The estimated prevalence of PA was 35 patients/23057427 population years in the hospital between 2011 and 2021. PAs are more common in males than in females and tend to occur more frequently among older patients, which was similar to previous reports [6, 7].
PAs may be divided into primary and secondary abscesses according to the pathogenesis. Primary PAs occur as a result of haematogenous or lymphatic seeding from a distant site (which may be occult). Previous studies have reported risk factors including diabetes, intravenous drug use, HIV infection, renal failure, and other forms of immunosuppression [1, 8], whereas our study only showed diabetes as a predisposing factor.
Secondary PAs occur as a result of the direct spread of infection to the psoas muscle from an adjacent structure reported to be more likely in older patients [9]. However, this was not evident in our study. Both the incidence and the average age of patients with primary and secondary abscesses were similar.
In contrast to data from other studies, in this study, infective spondylitis was found to be the most common aetiology of secondary PA, and no cases were deemed related to Crohn’s disease, which was considered the most common aetiology. Most reported cases of PAs related to aortic infection tended to occur in the abdominal aorta [10, 11]. In this study, stent graft infections were found to be the causative organism in two of the seventeen patients with secondary PAs. One patient relapsed and died of uncontrolled sepsis. PA was also described in the infected stent graft material in previous studies. There were multiple bacterial species cultured from pus [12, 13]. We should pay attention to patients who received a stent graft. Because PA is often diagnosed prior to the detection of the primary disease, clinicians must be careful to avoid overlooking these lesions.
In our study, the most common bacterial cause was S. aureus in both primary and secondary PAs, including MRSA, which was only discovered in primary PAs. Polymicrobial infections tended to occur in primary PA, which was different from previous studies showing that primary PAs were most frequently due to infection with a single organism [14]. Almost 46% of pathogens were unknown. The percentage increased compared with previous reports that pathogens were not identified in 40% and 25% of cases [2, 7]. The microbiology of PA varies with geography and pathogenesis of infection. Clinicians should consider pus and blood cultures along with antibiotic sensitivity testing by polymerase chain reaction (PCR), gene sequencing and other molecular tests as supportive evidence for the diagnosis.
Rarely, some abscesses are related to malignant metastatic disease, as described in the literature. Most cases were thought to be secondary PA that had metastases of cervical carcinoma or retroperitoneal metastatic germ cell tumour as the primary diagnosis [15, 16].
In our report, metastatic involvement of skeletal muscles exclusively among patients was affected by primary lung malignancy. Cytology smears from the swelling in the psoas muscle showed metastatic squamous cell carcinoma that was considered primary PA. One report showed that the estimated prevalence of PA in lung cancer patients was 2.62% at disease presentation and/or diagnosis [17]. Biopsy material should be sent for histopathology evaluation, as this may be useful for diagnosis.
Lumbar spinal tuberculosis was discovered to be one of the common complications in patients with PA in our study, and the proportion was the same in primary PA and secondary PA. In contrast to previous reports, the high prevalence of TB in the aetiology of PA highlights the importance of TB control to help reduce transmission [18].
Moreover, patients with secondary PAs had longer hospital stays for the treatment of their underlying pathologies. Recurrence may be associated with inadequate drainage or inadequate antimicrobial therapy.
The diagnosis of a PA may be suspected on clinical grounds and confirmed on imaging studies. In this report, PA tended to occur on the right side rather than the left, while previous studies showed a roughly equal frequency. Bilateral PAs are uncommon. However, some authors have reported that bilateral abscesses have been increasing in number in previous case series [2, 19]. In these cases, none presented the classical triad of fever, back pain, and a groin or flank mass simultaneously, so PAs were frequently missed at the initial presentation.
It has been reported that ultrasound imaging can aid in the early diagnosis of PA and may be diagnostic in up to 50% of cases. This modality may miss diffuse phlegmon or small lesions. The patients we enrolled were diagnosed by CT or MRI imaging, and one received PET-CT. Although the sensitivity may be limited early in the course of disease, in most cases, an abscess is obvious. Findings may include a focal hypodense lesion, infiltration of surrounding fat, and gas or an air fluid level within the muscle that should be distinguished from a neoplasm or haematoma in the muscle. CT enhancement of the rim of the abscess with contrast is a more specific feature [20]. We should increase suspicion for TB according to the evidence of bony spinal infection. Spinal TB is usually insidious in onset, and the disease progresses at a slow pace [21]. Lumbar spinal TB complicated with PA was also found to present atypical symptoms. In this circumstance, magnetic resonance imaging (MRI) may allow improved definition of soft tissues and adjacent structures [22, 23]. There are limited reports of the use of PET-CT. In one report, PET-CT was useful in the follow-up assessment of a patient with a tuberculous PA [24].
Treatment of PAs consists of drainage and the prompt initiation of appropriate antibiotic therapy. Percutaneous drainage either by ultrasound or CT guidance is an appropriate initial approach. In our study, this technique was successful in 90% of cases, which was consistent with that in previous studies [3, 25]. A pigtail catheter may be placed in situ to allow further drainage following needle aspiration in some cases. Surgical drainage may be warranted in the setting of PA accompanied by lumbar TB. After successful drainage, disease activity can sometimes remain, and recurrence may occur. Antitubercular medication regimens of appropriate duration are still necessary, and we suggest that CT be useful for evaluating disease activity in the context of this condition. In our series, mortality due to primary and secondary abscesses was the same, whereas mortality rates of approximately 2.4% for primary PA and up to 19% for secondary PA have been documented [1]. Risk factors for mortality included advanced age, diabetes, renal failure, immunosuppression, previous PA, inflammatory bowel disease, and delayed or inadequate treatment [26]. We suggest that a thorough medical history and subsequent imaging studies can be helpful in selecting effective therapies.