Specific Imaging Findings

Subcortical band heterotopia (SBH) on MR imaging shows a symmetric band of heterotopic gray matter located deep to the cortical mantle and embedded within well-defined, smoothly marginated layers of normal-appearing white matter that separate it from both the overlying cerebral cortex and the underlying lateral ventricle. The heterotopic band is isointense with gray matter on all MR sequences. The cerebral cortex is usually normal; however, it can be thickened with a pachygyric configuration. A more severe malformation that overlaps with classic lissencephaly and band heterotopia is characterized by SBH in the occipital regions and pachygyria in the frontal regions. There is a gross correlation between the thickness of the heterotopic band and of the overlying cortex - patients with pachygyria tend to have thicker heterotopic bands. The hemispheric extent of the heterotopias is variable, ranging from complete to partial (sparing either frontal or posterior regions).

Pertinent Clinical Information

Most patients with subcortical band heterotopia are females who present with seizures and a variable degree of cognitive impairment. However, about 25% of affected children possess normal or near-normal intelligence. The malformation is sometimes discovered in mothers of boys with X-linked lissencephaly when they are investigated after the malformation has been diagnosed in their offspring. There is a gross relationship between the degree of disability and the thickness of the heterotopic band of neurons. Patients with a well-developed cortex may have a better clinical picture, and the onset of epilepsy occurs later. The lifespan is likely to be shortened in patients with severe mental retardation, intractable epilepsy, or both.