Sonographically occult perineal abscess in children was surprisingly common, 28% (61/217). More than half had I&D within 4 hours of a "negative" radiology-obtained ultrasound. Symptoms < 4 days, age < 4 years, and history of MRSA were all associated with false negative ultrasound.
Why does this matter?
Cellulitis of the skin or perineum, as in this study, is treated medically. Abscess is treated surgically. It really matters if you can't rely on the accuracy of ultrasound in children with this type of infection. In the MRSA era, true abscesses may appear sonographically negative, due to the different toxins produced by community acquired MRSA.
Why would it be hard to image this area in kids? I don't get it.
This was a retrospective study of 217 children with perineal infection who underwent ultrasound in radiology. This was not bedside ED ultrasound. They defined sonographically occult abscess as one in which a wound culture collected within 48 hours of a "negative" ultrasound grew a pathogenic organism. They found the miss rate was quite high, 28%, and more than half had an I&D within 4 hours. It is not easy to perform ultrasound of the perineum in children with a painful skin infection. They do not hold still and the exam is uncomfortable, which may have led to the high rate of missed abscesses. Also, community acquired MRSA forms thicker purulent material (isoechoic on ultrasound) and may have led to more false negative ultrasounds. They found on multivariate analysis that age < 4 years, history of MRSA, symptoms < 4 days, and Hispanic ethnicity were associated with an increased risk of false negative ultrasound.
Sonographically Occult Abscesses of the Buttock and Perineum in Children. Pediatr Emerg Care. 2017 Sep 25. doi: 10.1097/PEC.0000000000001294. [Epub ahead of print]
Peer reviewed by Thomas Davis, MD.