hello
mrdoctor
Back to Health A-Z

Psoas Abscess

Authored and reviewed by medical professionals

Psoas abscess is a collection of pus in the psoas muscle, usually caused by infection, leading to back, flank, or hip pain and difficulty walking.

Definition

The psoas muscle abscess can be defined as a fluid (pus) collection, which is located in the retrofascial space. The abscess is found in this space and not in the retroperitoneal space, due to the location of the psoas muscle in the iliopsoas compartment. This compartment is found posterior to the transversalis fascia, the latter being one of the boundaries defining the retroperitoneum (posterior).

The appearance of pus in the iliopsoas compartment is rare but the advancements in the imaging diagnosis (CT in particular) have allowed for this diagnosis to be more frequently made. In the past, the majority of the cases were diagnosed during the autopsy. Unfortunately, there are still many situations in which the diagnosis is missed or made when it is too late (potential for severe morbidity).

Anatomy

In order to understand how the psoas abscess develops, you must also be familiar with the anatomy of the region. First of all, the psoas muscle stems from an area that is located between the 12th thoracic and 5th lumbar vertebrae (transverse processes and lateral aspects of the said vertebral bodies). Leaving its origin, it goes along the pelvic brim and passes deep from the inguinal ligament. When it reaches the anterior part of the hip joint capsule, it forms a tendon and it inserts itself into the lesser trochanter of the femur. It is the joined by the iliacus muscle, which inserts itself using the same tendon. The main purpose of the psoas and iliacus muscles is to promote the flexion of the hips. In 30% of the people, the psoas muscle is accompanied by a smaller muscle. This is known as the psoas minor muscle, being located anterior to the psoas major.

The two muscles unite, resulting in the iliopsoas muscle – this is located in the compartment bearing the same name (extraperitoneal location). The tendon of the muscle is actually separated from the capsule of the hip by the iliopsoas bursa. In 15% of the people, the iliopsoas bursa communicates with the hip joint space. This is considered as one of the factors that facilitate the spread of the infection from one site to the other.

There are a number of important structures that are located in the vicinity of the iliopsoas muscle, including the iliac lymph nodes, hip joint, appendix, sigmoid colon, abdominal aorta and vertebral bodies. These structures might also facilitate the spread of the infection to the psoas muscle, leading to the appearance of the abscess.

Symptoms of Psoas Abscess

These are the most common symptoms that are encountered in patients who suffer from the psoas abscess (non-specific at the debut):

  • Fever
  • Pain

    • More often – flank and general, abdominal pain
    • The pain increases in severity with the passing of time
    • May also affect other parts of the body, such as the lower back, buttocks, groin or hips
    • In the situation that the pain has radiated to the hip or thigh, this means that the L2-L3-L4 dermatomes have been affected (source of infection)

  • Limping
  • Other symptoms

    • Nausea
    • State of general malaise
    • Weight loss

It should also be mentioned that the psoas abscess can lead to inflammation in the upper medial part of the thigh. This occurs because the iliopsoas has a proximal attachment and it can drain in the respective area, thus leading to the above-mentioned swelling. Such changes are generally encountered in patients who also suffer from lumbar tuberculosis.

Pathology

The abscess located at the level of the psoas muscle can be classified into primary or secondary. While the primary psoas abscess appears on its own, the secondary one is always accompanied by an underlying medical problem.

It is known that the primary psoas muscle abscess appears as the infection from another part of the body reaches it (hematogenous spread). Patients who present the following medical problems can suffer from the primary form of psoas abscess: reduced state of immunity (immunosuppression), renal failure, AIDS, intravenous drug abuse and diabetes mellitus.

On the other hand, the secondary psoas muscle abscess is caused by the spread of the infection caused by a gastrointestinal disease. Among the most common causes that lead to the appearance of such problems, there are: perforated colon carcinoma, Crohn’s disease, diverticulitis, appendicitis and vertebral osteomyelitis. Both renal disease (pyelonephritis) and spondylodiscitis can lead to the appearance of secondary psoas abscesses.

Among the most common infectious organisms that can lead to the appearance of the psoas abscess, there are: staphylococcus, streptococcus, gram negative enteric bacteria and tuberculosis bacteria. The staphylococcus is, by far, the most often found when it comes to the psoas abscess. On the other hand, the tuberculosis bacteria is either found in those who are immunosuppressed or in endemic areas.

There are a number of risk factors to be considered when it comes to the appearance of the psoas abscess, such as: immunosuppression (encountered in patients who suffer from HIV or different forms of malignancy), tuberculosis (also because of the immunosuppression, with a high risk of the infection spreading from one part of the body to the other), local recent surgery, trauma in the area, infected hematoma at the level of the psoas muscle, hemophilia (this is related to the fact that these patients can develop hematoma at the level of the psoas, which can become easily infected – thus leading to the psoas abscess).

Diagnosis of Psoas Abscess

These are the most common methods used for the diagnosis of the psoas abscess:

  • Medical history

    • Trauma
    • Musculoskeletal injuries (recent)

  • Physical examination

    • Identification of fever (high-running)
    • Point tenderness at the level of the back (at the debut – this may be absent or diffuse)
    • Psoas irritation is present

      • The patient assumes a position of comfort – in supine, with the knee maintained in a moderate flexion and the hip in a mild external rotation
      • Movement, such as the resisted hip flexion or weight bearing elicits a lot of pain at the level of the hip

  • Laboratory studies

    • Glucose (suspicion of diabetes mellitus)
    • FBC (leukocytosis)
    • CRP (C-reactive protein)
    • UEC (urea, electrolytes and creatinine)
    • Septic screen – includes cultures of the blood and also urine analysis (so as to exclude the urinary tract infection)

  • Imaging studies

    • Both the CT and MRI can be used for the confirmation of the diagnosis (diagnostic in 80-100% of the cases)
    • The CT might excluded other potential diagnoses
    • The MRI is considered the most faithful investigation; best test to exclude the epidural abscess
    • The ultrasonography can also be used for the detection of the psoas abscess, even though it is not as faithful as the CT or MRI (diagnostic in 60% of the cases).

The suspicion for the psoas abscess increases if the patient does not present any history of trauma or musculoskeletal injuries.

Complications

These are the most often encountered complications of the psoas abscess: epidural abscess, osteomyelitis, pyelonephritis, septicemia and septic shock.

Treatment

These are the most common treatment approaches employed for the patients suffering from the psoas abscess:

Antibiotics

  • Elimination of the infectious agent (bacteria)
  • Brings the necessary relief from the symptoms
  • Empiric antibiotics are generally recommended – flucloxacillin, vancomycin
  • Specific antibiotics for the treatment of tuberculosis (along with supportive care, if necessary)
  • Long-term treatment might be necessary in order to eliminate all sources of infection.

Drainage of the psoas abscess

  • Image guided percutaneous drainage (CT) may be used for the actual drainage process, due to the location of the psoas abscess (retroperitoneal)
  • The surgical drainage is recommended in the patients in which the image-guided percutaneous drainage has failed to provide the desired results (10% of the cases).

Severe cases can benefit from:

  • Supportive care and monitoring
  • Resuscitation (rarely required) – performed in patients who suffer from septicemia and who have entered into the state of septic shock.

Related Health Topics