This patient has presented with subacute right lower abdominal pain radiating to the ipsilateral hip, in conjunction with marked constitutional symptoms. Examination shows that he prefers to keep the hip of the affected leg slightly flexed, and also reveals a positive psoas sign. When the above clinical findings are considered together, the first differential diagnosis which comes to mind is a psoas abscess; however, retrocecal appendicitis, inflammatory bowel disease (particularly of the cecum and ascending colon), and septic or tuberculous arthritis of the hip should also be considered. Retrocecal appendicitis does not demonstrate the classical symptoms of acute appendicitis, and can give rise to a positive psoas sign because the inflamed appendix irritates the adjacent psoas muscle. However, these patients also typically report pain in the retrovesical pouch upon rectal examination. With both septic and tuberculous arthritis of the hip, one would expect pain upon active and passive movements, as well as restriction of movement; thus, this too is clinically less likely. Inflammatory bowel diseases are also unlikely, given the absence of concomitant gastrointestinal symptoms. Contrast computed tomography (CT) of the abdomen is the gold standard for diagnosis of a psoas abscess, and should be attempted here; note that the normal ultrasound findings do not exclude any of the etiologies listed above, as sonography is only 60% sensitive for detection of a psoas abscess. CT in turn reveals a rim-enhancing lesion with a hypodense center in the right psoas muscle, clinching the diagnosis; the otherwise unremarkable findings also help rule out the other differential diagnoses mentioned above. The next step is to determine if the psoas abscess is primary or secondary in origin. In this regard, the normal imaging appearance of the adjacent structures favors the former; very likely, there is an occult infectious process in the body Blood cultures should be drawn to identify the causative agent; empirical antibiotics should be started immediately following this, with coverage of Staphylococcus aureus being vital, as this is the major causative organism of primary psoas abscesses. Drainage of the abscess is essential for the further management; as the abscess appears to be solitary and without gas formation, percutaneous techniques are a better option here. Etanercept therapy is associated with an increased risk for infections; it should be stopped until the abscess has been treated. Steroids have no role in the management of these patients, and may worsen their condition. Note also that there is no indication to perform a colonoscopy, given the lack of signs and symptoms suggestive of a bowel pathology; furthermore, plain x-rays of the abdomen, even when performed along with contrast administration, have a very low sensitivity for detection of psoas abscesses.
A psoas abscess (PA) occurs when a suppurative collection forms within the psoas compartment; while rare, it can potentially be life-threatening. In the United Kingdom, the incidence of PA is reported to be 0.4/100,000 persons, with males predominantly affected. While earlier thought to mainly occur in persons less than 20 years of age, recent studies have show that PA are encountered in older age groups as well. Based on origin and pathophysiology, PA can be broadly classified into two main types: primary and secondary. Primary PA occurs due to hematogenous or lymphatic spread of the causative organism from a distant site elsewhere in the body. Secondary PA occurs in association with an underlying condition such as intestinal, genitourinary, spinal, or skeletal infections. Note also that primary PA is the dominant form in developing countries, accounting for >90% of cases in those geographies. The developed world shows an opposite pattern, with >80% of cases being secondary in origin. The anatomy of the iliopsoas compartment plays a key role here; the rich vascular supply of the psoas muscle is responsible for primary PA, while its anatomical position (being adjacent to several organs) is the main cause of secondary disease. In earlier times, tuberculosis was the major pathogen implicated in both primary and secondary PA; however, nontuberculous PA is now the predominant form, with Staphylococcus spp. being the most common causative organism, followed by Streptococcus spp., Escherichia coli, and Klebsiella spp. Gram-negative bacilli and anaerobic organisms are uncommon, but should be considered in cases where gas is present on imaging. Key risk factors for primary PA include immunocompromise (particularly HIV infection), diabetes mellitus, chronic kidney disease, intravenous drug abuse, and trauma to the iliopsoas muscle. The major causes of secondary PA include Crohn's disease, diverticulitis, appendicitis, and colorectal carcinoma; urinary tract infections and instrumentation of the urinary tract; and spinal infections and septic arthritis. Endocarditis and hepatocellular carcinoma are less common causes. The symptoms of PA are initially insidious, starting with low-grade fever and malaise. Over time, more specific findings develop, including abdominal or flank pain, and pain upon hip movements, especially flexion and eternal rotation. There may be referred pain to the hip and thigh. Other potential symptoms include weight loss, nausea, and presence of a groin mass, although all of these are nonspecific. Note that the classic triad of PA comprises flank pain, fever, and limitation of hip movement; however, this occurs only in ~30% of cases. The examination is usually non-specific; when supine the patient may favor a posture where the knee is moderately flexed and hip mildly externally rotated, while a positive psoas sign may be present. Note that the latter finding is not exclusive to PA, and may occur in other conditions, such as appendicitis. Another potential sign is a painless swelling below the inguinal ligament; this should not be confused with a hernia. Basic laboratory investigations should include a complete blood count, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and blood cultures; elevated levels of inflammatory markers will be present, while cultures, if positive, will help determine the causative organism. Imaging is essential, both to confirm the diagnosis, and also to determine the further management. Contrast computed tomography (CT) is the gold standard; this will typically show a homogenous collection with a thick wall which enhances with contrast within the psoas muscle. Magnetic resonance imaging (MRI) is an alternative; some experts attest to its superiority over CT, due to the ability to discriminate soft tissues, and visualize the abscess and surrounding structures without the need for intravenous contrast media. Ultrasound is inexpensive and does not carry a radiation risk, but is extremely operator dependent and only diagnostic in 60% of cases; x-rays, even with IV contrast, are not reliable for the diagnosis, although gas shadows in the retroperitoneal space may be seen in gas-forming abscesses. The principles of treatment include administration of antimicrobials, followed by drainage of the abscess; the latter can be via surgery or percutaneous drainage (PCD). In suspected primary PA, broad-spectrum antibiotics covering Staphylococcus should be commenced empirically until culture results are available; antistaphylococcal penicillins, clindamycin and aminoglycosides are suitable choices in this respect. Antibiotic therapy should continue for up to two weeks, even after complete drainage of the abscess. While some studies have mentioned the effectiveness of antimicrobial therapy alone in abscesses <60 mm in size, this has not entered general practice. The drainage technique used should be based on the type of abscess. PCD is a much less invasive option, and is preferred in solitary and non-gas forming abscesses. Surgery is preferred in gas-forming PAs, in cases where PCD fails, or where it is contraindicated (e.g. clotting disorders). Note also that where other intra-abdominal infections (e.g. ruptured appendicitis or an infected abdominal aortic aneurysm) are present, surgery is also the technique of choice. The key complications of PA include hydronephrosis due to compression of the ureter, and deep vein thrombosis (DVT) secondary to compression of the iliac vein. While the mortality of primary PA is relatively low, at 2.4%, this may be as high as 18.9% in secondary disease; sepsis is the usual cause of death in both forms. In particular, gas formation has been noted as a prominent factor in determining the ultimate clinical outcome.