A rare and unusual case was reported in China, where doctors surgically removed a malformed twin from the brain of a 1-year-old girl.
The little girl had shown signs of developmental delays and had an unusually large head, leading to the discovery. The mass was actually a partially formed twin, with bones like a spine, arms, and even finger-like buds.
This rare condition, known as fetus-in-fetu, happens in about 1 in 500,000 births, where one twin grows inside the other.
Fetus-in-fetu is often mistaken for a type of tumor, but it can be identified by the presence of actual body parts.
While most cases involve the mass growing in the abdomen, it’s incredibly rare for it to be found in the brain. Fewer than 200 cases have been documented worldwide, and this adds to the small number of cases where the twin developed in such an unusual location.
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.
Figure 1 Imaging of Fetus-in-Fetu. 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).
Figure 2 Copy Number Duplication in Fetus-in-Fetu, 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
