The Role of Interventional Management for Urinary Tract Calculi

The treatment of urinary tract stone disease has been revolutionized over the last 3 decades with the major technological advances of extracorporeal shock wave lithotripsy, percutaneous renal surgery, and ureteroscopy. These have not only made open surgery virtually obsolete but undoubtedly have lowered the threshold for intervention.

The principles and technical aspects of these modern minimally invasive methods for treating calculi are discussed and then how these modalities are applied to specific stone scenarios are described.

Extracorporeal Shock Wave

Lithotripsy (ESWL)

The concept of using shock waves to shatter kidney stones was proposed by Chaussy et al. in 1980.1 As a result of the clinical introduction of this technique in the early part of that decade, the management of urinary tract stone disease was transformed. There are three major types of shock wave generator in common usage: electrohydraulic, piezoelectric, and electromagnetic.

The general principle common to all is the generation of a shock wave in a fluid. Typically, the shock wave is characterized by a rapid positive pressure followed by a shorter and shallower negative pressure component. The first

focal point (F1) is at the site of shock wave generation. A focusing device is required to converge the shock waves onto the stone for fragmentation (F2).

Electrohydraulic systems rely on the discharge of a high-voltage current across a spark gap. This results in an expanding gas bubble which creates a shock wave. This is focused by a metallic semiellipsoid which surrounds the generator. The pressures achieved do vary (perhaps by as much as 45%), and the spark gap electrode has a finite life span. There is a risk of cardiac arrhythmias with this shock wave generator and to avoid this ECG gating is required. This ensures that the shock wave occurs after the R wave during the refractory period of the cardiac cycle.

Piezoelectric lithotripters depend on the property that ceramic crystals have a characteristic frequency of resonance. When a highvoltage current is applied, the crystal vibrates producing a shock wave that can be propagated in water. This type of lithotripter has a large number of such elements in a hemispherical array which acts as a self-focusing device. The analgesic requirements are less with this technology and it is accepted that stone fragmentation is slower with a greater retreatment rate.

The basic principle of electromagnetic lithotripsy is the application of an electric current to an electromagnetic coil within a water-filled cylinder. The ensuing magnetic field repels a metallic membrane producing a shock wave

PCNL Access

Variations in patient position are described with the commonest being the prone position. The safest and easiest access is via a posterior lower pole calyx, but the route into the pelvicalyceal system will be determined by the configuration of the stone and is also facilitated by knowledge of the anatomy. Intravenous urography does give valuable information for both aspects. However in complex cases (e.g., staghorn calculi), special circumstances such as anatomical variants (e.g., horseshoe kidney) or certain patient types including spina bifida, CT scanning can be invaluable. Preand postcontrast CT with reconfiguration allows the access strategy to be planned in advance.

A number of radiological techniques are popular in facilitating access to the pelvicalyceal system for PCNL.

In the first method, a retrograde catheter is passed cystoscopically into the collecting system and this allows injection of a mixture of contrast and visible dye such as methylene blue. The former opacifies the system for x-ray screening and the latter gives the operator visible confirmation that the pelvicalyceal system has been punctured.

The second method depends on direct puncture of the pelvicalyceal system under ultrasound guidance. Third, an intravenous injection of contrast may be given and, after the pelvicalyceal system is opacified by any of these methods, access may proceed.

When the pelvicalyceal system is visible, it is punctured as previously planned,ideally through calyx and a guidewire may then be introduced through the needle into the kidney. It is recommended that every effort should be made to feed the wire into the ureter ideally as far as the bladder to prevent loss of the wire. If this is not possible, the wire is coiled within the collecting system. Some advocate the use of a second safety wire as a precaution in case the first wire is displaced. The tract is then dilated using one of a number of methods: serial dilators, the coaxial metal Alken dilators, and balloon dilatation. The latter has the advantages of speed and possibly less blood loss. However, the tapered configuration of the balloon can leave the dilated tract short of the target in certain circumstances (calyceal diverticulum, complete staghorn), and in these circumstances coaxial dilators have an

advantage as they are not tapered. Typically, the tract is dilated to 30 F and access to the system is achieved by placing a 30 F Amplatz sheath over the dilator and leaving it in situ. In complex cases, a second tract may be created.

Refinements such as the mini-PCNL using smaller caliber instruments (15 F access and 12 F Nephroscopes) are proposed for smaller stones and ESWL failures.5 Other than the potential for reduced complication rates no clear benefit of this technique has been shown. The disadvantages may include a longer operating time and the need to acquire further specialized instruments.

Instrumentation for PCNL

Rigid nephroscopes with an offset lens are standard for PCNL. These have large instrument channels allowing deployment of fragmentation probes and retrieval instruments. Flexible cystoscopes inserted via the Amplatz can also be helpful for cases such as complex staghorns with multiple calyceal extensions and may avoid the need for a second tract. A flexible ureteroscope can also be used antegradely to access calyces and the ureter via a PCNL tract.

Nephrostomy Drains Post PCNL

Leaving a nephrostomy drain post PCNL allows tamponade of the tract to minimize hemorrhagic complications and gives the possibility of facilitating a second-look procedure. Furthermore, if ureteric fragments are causing obstruction, the presence of a tube allows the surgeon to temporize and consider options.The drains are however uncomfortable and there has been interest therefore in tubeless PCNL. The nature of the case will determine if this approach is appropriate and a consequence may be a higher rate of stenting.

Contraindications to PCNL

There are few instances when PCNL may be inappropriate and most are patient related. The procedure can be lengthy and therefore fitness for anesthesia is important. In the morbidly obese, there can be problems in reaching the stone and skeletal problems can make access a challenge in spina bifida patients. Stone in a solitary kidney should not be considered a contraindication.