Features and treatment modality of iliopsoas abscess and its outcome: a 6-year hospital-based study

Iliopsoas abscess (IPA), a suppurative collection within the compartment of the psoas and iliacus muscles, was first reported by Mynter et al. in 1881 [1]. It is an infrequent but potentially life-threatening infectious disease. Prior to introduction of effective anti-tuberculosis drugs, IPA was once a common complication of tuberculous spinal infection. With the common application of anti-tuberculosis drugs, non-tuberculous pyogenic IPA has become the predominant form. Clinically, IPA is classified according to its origin [2]. Primary IPA is thought to be secondary to an unrecognized staphylococcal bacteremia, while secondary IPA is caused by underlying conditions such as gastrointestinal or genitourinary tract diseases, spread of infection from postoperative aortic aneurysm, spondylodiskitis, osteomyelitis, septic arthritis, and infection subsequent to renal surgery [1, 3]. Primary IPA is often seen in younger patients and in developing or tropical countries, while secondary IPA occurs more frequently in developed countries with mixed enteric flora [2]. In recent decades, the incidence of primary IPA has gradually increased as numbers of intravenous drug-abusers and those afflicted with human immunodeficiency virus (HIV) infection increase [1, 4].

The psoas-muscle sign, the triad of fever, flank pain, and limitation of hip movement, is noted in only 30% patients [5]. Because of the rapid progress in advanced imaging techniques, such as gallium-67 scanning, computed tomography (CT), and magnetic resonance image (MRI) in recent years, early diagnosis of IPA has become easier despite initial clinical presentation that is frequently ambiguous [6–8]. Although origin-based IPA classification has been adopted in clinical practice, it does not help clinicians make appropriate treatment decisions. Currently, whether percutaneous drainage (PCD) or surgical intervention should be used in IPA patients remains controversial because few studies with large sample sizes have been performed. At present, PCD is considered preferable to surgical intervention for the treatment of IPA [9–11].

In this study, we compared demographic information, laboratory data, microbiological distribution, origin of infection, treatment modality (i.e., antibiotics alone, PCD, and surgical intervention), and clinical outcomes between patients with gas-forming IPA and non-gas forming IPA. From these results, we established an evidence-based algorithm for selecting an appropriate treatment modality for IPA.

This retrospective study was approved by the institutional review board of Taichung Veterans General Hospital (No. CE13129). The data were retrieved from the electronic clinical database of Taichung Veterans General Hospital, a 1,520-bed referral medical center in central Taiwan. The diagnosis of IPA was confirmed by (1) documentation of abscess within the iliopsoas muscle by either PCD or surgery; or (2) classic CT or MRI finding of IPA with compatible clinical presentation, laboratory data, and blood cultures. Typical CT or MRI images in IPA patients included the following features: (1) enlargement of affected muscle, (2) rim enhancement of the abscess wall with lower central attenuation, and (3) gas presence either as an air–fluid interface or mottled air bubbles. All CT and MRI images were reviewed by the primary study investigators and one radiologist.

The primary outcome of this study was the intra-hospital mortality of treated IPA patients. We defined success of treatment as improvement in clinical condition, decrement of IPA size in follow-up images, and discharge alive after treatment with PCD, surgery, or antibiotics alone. Failure of treatment was defined as mortality during hospitalization or deterioration of a patient’s clinical condition with accompanying non-decrement of IPA size on follow-up images necessitating another advanced treatment modality (i.e., antibiotics alone plus PCD, antibiotics alone plus surgery, or PCD followed by surgery). All the patients accepted a minimum follow-up period of 2 months. Cases were excluded if the patient was < 18 years old or had an incomplete treatment course.

