Abstract
OBJECTIVE To determine long-term outcome for rhesus macaques (Macaca mulatta) with endometriosis that underwent surgical treatment and identify factors potentially associated with long-term outcome.
DESIGN Retrospective case series.
ANIMALS 11 female rhesus macaques.
PROCEDURES Medical records of female rhesus macaques in which endometriosis was diagnosed between 2007 and 2011 and that underwent abdominal exploratory surgery were reviewed.
RESULTS In 5 macaques, the only clinical abnormality was a caudal abdominal mass identified during a routine physical examination, and in 6 macaques, overt clinical signs of endometriosis, including anorexia, dysmenorrhea, and lethargy during menses, were reported. Five macaques had histologically confirmed complete ovarian removal, and another 5 had incomplete ovarian removal (ovarian tissue was not examined histologically in 1 macaque). Nine animals survived at least 12 months after surgery, and 6 survived at least 60 months after surgery. Macaques that did not have overt clinical signs were significantly more likely to survive at least 60 months after surgery. However, extent of ovarian removal was not significantly associated with survival 12 or 60 months after surgery.
CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that, in select situations, surgery (ovariectomy or ovariohysterectomy) may be curative in macaques with endometriosis and may result in long-term survival. Further, findings suggested that monitoring until clinical signs appear before performing surgery is not warranted in adult female macaques suspected to have endometriosis that only have a caudal abdominal mass and no other overt clinical signs.
Spontaneous endometriosis has been reported in multiple nonhuman primate species, including, most commonly, macaques and baboons.1,2 Prevalence rates as high as 30% have been reported in female macaques from several institutions,3–5 making endometriosis an important disease in any facility with an adult macaque population. Endometriosis is characterized by ectopic endometrial tissue (ie, endometrial stroma and glands) outside of the uterus that advances and regresses under hormonal influence.6 The location and histologic appearance of lesions in nonhuman primates are similar to those in humans.2 In macaques, endometriosis most commonly leads to dysmenorrhea and anorexia, but it may also cause signs of abdominal pain secondary to adhesions. Endometriosis has also been reported to cause death in nonhuman primates and humans and is a major risk factor for spontaneous hemoperitoneum in pregnancy, a life-threatening condition.7,8
The optimal treatment for endometriosis has been the subject of much discussion. Ovariectomy is considered curative in macaques because estrogen is required for the ectopic tissue to proliferate.3 However, the success rate following surgery has been reported to be as low as 60% in macaques, and success rate has been suggested to be correlated with severity of disease at the time of surgery.9,10 In animals with advanced disease, there is a high risk that ovarian remnants will be left in the abdomen because of adhesions and difficulties associated with identifying the ovaries in their entirety. In addition, there is little published information on long-term outcomes of macaques treated surgically.
Medical management is often used as an alternative to surgery in nonhuman primates with endometriosis, but many hormonal treatments can have undesirable effects or are difficult to administer properly.3,10–13 Medroxyprogesterone acetate has become the standard treatment at many facilities managing nonhuman primates with endometriosis. Importantly, this treatment has been shown to alter glucoregulatory function in macaques,5 although the clinical importance of these effects in macaques with endometriosis is not known.14
Given the advantages and disadvantages of the various therapeutic approaches, there appears to be no single best treatment for endometriosis, and the choice between surgical and medical treatment typically varies on an institutional basis because of personal preferences, familiarity or a lack of familiarity with surgical treatment, and study factors. The present study, therefore, was designed to elicit information on optimal treatment decision-making for rhesus macaques with endometriosis. Specifically, the purposes of the study reported here were to determine long-term outcome for rhesus macaques with endometriosis following surgical treatment and identify factors potentially associated with long-term outcome.
Materials and Methods
Adult female rhesus macaques (Macaca mulatta) were housed for research purposes at the University of Michigan in accordance with the Animal Welfare Act and the Guide for the Care and Use of Laboratory Animals. Animal protocols were reviewed and approved by the University of Michigan Institutional Animal Care and Use Committee. All animals underwent semiannual evaluations that included a thorough physical examination, a tuberculosis screening test, and an evaluation for alopecia.
