Extracerebellar ectopic brain tissue in the posterior fossa

Intracranial extracerebral ectopic brain tissue (also called glioneuronal heterotopia) is encountered in rare developmental disorders. These lesions are mainly located at the base of the brain in the supratentorial region and are frequently associated with craniofacial anomalies. We report the prenatal sonographic and magnetic resonance imaging (MRI) findings of isolated extracerebellar ectopic brain tissue in the fetal posterior fossa, which was confirmed by postnatal MRI examination in a healthy infant. A 27-year-old woman at 28 weeks' gestation was referred to our department with the diagnosis of a round mass in the fetal posterior fossa located in the cisterna magna, which had been identified on a routine ultrasound scan. Ultrasound examination (Antares, Siemens, Munich, Germany) confirmed the presence of a round echogenic non-vascularized mass of 12 mm in diameter located in the midline of the cisterna magna posterior to a normal vermis (Figure 1). The cerebellum was anatomically normal with a normal transverse cerebellar diameter for gestational age (32 mm). The size of the cisterna magna was also within normal ranges. No other structural cerebral or extracerebral abnormalities could be seen. Biometric parameters were normal for gestational age. Fetal cerebral MRI showed a round and apparently solid mass, posterior to the vermis, with a signal similar to that of normal brain tissue on T2-weighted imaging (Figure 2) and with no hemorrhagic changes on T1-weighted imaging. Vermian and hemispheric cerebellar anatomy were normal. Although this lesion has not been described previously in this location, imaging findings were suggestive of ectopic/heterotopic cerebral tissue. On the basis of normal cerebellar anatomy with a complete vermis and otherwise normal cerebral and extracerebral anatomy and, following prenatal counseling with neuropediatricians, the couple decided to continue with the pregnancy. Follow-up at 33 weeks' gestation found the MR signal, size and echogenicity of the lesion to be unchanged. Since the patient was lost to follow-up for delivery and early pediatric examinations for a period of 1 year, postnatal MRI could only be performed at the age of 12 months. T1- and T2-weighted images confirmed the prenatal diagnosis of ectopic brain tissue, with small gyri visible at its surface. The signal of the lesion was similar to that of normal brain (Figure 3). Clinical neurological examination of the child was normal. Axial (a) and sagittal (b) ultrasound views at 28 weeks showing an echogenic well-circumscribed round lesion located in the midline of the cisterna magna posterior to a normal vermis. Fetal magnetic resonance imaging at 29 weeks showing a non-hemorrhagic solid mass in the infratentorial subarachnoid spaces on T2-weighted infratentorial axial (a) and sagittal (b) images, with a signal similar to that of normal brain tissue. Postnatal magnetic resonance imaging at 12 months of age confirmed a lesion with a signal similar to that of normal brain tissue on T1-weighted axial (a) and T2-weighted sagittal (b) images. On T2-weighted sagittal imaging small gyri were visible on the surface of the lesion. The main abnormalities of the posterior fossa that are observed on antenatal ultrasound examination include a small posterior fossa associated with a neural tube defect (Chiari II malformation), increase in size of the fluid-filled space of the posterior fossa, small cerebellar biometry and a heterogeneous group of rare focal parenchymal cerebellar abnormalities1. Prenatal diagnosis of an infratentorial subdural hematoma has been described in one case2. One case of prenatally-diagnosed dural sinus malformation has also been reported at 24 weeks as a huge midline dural lake in the posterior fossa with absence of signal intensity on MR imaging3. In our case, MRI showed no signal suggestive of hemorrhage or thrombosis, although the lesion was hyperechogenic on ultrasound examination. Moreover, this focal abnormality was within the cisterna magna and away from the dural sinuses. MR findings were highly suggestive of heterotopic or ectopic brain tissue, which were confirmed on postnatal imaging. True ectopia in the posterior fossa should be distinguished from the more frequent focal cerebellar cortical dysplasia or heterotopia (typically small clusters of cells located within the cerebellum), or heterotopic cortex and heterotaxy4. These intraparenchymal abnormalities are identified in the cerebellar white matter and are mainly recognized as incidental postmortem findings that may represent normal variants4. In our case, the abnormality was a true ectopia of extracerebellar brain tissue. These extremely rare developmental disorders have been recently reviewed by Oya et al. and are grouped under the term of ‘intracranial extracerebral glioneuronal heterotopia’ (IEGH)5. Oya et al. reported a case of IEGH, depicted as a massive intracranial lesion in the left anterior fossa, diagnosed at 33 weeks' gestation and reviewed ten cases in the literature5. In two cases, IEGH were also found in the prenatal period. The first, at 24 weeks' gestation, appeared as an intracranial cyst of the anterior cranial fossa and led to termination of the pregnancy6. The second, at 32 weeks' gestation, appeared as enlarged cephalic biometry due to a large lesion of the middle cranial fossa and resulted in fetal death7. All reviewed cases but one were located in the supratentorial region, mainly in the anterior and middle cranial fossa (n = 6). The authors also underlined a significant association between IEGH and craniofacial anomalies (n = 6). All infants underwent surgical resection of the lesion. Our case is atypical, since the lesion was located in the subarachnoid spaces of the infratentorial region and was not associated with any other abnormality. Since the child had developed normally and was asymptomatic, surgical resection was not performed; however, clinical follow-up was scheduled with a neuropediatrician. The pathogenesis of these rare developmental disorders remains speculative5. In the protrusion theory, IEGH is hypothesized to result from the development of heterotopic glioneuronal tissue in the subarachnoid space through a pre-existing pial defect. In the aberrant migration theory, such lesions are thought to result from the maturation of abnormal embryonic neuroepithelial elements that have migrated into the subarachnoid space. Finally, in the separation and detachment theory, IEGH results from an aberrant division of the forebrain vesicle, leading to the development of an ‘accessory brain’6. The infratentorial location of the legion in our case did not support the latter hypothesis, although imaging findings were indeed suggestive of an ‘accessory brain’.