Q. Describe the various components of the shockwave lithotripter?
A. Whatever the device used, the lithotripter will have four main components: an energy source, a media for transmission of the energy (e.g. water), a focusing device and an imaging modality.
The first machines used were the Dornier lithotripters. These used electrohydraulic lithotripsy (EHL), whereby a spark is produced between two electrodes under water, which results in the rapid expansion and collapse of a gas bubble and subsequent energy transmission. A metal hemi-ellipsoid reflector is used to focus the energy. This modality produces the most effective shocks but can be painful, and the intensity of the shock wave is variable. An example of such a machine used today would be the Dornier Lithotripter S II.
A second form is the electromagnetic lithotripter. This relies on a cylindrical electromagnetic source, and energy is focused by an acoustic lens. An example of this would be the Storz Modulith SLX-F2.
Third, piezoelectric technology may be used to produce the energy. Piezoelectric materials consist of ceramic or crystal elements that produce an electrical discharge under stress or tension (the direct effect). Energy transmission in this lithotripter relies on the ‘converse piezoelectric effect’, whereby energy is produced via the movement of the source when electricity is passed through it. An example of a piezoelectric lithotripter would be the EDAP LT 02.
The acoustic shockwave produced has two main phases. First a short positive phase causes erosion at the entry and exit points of the calculus. The stone also shatters internally due to the compressive effect of this wave. The effect of compression/tension induced cracks is sometimes referred to as ‘spallation’. Second, a longer negative pressure phase component to the wave results in the formation of microbubbles, and the collapse of these microbubbles causes further erosion of the stone surface via the formation of ‘microjets’. The two phases are demonstrated schematically in Figure 14.3.
Figure 14.3 The two phases of a shockwave (shockwave pressure profile).
Q. What are the indications for ESWL?
A. In general the indications are
Renal pelvis stones <20 mm
Lower pole stones <10 mm, or 10-20 mm if favourable factors (see stone chapter)
Upper or distal ureteric stones <10 mm
Sandwich therapy in conjunction with PCNL
Indications vary according to patient factors and choice.
Q. What are the contra-indications for ESWL?
A. The contra-indications can be divided into absolute and relative:
Absolute
Uncorrected coagulopathy
Sepsis or active urinary tract infection (UTI)
Distal obstruction
Pregnancy
Arterial aneurysm in vicinity of stone
Relative
Hard stones (cystine or Ca oxalate monohydrate)
Morbid obesity (>135 Kg) or skeletal malformations that prevent targeting of the stone Abdominal pacemaker
Q. How would you consent a patient for ESWL?
A. Informed consent for ESWL would involve a description of the procedure, discussion of alternative treatments and an explanation of potential complications [4]. Patients should be warned that repeated ESWL treatments may be required and stones may recur. In addition, the complications below are taken from the BAUS 2016 patient information leaflet.
Common complications (greater than 1 in 10):
Haematuria
Renal/ureteric colic
UTI needing antibiotic treatment
Skin bruising
Stone will not break as too hard, requiring an alternative treatment
Occasional complications (between 1 in 10 and 1 in 50):
Steinstrasse requiring surgical treatment
Rare complications
Perinephric haematoma; bleeding may require further treatment
Severe infection requiring intravenous antibiotics +/- nephrostomy
Adjacent organ damage
Hypertension (not on BAUS form but well-described)
Arrhythmias