Aetiological causes of EVF are shown in Table 1. Diverticulitis is indisputably the most common cause, accounting for up to 80% of cases, the vast majority of which are colovesical in origin and involving the sigmoid colon[3-8]. Although the cause of CVF is predominantly diverticular, it has a relative risk of only 2%-4% in those with diverticular disease[6,7,9]. CVF are postulated to form via the direct extension of an inflamed, ruptured diverticulum, or erosion through the bladder wall by a pericolic abscess/inflammatory process[3,7,10].

Crohn’s disease (CD) is the most common cause of small bowel EVF, accounting for up to 10% of all EVF cases. It is the most frequent cause of ileovesical fistulae; however, it is rare, with a reported incidence of just 2%-6% in CD[11,12]. They are caused by transmural fissures extending from the diseased bowel segment to the normal bladder[12], and may be preceded by the patient suffering from subacute small bowel obstructive episodes secondary to an associated Crohn’s stricture.

Other inflammatory causes include appendicitis[13], Meckel’s diverticulum[14], genitourinary coccidioidomycosis[15], tuberculosis[16] and syphilis.

Malignancy is the second most common cause of EVF and accounts for 10%-20% of cases[2-8]. Colorectal carcinoma, particularly sigmoid tumours, are the most commonly associated neoplasms, and occur due to direct invasion of the tumour into the wall of the urinary bladder[17]. Fistulae caused by transitional cell bladder cancer are much rarer; however, certain studies have reported an incidence of 3%-8%[2,6,7]. Transitional cell carcinoma is a far more common cause than squamous cell carcinoma of the bladder[18], due to its greater prevalence. There have also been case reports of lymphoma associated fistulae[19,20].

EVF are very rarely induced by surgical procedures; with radical prostatectomy accounting for a significant proportion of rectourethral fistulae. Palliative endoscopic stenting of malignant obstruction caused by colorectal tumours is also a well-documented cause[21,22]. Other surgical procedures known to be implicated in EVF formation include complex rectal resections, and cases have been reported of fistulae occurring secondary to laparoscopic inguinal hernia repair, albeit rarely[23,24].

EVF development post pelvic radiation are rare but well described, and are discussed later in this article. They are most often seen years after treatment, with many occurring without primary tumour recurrence[25-28].

Other rare but documented causes of EVF include spilled gallstones during cholecystectomy[29,30], penetrating abdominal trauma from gunshot wounds[31], and the presence of foreign bodies in pelvic organs such as chicken bones[32].

Although the aetiology of EVF is almost exclusively intestinal in origin, symptoms are generally urological in nature (Table 2[2-4,6-8]). This is accounted for by the mechanical properties of both the bowel and the bladder. The bladder is a highly compliant organ, with a relatively low intraluminal pressure when compared to the colon, which favours the flow of contents from the bowel to the bladder[10,33].

Terminal pneumaturia (64%-95%) and faecaluria (36%-82.5%)[3,4,7,8] are the most common symptoms, with urine flow from the rectum found in 15% of patients. Pneumaturia, generally occurring at the end of micturition due to the gravitational effect on gas in the bladder, is a non-specific symptom and as such, other causes must be investigated and excluded. Infections with gas-forming bacteria such as Pseudomonas, rare types of Escherichia coli[34] and yeast infections in diabetic patients with persistent glucosuria[35] must be excluded, along with recent bladder instrumentation and, although exceptionally rare, emphysematous cystitis[4]. The vast majority of patients (up to 90%)[6] will describe these pathognomonic features, as well as recurrent Urinary tract infections (45%-87.5%)[3,4,6-8] or episodes of urosepsis, with infections being almost exclusively caused by gut commensals such as Escherichia coli, enterococci and other coliform bacteria[4,6].

Delayed diagnosis can occur due to non-specific symptoms at presentation such as; urinary frequency, urinary urgency, dysuria, altered bowel habit and non-specific abdominal pain[6].

The diagnosis of an EVF can be challenging, and thus a high index of suspicion is required in non-specific presentations. Investigations aim to confirm its presence firstly, and then to establish the underlying aetiology as well as anatomical location.

