MINIMALLY INVASIVE STONE SURGERY (Ureteroscopy)
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Evolution of Technique
The fascinating history of urology with its captivating depictions, texts and stories rivals any other field in medicine. In particular, the ancient past of lithotomy (treatment and removal of urinary stones) dates back to Mesopotamia (3200 BC), and descriptions and treatments for urinary stone disease have been identified from ancient cultures including Hindu, Greek, and Egyptian. The treatment of stones, which at the time most commonly occurred in the bladder, was very dangerous, often lethal. As such, it led to the development of one of the first medical subspecialists, the lithotomist, who opened the urinary tract and removed stones directly. Recognition of this unique set of skills earned a distinction in the Hippocratic oath, still recited by graduating students: “I will not cut for the stone, but will leave this to be done by practitioners of this work…”
Today, “cutting for stone,” otherwise known as open stone surgery, has been replaced by alternative minimally invasive techniques. Critical to this change has been the introduction of the endoscope. Whereas early pioneers such as Howard Kelly (depicted above in the early 1900’s using a wax tipped catheter in the ureter to detect a ureteral stone) relied heavily on intuition, feel, and clinical suspicion, advances in endoscopy have made it possible to inspect all parts of the urinary tract directly.
The particular endoscope that has revolutionized our ability to treat kidney stones in a minimally invasive fashion is the ureteroscope. Incorporation of its use into urologic practice has led to the development of a unique subspecialty, endourology, that focuses on minimally invasive and endoscopic surgical treatments.
A ureteroscope, as its name implies, is an endoscope designed to visualize and work within the ureter. Other scopes commonly used by urologists and endourologists include the cystoscope (used in the bladder) and the nephroscope (used in the kidney).
Although such scopes now enable modern surgical stone treatment, they have only been routinely utilized for the past several decades. Minimally invasive kidney stone treatment began in the mid-1980’s with shock wave lithotripsy and percutaneous nephrolithotomy. Ureteroscopy for treatment of ureteral stones was not performed routinely until then as well, and it was not until the late 1990’s and early 2000’s that ureteroscopes could routinely access and treat stones in the kidney. In the United States we often take such technologies and treatment options for granted, but open stone surgery is still practiced in many parts of the world where modern technology remains limited.
There are two types of ureteroscopes, rigid and flexible. Rigid ureteroscopes, as their name implies, are firm and preferred for treatment of stones lodged in the lower ureter which can usually be accessed in a straight path. When stones are located in the upper ureter and/or kidney, they often require
flexible ureteroscopes that accommodate to the shape of the ureter and renal collecting system (figure on right). Though more difficult to maneuver, flexible scopes allow the urologist to inspect nearly the entirety of the inner kidney to find stones, treat them, and remove them using a variety of techniques. Innovation this area is brisk and modern digital ureteroscopes offer increasingly superior image quality, lighting and wide fields of view.
(Figure – Left ) Ureteroscopic image of the renal collecting system using older generation fiberoptic ureteroscope (left) and newer generation digital ureteroscope (right). Note the superior image quality, increased light intensity and wider field of view. Panels A and B feature small stones attached to a renal papilla. Panels C and D feature the endoscopic appearance of Randall’s plaque.
Irrigation and Access Sheaths
In order visualize anything at all, fluid must run continuously through the scope and into the kidney to wash away blood, debris, and crystals that impair visualization. To accomplish this, the fluid (most commonly saline) is hung from a bag and run through tubing directly into the body of the scope where it comes out the tip and into the kidney.
This inflow is harmless, but excess filling of the kidney because of high pressure inflow or an inability of the fluid to freely exit around the scope can overstretch the kidney and cause fluid absorption, leakage, bleeding, and small tears within the kidney itself, a term we call extravasation.
To prevent this, many urologists use a device called a ureteral access sheath. This can be thought of as a temporary tunnel from the bladder to the kidney which allows any fluid that is irrigated into the kidney to quickly wash out around the scope, optimizing visualization and decreasing the likelihood of extravasation. The scope can be passed back and forth without rubbing against and potentially damaging the lining of the ureter, which is generally around the same small diameter as the scope itself. When used appropriately, temporary sheaths are safe and decrease postoperative infections and sepsis. While the majority of urologists routinely use them during ureteroscopy, debate does exist within the community as to whether or not they are required in every case.
(Figure on Left – Boston Scientific Navigator HD TM, Marlborough, MA)
The concern with routine use of a sheath is trauma and injury to the ureter when the sheath is passed. Larger diameter sheaths are preferable for procedures, but the ureter is quite narrow and often will not accommodate a large sheath and sometimes any sheath at all. In such cases, surgeons have several options. If they deem a sheath necessary, they can place a temporary ureteral stent that lets the ureter passively dilate and facilitate sheath placement at a later time. Alternatively, they can dilate the ureter during the procedure using either balloon or serial dilators in order to get the sheath up.