Case selection
Medical records of all female rhesus macaques housed at the University of Michigan were searched to identify individuals in which endometriosis had been diagnosed between January 2007 and December 2011. Animals were considered eligible for inclusion in the study if they had undergone ovariectomy or ovariohysterectomy for treatment of endometriosis and the diagnosis had been confirmed by means of histologic examination.
Medical records review
For animals included in the study, semiannual examination, diagnostic test, surgery, biopsy, and necropsy reports; logs of food intake and menses; and research logs were reviewed. Specific to the diagnosis of endometriosis, information retrieved from the medical records consisted of signalment, initial clinical signs, operative findings, and postsurgical outcome. In addition, information was obtained on any recurrence of disease and survival time after surgery.
Surgical procedure
For surgical treatment of endometriosis, macaques were sedated with ketamine or tiletamine-zolazepam. An endotracheal tube was placed, and anesthesia was maintained with isoflurane. An IV catheter was placed for fluid administration during surgery.
An exploratory laparotomy was performed, and the abdominal organs were systematically examined for masses and adhesions. The ovaries were identified and removed if possible. If the ovaries could not be identified, any suspect tissue was removed. In animals with masses involving the uterus, partial or complete hysterectomy was performed. Endometriomas (so-called chocolate cysts) were removed when possible or were drained and debulked. Any adhesions constricting parts of the intestinal tract were carefully broken down. All tissues that were removed were submitted for histologic examination.
The surgical wound was closed routinely. At the surgeon's discretion, a splash block with bupivacaine was performed prior to skin closure. Following surgery, all macaques were treated with carprofen, cefazolin, and buprenorphine.
Statistical analysis
Continuous data were summarized as median and range. Fisher exact tests were used to test whether initial clinical signs (incidental vs overt) or extent of ovariectomy (complete vs incomplete) was significantly associated with survival 12 months after surgery (yes vs no) or survival 60 months after surgery (yes vs no). For these analyses, clinical signs were considered incidental if the only abnormality was a caudal abdominal mass that was identified by means of palpation during a routine physical examination; clinical signs were considered overt if anorexia, lethargy, or signs of pain during menses were reported. Ovariectomy was classified as complete if histologic examination revealed 2 intact ovaries and was otherwise classified as incomplete. Statistical analyses were performed with standard software.a Values of P < 0.05 were considered significant.
Results
Endometriosis was diagnosed in 13 of the 44 (29.5%) female rhesus macaques housed at the institution during the study period. However, the diagnosis was made only at necropsy in 1 animal. In a second animal, a presumptive diagnosis was made on the basis of clinical signs (ie, dysmenorrhea, anorexia, and signs of caudal abdominal pain) during menses that were ameliorated with MPA; however, the diagnosis was not confirmed. These 2 animals were excluded from the study. The remaining 11 met the criteria for inclusion.
For 10 of the 11 macaques included in the study, median age at the time of surgical treatment of endometriosis was 17 years (range, 15 to 24 years); age of the remaining macaque was not known. The 11 macaques had been at the University of Michigan facility for a median of 7.3 years (range, 1 month to 20.3 years). None had been born at this institution, and records prior to arrival at the institution were incomplete. However, none of the macaques had any history of previous abdominal surgery or pregnancy. The macaques were enrolled in several pharmacological studies for which they were catheterized for administration of addictive substances (typically opioids) and then underwent behavioral tests. No macaque was catheterized at the time endometriosis was diagnosed.
For 5 of the 11 macaques, the only clinical abnormality was a caudal abdominal mass that was identified by means of palpation during a routine physical examination. In these 5 animals, surgery was performed before the development of any other signs of endometriosis. In the remaining 6 macaques, overt clinical signs of endometriosis, including anorexia, dysmenorrhea, and lethargy during menses, were reported. Two of these 6 macaques initially had only a caudal abdominal mass, and these macaques had been monitored until clinical signs of anorexia and discomfort during menses developed, at which time surgery was performed. The 4 remaining macaques with overt clinical signs initially had anorexia and discomfort during menses, and a caudal abdominal mass was subsequently identified in all 4.
