Urology Specialties, Conditions, Treatments & Technology

ESWL – Procedure

Extracorporeal Shock Wave Lithotripsy (ESWL) is a well-established and most common procedure for the treatment of urinary stones. In general, it ensures a high success rate with low adverse effects.

The success of ESWL is dependant upon several factors such as stone location, stone composition, stone size, patient’s body characteristics and risk factors, pain management, treatment strategies, and the shock wave parameters. The effectiveness of the shock wave is also related to the absence of anatomical structures within the shock wave path and the accuracy with which the stone is located and aligned in the therapy focus of the shock wave source.

Symptoms:

Kidney stones are usually undetected during formation. As the stone moves into the urinary tract they are often accompanied by the following symptoms:

• Renal colic
• Frequent, burning or slow urination
• Blood in the urine
• Fever, nausea and/or vomitting

Incidence and Causes:

About 5 % of all women and 10 % of men have a kidney stone once in a life time. Kidney stones usually appear in people between the ages of 20 -45, and occasionally develop in children. After the age of 50 the incidence rate declines. Kidney stones are especially common in dry, hot countries and are seen more often in Caucasians than people of African descent.

When the urine becomes too concentrated with impurities, small crystals may form and develop into stones. Calcium oxalate stones are most common. They often develop due to insufficient fluid intake. Other types of stones include calcium phosphate, struvite, uric acid and cystine stones.

The most common causes of calculi are dehydration and poor nutrition. Certain medications such as antacids and protein supplements have also been linked to calculi formation. Cystine stones are linked to heredity and although rare, have been seen in children.

Patient Selection:

Contraindications to ESWL include:

• Pregnancy
• Coagulation abnormalities as indicated by abnormal prothrombin time (PT), partial thromboplastin time (PTT), or bleeding time; patients on anticoagulants;
• Cysts, arterial aneurysms, bodies of vertebrae, large bony areas, arterial calcification or hemangioma in the shockwave path;
• Obstruction of urinary tract distal to the stone
• Anatomic anomalies preventing adequate positioning
• Stones which cannot be clearly located and localized on the imaging system of the lithotripter

Procedure:

Stones can either be localized with ultrasound or X-ray. The main benefits of ultrasound are real time monitoring of the disintegration process and the absence of ionizing radiation. Flouroscopic imaging usually is very fast and precise. It visualizes areas that are not seen with ultrasound. Dornier lithotripters are designed for dual-mode imaging offering the choice to apply both imaging modalities simultaneously.
Stone fragmentation is primarily caused by high local tensile and shear waves created by the focused shock waves hitting the stone. Spherical shock wave fronts contribute to compression-induced tensile cracks or spalling at the posterior surface of the stone. Cavitation is also important for stone comminution. The rapid collapse of cavitation bubbles on the surface of the stone or in liquid-filled cracks within the stone produces shock waves that cause microfractures in the stone.

Effective Disintegration Energy:

Stone fragmentation correlates quite well with the shock wave energy delivered into the focal zone. The acoustic energy of a shock wave pulse is determined within an area having a diameter that corresponds to the average size of urinary stones. The effective energy contributing to stone disintegration is generally defined as energy delivered to an area of 12 mm diameter in the focal plane. The 12-mm area corresponds to most stones indicated for ESWL monotherapy. Provided the stone is precisely targeted, most of the effective energy contributes to fragmentation except for the portion that does not hit the stone. The surrounding tissue absorbs the energy that misses the stone. For larger stones, the full effective energy is applied to the stone for fragmentation. This is important when selecting a safe energy dose, i.e., the number of shock waves times the effective energy, to disintegrate the stone.

The energy dose Etot (12mm) is defined as Etot(12mm) = n * Eeff(12mm) where n is the number of applied shocks and Eeff(12mm) is the disintegration energy, i.e. the acoustic energy per shock wave delivered to an area of 12 mm diameter in the focal plane. The applied energy dose determines treatment success in terms of fragmentation and side effects. Thus, the success of ESWL treatments at different intensity levels seems to remain the same when the number of shots is in the range receiving equivalent energy dose. The energy dose for kidney stones typically is in the range of 100 to 200 J per treatment.

Pulse Repetition Frequency:

Cavitation bubbles in the shock wave path have a noticeable effect on fragmentation efficacy. They attenuate the shock wave and fragmentation efficacy is reduced. Studies performed in vitro suggest that the PRF influences fragmentation efficacy due to cavitation effects. At higher PRFs shock waves become less effective. For this reason PRF should be kept as low as possible.

Post-Procedure Protocol:

Most patients only experience minimal side effects after the procedure. Side effects may include mild discomfort in the abdominal region, along with redness or bruising at the treatment site, and blood in the urine. Patients can return to a normal routine within twenty-four hours of the treatment.

Side Effects:

Even though it has not been scientifically proven, the risk of side effects from a shock wave exposure may increase due to factors such as medication, age, hypertension, cardiac rhythm dysfunction, vascular debility, diabetes or obesity. Tender vessels in the renal parenchyma are particularly endangered, and high shock wave intensities can lead to a serious hematoma if the patient presents a combination of these risk factors. Clinically relevant perirenal hematomas are observed within 1 – 3% after ESWL as confirmed by ultrasound imaging, and most are treated conservatively.