It is believed that secondary milia originate from the eccrine glands. They are secondary to various processes and appear over time. This is a localised form that can be associated with a disease, medication or trauma.
Trauma.
Traumas can lead to the development of secondary milia. A study conducted by Samuel MONASH has shown that theuse of an abrasive treatment for acne scars could result in the formation of milia. In 25% of patients treated more than once, milia formed two months after the second or third treatment.
Two possible reasons have been identified: some follicles may close due to the abrasive action of the treatment, or pieces of epithelium may have become embedded in the skin during the treatment, which would block the hair follicles. Similarly, the use of cosmetic products that are harsh or abrasive actions on the skin can cause milia.
Exposure to UV rays.
Exposure to UV rays could also contribute to the formation of secondary milia. Similar to trauma, UV rays have numerous abrasive effects, including the induction of cellular lesions and the death of keratinocytes. These situations weaken the skin barrier and hinder cellular renewal, which would promote the appearance of milia.
Long-term application of topical corticosteroids.
It is well-known that the long-term application of topical corticosteroids can lead to skin atrophy. Indeed, corticosteroids cause a loosening and marked degeneration of collagen and elastic fibres around hair follicles, which may be the root cause of milia formation. However, few studies have been conducted and little evidence has been observed.
Dermatological diseases.
Dermatological diseases, such as theepidermolysis bullosa, can be the root cause of the appearance of milia, as previously mentioned. Epidermolysis bullosa is an autoimmune disease. The body, following a disruption of the immune system, produces antibodies against itself, and particularly against the type VII collagen that maintains dermal cohesion. This results in significantskin fragility. This leads to skin fracturing and, consequently, skin detachment at the slightest trauma. The skin's healing will be poor and it is possible to see milia forming. The precise causes, however, remain unknown.