A psoas abscess is a relatively rare condition that usually presents with nonspecific signs and symptoms. Without prompt diagnosis and treatment, it may cause serious complications.
A classic triad consisting of psoas spasm, fever and back pain is characteristic of psoas abscess but is only present in 30% of patients. Nonspecific symptoms such as abdominal and flank pain, fever, malaise, weight loss and nausea predominate and complicate the picture. Back pain remains the most common symptom, appearing on average 10 days before the patient usually presents to the clinician. Pain associated with psoas abscess tends to radiate to the anterior sides of the thigh and the hip, as a result of the innervation of the muscle with L2, L3 and L4 nerves.
The physical exam is very important in the diagnostic workup of the disease. Signs that are particularly suspicious include a palpable mass in the inguinal area, a limp and severe pain after the performance of certain maneuvers. Pain that occurs after the patient attempts to lift his or her thigh against pressure caused by the examiner's hand is especially suspicious. Furthermore, pain that occurs while the patient hyperextends the hip while lying on the non-affected side is also suggestive . It is worth noting that these two tests tend to be also positive in appendicitis or any condition that involves inflammation of the psoas muscle. Another important sign is when the patient favors the supine position while keeping the knee slightly flexed and the hip externally rotated.
A psoas abscess can be difficult to diagnose and is commonly confused with lumbar strain, arthritis, osteomyelitis of the vertebra, as well as other urologic and abdominal conditions. Furthermore, patients who present with septic shock or with evidence of a Mycobacterium tuberculosis infection do not commonly present with classic symptoms and diagnosis may be very difficult to establish.
Workup is broad and includes blood tests, although findings on the latter can be variable. Nonetheless, a Johns Hopkins study showed that almost all patients will have an elevated ESR (erythrocyte sedimentation rate) with a mean of 90 mm/hr, elevated blood urea nitrogen (BUN) with a mean of 30.5 mg/dL and leukocytosis with a mean count of 15,900/mm3 . Furthermore, pyuria is occasionally reported .
Imaging tests are critical in establishing the diagnosis. CT is generally used to confirm diagnosis and is considered more sensitive and specific than ultrasound. Ultrasound may only be able to diagnose 60% of patients, whereas CT can detect and confirm a psoas abscess in 80 to 100% of all cases . Magnetic resonance imaging (MRI) is not part of the workup of the condition. This is due to the associated high costs, the high level of discomfort for the patient, in addition to the fact that it does not improve sensitivity and specificity relative to CT.
On the other hand, the gold standard for diagnosis remains blood culture or direct aspiration of the abscess. The latter in general has better sensitivity and specificity.
Treatment of a psoas abscess consists of antibiotic administration and drainage of the abscess. The choice of the antibiotic is based initially on the most likely pathogens involved and is subsequently adjusted according to culture sensitivity tests.
The choice of empiric antibiotic treatment depends on whether the condition is primary or secondary. Treatment for primary psoas abscess is targeted initially against S. aureus. Antistaphylococcal antibiotics are initiated even before diagnosis is conclusively established, given that approximately 90% of all cases of primary psoas abscess involve S. aureus. The chosen antibiotic should also act against methicillin resistant S. aureus and is usually one of the following: vancomycin, linezolid and clindamycin.
On the other hand, a secondary psoas abscess is most commonly associated with enteric bacteria, with involvement of both anaerobic and gram negative organisms. Antibiotics that are frequently administered in such cases include fluoroquinolones, third and fourth generation cephalosporins, metronidazole and anti-pseudomonal penicillins .
It is important to note that these are broad guidelines and that many cases involve enteric bacteria in primary cases and S. aureus in secondary cases . Appropriate broad spectrum antibiotics such as aminoglycosides, clindamycin or an antistaphylococcal penicillin should be used in either form .
Abscess drainage can be performed either percutaneously with CT assistance or open surgery. Indications for surgical management include patients in whom percutaneous intervention cannot be performed (such as the presence of coagulopathies), failure of CT-assisted percutaneous drainage or the concomitant presence of a pathology within the abdomen that also necessitates open surgery. CT-assisted percutaneous drainage is generally the favored approach because it is a non-invasive procedure, is associated with low morbidity and mortality and can effectively treat uniloculated as well as multiloculated psoas abscesses . Nonetheless, some patients may require repeated aspirations for effective treatment, defined as clinical improvement and complete obliteration of the abscess .
Prompt diagnosis and treatment improves outcomes. Drainage of the abscess and antibiotic administration are the most important interventions that have proven to reduce both mortality and morbidity.
Causes responsible for the development of a psoas abscess can be divided into primary and secondary. Primary abscesses are not caused by an underlying disease but usually result from hematogenous spread of bacteria from sources of infection in other locations. Secondary psoas abscesses most commonly occur in the context of Crohn's disease. Interventional procedures in the hip, groin and lumbar regions can also substantially increase the risk.