(The ureteral access sheath (white arrow) is passed under fluoroscopic guidance over a wire (black arrow) that is positioned from outside the body and extends through the ureter to the kidney.)
Because the ureter is delicate and easily torn and injured when overstretched, there is risk. In a recent publication by Traxer et al. that describes 359 consecutive cases of ureteroscopy where a ureteral access sheath was used, nearly half (46.5%) of the patients had some degree of ureteral injury. Most injuries were superficial, but severe injuries were found in 13.3%. The main figure (below) from this paper often presented at society meetings, particularly by those who do not favor sheath use.
While these images raise concerns, there is no evidence to date that when managed properly with placement of a temporary ureteral stent these injuries produce any long term consequence. For example, there is no published evidence of an association between ureteral access sheath use and ureteral stricture. Most studies have shown that intraoperative complication rates during flexible ureteroscopy are no different with or without an access sheath.
Alternatively, urologists have the option of performing ureteroscopy without using a sheath at all. One advantage of this approach is that the ureteroscopes themselves are thinner than the sheaths and thus more likely to be able to be passed up the naturally thin ureter without trauma or need for dilation. This approach does have limitations though including poorer visualization, higher potential for excess intrarenal pressure, and a limited ability to make multiple passes back and forth between the kidney and bladder to remove stones.
Lasers and Baskets
Many instruments, all are extremely thin, can be passed through the small working channel that runs from the back of the ureteroscope outside of the body, to the tip of the scope in the kidney. They are used to fragment and remove stones.
Stone retrieval baskets, pictured below, (Cook Medical, Bloomington, IN) are opened and closed by an assistant to ensnare stones and remove them from the kidney. They come in various shapes, sizes and configurations. When stones are roughly 2-4 mm the basket is able to remove them intact. When stones are larger, they must be fragmented into smaller pieces in order to be removed or pass.
The most commonly utilized method to fragment stones during ureteroscopy is laser lithotripsy. Several lasers exist for the purpose but the most popular one by far is the Holmium laser. The Holmium laser can safely fragment any type of stone. The laser delivers short bursts of light energy through water that destroys the stone’s structure by creating a micro explosion of intense heat, pressure, and cavitation bubbles. Because the laser energy penetrates less than half of a millimeter deep it causes minimal damage to surrounding tissues.
(Dornier Medtech, Friedrichshafen, Germany)
Ureteroscopic Stone Treatment Techniques
As ureteroscopic technology and familiarity with this approach to treat stones continues to grow, the relative number of stones being treated in this fashion has been growing rapidly as well. The latest estimates of surgical practice from the past decade show that ureteroscopy has replaced shock wave lithotripsy as the most commonly performed treatment of kidney stones, accounting for 54% of upper urinary tract stone procedures. Along with the widespread adoption of this procedure, two main techniques have evolved into practice, the optimal one of which remains unclear.
Active extraction means to remove every piece of stone from the kidney so that at the end of the procedure there are no remaining stones and patients do not pass any stones postoperatively. Critics raise concerns that this technique requires a ureteral access sheath, an assistant to operate the basket, and potentially increases the operating time. A video description of this technique prepared for this article can be seen below.
Stone dusting is conceptually different from active extraction because the goal of the procedure is not to remove all stones directly but rather fragment them into minute pieces (dust) that can then be passed after the procedure, painlessly and without consequence.
Because his technique needs no access sheath, being performed with single pass of the ureteroscope, it benefits patients with narrow ureters. The surgeon can dust with only a laser and avoid a basket with its cost of disposable equipment and need for a skilled assistant. Here is a video of the procedure prepared by Lumenis Ltd, Israel.
Which Technique is Best?
Urologists debate whether active extraction or dusting is the better treatment approach, and both sides have their staunch advocates. As is common, the intensity of debate reflects the paucity of studies that compare the two techniques.
Interim results from an ongoing study by the Endourology Disease Group for Excellence (EDGE) comparing dusting to active extraction (table) indicate that dusting is associated with a shorter operative time (40.5 vs. 59.8 min) despite being used to treat larger stones (113 mm2 vs 81mm2).
On the other hand, a greater percentage of patients treated with a dusting had residual (leftover) stones after the procedure than those treated using active extraction (39.1% vs 11.1%).
These residual stones were not just dust; 44% of the stones were greater than 4 mm.
Despite the higher rate of residual fragments in the dusting group, only one patient in each arm suffered symptoms from a residual fragment at short term follow-up, raising the question of whether or not small residual stones are clinically significant.
But these ‘clinically insignificant residual fragments (CIRFs)’ may not be clinically insignificant.
In one recent study, the EDGE working group found that 44% of 232 patients who had ureteroscopic stone treatment and were found to have residual fragments experienced a “stone event” (defined as a composite of stone growth (at least 1 mm), stone passage, re-intervention or postoperative complications) at mean follow-up of nearly 17 months. Of these 29% required a surgical intervention.
