LAUSR

I wish to present a case of foreign body within an abscess, in order to highlight the importance of routine point-of-care ultrasound (POCUS) in these cases. A 9-year-old girl presented with an area of induration and redness of the left thigh. She denied a history of trauma to that area. She had an area of induration at that same location 1 year earlier treated successfully with oral antibiotic. On POCUS, an anechoic hypovascular abscess cavity was seen to have a distinct hyperechoic object in its center (Figure 1). Drainage and extensive exploration under sedation revealed a keratinaceous material compatible with keratinous cyst content. The foreign body was consistent with the structure seen on POCUS, likely a remnant of an epidermal inclusion cyst. Superficial masses, including abscesses, are best imaged with a linear high frequency transducer. Color Doppler should be applied with minimal pressure to view vascularity. Copious gel can be applied, and the hand holding the transducer can be stabilized with the contralateral hand, in order to avoid putting pressure on the region and affecting flow to the area. Poorly defined borders, septation, and swirling of fluid content under the pressure of the transducer can support clinical examination findings of erythema and tenderness consistent with an abscess. Keratinous cysts, more commonly known as epidermal inclusion cysts, can be congenital and can be seen after local trauma. They are typically subcutaneous but extension into the dermis can be seen. Less than 10% occur on the extremities. A ruptured epidermal inclusion cyst can have irregular borders similar to the image seen in our case. The keratin acts as a foreign body, and can impair healing of the abscess cavity or cause recurrence. Had POCUS not been performed, the cavity would have been opened and drained, but keratinous material might have been retained. The POCUS image revealing this foreign body within the abscess cavity caused the treating physician to ensure that the cavity was thoroughly probed and irrigated until the foreign body was retrieved. This case highlights the importance of POCUS prior to abscess drainage.