Blood investigations are of little value in the diagnostic algorithm of an EVF as they tend to be within normal limits.

Urinalysis and urine culture can be useful in the diagnostic process, with a positivity rate of > 90% being reported, with both coliform and anaerobic bacteria identified[6]. Urine centrifuge has also been used to aid in the diagnosis, revealing the urinary presence of faecal and vegetable material in one study[4].

The utilisation of dyes and colouring agents has been described in the attempted diagnosis of EVF, with the aim being to identify these in the urine following rectal instillation, which would be considered diagnostic. The instillation of methylene blue per rectum has been trialled, with the presence of blue coloured urine being considered diagnostic[6]. Doubt has been cast over the diagnostic accuracy of this method; however, as methylene blue is absorbed by gastrointestinal mucosa and excreted by the kidneys leading to inaccurate results. The use of orally and rectally administered indocyanine green has also been evaluated and has been shown to be very specific in early studies[36]. Most recently, a study assessing indocyanine green as an alternative investigation tool in the assessment of EVF, has demonstrated a sensitivity of 92%[37].

The charcoal test is a bedside test that involves the ingestion of activated charcoal. It has shown to have a sensitivity of 100% in confirming the presence of a fistula if the presence of charcoluria is present within 24 h[38]. The poppy seed test has also been well described and involves the consumption of 50 grams of poppy seeds, followed by evaluation of urine for their presence 48 h later. Studies have shown it to have a sensitivity of 95%-100% for the presence of an EVF[39]. Both diagnostic investigations are cheap and accurate but do not delineate the aetiology or anatomy of the fistula any further. In addition, modern imaging modalities are extremely accurate, readily available and can be performed in an outpatient setting.

The aim of imaging in the assessment of EVF is to determine and visualise the anatomical location of the fistulous tract. Historically, the use of plain abdominal x-ray, barium enema and intravenous urography have all been used to establish the anatomical location of fistulae or assess for secondary signs suggestive of a fistula. However, with advances in both computed tomography (CT) and magnetic resonance imaging (MRI), as well as improved accessibility of both modalities, conventional radiology is no longer advocated.

CT is considered the imaging modality of choice for diagnosing EVF, having a vastly superior sensitivity when compared to conventional radiology. It is widely available, non-invasive and can be performed and interpreted quickly and accurately. With a reported diagnostic accuracy of 61%-100%[2,33,40] it is recommended by the American College of Radiology as the first-line imaging modality in cases of suspected EVF. CT findings which suggest the presence of an EVF include air in the bladder without a history of recent bladder instrumentation, perivesical colonic thickening adjacent to a locally thickened bladder suggesting adherence, and the presence of oral contrast medium in the bladder on a scan without the use of intravenous contrast medium[7,41]. Unfortunately, these are indirect signs, and CT still remains suboptimal in the delineation of the fistulous tract, with the pathognomonic sign of intravesical air contributing massively to the diagnostic accuracy of the test[17]. CT has been reported to delineate the fistula directly in only 64% of cases[42]. In order to opacify the fistulous tract on CT, a hydrosoluble enteral contrast agent (Gastrografin) must be given, which can become more dilute and therefore less opaque as it transitions through the bowel. This can lead to false-negative examinations due to this dilutional effect, and in cases where the tract has been occluded due to inflammatory oedema. This is essential in determining the presence of a fistula, as the presence of contrast in the bladder must come directly from its passage through the fistula. Intravenous contrast can render the scan obsolete as it is excreted via the kidneys and thus will be present in the bladder, obscuring results.

As the pathological processes that result in EVF are generally extraluminal (peridiverticular inflammation, pericolic abscess development), CT is advantageous in that it assesses for the presence of extraluminal pathology[4,7,41], and also allows diagnosis and staging if a malignant process is suspected, by evaluating all visceral abdominal organs. Visualisation of extraluminal pathology also aids in surgical decision making pre-operatively as the presence of extensive metastatic disease or a pelvic abscess will alter the surgical plan[3]. Three-dimensional CT imaging, when compared to traditional axial CT imaging provides a more detailed assessment of the anatomical relationship between the bladder and bowel in EVF, which can further aid in diagnosis, provide more detailed evaluation of anatomy and ultimately improve surgical planning[43,44]. Most modern CT scanning software contains algorithms to reconstruct raw data into three-dimensional images without additional cost[17]. Thus, CT scanning with the aforementioned symptoms is often sufficient to establish the diagnosis.

