What Are These White Papules With an Erythematous Base on an Older Man’s Arm?

Answer: B. Eruption of secondary milia

Due to the morphology of the lesions and recent history of exposure to poison ivy, the diagnosis favored an eruption of secondary milia following allergic contact dermatitis from poison ivy.

Discussion. Milia are small, white, superficial keratinous cysts that form when keratin becomes trapped beneath the skin’s surface.¹Milia can be primary or secondary to another process. Primary milia are much more common and occur most often on the face.¹ Secondary milia can occur after various blistering or traumatic skin conditions, such as porphyria cutanea tarda, bullous pemphigoid, pemphigus vulgaris, epidermolysis bullosa, herpes zoster, second-degree burns, dermabrasion, radiotherapy, and, less commonly, allergic contact dermatitis.¹ Secondary milia have also been reported after long-term topical corticosteroid therapy with underling atrophy,² but the patient’s short 2-week trial with a steroid cream made this diagnosis less likely.

The pathogenesis of primary and secondary milia differ. Primary milia are thought to originate from the sebaceous collar of vellus hairs, whereas secondary milia are believed to derive from eccrine ducts.¹ It has been hypothesized that formation of secondary milia following blistering skin rashes are secondary to disruption of the dermal-epidermal junction, followed by a regeneration process of the disrupted sweat glands or hair follicles.³ Another theory is that these milia are caused by an aberrant interaction between the hemidesmosomes and the extracellular matrix components beneath the hemidesmosomes.³ There also seems to be a correlation between the severity of the initial blistering rash and the development of secondary milia, with more severe blistering, leading to a higher likelihood of milia development. The exact pathogenesis for this is unclear, but a running hypothesis is that the blisters would have to extend deep enough to disrupt the dermal-epidermal junction to trigger milia formation.⁴

Diagnosis of secondary milia, similar to primary milia, is largely based on the history and physical examination. Upon examination, milia will often appear as small white or yellow papules measuring less than 3 mm in diameter with a smooth dome shape.² The differential diagnosis may include molluscum contagiosum, closed comedones, osteoma cutis, eruptive vellus hair cysts, and other cysts. If unable to determine by history and physical examination alone, a biopsy could be performed, but it is not required for diagnosis.²

Histologically, milia resemble miniature infundibular cysts, with several layers of stratified squamous epithelium and a granular cell layer.² Secondary milia typically resolve within a few months without intervention, but if they persist or are particularly bothersome to the patient, they may be treated with manual extraction, such as by nicking them with a scalpel blade.¹ The patient in this case experienced near resolution of his symptoms without any intervention by his 6 month follow-up appointment. Secondary milia should be kept on the differential diagnosis whenever patients present with milia following a blistering or erosive rash so that they can be reassured of this benign condition.

References

1. Berk DR, Bayliss SJ. Milia: a review and classification. J Am Acad Dermatol. 2008;59(6):1050-1063. https://doi.org/10.1016/j.jaad.2008.07.034

2. Gallardo Avila PP, Mendez MD. Milia. In: StatPearls. StatPearls Publishing. Updated October 1, 2020. Accessed April 6, 2021. https://www.ncbi.nlm.nih.gov/books/NBK560481/

3. Patsatsi A, Uy CDC, Murrell DF. Multiple milia formation in blistering diseases. Int J Womens Dermatol. 2020;6(3):199-202. https://doi.org/10.1016/j.ijwd.2020.03.045

4. Thormann H, Andersen KE. Milia as sequelae to allergic contact dermatitis. Contact Dermatitis. 2005;53(4): 239-240. https://doi.org/10.1111/j.0105-1873.2005.0670g.x