Psoas Abscess: A Primer for the Internist

, Department of Internal Medicine, Lincoln Health Center and Durham Regional Hospital, Durham, NC

South Med J. 2001;94(1) 

In This Article

Diagnosis

Symptoms are often nonspecific. Patients may present with fever, flank pain, abdominal pain, or limp. Because of the innervation of the psoas muscle by L2, L3, and L4, pain due to its inflammation sometimes radiates anteriorly to the hip and thigh. Other symptoms are nausea, malaise, and weight loss. These symptoms bring to mind other clinical syndromes that are more commonly seen by primary care physicians ( Table 2 ).

A good physical examination is critical for the prompt diagnosis of psoas abscess. The diagnosis may be given away, if the patient is noted to favor the position of greatest comfort. This is the supine position, with the knee moderately flexed and the hip mildly externally rotated. Rarely, psoas abscess is associated with painless subinguinal mass. There are well-described signs of psoas inflammation,[8] which the clinician should look for in every patient suspected of having psoas abscess ( Table 3 ). The premise of these tests is that the psoas muscle is the primary hip flexor. Flexion and stretching or contraction of the inflamed psoas muscle results in pain.

Laboratory tests are helpful in the evaluation of suspected psoas abscess. Leukocytosis (mean count, 15,900/mm3), elevated erythrocyte sedimentation rate (ESR) (mean, 90 mm/hr), and elevated blood urea nitrogen (BUN) (mean, 30.5 mg/dL) were reported in 100% of patients in the series from Johns Hopkins.[3] These are not universal findings. Pyuria is sometimes present.[9] As in most clinical problems, diagnosis is aided by appropriate radiologic testing. Before the availabilty of ultrasonography and CT, many cases of retroperitoneal abscess were diagnosed at autopsy. Whenever psoas abscess is suspected, CT should be done for definitive diagnosis. This has superceded ultrasonography as the radiologic test of choice. Ultrasonography is diagnostic in only 60% of cases of psoas abscess,[9] compared with 80% to 100% for CT.[10] Sensitivity and specificity of diagnosing psoas abscess is not improved by magnetic resonance imaging (MRI), and with its higher cost and greater patient discomfort, MRI has no role in the diagnosis of psoas abscess.

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