Diagnostic criteria for Rett Syndrome are proposed by the International Rett Syndrome Association and the Centers for Disease Control, Koger Center, F-37, Atlanta, GA. The criteria are separated into three categories: 1) necessary, 2) supportive, and 3) exclusion criteria. Female sex is not included as a necessary criterion because the possibility of undiagnosed male cases cannot be ruled out. Diagnosis is tentative unil 2-5 yrs of age, and the presence of one or more of the exclusion criteria is against the diagnosis, regardless of whether all of the necessary criteria have been met.
Necessary criteria include the following: 1) normal pregnancy, birth and psychomotor development through the first 6 or 18 months; 2) normal head circ at birth and deceleration of head growth between 5 mos and 4 yrs; 3) loss of purposeful hand skills between 6 and 30 months; 4) impaired language and psychomotor development; 5) stereotypic hand movements such as hand wringing; and 6) gait apraxia and ataxia between 1 and 4 years. Supportive criteria include breathing irregularities, EEG abnormalities, seizures, spasticity, scoliosis, growth retardation and small feet. Evidence of intrauterine growth retardation or perinatal acquired brain damage, microcephaly at birth, identifiable metabolic, degenerative or storage diseases are listed as exclusion criteria.
The clinical characteristics of Rett Syndrome and differential diagnoses are listed according to stages and age at onset: 1) Early onset deceleration stage, 6-18 mos; 2) rapid “destructive” stage, 1-3 yrs; 3) pseudostationary stage, 2-10 yrs; 4) late motor deterioration stage, 10+ years. 
COMMENT. Heller's dementia, an infantile dementia described in 1908, almost 60 years before the first description of Rett Syndrome, should be added to the differential diagnosis [2, 3], especially as the female sex is no longer considered a necessary diagnostic criterion for Rett Syndrome. At this stage of our understanding, the diagnostic criteria of Rett Syndrome should not be too strict and too exclusive .
 describes Rett Syndrome in a pair of monozygotic twin girls, pointing out that their development was delayed from birth with no period of normal progress in infancy and subsequent regression, findings at variance with the necessary diagnostic criteria listed above. He states that the cause is not necessarily genetic but could be explained by prenatal toxic or slow viral factors.
 reports scoliosis in eight of 10 females with Rett Syndrome treated at the Alfred I. DuPont Institute, Wilmington, Delaware. Curve progression occurred in four and posterior spinal fusion was performed in five. Scoliosis developed at an average age of 11 years and progression was rapid in adolescence. Early surgery is recommended to arrest curve progression and to obtain correction of the deformity.