Eighty-eight IPA patients admitted to the hospital between July 2007 and February 2013 were included in this study. Demographic data, laboratory studies, etiological pathogens, infection origins, management approaches, clinical courses, and patient’s outcomes were coded for further statistical analysis. Continuous variables were reported as mean ± SD, whereas categorical variables were described as the number and percentage of subjects. Patients were divided into gas-forming IPA and non-gas forming IPA groups. Further comparisons were performed using Mann–Whitney U test for continuous variables and chi-square test or Fisher’s exact test for categorical variables. A P value of less than 0.05 was accepted as statistically significant. Statistical analysis was performed using the Statistical Package for the Social Sciences Version 15.1 (SPSS Inc., Chicago, IL, USA).

Iliopsoas muscles are susceptible to infections from distant sites and contiguous structures because of their rich blood supply and overlying retroperitoneal lymphatic systems. IPA is often overlooked because of its insidious onset, and subsequent severe sepsis may be life-threatening. Although the exact incidence of IPA is unknown, more and more cases are being identified because of widespread application of CT in suspected IPA patients. Despite recent advances in diagnostic modalities, the mortality rate of IPA has not improved much (6.7% in a study by Ricci in 1986 and 5% in a study by Navarro in 2009) [2, 12]. In our study, the mortality rate in IPA patients (22/88, 25%) was higher than that reported in a meta-analysis conducted by Lai et al. (8%, 55/682, between 1986 and 2011) [9]. However, because those patients were from various medical settings in different countries, the results of that meta-analysis cannot be used as the gold standard for clinical comparison, and further clarification is needed. Among the 88 patients in our study group, 24 (27.3%) had already been hospitalized in other hospitals and were referred to us because of poor treatment response. The mortality rate among these patients was high (7/24, 29.2%). The remaining patients (64/88, 72.7%) were admitted via our emergency department (ED) with initial severe illness. In addition, severe sepsis was frequently observed in our total study group. The general poor health of the patients was probably the major cause of high mortality in the study group. In addition, at our hospital, more conservative treatment policies were adopted because of patients’ older age, which itself might have contributed to a higher mortality rate. Other possible explanations of the high mortality rate include large number of gas-forming IPA cases and predominance of secondary IPA.

The presentations of IPA in our study varied, including fever, flank pain or back pain, abdominal pain, thigh lumps, altered consciousness, and even shock. The initial nonspecific symptoms; e.g., nausea, general weakness, intermittent mild fever, or low back soreness, might easily be ignored by patients. We attempted to evaluate how long the patients had been ill via a complete medical record review. Pre-presentation symptoms could not be evaluated definitively in 24 of 88 patients (27.3%) who had stayed in other hospitals for periods of time. A higher mortality rate (7/24, 29.2%) was noted in this group. Additionally, the morphology of IPA pattern, according to the images in our hospital, might have been influenced by prior therapy. Sixty-four patients (72.7%) in our study were directly referred from another ED because of severe illness or initially sought medical assistance in our hospital. The majority of patients in this group (55/64, 85.9%) had a long illness prior to presentation; 12.55 days on average. Among these 55 patients, 17 (30.9%) had bilateral IPA and 49 (89.1%) had multiloculated IPA. The remaining 9 patients (9/64, 14.1%) in this group could not be evaluated because medical records were unclear or because patients had deterioration of consciousness upon initial presentation and ultimately expired. In brief, the late presentation of most of our patients may have contributed to the high total mortality.

IPA is clinically classified into primary and secondary origins according to its initial infection site. In 1986, Ricci et al. described 286 cases of primary IPA, mainly young patients from developing countries [2]. There were 90 cases of secondary IPA, which were almost exclusively from developed countries, and the most common etiologies were Crohn’s disease, followed by appendicitis and colon inflammation or cancer. Since then, more and more patients have been found to have secondary rather than primary IPA. In 2009, Navarro et al. reported 124 cases in a multi-center study, 78.2% with secondary IPA, and a single-center study by Carolyn et al. in 2001 demonstrated 80% of 61 cases having secondary IPA. The main sources of secondary IPA in the study by Navarro et al. were skeletal (50.5%), followed by gastrointestinal tract (24%) and genitourinary tract (17.5%). Our study revealed a similar trend with predominance of secondary IPA (76.1%) in which the most common infection origins were skeletal, followed by cardiovascular system and urinary tract. The higher prevalence of secondary IPA reported recently and in our study may be attributable to the widespread application of CT and MRI [13]. These imaging modalities can clearly delineate adjacent structures, especially the vertebrae and epidural space, and it is therefore likely that the infectious focus will be clarified. Furthermore, in our report, 11 patients of secondary IPA originated from cardiovascular system, including abdominal aortic aneurysm (AAA) post-stent implantation with subsequent infection, infected aortic aneurysm, and infective endocarditis, all of which have rarely been mentioned in the literature [14, 15].

