Cysts of the cutaneous appendages can occur anywhere on the skin, with most arising from the infundibular portion of the hair follicle.¹ Other cysts arise from sweat glands or within the tarsal meibomian glands or show morphological similarities to different portions of the folliculosebaceous apparatus, and these have been described under a wide variety of different names. Ackerman et al² suggested that cutaneous cysts should be named based on the normal epithelial structure of those portions of the folliculosebaceous unit that the lining of the cyst most closely resembles. Based on this approach, a trichilemmal cyst is an isthmic-catagen cyst and a primary epidermal cyst is an infundibular cyst. Sebaceous duct cysts and steatocystomas most closely resemble sebaceous gland duct epithelium,¹ so that both can be grouped under the term sebaceous duct cyst.

A variety of different cysts occur on the skin. In the periorbital region, these most commonly include apocrine and eccrine hidrocystomas, intratarsal keratinous cysts, and cutaneous keratinous cysts. Among the cutaneous keratinous cysts, a large number of different labels have been applied with little consistency in usage or definition, resulting in confusion of terminology in the literature. For this book, we adopt a simple definition and divide the more common cutaneous cysts into hidrocystomas (Chapter 70), cutaneous keratinous cysts (Chapter 77), intratarsal keratinous cysts (Chapter 78), and sebaceous duct cysts, all discussed in separate chapters. Hidrocystomas arise from obstruction of sweat glands. Keratinous cysts accumulate keratin of various types within the cyst cavity and derive from the epithelium of different portions of the pilosebaceous apparatus. Sebaceous duct cysts trap sebum produced from the occlusion of apocrine sebaceous glands. In 1930, Ormsby and Finnerud³ first recognized that the lining of steatocystomas resembles a sebaceous duct and proposed that these cysts arise by pathologic keratotic blockage of the duct at its entrance onto the skin surface. Ackerman et al⁴ noted that, in the diagnosis of steatocystoma, sebaceous lobules are not necessary, arguing that cutaneous keratocyst and steatocystoma be united under the term sebaceous duct cyst.

Sebaceous duct cysts (SDCs) are superficial cutaneous cysts caused by obstruction of the outflow duct of the sebaceous glands associated with the pilosebaceous follicular unit. It has also been referred to in the literature as steatocystoma simplex and keratocyst.⁵ They are nonheritable, solitary, benign adnexal lesions seen most often on the forehead, nose, scalp, neck, axillae, chest, upper limbs, back, legs, and the oral mucosa.⁶,⁷ Males and females are affected equally, and patients range in age from the second to the eight decades.⁶,⁷,⁸

SDCs are usually found on the skin, and eyelid involvement is rare.⁶,⁹,¹⁰,¹¹,¹²,¹³,¹⁴,¹⁵ But since the caruncle also harbors skin elements such as hair follicles, sebaceous glands, sweat glands, and accessory glands, the caruncle may develop SDCs similar to those found in the skin.¹²,¹⁶,¹⁷ A case was described of a follicular hybrid cyst of the tarsus, which included foci of pilomatricoma and some foci of steatocystoma.¹⁸

Steatocystoma multiplex (SM) is an uncommon benign disorder that shares similar pathological features and is considered a counterpart of steatocystoma simplex. It is seen most often on the chest and less commonly on the abdomen, upper arms, armpits, and face. Unlike steatocystoma simplex which arises as a sporadic lesion, SM is generally inherited in an autosomal dominant fashion. Lesions are thought to arise from an abnormal lining of the sebaceous duct with onset at puberty, likely due to the hormonal stimulus of the pilosebaceous unit.¹⁹,²⁰ The cysts are generally 2 to 4 mm firm bumps that contain an oily, yellow liquid and can sometimes contain one or more hairs.