In some cases, normal development is observed up to about 1½ years of age. The progress of the disorder characteristically occurs in phases:

• The initial stagnation phase occurs between 6 and 18 months, when slow growth of the head, hypotonia and a loss of interest in play are observed.

• The phase of fast regression occurs between the first and the second year of age of the child. During this phase, interest in the surroundings diminishes, communication ceases, stereotypical reactions to stimuli begin to appear, as well as significant timidness, and loss of existing speech and language skills. What is typical is also the loss of existing purposeful movements of hands, emergence of stereotypical twisting arm motions, frequent episodes of hyperventilation, abnormal chewing of food, and incontinence caused by the inability to control one’s sphincters. Development of social skills and play remains on the level of 2 to 3 years of age, with a tendency to preserve social interests.

• During the pseudo-stagnation phase (3rd to 4th year of age), signs of ataxia and apraxia appear (the inability to control one’s limbs or other parts of the body at will), which become more severe in situations causing increased stress.

Rett syndrome is associated with progressive motor disorders, back atrophies, and severe motor disorders. With progressing age, the child loses muscle matter, and changes in joints and the spine are observed. Children who have already started walking, often lose this ability. Assessing the level of intellectual skills tends to be difficult with these children, as their speech and the ability to control their own movements through will are impaired. Girls with this syndrome are usually diagnosed with mild to profound mental retardation. The developmental regression affects thinking, learning and understanding speech.