Gas-formation is an important predictor of clinical outcome for patients with liver abscess and acute pyelonephritis [16, 17]. Furthermore, in patients with acute pyelonephritis, the treatment policy depends mainly on whether the gas-forming (emphysematous) change is detected within the urinary tract. The major components of the gas in emphysematous pyelonephritis are nitrogen, hydrogen, carbon dioxide, and oxygen, and mixed acid fermentation has been proposed as the major mechanism of gas production because of the hydrogen content [17, 18]. Other authors have indicated that rapid tissue catabolism complicated by impaired transport of end products around the infection site produces the gas [19]. Based on the clinical consensus that in acute pyelonephritis gas formation is the major criterion for further treatment decisions, we propose a new algorithm for determining treatment modality in IPA patients according to the presence of gas. In our study, GNB and anaerobic infections were more often discovered in patients with gas-forming IPA than in those with non-gas forming IPA. This phenomenon may explain the mechanism of gas formation and the development of a fulminant course of IPA.

Abscess drainage plus antibiotic treatment is essential for appropriate management of IPA patients. Options for drainage are surgery or PCD. Generally, IPA patients with concurrent intra-abdominal or retroperitoneal abnormalities such as ruptured appendicitis or ruptured infected aortic aneurysm require surgery to clean out or repair the infected foci. Surgical intervention may provide more effective drainage than PCD, especially in patients with multiloculated IPA [4]. IPA patients were often treated with surgical debridement in the 1980s, when imaging techniques were still uncommon [2]. Afaq et al. also reported no mortality in 72 cases in Nepal in which all of the patients received surgical intervention as the first choice of treatment [20]. Recent studies have suggested that IPA can be successfully treated with antibiotics plus PCD [11, 21], but not all of the cases in our study could be cured by PCD, especially when the IPA was gas-forming. If there are strong indications for primary operation, such as ruptured infected aortic aneurysm, ruptured appendicitis, or epidural abscess with spinal cord compression, surgical intervention should not be delayed. Furthermore, in our study, surgical intervention was preferred when gas-forming IPA was observed because of the higher failure rate of PCD. However, PCD plus appropriate antibiotics is adequate for treating patients with non-gas forming and solitary IPA. In addition, PCD remains an option for IPA patients who are not suitable for operation under general anesthesia. The suggested treatment algorithm based on the results of our study is delineated in Figure 3.

The major strength of this study is the use of a large number of IPA patients from a single center. Although previous multi-center study used larger sample sizes, many relevant variables could not be evaluated because of differences in record format across medical centers. However, there are several limitations to this study. First, there is no international consensus for IPA treatment; treatment policy depends on the treating physicians, whose training and clinical experience may play roles. Second, no matter which modality of treatment is chosen, the origin of infection may affect the clinical outcome, and this could not be evaluated in this study. Third, recurrence was not evaluated in this study. Thus, it is unknown whether repeated PCD or surgical intervention was performed. Fourth, the complexity of the disease varied in our sample.

Our findings suggest that classification of IPA patients into gas-forming IPA or non-gas forming IPA groups may help physicians make early and definitive decisions with regard to treatment choices. Our study showed that PCD treatment for patients with gas-forming IPA resulted in a high failure rate and increased intra-hospital mortality rate. We recommend early surgical intervention with appropriate antibiotic treatment for gas-forming IPA patients. PCD is adequate for the management of patients with solitary and non-gas forming IPA, especially for primary IPA.