A pair of monozygotic twins, aged 2 years, presented with a congenital bald patch.
Initial Presentation: Monozygotic twins, aged 2 years, were referred to our outpatient clinic because of a hairless area 2 cm × 2 cm in a similar point on their occipital vertices (figure).
Description: A pair of monozygotic twins, aged 2 years, presented with a congenital bald patch. The lesions had no areas of ulceration, redness, or any other symptoms.
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The twin boys were born prematurely after 33 weeks and 3 days of gestation and it was initially thought that the hairless regions, which also had no skin cover, were caused by a fetal scalp electrode. The twins had otherwise developed normally, with no associated anomalies or syndromes, and the defects on their scalps had healed over with scar tissue. A detailed history was taken from the children’s mother, who said that the pregnancy had been uncomplicated and that there was no history of skin conditions in the family. Notably, an ultrasound scan done at gestational week 8 showed an empty yolk sac which suggested that a third fetus had been present and was reabsorbed in utero.
What is the most likely diagnosis?
- Alopecia areata
- Discoid lupus erythematosus
- Aplasia cutis congenita
- Nevus sebaceous
Answer: Aplasia cutis congenita
Breakdown: After considering the history and examining the lesions, we made a diagnosis of aplasia cutis congenita (ACC)—simply a condition with a congenital absence of skin. Vanishing twin syndrome, which perhaps in our case ought to be described as vanishing triplet syndrome—where usually twin fetuses convert to a singleton—is a rare but well recognised phenomenon that is associated with widespread and predominantly truncal ACC. Non-syndromic ACC of the scalp has been associated with a mutation in the ribosome biogenesis factor (BMS1) gene. The suspected mechanism by which this mutation produces ACC is through an increase in the p21 level, which has been linked to an inhibition of cell proliferation and the formation of scar tissue. Scalp ACC in twins is very rare and we found only two other reported cases after a literature search.
At first glance, the scalp defect is easily confused with other conditions causing areas of hair loss, such as alopecia areata or naevoid conditions. The absence of skin poses a slight risk of CNS infection or of sagittal sinus haemorrhage before the healing occurs. Otherwise, ACC is considered benign once the scalp defect has healed and as such does not require further treatment. Therapeutic options include conservative management or surgery for aesthetic purposes, larger defects, or underlying bone defects. Since this condition is very rare, physicians outside of the dermatological arena should not be chastised for missing this spot diagnosis.
Source: All content for this “Diagnostic Puzzle” was sourced from The Lancet. The article was written by Zahràa Chayed, Shailajah Kamaleswaran, MD and Prof Anette Bygum, DMSc
Original article is available at:
https://www.thelancet.com/article/S0140-6736(19)32006-9/fulltext
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