What is the posterior fossa abnormality? What is the main differential?
A large arachnoid cyst. The main differential is that of a mega cisterna magna, however they are located lower and do not have positive mass effect.
What is the supratentorial abnormality?
Extensive periventricular grey matter heterotopia.
What is the genetic basis for periventricular heterotopia?
Although the majority of cases are sporadic, some are X-linke recessive (Xq28). In X-linked cases the abnormal gene product is filamin-1, a protein which cross-links intracellular actin.
What does MR spectroscopy of heterotopic grey matter demonstrate?
In general MR spectroscopy demonstrates a decrease in NAA/Cr ratio in the heterotopic grey matter compared to normal control subjects. It is important to note that comparing the spectrographic trace to the 'normal appearing contralateral side' is not necessarily valid as a control as metabolic abnormalities in patients with malformations of cortical development may be widespread
Pre and post contrast multiplanar multisequence MRI demonstrates a large nonenhancing extra axial collection at the posterosuperior aspect of the posterior cranial again demonstrates no enhancement or diffusion restriction and is consistent with a large arachnoid cyst. A few thin internal septations are noted. It results in mild positive mass effect on the cerebellum and tentorium, extending anteriorly to the quadrigeminal plate cistern and resulting in marked remodelling of the occipital bone. The fourth ventricle maintains normal calibre and there is no hydrocephalus.
Extensive periventricular grey matter heterotopia is again demonstrated. A few small nodules of grey matter are also noted in the left anterior corona radiata. Although a formal T1 FSPGR study has not been performed, there are multiple areas where normal cortical gyral pattern appears disordered but there is no definite focal cortical dysplasia.