Epidermal inclusion cyst of the skin
Asymptomatic left flank swelling for a few months. Approximately 4 x 3 cm tubular structure.
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Targeted superficial high resolution ultrasound for the left lateral abdominal wall at the site of the patient's clinically palpable swelling has been performed revealing a well-defined encapsulated predominantly hypoechoic lesion seen in the subcutaneous fat planes measuring around 4.8 cm in transverse diameter, 2.4 cm in depth and 3.0 cm in maximum craniocaudal diameter. It shows a pseudotestis appearance with a few internal echogenic linear foci and filiform hypoechoic bands. It shows posterior acoustic enhancement with lateral wall shadowing consistent with sonographic picture of cystic appearance. No nearby collections. No significant intralesional or perilesion vascularity on color Doppler imaging. The overlying skin and underlying muscular planes appear unremarkable. No calcifications.
These subcutaneous cysts are formed by the implantation of epidermal components in the dermis and subcutaneous tissue. Thus, the causes can be embryonic, traumatic or secondary to surgical procedures. Epidermal cysts are covered by stratified squamous epithelium with a granular layer.
On ultrasound, they show a variable appearance according to the phase of the cyst. If they are intact, the sonograms will usually show a well defined, rounded- or oval shaped, anechoic or hypoechoic structure located in the dermis and subcutaneous tissue. Also, these cysts may present inner echoes (debris) and sometimes show a “pseudotestis appearance” (brighter inner echoes and anechoic filiform areas) as the result of highly compacted deposits of keratin and cholesterol. Commonly, a connecting anechoic tract to the epidermis (punctum) can be detected on ultrasound.
If complication occur with a rupture, the keratin is spread into the surrounding tissue and inflammation and a foreign body reaction happens which may cause hypoechoic collections, adjacent to the cysts or a big change in the morphology of the cysts that turn into ill-defined hypoechoic structures. Posterior acoustic enhancement, a classic artifact of cystic lesions, is usually conserved during all the phases. A color Doppler ultrasound may show increased blood flow in the periphery of the cysts during the phases of inflammation and rupture, frequently with low- flow vessels.