Effects of PKU on the brain

The cause of neurological disorder caused by PKU  could be due to the myelination or de-myelination as myelination and white matter are affected in the early stage of development. A fall in the myelination levels of neurons and glial cells have been detected. This fall is due to the transformation of oligodendrocytes(a variety of nruroglial) to non-myelinating phenotypes. In periventricular region and the forceps major and carpus callosum minor, the loss of white matter is more visible.In the early development stage, dendrite growth is also affected. 

The PKU patients' gray matter reduce greatly in motor and pre-motor cortex, hippocampus and thalamus but increase in striatum ventral regions. These explain for mental and cognitive abnormalities in PKU patients as damaged motor and pre-motor cortex can lead to damaged motor functions and reduce volume of nueronal cells in the thalamus and hippocampus.

Reference:
Nariman Hossein-Javaheri. (2012). Phenylketonuria. Tangient LLC. Retrieved July 10, 2012, from https://neurowiki2012.wikispaces.com/Phenylketonuria