For 9 of the 11 macaques, time from initial observation of clinical signs to surgical treatment of endometriosis was ≤ 2 months. The remaining 2 macaques had surgery 12 and 18 months after initial clinical signs were noted. Diagnostic testing performed between the initial observation of clinical signs and surgery varied. Eight macaques underwent abdominal radiography, with 7 of these 8 animals also undergoing abdominal ultrasonography. Radiography revealed soft tissue opacities in the caudal part of the abdomen, or results were inconclusive. Ultrasonography most often revealed a mass that was usually associated with the uterus and sometimes fluid filled. In 9 macaques, a CBC and serum biochemical panel were performed between the initial observation of clinical signs and surgery. Any abnormalities that were detected were mild and not considered clinically important.
In 6 of the 11 macaques, the ovaries were easily identifiable and grossly normal in appearance, and in 1, the ovaries were easily identifiable but grossly abnormal in appearance and were later confirmed on histologic examination to be cystic. In the remaining 4 macaques, 1 or both ovaries were not identifiable during surgery. Of the 7 macaques with easily identifiable ovaries, 5 had histologically confirmed complete ovarian removal, and 1 had histologically confirmed incomplete removal of ovarian tissue. In the remaining macaque, ovarian tissue was not examined histologically.
Endometriomas identified during surgery varied from small masses with focal adhesions in the caudal portion of the abdomen to large hemorrhagic cysts that required drainage or debulking. Adhesions ranged from small focal areas (sometimes associated with a mass) in the caudal portion of the abdomen to widespread involvement of the entire abdomen. Extensive caudal abdominal adhesions were a contributing factor in animals in which there was difficulty identifying ovarian tissue. Although most animals had adhesions localized to masses or the caudal portion of the abdomen, 2 macaques had adhesions involving the upper gastrointestinal tract as well.
Six of the 11 macaques survived for at least 60 months after surgery, and 5 of these 6 animals had no additional reports of menses or clinical signs related to endometriosis at the time of the study (74, 78, 80, 82, and 87 months after surgery). The remaining macaque that survived at least 60 months had an episode of anorexia and vomiting 1 year after surgery. Treatment with quarterly injections of MPA was instituted at that time, and the macaque was being maintained on that treatment at the time of the study, 103 months after surgery.
Five animals survived < 60 months after surgery. These macaques had been returned to research studies but had progression of their disease. Four of the 5 macaques were treated with MPA a median of 3.5 months (range, 0.5 to 6 months) after surgery because of a recurrence of clinical signs or menses. All were euthanized because of chronic endometriosis (n = 2; euthanized 4 and 16 months after surgery) or acute decompensation secondary to cyst rupture and subsequent hemoabdomen (3; euthanized 3, 14, and 18 months after surgery). For these 5 macaques, median survival time after surgery was 14 months (range, 3 to 18 months).
All 5 macaques for which initial clinical signs were classified as incidental survived at least 12 months after surgery, compared with 4 of the 6 macaques that initially had overt clinical signs. In contrast, although all 5 macaques for which initial clinical signs were classified as incidental survived at least 60 months after surgery, only 1 of the 6 macaques that initially had overt clinical signs survived this long. Initial clinical signs (incidental vs overt) were not significantly (P = 0.454) associated with survival 12 months after surgery but were significantly (P = 0.015) associated with survival 60 months after surgery.
Extent of ovariectomy (complete vs incomplete) was not significantly associated with survival 12 months after surgery (P = 0.444) or survival 60 months after surgery (P = 0.206). All 5 macaques with complete ovariectomy survived at least 12 months after surgery, and 4 of the 5 survived at least 60 months after surgery. Of the 5 macaques with incomplete ovariectomy, 3 survived at least 12 months after surgery and only 1 survived at least 60 months after surgery.
Discussion
Results of the present study suggested that, in select situations, surgery (ovariectomy or ovariohysterectomy) may be curative in macaques with endometriosis and may result in long-term survival. In addition, macaques that had only incidental clinical signs (ie, a caudal abdominal mass identified by means of palpation during a routine physical examination) were more likely to survive at least 60 months after surgery than were macaques with overt clinical signs (eg, anorexia, lethargy, or signs of pain during menses).
