The term FIF was first described in 1800 by the German Anatomist Johann Friedrich Meckel, the first case report had been published by Young in 1809, and the first approach of definition by Willis in 1953 [2, 5].
FIF results from the abnormal embryogenesis of a diamniotic monochorionic twin pregnancy leading to asymmetric division and abnormal anastomosis of the yolk circulation [5]. It is a rare pathology, with a prevalence of 1/500,000 births; less than 200 cases have been reported worldwide [6]. FIF occurs in different age groups with a predominance in early childhood and in the majority of cases diagnosed in patients younger than 18 months with a presumed male prevalence [6, 7]. The clinical symptoms of FIF are related to the compression of neighboring organs by the mass (abdominal distension, feeding difficulties, vomiting, respiratory distress), but it can be asymptomatic [3, 5, 6]. Our patient presented with respiratory distress preceded by recurrent lung infections. She improved markedly after thoracic drainage and mass exeresis. The preoperative diagnosis of FIF is made in 16.7% of cases [3, 5]. In our case, the diagnosis of FIF was made intraoperatively. The chest x-ray points to the diagnosis, when it shows the axial skeleton, while computed tomography and MRI confirm the diagnosis, relations, and the mass effect [2, 5]. All these radiological specificities had been found in our case.
FIF can be the subject of prenatal diagnosis; however, a limited number of cases (7%) of FIF were diagnosed before birth [6]. In our case, the diagnosis was made after the birth intraoperatively. In most cases, the localization is abdominal; however, it can be detected in the brain, mouth, skull, neck, mediastinum, liver, scrotum, and even the sacrococcygeal region. The thoracic localization of FIF is extremely rare. Only 6 cases have been reported; in three patients, the mass was in the mediastinum, one in the lung parenchyma, one in the left hemithorax, and one in the anterior chest wall [3, 6]. In our case, the FIF straddled the left hemithorax and mediastinum.
Macroscopically, the FIF appears as a mass suspended by a vascular pedicle, within a sac containing liquid, or vernix caseosa. A vascular anastomosis with the host’s vessels is identifiable. FIF is generally anencephalic and acardiac. It has a varying degree of organogenesis with an axial skeleton and rudimentary limbs. The size of the FIF is related to the blood supply, and the weight varies between 13 and 2000 g. Most often, it is unique, but the presence of 2 or 3 FIFs, or even more, has been observed; Kimmel found more than 5 FIF intracerebral [5, 8]. Spencer recommends that a FIF should meet one or more of the following criteria: (a) be enclosed in a separate bag containing liquid or vernix caseosa, (b) be partially or completely covered with normal skin, (c) have roughly recognizable anatomical parts, and (d) be attached to the host by one or a few relatively large blood vessels [8]. Our clinical case met most of these criteria. AFP, BhCG, and CEA may be normal or elevated [5, 9]. In our case, AFP and BhCG were elevated, and CEA was normal.
The main differential diagnosis of FIF is teratoma due to the risk of malignancy associated with the latter. FIF is almost always benign with only one reported case of malignancy. The presence of a spine is a differentiator between a FIF and a teratoma. However, radiologists and pathologists have concluded that FIF can be retained even in the absence of a vertebral axis. Although fewer than 200 cases of FIF have been reported, the presence of a well-formed spine has only been documented in a limited number of cases. In about 9% of FIF cases, no spine was found [2, 3, 5, 10]. Gonzalez-Crussi suggested that the term FIF be applied to any structure in which the fetal form has highly developed organogenesis or the presence of a vertebral axis [8, 9].
There is a theory of FIF as being “a highly differentiated mature teratoma” and the theory of “parasitic twins” [2, 5, 6].
Analysis of genetic markers, such as blood group, karyotype, and DNA, shows no genetic difference between host and FIF. Therefore, this result is compatible both with the theory of “parasitic twins” and the “monozygous teratoma” theory [6, 8]. The final diagnosis of FIF is not made until the pathological analysis. The recommended surgical treatment is complete surgical excision. The design of the type of surgical approach is an important preoperative consideration. Close postoperative follow-up by ultrasound and serology (AFP or beta-HCG level) is recommended to exclude even rare cases of recurrence [6, 9].