Glial heterotopia is a rare congenital disease. It is not a true tumor, but rather glial tissue located outside the central nervous system. It has previously been called glial hamartoma, brain heterotopia, and glial branching tumor [
6]. At present, the pathogenesis of glial heterotopia remains unclear. However, there are three main hypotheses that attempt to explain it. In the first hypothesis, the descending brain tissue bulges outward through the defective skull. Once the defective skull is closed, the bulging brain tissue eventually becomes glial heterotopia [
7]. In the second hypothesis, the displacement of neuroectodermal cells is caused by the failure of anterior foramen closure. Abnormal migration of olfactory bulb glial cells then leads to glial heterotopia [
8]. There are mature and immature types of ectopic glia, which are mainly composed of neurons, meningeal tissue, choroid tissue, and ependymal epithelial cells in different proportions [
9].
Based on statistical data, glial heterotopia is typically diagnosed in newborns and infants. The nasal cavity is the most common tumor origin, and its clinical manifestations depend on tumor size and location. Most cases are discovered after birth based on body surface mass or airway obstruction. Some patients with exogenous glial heterotopia are often misdiagnosed with hemangioma because of reddish or purple skin color. Glial heterotopia occurs deep in the tissue and remains hidden. It is usually noticed after the tumor grows enough to cause the surrounding organs to be compressed.
Looking back at our case, no abnormal mass was found in the pharynx of the fetus during the first and second trimesters. An indirect sign of polyhydramnios was discovered in the third trimester. However, its causes were complicated and unclear, and only some of them have been previously reported. Generally speaking, common causes include fetal malformation and chromosomal abnormalities, such as fetal nervous system defects, digestive system defects (esophageal atresia, duodenal atresia), abdominal wall defects, diaphragmatic hernia, frontal malformation, urinary system malformation, chromosomal abnormalities (trisomy 18, 21, and 13), and maternal factors, such as gestational diabetes mellitus and idiopathic polyhydramnios. In addition, the stomach bubble tension in the fetus is low, so that the clinical diagnosis tends to be that of fetal digestive tract malformation. Finally, the upper digestive tract of the fetus was carefully explored again before the pharyngeal lump was found. MRI findings suggested benign lesions, but there were many uncertain factors due to the rarity of the fetal neck masses and the complexity and diversity of developmental malformations. Common neck masses include lymphatic deformity, teratoma, branchial anomaly, and vascular deformity [
10]. Moreover, the mass in this case exerted pressure on the esophagus and airway of the fetus. In order to save the fetus, it was necessary to perform a tracheotomy immediately after the cesarean section to ensure the respiratory circulation of the fetus. The pharyngeal mass in the fetus then had to be removed. In the end, the pregnant woman gave up the fetus and chose to induce labor. Postoperative pharyngeal mass was diagnosed as glial heterotopia via pathological examination. Since fetal death due to the lack of knowledge about glial heterotopia is not a desirable outcome, it is particularly important to fully understand the imaging features of glial heterotopia for prenatal diagnosis, which is helpful to formulate treatment strategies and deal with fetal respiratory distress and feeding difficulties after birth [
11]. Moreover, glial heterotopia may also be combined with other deformities, such as cleft palates, Pierre Robin syndrome, congenital heart disease, pectus excavatum, and micrognathia [
12], which cannot be ignored. Based on previous reports, glial heterotopia is a round or quasi-round solid mass, and its computerized tomography (CT) density and MRI signal are related to the maturity of heterotopic nerve tissue. The components of pharyngeal glial heterotopia can be solid, cystic, or cystic-solid, and the solid signal or density is similar to that of cerebral gray matter. For the solid mass, the CT results generally show that the mass boundary is clear, while contrast enhancement is uniform. In addition, a CT scan can clearly show the relationship between the lesion and the adjacent bone tissue. Imaging diagnosis can distinguish glial heterotopia from other tumors by the presence or absence of bone destruction, because most glial heterotopias are characterized by bone compression. Due to the high resolution of the soft tissue images, MRI can show the location of the lesion and the surrounding tissue more clearly and evaluate the compression of the trachea and esophagus. It also shows that the T1WI signal is similar to that of gray matter, while the T2WI signal is slightly higher [
13], with a clear boundary with surrounding soft tissues, no edema present, slight uniform enhancement of the lesion, and more obvious edge on contrast enhancement. However, some scholars believe that ectopic nerve tissue dysplasia is usually not enhanced, while the compressed tissue can be slightly enhanced [
6]. Whether glial heterotopia is enhanced or not may be related to the fibrous blood vessels [
3]. For cystic components, glial heterotopia can be single or multi-cystic. Contrast enhancement shows that the cystic wall is slightly or obviously enhanced, which is similar to a lymphatic malformation [
14]. Pathology analysis has shown that the cystic component is actually cerebrospinal fluid, which is produced by the functional choroidal plexus in ectopic glial tissue. A prior study has shown that the thinning or erosion of the middle cranial fossa and adjacent bones under pressure is often a characteristic manifestation of glial heterotopia [
15], which can be used as a sign to differentiate it from a lymphatic malformation. In the present case, glial heterotopia presented as a solid mass, and the MRI findings were consistent with the description of previous studies. It was also preliminarily diagnosed as a benign lesion. Pharyngeal masses can be diagnosed based on combined relevant ultrasound and MRI findings. Ultrasound is a simple and non-radiation tool that is particularly important for prenatal diagnosis of glial heterotopia. It can help to observe the location and size of the mass and other malformations, which plays a very important role in fetal monitoring. In the present case, both the ultrasound and MRI examinations played an important role in the preliminary diagnosis of glial heterotopia. Therefore, ultrasound and MRI can complement each other in prenatal imaging diagnosis of glial heterotopia.
Surgical resection can be used for the treatment of glial heterotopia and the recurrence probability then becomes extremely low. Because it is not a true tumor, the long-term prognosis is satisfactory. Therefore, the advantages of different imaging examinations and their related features need to be fully understood, especially in prenatal glial heterotopia. Different diagnoses and decisions may have completely different results. In the present case, if the fetus was allowed to survive and a detailed treatment strategy was formulated for the pharyngeal glial heterotopia, a satisfactory outcome may have occurred.
Prenatal diagnosis of glial heterotopia has some challenges. The imaging manifestations of glial heterotopia are benign tumor-like changes, and pharyngeal glial heterotopia can be diagnosed exclusively via ultrasound and MRI examinations. Although the disease is extremely rare, clinicians need to further understand the main features of the disease, thereby reducing the possibility of misdiagnosis and improving the odds of a good prognosis.