Milia are small, benign, superficial keratinous cysts that resemble miniature infundibular cysts on histopathology.¹ Hubler et al² followed the Epstein and Kligman³ classification in dividing milia into primary spontaneous milia and secondary milia associated with various skin conditions or trauma, but they also proposed the additional term “milia en plaque” (MEP) for unusual cutaneous lesions consisting of multiple milia on an erythematous edematous base. Primary milia have been further classified into several different forms based on demographic and clinical characters. These are congenital, benign primary of children and adults, en plaque, and multiple eruptive milia.

Congenital milia are common in newborns. In various studies of cutaneous lesions in newborn infants with patient databases of 420 to 5387 infants, the incidence of milia was found to be between 7.5% and 36%.⁴,⁵,⁶,⁷,⁸,⁹,¹⁰,¹¹ Congenital milia present with a few or numerous lesions and most occur on the face, scalp, upper aspect of the trunk, and upper extremities. There are no significant racial or gender differences. Lesions tend to resolve spontaneously within weeks to several months.

Benign primary milia of children and adults develop spontaneously. Unlike congenital primary milia, they occur most often on the cheeks, eyelids, forehead, and genitalia, and lesions tend to be more persistent than congenital lesions, without spontaneous resolution.¹

MEP is a relatively rare variant characterized by erythematous plaques containing numerous milia.²,¹²,¹³,¹⁴,¹⁵,¹⁶ Lesions are generally unilateral, may be indurated, and are usually located on the head and neck, the periorbital region, the nasal bridge, or the trunk. MEP is more common in middle-aged adults with a female predominance.¹ It can be associated with pseudoxanthoma elasticum,¹⁷ discoid lupus erythematosus,¹⁸ lichen planus,¹⁹ trauma, and renal transplantation,²⁰ but also arises in healthy individuals. Dogra et al²⁰ suggested that cyclosporine immunosuppression may predispose patients to MEP.

Multiple eruptive milia occur spontaneously in very large numbers.²¹,²²,²³,²⁴,²⁵ Cases have been reported in patients from teens to old age, occurring most often on the head, upper trunk, and upper extremities. Lesions continuously erupt over weeks to months. Ratnavel et al²⁶ described a variant of eruptive milia restricted to the eyelids in a family over three generations.

Milia can be associated with several genodermatoses, rare inherited disorders with skin manifestations.¹ Bazex-Dupre-Christol syndrome (BDCS) is an X-linked dominant disorder consisting of congenital hypotrichosis, follicular atrophoderma, and basal cell carcinoma, and milia are seen in 75% of cases.²⁷ Rombo syndrome resembles BDCS with atrophoderma vermiculatum of the face, multiple milia, telangiectases, acral erythema, and a propensity to develop basal cell carcinomas.²⁸ Atrichia with papular lesions is an autosomal recessive disorder,²⁹ characterized by alopecia and widespread milia, especially on the cheek, scalp, elbows, and knees.³⁰ They tend to appear during the first decade of life and lesions may increase in number with age or partially regress.³¹ Hereditary vitamin D-dependent rickets type IIA is an inherited disorder in which 50% of patients have milia on the face, scalp, and upper trunk, appearing in childhood. Basal cell nevus syndrome (Gorlin syndrome) is an autosomal dominant disorder characterized by numerous basal cell carcinomas, odontogenic keratocysts, rib abnormalities, abnormal facies, spina bifida occulta, calcified falx cerebri, comedones, palmoplantar pitting, chalazion, and cleft palate or lip. Milia occur in about 30% of patients,³²,³³ mostly located in the periorbital region or on the forehead.

Secondary milia represent a localized form of milia that may be associated with systemic disease, medication, or trauma. Diseases reported with secondary milia include bullous pemphigoid, herpes zoster,³⁴ contact dermatitis,³⁵ bullous lupus erythematosus,³⁶ lichen sclerosus,³⁷ Stevens-Johnson syndrome,³⁸ bullous erysipelas,³⁹ bullous amyloidosis,⁴⁰ lichen planus,⁴¹ leprosy,⁴² pseudoxanthoma elasticum,¹⁷ lupus erythematosus,⁴³ phototoxic reactions,⁴⁴ and allergic contact dermatitis.⁴⁵

Several medications have been associated with secondary milia. These include sorafenib for the treatment of unresectable hepatocellular carcinoma,⁴⁶ benoxaprofen,⁴⁷ topical steroids,⁴⁸ 5-fluorouracil,⁴⁹ cyclosporine,²⁰ and penicillamine.⁵⁰

Superficial trauma is a common cause of secondary milia in children. Other traumatic injuries causing secondary milia include dermabrasion,⁵¹ radiotherapy,⁵² chemical peels,⁵³ skin grafts,⁵⁴ and ablative laser therapy.⁵⁵