Fetus in Fetu: The Rare Case of a One-Year-Old Girl with her Twin Inside Her Skull

Fetus in Fetu: The Rare Case of a One-Year-Old Girl with a Parasitic Twin Inside Her Skull

Medical anomalies often present some of the most fascinating cases for study and discussion. One such rare phenomenon is "Fetus in Fetu" (FIF), a condition where a malformed parasitic twin is found within the body of its otherwise healthy sibling. While FIF typically occurs in the abdominal cavity, there have been extraordinary cases where it is found in the cranial cavity, particularly within the skull of an infant. This article delves into the intricacies of this rare medical condition, exploring its etiology, diagnosis, treatment, and implications for medical professionals.

Recent Case Report from China
A very rare case of intraventricular fetus-in-fetu was surgically removed from a one-year-old girl in China. The child presented with motor delay and an enlarged head circumference. The fetiform mass was identified as a malformed monochorionic diamniotic twin. Genetic sequencing revealed identical single-nucleotide variants in both the host child and the fetus-in-fetu, with extensive de novo copy number gains in the parasitic twin. This suggests the significant role of copy-number variation during embryogenesis.


Understanding Fetus in Fetu (FIF)
Definition and Etiology
Fetus in Fetu is an extremely rare condition, occurring in approximately 1 in 500,000 live births. It is characterized by the presence of a malformed, parasitic twin inside the body of its host twin. This anomaly is believed to result from an aberration during the early stages of embryonic development, specifically during the blastocyst stage. The exact mechanism remains unclear, but it is hypothesized that incomplete twinning occurs, where one twin envelops the other, resulting in one twin growing within the other.

Pathogenesis
In the case of FIF within the cranial cavity, the parasitic twin lodges itself within the developing brain of the host twin. This condition is even rarer than abdominal FIF and presents unique diagnostic and therapeutic challenges. The parasitic twin, typically lacking fully developed organs and structures, is sustained by the blood supply from the host twin, which can lead to complications.

Clinical Presentation and Diagnosis
Symptoms and Initial Presentation
The clinical presentation of a fetus growing inside the skull of an infant can vary widely. Common symptoms include:

• Increased intracranial pressure: Due to the space-occupying nature of the parasitic twin, symptoms such as vomiting, irritability, and a bulging fontanelle may occur.
• Neurological deficits: Depending on the location and size of the parasitic twin, the host twin may exhibit signs of developmental delay, seizures, or other neurological impairments.
• Cranial deformities: An abnormal head shape or increased head circumference can be a sign of an intracranial mass.

Diagnostic Imaging
The diagnosis of cranial FIF typically involves advanced imaging techniques. The most common modalities include:

• Ultrasound: Often the first imaging modality used, particularly in infants with a bulging fontanelle or abnormal head circumference.
• Computed Tomography (CT): Provides detailed images of the intracranial structures and can help differentiate between a tumor and a parasitic twin.
• Magnetic Resonance Imaging (MRI): Offers superior soft tissue contrast and can provide detailed information about the anatomy and potential involvement of surrounding brain structures.

Case Study
A notable case reported in the Journal of Neurosurgery involved a one-year-old infant presenting with an enlarging head circumference and delayed developmental milestones. MRI revealed a well-defined mass with features suggestive of FIF within the cranial cavity. The parasitic twin had rudimentary limb buds and a partially formed vertebral column, confirming the diagnosis (https://thejns.org/pediatrics/view/journals/j-neurosurg-pediatr/13/1/article-p96.xml).

Surgical Management
Preoperative Planning
Surgical removal of the parasitic twin is the definitive treatment for FIF. Preoperative planning involves a multidisciplinary team, including neurosurgeons, pediatricians, and radiologists. Detailed imaging studies are crucial to map the extent of the parasitic twin and plan the surgical approach.

Surgical Procedure
The surgical procedure aims to excise the parasitic twin while preserving the integrity of the host twin's brain structures. This requires meticulous dissection and hemostasis. In some cases, parts of the parasitic twin may be left behind if complete removal poses a risk to the host twin.

Surgical Outcomes
Postoperative outcomes largely depend on the extent of the parasitic twin and its impact on the host twin's brain. Successful removal can alleviate symptoms of increased intracranial pressure and potentially improve neurological outcomes. However, long-term follow-up is essential to monitor for any residual effects or complications.

Ethical and Psychological Considerations
Parental Counseling and Consent
Given the rarity and complexity of FIF, parental counseling is a critical component of care. Parents need to be informed about the nature of the condition, the risks and benefits of surgery, and potential long-term outcomes. Informed consent should be obtained after thorough discussions with the medical team.

Psychological Impact
The psychological impact on the family and the child cannot be underestimated. Families may require support from mental health professionals to cope with the diagnosis and treatment process. Additionally, as the child grows older, they may need psychological support to understand their medical history and any long-term implications.

Implications for Medical Professionals
Importance of Early Diagnosis
Early diagnosis and intervention are crucial for improving outcomes in cases of FIF. Medical professionals should be aware of this condition and consider it in the differential diagnosis of infants presenting with unexplained intracranial masses or increased head circumference.

Multidisciplinary Approach
Managing FIF requires a collaborative approach involving multiple specialties. Neurosurgeons, pediatricians, radiologists, and mental health professionals all play vital roles in the diagnosis, treatment, and follow-up care of affected children.

Ongoing Research and Education
Given the rarity of FIF, ongoing research and case reporting are essential to enhance our understanding of this condition. Medical professionals should stay informed about the latest developments and share their experiences through medical journals and conferences.


According to Chunde Li, MD, of Beijing Tiantan Hospital, and colleagues, "The intracranial fetus-in-fetu is proposed to arise from unseparated blastocysts. The conjoined parts develop into the forebrain of the host fetus and envelop the other embryo during neural plate folding."

This case emphasizes the importance of distinguishing FIF from teratomas, as FIF can be differentiated based on the younger age of presenting patients and the presence of vertebrae or internal organs.

Conclusion
Fetus in Fetu within the cranial cavity is a rare and fascinating medical condition that presents significant challenges and opportunities for the medical community. Understanding its etiology, clinical presentation, diagnostic methods, and treatment options is crucial for improving patient outcomes. By maintaining a high index of suspicion and employing a multidisciplinary approach, medical professionals can provide the best possible care for affected infants and their families.