A wide range of bacterial species are involved in the etiology of the disease and usually vary depending on whether the abscess is primary or secondary. Around 80% cases of primary psoas abscesses involve Staphylococcus aureus, including methicillin resistant S. aureus . Other bacterial species, such as Pseudomonas aeruginosa, Haemophilus aphrophilus, Proteus mirabilis and Serratia marcescens, have also been reported  . On the other hand, enteric bacteria play the most important role in the secondary form of the disease, particularly Salmonella enteritidis, Escherichia coli and Streptococcus and Enterobacter species. Tuberculosis has also been isolated in cases where the disease is still prevalent.
Studies have found that the prevalence of psoas abscess has increased worldwide from 3.9 cases to 12 cases per year from 1985 to 1992 . These studies also report that epidemiological characteristics vary depending on the underlying cause. Primary psoas abscess is more common in young patients. Around 83% of all cases are diagnosed in patients younger than 30. On the other hand, 40% of all cases of secondary psoas abscess are found in patients older than 40. Elderly patients rarely present with either form of the condition .
Nonetheless, more recent studies contest prior findings. A case series of 18 patients in the Johns Hopkins University School of Medicine found that secondary psoas abscess is more frequent than primary psoas abscess, with patient age ranging at diagnosis from 2 to 78 years, in comparison to an age range of 27 to 81 years in primary psoas abscess.
HIV infection and drug use are significant risk factors for primary psoas abscess. Around 57% of all patients are HIV positive and up to 86% have a history of IV drug use . These associations have not been reported in patients with secondary psoas abscess. Studies have also found other important associations that include diabetes, renal failure and immunosuppression .
The psoas muscle is a very well vascularized structure, thus explaining hematogenous spread of infectious organisms from an original nidus of infection. Furthermore, the psoas muscle is located very close to numerous vital organs and structures such as the pancreas, the colon, the jejunum, the aorta, the renal pelvis, the ureters, the spine and the iliac lymph nodes. Consequently, infections in these sites can easily reach the psoas muscle.
Primary psoas abscess occurs subsequent to spread from the lymphatic or vascular systems and tends to be more common in patients with AIDS, diabetes mellitus, IV drug abuse or renal failure. On the other hand, secondary psoas abscess results from direct spread from nearby structures. It is most frequently related to gastrointestinal disease and infections such as perforated colon carcinoma, Crohn's disease, appendicitis and diverticulitis. Kidney disease is also an important cause.
Prevention is mostly directed at addressing the complications before they arise, through early diagnosis and prompt and effectice treatment. The most relevant complications associated with psoas abscess are sepsis and pulmonary emboli.
Psoas abscess is a medical condition first identified by Mynter in 1881 . It can be either primary or secondary. Primary psoas abscess results from direct hematogenous or lymphatic spread from other sources of infection in the body whereas a secondary psoas abscess occurs subsequent to spread from adjacent structures and organs. The most common organism involved in primary psoas abscess is S. aureus, in contrast to enteric bacteria in secondary psoas abscess. Patients usually present with nonspecific symptoms that include nausea, vomiting, weight loss, abdominal and back pain, fever, a limp and malaise. A classic triad consisting of fever, back pain and a psoas spasm can be identified in only 30% of patients. The pain tends to radiate to the anterior surfaces of the thigh and the hip due to innervation by the L2, L3 and L4 nerves which also innervate the psoas muscle. The physical exam is very important in the workup. In particular, pain elicited with specific procedures may be strongly suggestive of the disease. In addition, the patient usually prefers to lie in a supine position with slight external rotation of the hip to alleviate the pain. Diagnostic workup further includes blood and imaging tests. Most patients will show leukocytosis, elevated erythrocyte sedimentation rate and blood urea nitrogen. Computed tomography (CT) is the imaging test of choice and has higher sensitivity and specificity than ultrasound. Magnetic resonance imaging (MRI) is rarely used because it is associated with discomfort and costs more than other tests without an increase in specificity or sensitivity. Treatment of psoas abscess consists of antibiotic administration and surgical drainage. The type of antibiotic administered depends on whether the condition is primary or secondary, and is subsequently adjusted when the culture and sensitivity tests are performed. Drainage of the abscess can be surgical or percutaneous under CT guidance. The latter method is preferred because it is non-invasive and is associated with decreased mortality and morbidity. Early diagnosis and treatment is critical to avoid serious complications like septic shock or pulmonary embolism.
A psoas abscess is a medical condition in which a collection of microorganisms form pus over the psoas muscle, which sits in the lumbar - pelvic area. This collection usually has well defined borders and is referred to as an abscess. It can result from spread of infection through the blood from other sites in the body or due to extension from structures that sit close to the psoas muscle. Patients will usually present with very nonspecific symptoms such as malaise, nausea, vomiting, pain in the abdomen, malaise and a limp. The pain tends to also occur in the anterior part of the thigh, because many of the nerves that innervate the psoas muscle also innervate the anterior thigh. The physician will perform specific maneuvers that elicit pain and that may help in establishing the diagnosis. In addition, patients usually like to lie in the supine position with a slightly externally rotated hip to decrease the pain associated with the condition. The physician will also perform particular laboratory and imaging tests. Computerized tomography (CT) is considered the imaging test of choice and is used to establish the diagnosis. Treatment of psoas abscess is managed with antibiotics and drainage. Drainage of the abscess is performed either with open surgical intervention or percutaneously, with CT assistance. The latter method is preferred because it is associated with less complications.