Dermatology MCQ - Infiltrative and Neoplastic Disorders - Vellus hair cysts

An adolescent patient presents for evaluation of an asymptomatic eruption on the chest, characterized by numerous, small, 1-3 mm, follicular, skin-colored to hyperpigmented papules. Some of the papules have a central umbilication. A single lesion is incised, and a small amount of cheesy material and tiny vellus hairs are expressed. What is the most likely diagnosis and its typical inheritance pattern?

A. Steatocystoma multiplex, which is often sporadic.
B. Eruptive vellus hair cysts, which are often sporadic but can be autosomal dominant.
C. Keratosis pilaris, which is often autosomal dominant.
D. Favre-Racouchot syndrome, which is acquired from chronic sun exposure.
E. Trichostasis spinulosa, which is an acquired condition.

Correct Answer: B. Eruptive vellus hair cysts, which are often sporadic but can be autosomal dominant.

Answer and Explanation

The correct answer is B. This question describes the classic presentation of eruptive vellus hair cysts (EVHC). The key clues are the age of onset (childhood/adolescence), the location (chest is most characteristic, but also limbs), the appearance of numerous small, follicular papules, and the pathognomonic finding of vellus hairs within the expressed cyst contents. EVHC can be sporadic, but familial cases with an autosomal dominant inheritance pattern are well-documented.

Why the Other Options are Incorrect:

• A. Steatocystoma multiplex: This is the closest mimic. Steatocystomas are also cysts that appear in adolescence on the chest and upper arms. However, they typically contain a yellowish, oily liquid rather than cheesy keratin and vellus hairs. Histologically, steatocystomas are lined by a thin epidermal wall with sebaceous glands in the cyst wall, which is diagnostic.

• C. Keratosis pilaris: This is extremely common and presents as rough, follicular papules, often with surrounding erythema, typically on the upper arms and thighs. The papules are due to keratin plugs, not cysts, and do not contain expressible vellus hairs or cheesy material.

• D. Favre-Racouchot syndrome: This is a condition of elderly, sun-damaged skin, characterized by large open comedones and cysts on the periorbital area, not an eruptive condition in an adolescent.

• E. Trichostasis spinulosa: This presents as dark, follicular spicules that are actually bundles of vellus hairs trapped within a dilated follicle. It is most common on the nose and central face of older adults. While it involves vellus hairs, it is not a cystic condition and the lesions are spiny, not papular with central umbilication.

Additional High-Yield Information for Exams:

• Histopathology: The definitive diagnosis is made by histology. The cyst is located in the mid-dermis and is lined by stratified squamous epithelium. The crucial finding is that the cyst lumen is filled with laminated keratin and numerous transversely and obliquely cut vellus hairs.

• Pathogenesis: EVHC are thought to arise from the midportion of the hair follicle (the isthmus), where the cyst forms and traps vellus hairs.

• Differential Diagnosis: As outlined above, the main differential is steatocystoma multiplex. The table below highlights the key differences:

FeatureEruptive Vellus Hair Cysts (EVHC)Steatocystoma MultiplexContentsCheesy keratin & vellus hairsOily, yellow liquidCyst LiningStratified squamous epitheliumThin, eosinophilic lining with flattened sebaceous glandsClinical ClueOften hyperpigmented, especially in darker skinCysts may be more translucent/yellowish

• Prognosis: The course is variable. Lesions may persist, resolve spontaneously, or inflame. Spontaneous resolution, sometimes with post-inflammatory hypopigmentation, is not uncommon.

• Management & Rationale:

  • Rationale: Treatment is challenging and primarily for cosmetic reasons, as the condition is benign. There is no universally effective cure.

  • First-line: Reassurance about the benign nature of the condition.

  • Treatment Options: If treatment is desired, options include:

    • Incision and Expression: Effective for individual lesions but impractical for widespread disease.

    • Topical Therapies: Topical retinoids (tretinoin) may help by reducing keratin plugging and promoting extrusion of cyst contents, but results are often modest.

    • Ablative Lasers: CO2 or erbium:YAG laser can be effective by vaporizing the cyst roof and allowing the contents to be expelled.

    • Medical Therapy: Oral isotretinoin has been used with some success in severe cases, but cysts often recur after discontinuation.