MRI can also be used in the assessment of EVF, and its use is well established. MRI has superior soft-tissue resolution when compared with CT and can also provide multiplanar imaging sequences[45]. MRI is advantageous as the fluid within the fistulous tract acts as a natural contrast agent. Multiple imaging sequences are generally performed as visualisation of the tract depends on its content, i.e., fluid vs air[17]. T2-weighted imaging shows high signal fluid within the fistulous tract, as well as extraintestinal fluid collections and localised inflammation within the muscle wall of the bladder[46,47]. T1-weighted imaging can characterise the fistulous tract with regard to adjacent hollow viscera and also demonstrate inflammatory changes in surrounding fat planes[17,48].

MRI has a sensitivity and specificity of up to 100% in detecting EVF and has similar accuracy in identifying the underlying aetiology[47,48].

Direct visualisation of a fistulous tract at endoscopy can be difficult; however, an endoscopic examination allows for the determination of the aetiology of the fistula.

Cystoscopy can suggest the presence of a fistula by evaluating for secondary changes within the bladder. It is beneficial in excluding urological malignancy as the causative factor of the fistula[3]. A localised area of erythema, oedema and congestion within the bladder mucosa are suggestive of the presence of a fistula. Other reported findings include cystitis glandularis, localised polypoid lesions, ulceration and the presence of faeces[4]. The sensitivity of cystoscopy in directly visualising a fistulous tract is reported at 46%-60%[3,49], with some studies advocating cystoscopy as the first-line investigation in suspected EVF[4].

Colonoscopy is poor at visualising the fistula; however, it is essential in diagnosing the causative bowel pathology, and thus an integral part of the diagnostic algorithm. It has been reported that colonoscopy can have a sensitivity of over 50% in directly visualising the fistula tract[2]. Colonoscopy is recommended as the first-line investigation if malignancy is the suspected cause of the fistula following CT evaluation[7]. In addition to excluding a neoplasm, colonoscopy will allow assessment of the extent of the diverticular process, the area affected by hypertrophy, determine if there is an associated stricture and it will help to determine the extent of colonic resection required if surgery is deemed suitable.

Table 3[2,3,49,6,33,36,38-40,47,48] summarises the sensitivities of the various diagnostic test used in the diagnosis of an EVF.

Conservative management is generally reserved for those deemed unfit for surgery, those with minimal symptoms and those who do not want surgical intervention despite associated symptoms. Some authors advocate for a conservative approach in confirmed cases of benign fistulae, and that surgery in this cohort should be carried out based on a quality of life basis[6]. In a 50 patient series, there was found to be no significant difference in disease-specific mortality between the group who underwent surgical intervention and the group managed conservatively. It was also found that amongst their cohort, untreated CVF were present for a cumulative total of 3254 wk, with patients symptomatic of urinary tract symptoms but none developing overt urosepsis. Furthermore, in this study, 32 patients had an untreated CVF for a six-month period without significant evidence of a decline in renal function during that period[6].

Conservative management is generally associated with high morbidity and mortality, particularly in younger patients with a longer life expectancy, and thus potential to be affected by related symptoms. In a review series of 90 cases of EVF, 18 patients were managed conservatively, with one-year follow-up revealing seven deaths, 2 of which were attributed to urosepsis[3]. Morbidity in the literature ranges from 4%-45%, with mortality ranging from 0%-60%[2,3,50] in certain series. Patients treated conservatively tend towards higher mortality rates, which may have an associated bias as this patient cohort are often in poor physical condition and have associated co-morbidities. There is also the risk of an undiagnosed malignancy carrying the potential for progression. Patients electing to follow a conservative pathway should be warned about the potential for urosepsis or development of significant intra-abdominal sepsis[2].

