Which of the following is true regarding extracorporeal shockwave lithotripsy?
ESWL relies on shockwave generation, a shockwave focussing, a coupling mechanism and a means of radiological imaging to localise the stone. Shockwaves may be generated at focal point F1 using electrohydraulic, piezoelectrical or electromagnetic lithotriptors. They are focussed, often by an ellipsoid dish, and converge on focal point F2, where stone fragmentation occurs.
Anticoagulation with warfarin is an absolute contraindication to ESWL and aspirin therapy should be withheld. Pregnancy, evidence of urinary sepsis and the presence of a calcified arterial aneurysm in the vicinity of the stone are also contraindications to ESWL.
Stone hardness is an important factor in predicting the likelihood of success of ESWL and can be estimated according to Hounsfield Units on non-contrast CT. Stones with density >1,000 HU are less likely to fragment and it may be worth considering alternative treatment modalities in these cases. Cystine stones either respond well to ESWL or poorly which may reflect different stereoscopic crystalline structures.
According to a recent EAU/AUA meta-analysis, stone-free rates were significantly better for distal ureteric stones <10 mm and >10 mm and for proximal ureteric stones >10 mm treated with ureteroscopy (URS) compared to ESWL. Stone-free rates for mid-ureteric stones treated by URS and ESWL did not differ significantly.
A meta-analysis has shown that a shockwave rate of 1 Hz is optimal for stone fragmentation. Data regarding the risk of developing hypertension and diabetes are controversial and insufficient to allow specific recommendations.
The following are true of stones in horseshoe kidneys except:
Horseshoe kidneys represent a common congenital renal anomaly with an incidence of 1:400. They are twice as common in men and are associated with certain genetic conditions such as Turner’s syndrome. Persistence of an isthmus, typically between the lower renal poles, arrests renal ascent usually at the level of the inferior mesenteric artery. As the kidney ascends, it takes a highly variable arterial blood supply from the aorta, iliac vessels and inferior mesenteric artery and this predisposes to a greater risk of bleeding with PCNL. An arteriogram should be considered prior to undertaking open surgery on a horseshoe kidney. There is also a failure of medial rotation of the kidneys that results in a medial orientation of the renal calyces and the isthmus causes the lower pole calyces to be deviated inwards.
ESWL is not contraindicated although the passage of stone fragments may be restricted by concomitant PUJ obstruction that is often seen in horseshoe kidneys as a result of high and medial insertion of the ureter into the renal pelvis. URS and PCNL are also treatment options for stones in horseshoe kidneys. The medially placed calyces should be considered when obtaining access for PCNL, and the incidence of colonic injury may be higher (figure below).
Axial computerised tomogram showing a horseshoe kidney. Note the position of the colon with respect to percutaneous renal access.
The following are true of cystinuria, except:
Cystinuria like most inborn errors of metabolism is inherited in an autosomal recessive pattern with the gene defect located on Chromosome 2. It has been classified into three types (I, II and III) according to the specific gene mutation although this differentiation is of little clinical relevance. The incidence of heterozygous cystinuria is about 1 in 20,000 and these individuals are at high risk of recurrent cystine urolithiasis. Patients with cystinuria have defective absorption in the jejunum of cystine, and the other dibasic amino acids ornithine, lysine and arginine. The reabsorption of these amino acids in the proximal convoluted tubule of the kidney is also abnormal leading to high levels in the urine. Cystine, in contrast to ornithine, lysine and arginine, is relatively insoluble at physiological urine pH and has a pKa of 8.3. At pH < 7.0 the solubility of cystine is approximately 250 mg/L but at pH > 7.5 its solubility increases considerably to more than 500 mg/L. Patients with heterozygous cystinuria excrete <200 mg/day and usually do not form stones whereas cystine excretion in homozygous cystinurics is typically 600–1400 mg/day.
Cystinurics usually present with their first stone episode in the second or third decade of life and represent a particularly challenging group of patients to treat. Cystine stones may be managed surgically by ESWL, URS, PCNL although ESWL may be less effective because they are hard. Also, cystine stones are poorly radio-opaque as they do not contain calcium and are visible only because of the disulphide bonds between the cysteine molecules.
Prevention of stone formation is the primary objective and patients should be advised to maintain a fluid intake of 2.5–3.0 L/day. They should also be referred to a dietician who will recommend a low methionine diet. Methionine, which is metabolised to cystine, is found in high concentrations in animal protein which should therefore be restricted. Patients are advised to alkalinise their urine with potassium citrate solution (10 mL tds) aiming for pH 7.0–8.0. However, potassium citrate solution is unpalatable and often poorly tolerated. An alternative, although not readily available in the UK, is potassium citrate tablets. When these measures fail to adequately prevent stone formation, drugs which bind to cystine and thereby increase its solubility may be considered. D-Penicillamine binds cystine but is associated with considerable side effects such as skin rash, oral ulceration and gastrointestinal upset and has largely been superseded by tiopronin (Thiola) 1000 mg/day in divided doses. Vitamin C may increase the solubility of cystine but in high doses can cause hyperoxaluria that may predispose to stones of other types.
The following are true of bladder calculi, except:
The incidence of bladder calculi in the Western world has been declining because of improved nutrition and better control of urinary tract infection. However, in the developing world, bladder stones remain a significant problem especially in the paediatric population because of malnutrition and low phosphate diets.
Certain conditions predispose to bladder calculi formation in the adult population. Bladder outflow obstruction leading to incomplete bladder emptying and urinary stasis is a common cause of bladder stones and was once considered an absolute indication for bladder outflow surgery. However, a recent study that evaluated men with bladder calculi using pressure-flow studies found that only about half had bladder outflow obstruction . Moreover, these findings were maintained even after stone surgery suggesting that the presence of the stone within the bladder did not affect the urodynamics results. Patients with long-term indwelling catheters, spinal cord injury patients and those with enteric bladder augmentations are also at higher risk of bladder calculi.
Open cystotomy is still considered by many urologists to be an excellent operation for large (>3 cm) bladder stones and may be combined with an open prostatectomy. However, the holmium laser now allows fragmentation of even very large bladder calculi and where bladder outflow surgery is also required it can be used to resect or enucleate the prostate.
The following are true regarding analgesia in patients presenting with acute ureteric colic, except:
Analgesia should be offered as a priority to patients presenting with severe pain consistent with a diagnosis of acute ureteric colic. NSAIDs should be considered initially as they provide effective relief from the pain of ureteric colic. NSAIDs inhibit cyclo-oxygenase and thereby inhibit prostaglandin synthesis from arachidonic acid. Prostaglandins mediate inflammation as well as pain through the sensitisation of nerve endings. Prostaglandins also act on afferent arterioles in the kidney causing vasodilatation. Inhibition of prostaglandin synthesis by NSAIDs therefore causes afferent arteriolar vasoconstriction which reduces renal blood flow, glomerular filtration rate and intrarenal pressure. Therefore, it may relieve the pain caused by distension of the collecting system associated with ureteric obstruction.
A Cochrane review and meta-analysis demonstrated lower pain scores with NSAIDs in 10 of 13 studies that compared NSAIDs with opioids for acute ureteric colic. Furthermore, patients treated with NSAIDs required less ‘rescue’ medication (need for further analgesia within 4 hours of administration) than those treated with opioids. Most studies showed a lower incidence of adverse effects with NSAIDs and vomiting was significantly less than in patients treated with opioids (RR 0.35, p < 0.00001). Pethidine particularly was associated with a higher rate of vomiting and should therefore be avoided in instances where an opioid is to be used. A further Cochrane review in 2015 was unable to determine which NSAID was the most effective.
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