Many different metabolic, anatomic, genetic, and environmental factors may play a role in stone formation, but children presenting with nephrolithiasis have a higher likelihood of having a genetic cause. Presenting symptoms can vary, but initial workup should include a renal and bladder ultrasound, 24-hour- urine collection, and serum studies. Treatment options should start with fluid repletion, but other acute interventions can involve both medical and procedural management. Each can have their own complications, and thus suggested follow-up is within 3–6 months with a subsequent urinalysis.

References

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Última actualización: 2023-02-22 15:40