In another study, 8 out of 12 patients managed conservatively either due to patient preference or medical co-morbidities died from sepsis during the subsequent follow-up period[51]. Thus, the option of conservative management depends on a myriad of factors including patient co-morbidities, patient preferences and associated symptoms. It is also important to avoid the patient re-presenting in the acute setting which may then necessitate a Hartmann’s procedure.

Due to the high morbidity and mortality associated with EVF, the presence of this diagnosis should generally be considered an indication for surgery in all patients deemed fit for it and whom understand the associated surgical risks. The reported rate of spontaneous closure of these fistulae is about 2%[4,51]. This, coupled with the risk of sepsis and ongoing quality of life issues, favours surgical management where possible. Many patients are initially abhorrent to the concept of surgery with the associated risk of stoma formation, but ensuing symptoms and poor quality of life often results in a change in perspective.

The surgery performed generally depends on the underlying pathological process, the anatomical location of the lesion and the patient’s premorbid condition. Treatment can involve a single-stage or multi-stage approaches.

Initial surgical approaches to benign EVF advocated for the formation of a proximal defunctioning colostomy as the only intervention, or prior to definitive pelvic dissection[52,53]. This afforded patients symptomatic relief and minimised surgical trauma, but did little to treat the causative problem, as the rate of spontaneous fistula closure is reported at only 2%[4,51]. These patients are also still prone to subsequent attacks of urosepsis. As surgical practice has evolved, it is now standard practice to resect the involved segment of bowel and associated fistula in order to prevent a recurrence, and subsequently re-anastomose both ends to achieve best results. Current surgical options include one-stage, two-stage and three-stage procedures, depending on patient age, comorbid state, disease factors, and whether the patient presents as an emergency or electively[54].

A single-stage procedure is advocated for the majority of cases of EVF[3,5,40,50,55]. Evidence shows that resection of the involved bowel segment and primary anastomosis without a protective ostomy is possible in most instances. A review carried out by Woods et al[50] from 1960-1986 looking at 92 diverticular fistulae found that resection and primary anastomosis was possible in 62%, resection with primary anastomosis and covering ostomy in 24%, Hartmann’s procedure in 7% and a three-staged procedure in 7%. There was a trend towards more single-stage procedures in the latter stages of the study, and they found no significant difference in complication rates between single and multi-stage procedures. They also found a prolonged overall hospital stay in patients undergoing staged procedures[50]. Another review series of 90 patients with EVF established that resection and primary anastomosis was achieved in 92% of patients undergoing left-sided procedures. The use of a covering loop ileostomy was required just once. In the same series, the anastomotic leak rate was reported at 2.8% with no mortalities[3]. These figures are consistent with those from other studies which have found no significant difference between single-stage and multi-stage procedures with regard to complications[49,56], and report a primary anastomosis in 82% to 95% of patients[57]. Length of hospital stay has also been shown to be less in patients undergoing single-stage procedures[55]. Table 4[3,5,40,50,55]summarises the adoption of a single-stage approach over time, with associated anastomotic leak rate.

Staged procedures are recommended in the setting of gross faecal contamination, large pelvic abscesses, advanced malignancy, previous surgical procedures, intra-operative complications such as ureteric injury, post-radiation changes or in patients who would not survive an anastomotic leakage due to co-morbidities[5,55].

A two-stage procedure involves resection of the involved segment and primary anastomosis with an upstream, protective/diverting loop ileostomy. A two-stage procedure is recommended if there is a question about the integrity of the anastomosis[2]. This is endorsed instead of a Hartmann’s procedure as it is more readily and easily reversed. In most institutions, the reversal of a loop ileostomy is done through the ileostomy site once anastomotic healing is confirmed via a Gastrografin enema. In addition, some centres perform early closure of the ileostomy. Certain surgeons favour a defunctioning colostomy, but in the authors experience we wish to avoid any potential problems with the marginal blood supply at time of reversal. In contrast, in order to restore intestinal continuity post-Hartmann’s procedure, a further major surgical intervention is required and in a percentage of cases, a laparotomy may be necessary[9].

