• Aplasia cutis congenita (ACC) refers to a congenital defect that results in a localized area of absent skin at birth and is most commonly seen on the scalp (Fig. 1.1).

• Usually there is absence of the epidermis and dermis in the affected area but occasionally subcutaneous tissues, bone, and dura can also be missing.

• The exact cause of ACC is unknown and most cases are sporadic.

• Proposed theories include genetic factors, birth trauma, teratogens, and intrauterine infection.

• A popular hypothesis suggests that ACC is the result of compromised vasculature of the placenta.

• Most often ACC presents on the scalp as a single, localized area of smooth alopecia in close proximity to the hair whorl.

• Less commonly, ACC presents as multiple areas of alopecia on the scalp or can be found on the face, trunk, or extremities (Fig. 1.2).

• ACC can have a variable appearance at birth including a well-formed hairless scar, ulceration with a granulating base, a superficial erosion, or a translucent, glistening membrane (“membranous aplasia cutis”—uncommon variant) (Fig. 1.3).

• Lesions are usually sharply demarcated, oval, circular, or stellate, and measure 1 to 3 cm in diameter.

• ACC is an isolated defect in the majority of cases, but it can be associated with other developmental anomalies or a feature of a variety of syndromes.

• A ring of long, dark hair around membranous aplasia cutis (the hair collar sign) is thought to herald an underlying neural tube defect (see Fig. 1.2).

* This consists of a hand-held magnifier that allows inspection of skin lesions unobstructed by skin surface reflections.

• Diagnosis is clinical.