Babies appear normal in the first few months of life. However, in early infancy they begin to lose previously acquired skills, including seeing and hearing. In some children, the head becomes progressively enlarged (megalencephaly) as the brain swells and the bones of the skull fail to fuse. Babies may lose control of their muscles, including those supporting the head, so the neck becomes floppy and weak (known as hypotonia).
Sitting, standing and walking are rarely achieved by affected children. Although speech is delayed, infants are able to interact socially. Learning difficulties become more severe over time. As children become older, floppiness of the muscles gives way to spasticity (stiffness of the limbs). After the first or second year of life, feeding difficulties become apparent.