A three-stage strategy is generally not recommended unless the risks of surgery are very high, the patient has extensive co-morbidities, associated faecal incontinence or the patient presents emergently with associated sepsis[55]. In addition if the patient presents emergently under the care of a general surgeon with limited colorectal experience, a Hartmann’s type procedure would be reasonable and safe. Subsequent restoration of intestinal continuity could be performed under the direction of a colorectal surgeon. Nowadays, significant advances in post-operative intensive care are an important adjunct to patient care[50,58]. Three-stage procedures involve an initial defunctioning surgery, followed by resection and eventual restoration of intestinal continuity. Defunctioning of the fistula containing bowel segment; however, rarely leads to closure of the fistula, and there is a high chance of fistula recurrence should it lead to closure initially[4,51]. These patients are vulnerable to episodes of urosepsis as the connection between bowel and bladder remains patent[3]. Nevertheless, it allows stabilization of the process, thus allowing definitive surgical resection of the fistulized segment at a later stage under more optimal conditions.

Management of the bladder is not yet standardised, with variations in intra-operative evaluation of the bladder opening, its repair and post-operative care. Multiple methods have been trialled and published including suprapubic cystostomy, excision of the fistula and primary closure, simple closure alone, omental interposition and bladder drainage with a urinary catheter[50].

There is considerable variation in the management of the fistulous tract because it is often not visible or is associated with concurrent abscess. Depending on the underlying aetiology of the fistula; urinary drainage, simple closure or partial/total cystectomy can be performed[59]. Visualisation of the fistulous tract can be performed by distending the tract with methylene blue[60,61]; however, it is argued that if the tract is not visualised directly then repair is not necessary[62].

A study by Walker et al[5] following 19 patients found that a standardised protocol of catheter removal 10 d post-operatively revealed no urine leaks in patients who were managed with urinary decompression alone, and one urine leak in a patient who underwent a partial cystectomy for locally invasive colon cancer. At five year follow-up, there were no fistula recurrences; however, they did find that a significant proportion of their cohort died from non-related causes in the follow-up period[5].

More recently, Ferguson et al[62] carried out a retrospective review of 74 patients with benign fistulae who underwent a procedure for EVF, focussing on the management of the bladder specifically. They found that as most cases were attributable to diverticulitis or CD, resection of the bowel was adequate without the need for concurrent bladder repair or resection. If no bladder defect was visible or palpable by finger, then no repair was necessary. They proposed that an intensive search for bladder defects was not necessary and that bladder decompression with a Foley catheter for 7 d, without the need for cystogram prior to removal, was adequate for bladder healing. Large, full-thickness bladder defects were closed in two layers with Vicryl sutures. All defects healed within one week[62].

Partial cystectomy is necessary in cases of malignancy with an adequate healthy resection margin to lessen the risk of local recurrence[5,63]. If there is intra-operative concern about a potential positive margin vs inflammatory changes, a frozen section can be sent. If a malignant colovesical fistula involves the trigone of the bladder, then a complete cystectomy may be required with ileal conduit formation, or the creation of a neobladder. However, pre-operative staging generally will identify the type and extent of resection required. In those patients requiring a partial cystectomy, the bladder is often closed in two layers, as the defect can be directly visualised[62,63]. An omental flap, very much favoured by the authors, can be used for interposition between the bowel and bladder in cases of large bladder defects to prevent a recurrence, and provide an autologous buffer between the bowel anastomosis and bladder repair; however, there is minimal convincing evidence for its use[64,65]. Most large centres dealing with these complex cases will have Urology services available, who should be involved in the case or be available in scenarios where the fistula is more complicated, such as involving the bladder trigone, with the potential for damage or ligation of the ureteric orifices.

Guidance on post-operative Foley catheter removal is scarce, with no standard approach adopted in the literature[66]. Prolonged Foley catheterisation can lead to complications such as urinary tract infections, urinary retention and bladder atony[67], and thus an expedited catheter removal pathway would be favourable if definitively proven safe to do so. In a review performed by de Moya et al[66] looking at early (< 7 d) and late (> 7 d) Foley catheter removal in 45 patients, they concluded that in patients with CVF secondary to diverticulitis, the catheter can be safely removed early without any increased risk of complications[66]. Patients with their catheter in situ for > 7 d were more likely to encounter catheter-related complications and thus an increased length of hospital stay, however, this was not statistically significant.