The likelihood of stone events and reintervention were depended on the size of the residual fragment. Among patients with fragments greater than 4mm, reintervention was required 38% of the time and complications related to the fragment occurred 59% of the time. Second surgical procedures occurred sooner in patients treated with a dusting technique compared to those treated with active extraction but the statistical significance of this difference was uncertain.
Portis et al. found that among 218 patients treated with ureteroscopy and active extraction residual stones predicted need for retreatment. At a median follow-up of 4.1 years, patients with residual fragments less than 2mm in size required retreatment 2.4% of the time, compared to 19% among patients with residual fragments 2 to 4 mm in size, and 46% among patients with residual fragment greater than 4mm. Rebuck et al. found a nearly 20% likelihood of retreatment for residual fragments less than 4 mm after ureteroscopy at a mean of 1.6 years.
All in all, both sides in the debate can find reasons for their position. Active extraction leaves behind fewer fragments, but so far in a head to head trial need for less secondary surgery has not been documented. Dusting avoids the cost and complexity of the basket and the sheath, but in the long term may condemn patients to more followup surgery. Time and trial will tell, ultimately.
Follow-Up After Ureteroscopy
Oftentimes urologists will leave a temporary ureteral stent to prevent swelling of the ureter as a result of the procedure. This use of stents is a source of controversy worthy of a future post.
All agree one needs follow-up imaging of the kidneys to tell whether or not stones have been fully removed and whether or not a ureteral stricture (scar) has caused obstruction and kidney swelling (hydronephrosis), a complication that occurs 1-4% of the time.
Ultrasound is relatively inexpensive, requires no radiation, and offers very detailed images of the kidney to determine whether or not there is hydronephrosis. But it is not very accurate at identifying residual stones, particularly small ones. It is also operator dependent and results can vary based upon the experience of the sonographer performing the examination.
Ultrasounds are commonly combined with a kidney x-ray (commonly referred to as a KUB) which is also inexpensive and better at detecting stones with minimal radiation.The majority of published studies to date that have assessed stone free rate after ureterscopy have used KUB/US to determine the presence of residual fragments. Stone free rates using this imaging endpoint are commonly found to be in the mid 80-90%; even for large stones greater than 2 cm in size.
CT scans are the best test to identify stones, but are more expensive and are associated with higher degrees of radiation. Those studies that have used postoperative CT scans to identify residual stones demonstrate much lower stone free rates, commonly in the range of 50%. Interestingly the difference appears to be in the small stones <4 mm more likely to be seen on CT scan over KUB/US. If these stones are not taken into consideration, CT scan follow-up would otherwise be on par with that reported in the KUB/US series.
Although the question of whether KUB/US or CT is a better follow-up test is an important one, the more important question is whether or not routine imaging is being performed after the procedure at all. Despite recommendations by the American Urological Association that all patients should undergo at least a minimum of an ultrasound after ureteroscopy (see figure below) ,a recent study from the Cleveland Clinic found that fewer than half of all urologists routinely obtain any postoperative imaging whatsoever.
Take Away Messages
The goal of ureteroscopy should be to render the patient stone free
Regardless of the technique used, the ultimate goal of the procedure should be to leave the patient with no stones. Residual fragments are likely sources of future stone events. Therefore removing all stones should improve long term outcomes.
Urologists should be familiar with all ureteroscopic treatment techniques
Results of studies such as the EDGE group comparing dusting and extraction will determine the gold standard ureteroscopic treatment technique. In all likelihood though, we will come to realize that ureteral anatomy, width, and ability to pass an access sheath as well as the stone themselves will mandate one technique over another for any one patient.
Soft stones, for example, are more likely to fragment to true dust and thus may be more amenable to dusting. Harder stones are more likely to fragment into large pieces and thus benefit from active removal. Infection mandates complete stone removal, potentially harder to achieve with dusting. Alternatively dusting does not require an assistant and thus may be the only feasible option in instances where extra help is not available.
The ureteroscopy should not be the end of the story
Nephrolithiasis often manifests as an acute event ending in ureteroscopy; however, it is increasingly recognized as a chronic condition. As such, it is critical that surgery to remove the stone is not the end of the relationship with the patient.
Residual stones commonly lead to clinical events greater than one year from the time of the initial procedure including growth, passage, and need for retreatment.
Stone disease is chronic and recurrent, and the least desired outcome is formation of new stones after stones have been removed. As many as 50% of first time stone formers will recur within 10 years highlighting the significance and potential benefits to dietary counseling, metabolic testing, surveillance imaging, and other practices to prevent and detect stones over the long-run.
The main thrust of this entire site is toward prevention, before surgery is ever needed and with especial need after a successful surgery so that another need never be performed.