Introduction

An inguinal hernia is defined as a protrusion of an organ or tissues through the inguinal canal.¹ Inguinal hernias in dogs are classified as congenital or acquired^1,2 and traumatic or nontraumatic.³ Acquired, nontraumatic hernias predominate, especially in sexually intact bitches,^3,4 whereas congenital hernias are rare and tend to occur in male dogs.³

The omentum and fat are the most common contents of inguinal hernias, and unless other organs prolapse, most cases are asymptomatic for a long period without treatment.³ Herniation of the hollow organs is less common, and prompt surgical repair is recommended to avoid organ strangulation.^1,2 Herniated organs can include the uterus, intestine, and bladder,^1,2 but the incidence of each organ herniation has not been demonstrated.

The negative effects or consequences of inguinal hernias also depend on the dog’s sex; severe scrotal edema may result from incarcerated omentum,^5,6 and herniation with a nonviable small intestine is more common in young male dogs,³ whereas uterine herniation in bitches may be asymptomatic and possibly prevent intestinal herniation.³ However, there are sporadic reports of surgically repaired uterine hernias caused by pregnancy or uterine diseases.^7–10

Anatomical, hormonal, and metabolic factors contribute to the development of inguinal hernias; the diameter of the inguinal canal in bitches is relatively large, estrogen weakens the inguinal ring, and metabolic abnormalities may weaken the abdominal wall.² Additionally, there may be other factors because inguinal hernias reportedly occur more often on the left than the right side,³ but the reason for this difference has not been described.

Knowledge of the epidemiological, etiological, and clinical features of inguinal hernias, as well as postsurgical outcomes, are important considerations for surgical decisions. However, to our knowledge, only one published case series has evaluated long-term outcomes.³ The aim of the current retrospective study was to evaluate the clinical characteristics and outcomes of nontraumatic inguinal hernias (NTIHs) in dogs.

Materials and Methods

Results

Signalments, clinical findings, surgical procedures, and outcomes in the HOH (n = 22) and non-HOH (n = 20) groups are summarized in Table 1. Because one male dog developed HOH 2 yr after the first hernia repair for omental herniation alone, his status (age, weight), surgical procedures, and complications at both surgeries were evaluated in both groups.

TABLE 1 Summary of the Clinical Findings, Surgical Procedures, and Outcomes in 41 Dogs Classified as the Non-HOH and HOH Groups

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All 41 dogs were small-breed dogs weighing less than 10 kg (median, 5.0 kg), with miniature dachshunds accounting for 63% (26/41) of all the dogs. There were significantly more bitches in both the HOH (P < .05) and non-HOH groups (P < .01). Four spayed bitches (age at surgery: 10–15 yr) had undergone OVH at 8–13 yr of age. The median age at surgery was 9.0 yr (range: 0.4–13 yr), and 33 dogs (80%) were >5 yr old (Figure 1). The dogs in the non-HOH group were significantly younger (P < .05) than the dogs in the HOH group. All eight dogs younger than 6 yr of age were sexually intact bitches, only one (5 yr old) of whom was in the HOH group.

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The affected sides were the left in 30 (73%) dogs, right in 9 (22%) dogs, and bilateral in 2 dogs, with significantly (P < .01) more cases on the left than on the right, for all dogs, and in the HOH group. Regarding the onset timing, 12 dogs were classified as acute because clinical signs associated with hernia (n = 7), hernia detection (n = 3), or rapid hernia expansion (n = 2) occurred <7 days before surgery. The remaining 29 dogs were regarded as chronic because they were asymptomatic with longstanding hernias (n = 12: 1–24 mo of duration) or unknown disease duration (n = 17).

Nine dogs underwent herniorrhaphy alone; the other 32 dogs underwent herniorrhaphy and other concurrent procedures. OVH was performed in 27 of 30 intact bitches simultaneously.

HOH Group

Of the 22 dogs in this group, 11 were classified as acute disease onset, 7 of whom showed clinical signs associated with hernia: vomiting (n = 4), anorexia (n = 3), and pain (n = 3). The affected side, herniated organs, and onsets of clinical signs separated by sex are shown in Table 2. The rate of left-sided hernias was significantly (P < .01) higher in bitches than in male dogs.

TABLE 2 Number of Dogs in the HOH Group Separated by Sex and Clinical Findings

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Uterine herniation was observed in 15 bitches aged 5–13 yr old, with pregnancy (n = 2), pyometra (n = 2), and endometrial hyperplasia (n = 1). Intestinal herniation was observed in five bitches and five male dogs, and three of the bitches also had uterine herniation. Resection and anastomosis of the small intestine was required in two bitches; one had a 3 day history of loss of appetite, and the other had a 2 day history of vomiting (concurrent herniated uterus case). Protruded organs were identified by ultrasonography in all dogs, but one case of colonic herniation on the left side required radiography for confirmation. One male miniature dachshund had a recurrent inguinal hernia that had been operated with castration at another hospital 2 yr earlier, and this dog also had bilateral perineal hernias. Because a fistula in the recurrent area suggested suture-reactive inflammation, the wound was repaired with short-term absorbable suture (3-0 polyglactin 910), and no recurrence was observed for 570 days.

Of the 22 dogs in the HOH group, 21 were followed for a median of 887 days (range: 297–3165), during which time no recurrence was observed. As a short-term complication, partial wound dehiscence was observed in one dog.