In the present study, 13 of 44 (29.5%) macaques housed at our facility during the 5-year study period developed endometriosis, which was similar to incidence rates reported previously.3,15 Previous authors4 have suggested that there likely is a familial component to endometriosis in macaques, and increasing age and previous abdominal procedures have been suggested as possible risk factors for its development.16,17 It has also been shown that pregnancy ameliorated disease in cynomolgus macaques with experimentally induced mild endometriosis,18 suggesting that reduced parity could be a risk factor for the disease in macaques. Findings in the present study supported the suggestion that increasing age is a risk factor for endometriosis, in that all animals with endometriosis were older (age range, 15 to 24 years). However, because of the incomplete history for these macaques prior to arrival at our institution, we could not definitively state whether any had previously been pregnant or undergone abdominal surgery. In addition, we did not know the genetic backgrounds of these macaques and, therefore, could not comment on whether there were any familial relationships.
In recent years, research on the pathogenesis of endometriosis in humans has offered new understanding on risk factors and potential treatments. A combination of retrograde menstruation along with immune dysfunction is a commonly suggested pathogenesis, and researchers are now seeking to understand the contributions from factors such as epigenetic regulation19 and oxidative stress.20 Immune dysfunction in the form of increased concentrations of peritoneal proinflammatory markers and other immune system mediators have led researchers to categorize endometriosis in humans as an immunemediated or potentially autoimmune disorder.21,22 In addition, the knowledge gained regarding potential new treatments, such as immunomodulating drugs that do not affect hormonal cycles, in human patients with endometriosis could be relevant to nonhuman primates. Additionally, findings for human patients emphasize the importance of considering both medical and study history when evaluating a population of captive nonhuman primates with a high incidence of endometriosis. Participation in research studies that induce inflammatory profiles or cause changes to immune function may well play a role in the development of endometriosis in female macaques.
As a treatment for endometriosis, surgery has been falling out of favor at many institutions because of the low success rate. However, at our institution, the treatment of choice has been to perform ovariectomy or ovariohysterectomy, with the goal of keeping the macaques on study and healthy for their natural life span. For the macaques in the present study, 9 of 11 survived at least 12 months after surgery and 6 of 11 survived at least 60 months after surgery. In comparison, a previous study9 involving 35 rhesus macaques found that 77% were alive 12 months after surgery and 29% were alive 60 months after surgery.
Given the limited overall success of surgical treatment, a central question to consider in the management of macaques with endometriosis is whether and when to choose surgery over medical treatment. In the present study, we found that our macaques had 1 of 2 clinical presentations, with endometriosis identified either as an incidental finding of a caudal abdominal mass during routine examination or because of overt clinical signs. Importantly, macaques in which endometriosis was an incidental finding were significantly more likely to survive at least 60 months after surgery, compared with macaques that had overt clinical signs, and all 5 macaques that were euthanized ≤ 18 months after surgery had overt clinical signs of endometriosis. Therefore, we believe that surgery is an appropriate option for any adult female macaque with a caudal abdominal mass and no other clinical signs.
Interestingly, during our review of medical records, we identified 1 macaque that underwent abdominal exploration after a caudal abdominal mass was palpated during a routine examination but that did not have any gross or histologic evidence of endometriosis at surgery. The risk of performing unnecessary procedures may be higher when surgery is elected earlier in the disease course. However, we believe that the better prognosis for macaques with endometriosis that undergo surgery prior to the onset of clinical signs is worth the risk of negative exploratory surgery findings.
In the present study, we also sought to determine whether surgical findings could provide prognostic information. Complete removal of the ovaries is considered an important step in preventing progression of the disease in nonhuman primates with endometriosis. However, even with complete ovarian removal, lesions that are already present are unlikely to regress, and severe lesions can cause further complications.3 This is likely why we were unable to identify significant associations between extent of ovarian removal (complete vs incomplete) and proportions of animals surviving 12 or 60 months after surgery.
There were several limitations to the present study, mainly reflecting its retrospective nature. First, because the study relied on reviewing medical records, missing information could have affected our conclusions. Second, all of the surgical procedures were performed by a single board-certified veterinary surgeon, and outcomes may have been different if the procedures had been performed by multiple surgeons. Third, the number of cases included was relatively small.
Acknowledgments
Dr. Kennedy's postdoctoral training at the University of Michigan was supported by NIH grant R25-OD-010915.
Presented in part at the 40th Annual Work shop of the Association for Primate Veterinarians, Saint Paul, Minn, November 2012.
ABBREVIATIONS
| MPA | Medroxyprogesterone acetate |
Footnotes
GraphPad Prism, GraphPad Software, La Jolla, Calif.
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