A recent study by Dolejs et al[68] also concluded that catheter removal should occur by day seven at the latest, with an argument being made for removal on post-operative day five as they found that all patients with bladder leaks in their study were identified by day seven, and suspicion was raised by day five with the presence of cloudy, dark urine drainage or high output abdominal drain drainage[68]. de Moya et al[66] also suggested that in cases of simple bladder repair or bladder drainage only post fistula surgery, there is no necessity for routine cystogram; however, they still advocate for cystogram post complex (repair of trigone) bladder repair. There is limited supporting literature for the use of routine cystogram outside of trauma[69].

Dolejs et al[68], looking at the role of using an intra-operative bladder leak test in patients with diverticular CVF concluded that all patients undergoing repair should have an intra-operative leak test as, for all of those who had a negative leak test, none went on to develop a urine leak post operatively, and thus theoretically, none would require the need for a closed suction drain or cystogram post-operatively. They found that a positive leak test led to a change in patient management as it allowed for the repair of the bladder defect intra-operatively and thus spared patients the morbidity of prolonged Foley catheter drainage and closed suction drainage. They also determined that there may be benefit for the use of closed suction drainage in patients deemed high risk for urine leak post-operatively, as it allows for the assessment of the drain creatinine to serum creatinine ratio, which has a positive predictive value of 100% and negative predictive value of 93% in identifying bladder leaks. This may also reduce the requirement of post-operative cystograms in these patients[68]. Table 5[5,62,66,68] summarises the current evidence base for bladder management in the resection of EVF.

Conservative management encompasses all of the management strategies that do not involve direct surgical intervention of the fistula and generally involves urinary diversion +/- faecal diversion. Conservative management is indicated if there is a fistula < 2 cm, no previous radiation, no malignancy and no concomitant sepsis[99].

Transurethral or suprapubic catheterisation is the first step and should be used very selectively in patients with small, surgically induced fistulae[110]. Should the fistula not resolve after 3 mo then faecal diversion should be considered. In a systematic review of various surgical techniques for acquired rectourethral fistulae, the indications for faecal diversion varied considerably from series to series and included; ongoing infection, previous radiation/ablation, previous failed repair, trial of spontaneous resolution, large complex fistulae > 2 cm, persistent symptoms affecting quality of life and patients with multiple comorbidities[99].

In a study carried out in the Mayo Clinic, colostomy formation was performed in all patients with RUF as an initial management step; however, all patients subsequently required definitive fistula repair due to lack of spontaneous closure[111]. In another series containing 2447 patients who had undergone radical prostatectomy by Thomas et al[112], 13 developed RUF, 9 of which were managed with diverting colostomy and transurethral catheterisation, with 33% having spontaneous closure of their fistula[112].

In patients who undergo diversion therapy prior to definitive surgical management, the stoma is kept in place after surgery and healing confirmed with a Gastrografin enema or proctoscopy prior to reversal[107,113,114]. Temporary urinary diversion is generally removed following confirmation of closure with retrograde cystourethrogram, retrograde urethrography or cystoscopy[99]. Due to the lack of reported success with conservative management, most patients will require definitive surgical management[107]. Should the fistula fail to heal with faecal diversion, definitive surgical intervention should be planned[112]. This allows the adjacent tissues adequate time to heal prior to definitive repair.

Numerous different surgical approaches have been described for the repair of RUF, indicating the general lack of consensus around the best surgical approach.

Because of the rarity of the condition, there is limited surgical expertise on its management and approaches vary significantly, with an optimal approach still unclear. Approaches described include transperineal, transanal, transsphincteric, transabdominal, transvesical and posterior pararectal, as well as combinations of the above[105]. Today, most are carried out by anterior transperineal approach as it provides the best exposure and allows for muscle flap interposition between the rectum and urethra. These complex procedures are often performed as a collaboration between a urologist and colorectal surgeon, and may require a plastic surgeon with experience in flaps[107,113].