Colloid Milium

 

Salient features

 

·       A rare disorder in which dome-shaped, translucent papules develop in sun-exposed skin

 

·       Four variants are recognized: adult, juvenile, and pigmented colloid milium and nodular colloid degeneration

 

·       Histologically, amorphous, eosinophilic, granular deposits are found within the superficial dermis

 

Introduction and Epidemiology

 

Adult colloid milium is the most common form of the disease. It usually occurs in middle-aged, fair-skinned individuals who have had significant cumulative sun exposure, with a male-to-female ratio of 4: 1. The adult form has also been observed in outdoor oil refinery workers, suggesting an additional effect of petrochemicals. In general, juvenile colloid milium develops before puberty and both an autosomal recessive and an autosomal dominant inheritance pattern have been reported.

 

Pathogenesis

 

Although excessive sun exposure clearly plays a role in adult colloid milium, the precise pathogenesis is not known. Photo induced damage to dermal elastic fibers has been suggested. In juvenile colloid milium, ultraviolet light-induced damage to epidermal keratinocytes has been observed and this may reflect an inherited susceptibility.  Pigmented colloid milium is a variant of the adult form in which there is a combination of excessive sun exposure plus topical application of hydroquinone or phenols.

 

Clinical Features

In both adult and juvenile forms, multiple, dome-shaped, translucent to yellowish papules of soft consistency are seen in areas of chronic sun exposure, e.g. the face (especially periorbitally), ears, posterolateral neck, and the dorsal aspect of the hands and forearms. With minor trauma, lesions can become hemorrhagic. A gelatinous mass appears after incision.

 

In pigmented colloid milium, grouped to confluent gray–brown papules are present on the face, whereas in nodular colloid degeneration, skin-colored to yellowish nodules or plaques develop, often on the face.

 

Pathology

In adult colloid milium, nodules composed of homogeneous eosinophilic colloid material are seen in the papillary dermis, with a thin subepidermal grenz zone.  Within the nodules, there may be clefts and fissures. Although the colloid masses contain degenerated elastic fibers, they often exhibit a staining profile similar to amyloidosis, i.e. positive staining with crystal violet, Congo red and thioflavin T stains. In addition, PAS staining is positive. A negative pan-cytokeratin stain is helpful in distinguishing adult colloid milium from primary cutaneous amyloidosis (amyloid K). Additionally, amyloid fibrils are much thicker than the fibrils of colloid milium when electron microscopy is used.

In juvenile colloid milium, amorphous eosinophilic deposits are also found within the papillary dermis, but when compared to the adult form, there is less clefting and no grenz zone. As with colloid bodies (which are also keratinocyte-derived), nonspecific staining for immunoglobulins, complement, and fibrin is seen. In addition, staining for keratin is positive as in primary cutaneous amyloidosis.

 

In pigmented colloid milium, lightly pigmented islands of colloid material are found in the upper dermis. In nodular colloid degeneration, homogenized amorphous dermal collagen with small fissures and clefts is seen. Whether this entity is a variant of nodular amyloidosis is still under debate. Ultrastructurally, colloid milium is characterized by amorphous and granular material which lacks the straight, non-branching filaments seen in amyloidosis.

 

Treatment

Excision, curettage, dermabrasion, cryotherapy, chemical peels and ablation, using various laser modalities such as long-pulsed erbium-YAG- and CO₂-laser can be performed. Due to the localization, an association with UV exposure has been postulated. Additional topical retinoids such as tretinoin, as well as consistent sun protection, are thus recommended.