Discussion:
This case report describes a case of 2 urethral diverticula in a 26-year-old gelding. There is one other case that reports a urethral diverticulum in a breeding stallion (Diel de Amorim et al. 2014) and one report that describes a vesical diverticulum in a 6-year-old gelding (Odenkirchen et al. 1994).
The origin of the diverticula in the present case could not been determined but a congenital origin was strongly suspected. The horse was owned since the age of 2 years by the owner and no previous catheterization or trauma, except for the urine dribbling, were reported. Also, the diverticula were symmetrical, which made an acquired origin less likely.
However, in human medicine, in males, acquired diverticula are more frequently reported than congenital (Cinman et al. 2012; Popoolaet al. 2009; El Ammari et al. 2012; Mohanty et al.2014). The case described by Diel de Amorim et al . (2014) was suspected to have an acquired urethral diverticulum due to a breeding injury.
Congenital anomalies of the urinary tract in horses are rare although some are reported such as ureteral ectopia, ureteral defects, renal dys-/hypoplasia, polycystic kidney disease and rectovaginal or rectourethral fistula (Chaney 2007). If the lesion in this case was congenital, the reason why the horse did not show clinical signs before is unknown. In women however, it has also been described that a urethral diverticulum can be asymptomatic and an incidental finding (Crescenzeet al. 2015; El-Nashar et al. 2014).
The main hypothesis in order to explain the permanent dribbling of the urine is that, during voiding of the bladder, the diverticula fill up with urine and that there is a slow emptying caused by gravity afterwards. This was also a hypothesis in the case report of Henryet al . in a cat with urinary incontinence. The stagnation of the urine in the diverticula was also thought to have caused the urinary tract inflammation. Another hypothesis is that the urinary tract inflammation was responsible for the permanent dribbling of the urine but this theory seems less likely because the horse showed signs of urinary incontinence for 6 years and the signs of urinary tract infection were recent. Dysuria was probably caused by the urinary tract infection, which could have also been responsible for the reported signs of colic. Another hypothesis for the colic could have been pain, caused by urine accumulation but other none identified causes could not be excluded.
In human medicine, described techniques for diagnoses are cystourethroscopy, voiding cystourethrogram, MRI, CT and transperineal and transvaginal ultrasound.
Different reports in humans demonstrate a variable ability, 15-89%, to visualize the ostium with cystourethroscopy (Crescenze et al.2015; Romanzi et al. 2000; Kim et al. 1993; Ljungqvistet al. 2017). Diagnosis in this case was purely based on cystourethroscopy, as in the described case of a breeding stallion (Diel de Amorim et al . 2014).
Another modality often used in human medicine is ultrasound. Some studies however have shown that the sensitivity is depending on the skills of the sonographer, some studies even report that the sensitivity is lower than 50% when evaluating cases with confirmed urethral diverticulum (Crescenze et al. 2015; Reeves et al. 2014). Other studies showed a sensitivity of transvaginal and transperineal ultrasound of 95-100% (Crescenze et al. 2015; Stav et al.2008; Gugliotta et al. 2015). In the here reported case, we were however unable to identify the urethral diverticula with transrectal and transabdominal ultrasound. Visualization of the urinary tract by transperineal ultrasound in horses is not possible due to the pronounced muscle mass in this region. In one of the previous described cases in a kitten, transabdominal ultrasound did not reveal any abnormalities and also in this case, no transvaginal or transperineal ultrasound were performed (Henry et al.). In the stallion described by Diel de Amorimet al. (2014), the authors were able to identify the diverticulum with penile ultrasound, in our case the diverticula were localized too high for this technique
Voiding cystourethrogram is another described imaging modality with a reported sensitivity of 67-95% (Crescenze et al. 2015; Ockrimet al. 2009). This technique and retrograde contrast cystourethrogram were used successfully in small animals (Henry et al. 2018; Foster et al. 1999; Atilla 2018; Watanabe et al. 2015). In this case, the use of a contrast cystourethrogram could have helped to determine the extent of the diverticula. However, the use of abdominal radiographs in adult horses is limited to the ventral part of the abdomen to identify sand impaction and enteroliths.
The “gold standard” for diagnosis of urethral diverticula in humans is MRI (Pathi et al. 2013; Dwarkasing et al. 2011; Blanderet al. 2001). In none of the described cases in small animals, this technique was used and neither in this case. In one case of a dog, CT was successfully used to identify the diverticulum.
In this case, due to financial restraint and the age of the horse, a medical treatment with large spectrum antibiotics and anti-inflammatory drugs was attempted with resolution of the clinical signs according to the owner.
Treatment in human medicine consists mainly of surgical excision and reconstruction in symptomatic patients with high success rate of 84% to 98%. Re-operation rate has been reported to be 2% to 13% (Crescenzeet al. 2015; Reeves et al. 2014; Nickles et al.2014; Stav et al. 2008).
In asymptomatic patients, conservative treatment with prophylactic antibiotics can be attempted (Greimann et al. 2019). Surgical treatment was successfully performed in the 4 described cases in small animals (Henry et al. 2018; Foster et al. 1999; Atilla 2018; Watanabe et al. 2015Diverticulectomy has also been described to be successful in goats and calves (Temi̇zsoylu 2005; Bokhari 2013; Sylla et al. 2019; Abd-El-Hady 2014; Maiti et al. 2018). Surgical treatment by perineal urethrostomy and distal urethrostomy over the diverticulum was performed in the stallion described by Diel de Amorim et al. (2014).
To conclude, urethral diverticula are a rare condition in equids and should be considered in cases of colic with urinary incontinence and urinary tract infection. We were unable to identify the origin in this case but a congenital defect was strongly suspected.