Psoas muscle abscess

Last revised by Rohit Sharma on 26 Aug 2023

Psoas abscess is a collection of pus located in the iliopsoas muscle compartment. It can occur either via contiguous or hematogenous spread from distant sites.

It has a relatively rare incidence, nevertheless timely diagnosis and treatment is important as it significantly reduces morbidity and mortality.

The incidence is rare, with it being more common in males than females. Median age of incidence varies according to geographic location. In developed countries, median age is 44-58 years, however it can occur in younger patients in the setting of immunosuppression or intravenous drug use.

There is relatively equal frequency of psoas abscesses occurring on the right or left side. Bilateral abscess are rare, with varying reported incidence in the literature.

Timely diagnosis of psoas abscess is important, as the motility rate of first presentation is anywhere from 5-15% 1.

Psoas muscle abscess may present with the classical triad of fever, limp and back pain. However the triad accounts for approximately less than 30% of patients 2,3.

Pain is the most common sign, with up to 90% of patients localizing the pain to the back, flank or lower abdomen 3. Other signs include inguinal mass, anorexia weight loss, nausea, and limp.

The clinical presentation timeline often varies, as symptoms can either be nonspecific and subacute (weeks to months), or patients can present with acute deterioration due to septic shock.

The psoas muscle arises from the anterior surfaces of the transverse processes from T12 to L4-5. As it runs down the medial margin of the superior aperture of the lesser pelvis, it enters the thigh to pass posterio-inferior to the lesser trochanter of the femur. At its inferior part, it often fuses with iliacus to form the iliopsoas muscle.

Psoas abscesses typically form in this iliopsoas compartment, located in the retrofascial space (theoretical space posterior to the pararenal space containing the psoas muscle).

Psoas muscle abscess may be classified as primary or secondary depending on the presence or absence of underlying disease. Secondary abscesses are more common.

Primary psoas muscle abscess occurs probably as a result of hematogenous spread of an infectious process from an occult source in the body. Primary psoas muscle abscess can occur in patients with:

Secondary causes result from direct spread of infection from adjacent structures. Secondary psoas abscesses can occur in patients with:

A positive psoas sign can raise suspicion of pathology involving the psoas muscle, but is in itself non-specific.

Ultrasound has low sensitivity and specificity for the diagnosis of psoas abscess. Often it is obscured by bowel gas or the pelvic bone. It may be diagnostic in only 50% of cases.

CT is often the modality of choice for abscess detection. Radiographic features may include diffuse enlargement of the psoas muscle with an area of central low density (specifically on contrast studies) 4. Gas bubbles may be visualized, however are uncommon.

Other findings include indistinct margins, infiltration of surrounding fat and lesions can be uni or multi-loculated 4,5.

MRI can be used to diagnose psoas abscesses and simultaneously rule out other causes of back pain. However given its availability and scan time, MRI is often not the first modality of choice. Features are similar to CT with diffuse psoas major enlargement.

T2 and STIR can demonstrate a hyper-intense collection on the psoas region.

Appropriate antibiotics along with drainage of the abscess are the treatment of choice. This is often treated with image-guided percutaneous drainage, typically CT due to the retroperitoneal location. Nevertheless, ultrasound guided drainage is an option in the absence of CT.

Surgical drainage (either open or laprascopic) is an alternative option to image guided drainage. Indications for surgery may include multiloculated abscesses or failure of percutaneous drainage.

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