Factors Associated with the Affected Side

A significantly higher rate of left-sided involvement was detected in the uterine-herniated cases (P = .038) and in intact bitches (P = .0184) compared with other cases in each group, but no significance was identified between cases classified by onset or age (Table 3). Differences in the rates of left-sided herniation were not significant between the HOH and non-HOH groups (P = .1551) or between bitches and male dogs (P = .058).

TABLE 3 Forty-One Dogs Separated by the Affected Side and Clinical Findings

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Discussion

All dogs in this study were small breeds, and 80% were >5 yr of age. We encountered no traumatic inguinal hernias during the study period. The case composition suggested that acquired NTIH is more likely to occur in small breeds. In the present study, 63% of all dogs were miniature dachshunds, which is obviously higher than the percentage of this breed among the total dog population (16%, 455/2781 dogs) admitted to our hospital during the study period, suggesting the higher risk of this breed.

The predominance of female dogs was consistent with previous reports.^3,4 Anatomically, the inguinal canal in bitches is larger than that in male dogs.² Additionally, estrogen-induced weakening of the inguinal ring may contribute to inguinal herniation, which is supported by the higher percentage of intact than spayed bitches in this study (88%, 30/34) and in a previous report (68%, 13/19).³ Metabolic abnormalities such as obesity, hyperadrenocorticism, or diabetes mellitus are also presumed to contribute to NTIHs in both sexes,¹¹ but these were not noted in our study.

The most frequently protruded hollow organ was the uterus (68%), followed by the small intestine (41%) and the colon (5%), suggesting that the uterus tends to protrude before the intestine. Waters et al.³ suggested that the herniated uterus prevented further protrusion of the intestine because no simultaneous intestinal protrusion was observed in 12 herniated uterus cases (8 cases were reported in other studies^12–19). However, in the present study, three dogs had simultaneous protrusion of both the uterus and the intestine, one of whom required an intestinal resection. Concomitant uterine conditions, namely, pregnancy, pyometra, and endometrial hyperplasia, were seen in five bitches in the present study, and these conditions have been reported sporadically.^7–10 The uteri in the other 10 intact bitches were also sufficiently large to identify ultrasonographically, which indicates possible concurrent disease or estrus.¹ An enlarged uterus may widen the inguinal ring further and may even result in simultaneous intestinal prolapse. Therefore, early herniorrhaphy with OVH may be preferable in intact bitches with NTIHs.

A left-sided bias of unilateral hernias was reported previously (69%, 20/29),³ and the rate was even higher in the present study (77%, 30/39), particularly in the intact bitches (84%) and in uterine herniated cases (91%). The reason for the left-side bias cannot be explained by the anatomy of the inguinal canal or by systemic factors. The most likely reason may be a mechanical factor in which the descending colon pushes the left uterine horn ventrolaterally, which may be supported by the left-sided colonic herniation in one dog in our study. Physiological or pathological enlargement of the uterine horns pushed by the colon may result in greater pressure on the left inguinal ring. This partly explains the left-sided bias we saw in bitches but not in male dogs. However, seven of nine unilateral hernias in male dogs in the literature^{3–6,11,20,21} were on the left side, so further study is needed to derive firm conclusions.

Ultrasonography was useful in identifying the prolapsed organ, but bowel viability could be assessed only by intraoperative observation. Vomiting for 2–6 days is a reported predictor of nonviable small intestine.³ In this study, only one of the four vomiting dogs had a nonviable small intestine, probably because surgery was performed earlier.

Complications of inguinal herniorrhaphy are rare, and long-term postoperative results are excellent, as reported in a previous study.³ No seroma formation was observed and we did not use surgical drains in our study, which indicates the usefulness of walking sutures. When closing the hernia ring, monofilament prolonged absorbable or nonabsorbable suture is described.² We used short-duration absorbable multifilament polyglactin suture and experienced no recurrence, except for one male dog. These positive results may have been partly because of decreased risk factors related to OVH.

One male dog presented with a recurrent inguinal hernia with bilateral perineal hernias. The rate of simultaneous occurrence of both inguinal and perineal hernias in >3 yr old male dogs was 29% (2/7 male dogs) in the present study and 25–100% in previous reports.^3,4,11 Weakening of the muscles or ligaments is thought to be caused by estrogen from the aged testis and relaxin from the hypertrophic prostate.¹¹ In aged intact male dogs with inguinal hernia, greater attention may be needed regarding herniorrhaphy, and in severe cases, flaps from the cranial sartorius muscle² or tunica vaginalis²⁰ may be required.

There are several limitations in this retrospective study. First, our results reflect surgical cases only and do not reflect epidemiological features, namely, mild cases not treated surgically. Second, the possibility that factors specific to miniature dachshunds affected the overall results should also be considered. Third, the exact duration of each hernia was unknown in most cases, so small or asymptomatic hernias may have been present longer in cases classified as having an acute onset. However, because the acute cases were operated within <7 days after diagnosis, these were clinically regarded as having acute disease. Finally, the results of the statistical analysis should be interpreted while considering the small sample size, and larger-scale studies are needed to confirm our results.

Conclusion

NTIHs are more likely to occur in middle- to older-aged, female, small-breed dogs, and miniature dachshunds may be predisposed. The most frequently protruded hollow organ in our study was the uterus, which tends to protrude on the left side, and intestinal herniation may occur concurrently. Physiological or pathological enlargement of the uterus associated with aging and sex hormones appears to increase the risk of uterine herniation, but the long-term prognosis of herniorrhaphy combined with OVH is excellent. Therefore, even in asymptomatic NTIHs, early herniorrhaphy and OVH are recommended. The etiology and treatment for NTIHs in older male dogs may require further investigation.