Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.
A 59-year-old woman is evaluated for a 1-week history of increasing pain of the right foot. She recalls stepping on a nail about 1 month before her symptoms began. The patient has a 5-year history of heart failure secondary to idiopathic dilated cardiomyopathy. She has an implantable cardioverter-defibrillator, and her current medications are carvedilol, lisinopril, furosemide, and spironolactone.
On physical examination, vital signs are normal. Examination of the foot reveals tenderness and warmth directly below the proximal fifth metatarsal bone.
A radiograph of the right foot is normal.
Which of the following is the most appropriate next step to establish the diagnosis?
A: CT scan
B: Gallium scan
C: MRI
D: Three-phase bone scan
MKSAP Answer and Critique
The correct answer is A: CT scan.
The next study that should be performed is a CT scan of the foot. The clinical hallmarks of acute osteomyelitis are local pain and fever, particularly in patients with acute hematogenous osteomyelitis, but these symptoms may be absent in patients with chronic and contiguous osteomyelitis.
Given the limitations of physical examination findings in the diagnosis of osteomyelitis, radiologic studies are frequently used. In patients in whom radiographic results are negative but clinical suspicion for osteomyelitis remains high, MRI is indicated. MRI scans show changes of acute osteomyelitis within days of infection and are superior to and more sensitive (90%) and specific (80%) than plain films and CT scans; can detect soft tissue abscesses and epidural, paravertebral, or psoas abscesses possibly requiring surgical drainage; and can delineate anatomy before surgery. Nonetheless, false-positive MRI results may occur in patients with noninfectious conditions such as fractures, tumors, and healed osteomyelitis. In patients with a pacemaker or metal hardware precluding MRI or in those in whom MRI results are inconclusive, CT scans or (if metal hardware is likely to impair CT imaging) nuclear studies may be used instead of MRI. CT reveals excellent anatomic imaging details, and it is the imaging study of choice for patients with osteomyelitis when MRI cannot be obtained.
Nuclear imaging studies can reliably detect the presence of inflammation related to acute infection. However, such visualized abnormalities, which may be caused by bone turnover or inflammation, can also be from other noninfectious causes, including trauma, neoplasm, and degenerative joint disease. Gallium scanning, once a gold standard for cancer diagnosis, may still be used to visualize inflammation and chronic infections, partly because gallium binds to the membranes of neutrophils recruited to a site of infection. However, leukocyte-labeled nuclear scans have almost entirely replaced this imaging technique. Except in the setting of diminished blood flow to the affected area, a negative three-phase bone scan confers a high negative predictive value for osteomyelitis.
Key Point
- CT scan is the imaging study of choice for suspected osteomyelitis when MRI cannot be performed.
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