An intraventricular fetus-in-fetu, a malformed monochorionic diamniotic twin, was identified in a 1-year-old girl with motor delay and enlarged head circumference (Figure 1). After surgical removal, whole-genome sequencing revealed identical single-nucleotide variants in the host child and fetus-in-fetu, with extensive de novo copy number gains in the fetus-in-fetu (Figure 2, eMethods, links.lww.com/WNL/C529), suggesting the significance of copy number variation during embryogenesis.
Head CT (A) and T1-weighted image (B) of an infant showing hydrocephalus, compressed brain, and intraventricular mass with vertebral column, femur, and tibia. T1-weighted image of the mass revealing spinal bifida (C). The fetiform mass with upper limb and finger-like buds (D).
Red bands indicating copy number gain.
The intracranial fetus-in-fetu is proposed to arise from unseparated blastocysts. The conjoined parts develop into the forebrain of host fetus and envelop the other embryo during neural plate folding.1 Fetus-in-fetu can be distinguished from teratomas based on the younger age of presenting patients and the presence of vertebrae or internal organs.2
Study Funding
This project was supported by the National Natural Science Foundation of China (81801140 to Dr Ma and 81571110, 81771234 to Dr Zhao).
Disclosure
The authors report no relevant disclosures. Go to Neurology.org/N for full disclosures.
Footnotes
Go to Neurology.org/N for full disclosures. Funding information and disclosures deemed relevant by the authors, if any, are provided at the end of the article.
Submitted and externally peer reviewed. The handling editor was Resident and Fellow Section Editor Whitley Aamodt, MD, MPH.
Teaching Slides links.lww.com/WNL/C530
- Received June 17, 2022.
- Accepted in final form October 6, 2022.
- © 2022 American Academy of Neurology
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