Subependymal heterotopia appears as smooth, sometimes pedunculated, nonenhancing gray-matter-like ventricular masses. Seizures do not usually occur in neonates with subependymal heterotopia unless cortical abnormalities are also present. Subcortical focal heterotopia appears as islands of gray matter within the white matter. Subcortical focal heterotopia may be difficult to detect during the neonatal period because the signal intensities of the white and gray matter in the neonate are similar. Diffuse gray matter heterotopia appears as a band of gray matter within the white matter. Subcortical focal hetereotopia and diffuse gray matter heterotopia often produce seizures in the neonatal period.
Heterotopias may occur as isolated anomalies or as part of a syndrome. Isolated neuronal heterotopia is more frequent. Neuronal heterotopia may be associated with metabolic disorders (Zellweger syndrome, neonatal adrenoleukodystrophy, glutaric aciduria type II, nonketotic hyperglycinemia, Menkes disease, and GM2-gangliosidosis), neurocutaneous syndromes (neurofibromatosis, tuberous sclerosis, incontinentia pigmenti, hypomelanosis of Ito, and linear nevus sebaceous syndrome), dysmorphic syndromes (Smith-Lemli-Opitz syndrome, de Lange syndrome, oro-facial-digital syndrome, Meckel-Gruber syndrome, Coffin-Siris syndrome), chromosomal abnormalities (Trisomy 13, Trisomy18, Trisomy 21, 4p-syndrome), fetaltoxic exposure (carbon monoxide, isotretinoic acid, ethanol, organic mercury), congenital myotonic dystrophy, and Aicardi syndrome.