A milium is a small, superficial, keratin-filled cyst. Most patients present with multiple cysts and are thus diagnosed with milia.
Milia are off-white or pearly, superficial lesions that generally measure less than 3 mm in diameter . They are slightly raised above the skin surface.
With regards to the single entities, the following shall be stated:
- Congenital milia may indicate more severe pathologies, e.g., Bazex-Dupré-Christol syndrome, Brooke–Spiegler syndrome, oral-facial-digital syndrome type I, pachyonychia congenita  . Accordingly, neonates may present with the respective developmental defects.
- Many neonates develop primary milia not associated with hereditary disorders  . There are usually multiple lesions around the nose and in other facial areas, but the neonate's trunk and extremities may also be affected. Spontaneous remission is to be expected.
- Juvenile and adult milia largely resemble those observed in neonates and are very common. However, the condition is usually more persistent and it may take longer until spontaneous remission occurs. Predilection sites are periorbital regions, cheeks, nose, forehead and chest.
- Individuals affected by milia en plaque present with multiple milia grouped on an erythematous plaque . Patients suffering from discoid lupus erythematosus, lichen planus, or pseudoxanthoma elasticum have been suggested to be predisposed to this type of milia. Milia en plaque commonly manifest in the head and neck region, especially in periauricular, periorbital and nose areas.
- Multiple eruptive milia may be diagnosed if patients report the unexplained appearance of crops of milia within a short period of time . Head, neck, and trunk are most frequently affected.
A common method that a dermatologist will use to remove a milium is to nick the skin with a #11 surgical blade and then use a comedone extractor to press the cyst out. See also Eruptive vellus hair cyst Sebaceous hyperplasia Seborrheic keratosis [en.wikipedia.org]
Multiple eruptive milia Crops of numerous milia appear over a few weeks to months. Lesions may be asymptomatic or itchy. Most often affect the face, upper arms and upper trunk. Traumatic milia Occur at the site of injury as the skin heals. [dermnetnz.org]
Multiple eruptive milia presents as groups of numerous lesions which form over a long period, which may be accompanied by itching. These lesion groups typically appear on the upper arms, face, or upper torso. [ozarkderm.com]
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Milia are diagnosed clinically. A small incision into the lesion allows for the expression of keratinous material, which confirms the established visual diagnosis . Although not usually required, tissue samples may be obtained for histopathological analyses. Microscopically, milia correspond to epidermal, keratin-filled cysts delimited by several layers of stratified squamous epithelium. These cysts originate in the infundibulum of the hair follicle. In the case of milia en plaque, immune cell infiltrates can be observed in close proximity to milia .
A milium is a single, superficial, keratin-filled cyst. However, few patients present with solitary lesions and thus, research papers and case reports usually refer to milia. Besides cosmetic issues, milia are benign and not known to be associated with morbidity and mortality. Although they are very common, evidence regarding their etiology and pathogenesis is scarce. This particularly applies to primary milia, which arise spontaneously. In contrast, secondary milia develop after trauma, on sunburned skin, or sites affected by dermatological conditions related to the presence of blisters and bullae. The topical application of corticosteroids has also been related to the onset of milia . Milia have been reported to develop after radiotherapy .
The following types of milia may be distinguished:
- Congenital milia
- Neonatal, juvenile, and adult-onset milia
- Milia en plaque
- Multiple eruptive milia
- Trauma-associated milia
Of note, Epstein pearls and Bohn's nodules, i.e., gingival and palatal cysts, display certain similarities to milia and have repeatedly been referred to as the oral counterparts of the cutaneous condition. However, Spanish researchers could not confirm the association between the respective disorders  and they should be treated as individual entities .
- Tsuji T, Kadoya A, Tanaka R, Kono T, Hamada T. Milia induced by corticosteroids. Arch Dermatol. 1986;122(2):139-140.
- Lee A, Griffiths WA. Multiple milia due to radiotherapy. J Dermatolog Treat. 2002;13(3):147-149.
- Monteagudo B, Labandeira J, Cabanillas M, Acevedo A, Leon-Muinos E, Toribio J. Prevalence of milia and palatal and gingival cysts in Spanish newborns. Pediatr Dermatol. 2012;29(3):301-305.
- Lewis DM. Bohn's nodules, Epstein's pearls, and gingival cysts of the newborn: a new etiology and classification. J Okla Dent Assoc. 2010;101(3):32-33.
- Berk DR, Bayliss SJ. Milia: a review and classification. J Am Acad Dermatol. 2008;59(6):1050-1063.
- Rutter KJ, Judge MR. Profuse congenital milia in a family. Pediatr Dermatol. 2009;26(1):62-64.
- Kansal NK, Agarwal S. Neonatal milia. Indian Pediatr. 2015;52(8):723-724.
- Terui H, Hashimoto A, Yamasaki K, Aiba S. Milia En Plaque as a Distinct Follicular Hamartoma With Cystic Trichoepitheliomatous Features. Am J Dermatopathol. 2016;38(3):212-217.
- Gonul M, Benar H, Gokce A. Multiple eruptive milia on scalp. J Eur Acad Dermatol Venereol. 2015.
- Zuber TJ. Minimal excision technique for epidermoid (sebaceous) cysts. Am Fam Physician. 2002;65(7):1409-1412, 1417-1408, 1420.