I am sorry this happened to your family. Phe levels are pretty important when it comes to the PKU diet. When phe levels are high (especially in younger children) it can effect brain development. Some issues that can occur because of high phe levels include: lack of concentration and energy, poor balance, irritability, mood swings, skin problems such as eczema, behavior issues, and stunted growth. If infants do not follow the diet they can experience seizures and possible mental retardation. The diet is needed in order to prevent these problems in those who have PKU. I hope this helps!