People with Spinal cord injury (SCI) have a neurogenic bladder in about 70-84% of cases. The goals of treatment of patients with a neurogenic bladder include preservation of upper tract function, absence of infection, and maintenance of a low-pressure bladder that is both continent and capable of emptying well. When these goals are not met, complications occur and unfortunately when complications appear do so in a cascade whereby one complication leads to another and then to another

Urethral injuries are one of these complications that appear in a cascade because of other complications. One of these urethral injuries is the urethral fistula (UF), which can be defined as the abnormal communication of the urethra with other structures like the rectum, the skin, the bladder, and the male or female genital tract. 

This pathology although rare is serious due to the challenges of its treatment. Therefore, it is worth preventing it by acting on its risk factors. Urethral injuries in people with spinal cord injury result from other complications such as incontinence or urinary retention that require the use of urethral catheters especially indwelling catheters.

Another suspected risk factor is the presence of perineal pressure ulcers since the association of both lesions is not uncommon. Finally, given the presence of neurogenic bladder in these patients it would be very interesting to assess the type of bladder dysfunction associated with this pathology. Consequently, the objective of this study is to evaluate the clinical and urodynamic risk factors for urethral fistulae in patients with spinal cord injury.

A case-control study was carried out in a sample of 15 patients with UF (cases)
and 45 controls. The cases were adult patients (18 years or older) with SCI who had been diagnosed with urethral fistula. The diagnosis of urethral fistula was made by clinical examination followed by an imaging technique: urethrography and fistulography or computed tomography when necessary. The control group consisted of 45 adult patients from the same population as the cases without UF.

The urodynamic study consisted of a pressure flow study. The polygraph used was a Uro 2000 (MMS, Enschede, The Netherlands). The study was made according to the specifications of the International Continence Society (ICS) and the protocols of Good Urodynamic Practices (GUP) The diagnostic of BOO was made when the URA value was equal to or greater than 29 cm H2O.

The sample size was calculated according to the data obtained from the study by Singh et al. [1] With a probability of having urethral stricture of SCI patients with indwelling catheter of 0.25 and a probability of patients without indwelling catheter of 0.08, three control patients for each case, a type I error probability of 0.05 and a statistical power of 0.8, the total number of patients required was 60

The study consisted of a review of the clinical history and the results of the most recent urodynamic study. The urodynamic studies were performed according to the specifications to International Continence Society (ICS) with a Solar polygraph (MMS, Enschede, The Netherlands).

The clinical variables that sowed statistically significant differences between both groups were the traumatic cause of SCI (less frequent in patients with UF), the performance of CIC (less frequent in patients with UF), the presence of an indwelling catheter (more frequent in patient with UF), the presence of urinary incontinence (more frequent in patient with UF), and the history of pressure ulcers (more frequent in patients with UF) (Table1). 
The urodynamic variables are shown in Table 2. The variables that showed significant differences were the presence of stress urinary incontinence (more frequent in patients with UF) and the urethral resistance factor (URA) (lower in patients with UF).
The regression analysis showed that there are two independent clinical variables.  The performance of CIC, which is a protective factor (OR less than 1), and the presence of urinary incontinence that increases more than 22 times the risk of having UF (OR = 22.178)).

This study found several clinical risk factors that favour the appearance of UF. Some of them such as the presence of indwelling catheters and the absence of CIC have been already proposed. Others such as the urinary incontinence and a history of pressure ulcers have also been implicitly suspected, and finally the non-traumatic cause of SCI has not been reported to date. On the other hand, in this work it is proposed the urodynamic risk factors associated with UF for the first time.

In multivariate analysis we found that there were only two independent risk factors: the absence of CIC and mainly the presence of urinary incontinence. CIC has been proposed to be the best neurogenic bladder management method to avoid urinary complications in people with SCI. Urinary incontinence was shown to be the most important risk factor for UF. This complication was present in our series in 90% of patients with UF. Urinary incontinence was also related to urethral fistulae associated with pressure ulcers.

Unlike other pathologies of the urethra where infection and inflammation play a fundamental role, according to our results it is urinary incontinence that plays a paramount role in UF. We can hypothesize that the absence of CIC and especially the presence of indwelling catheters can lead to a decubitus as proposed by Raup et al [2]. This decubitus associated with the deleterious effect of urine leakage can injury the urethral and skin tissues causing UF. Consequently, to avoid the appearance of UF in people with SCI who cannot manage their urinary incontinence with CIC, the resolution of this incontinence would be very necessary, even using a suprapubic catheter when necessary.
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The urodynamic risk factors found in this study can be explained because of an injury to the sphincter mechanism. Indeed, most UFs are in the membranous urethra where the distal sphincter mechanism responsible for maximum urethral pressure is found. Its injury explains the high incidence of stress urinary incontinence among these patients. This also explain the lower urethral resistance in patients with UF because the urethral resistance is concentrated in a small segment located in the membranous urethra that acts as a flow controlling zone [3].

In conclusion we can state that urinary incontinence and the absence of adequate bladder drainage by CIC in patients with SCI are serious risk factors for UF and that lower urinary tract dysfunctions associated with this pathology are stress urinary incontinence and low urethral resistance

Singh R, Rohilla RK, Sangwan K, Siwach R, Magu NK, Sangwan SS. Bladder management methods and urological complications in spinal cord injury patients. Indian J Orthop. 2011 ;45(2):141-7
Raup VT, Eswara JR, Weese JR, Potretzke AM, Brandes SB. Urinary-cutaneous Fistulae in Patients with Neurogenic Bladder. Urology. 2015 ;86(6):1222-6.
Schaefer W. principles and clinical application of advanced urodynamic análisis of voiding function Urol Clin of Noth America. 1990; 17(3):553-556.