The essential feature of Rett's Disorder is the development of multiple specific deficits following a period of normal functioning after birth.   Individual have an apparently normal prenatal and perinatal period (Criterion A1) with normal psychomotor development through the first 5 months of life (Criterion A2).   Head circumference at birth is also within normal limits (Criterion A3).   Between the ages of 5 and 48 months, head growth decelerates (Criterion B1).   There is a loss of previously acquired purposeful hand skills between ages 5 and 30 months, with the subsequent development of characteristic stereotyped hand movements resembling hand-wringing or hand washing (Criterion B2).   Interest in the social environment diminishes in the first few years after the onset of the disorder (Criterion B3), although social interaction may often develop later in the course.   Problems develop in the coordination of gait or trunk movements (Criterion B4).   There is also severe impairment in expressive and receptive language development, with severe psychomotor retardation (Criterion B5).

      Rett's Disorder is typically associated with Severe or Profound Mental Retardation, which, if present, should be coded on Axis II.   There is no specific laboratory findings associated with the disorder.   There may be an increased frequency of EEG abnormalities and seizure disorder in individuals with Rett's Disorder.   Nonspecific abnormalities on brain imaging have been reported.   Preliminary data suggest that a genetic mutation is the cause of some cases of Rett's Disorder.

      Data are limited to mostly case series, and it appears that Rett's Disorder is much less common than Autistic Disorder.   This disorder has been reported only in females.

      The pattern of developmental regression is highly distinctive.   Rett's Disorder has its onset prior to age 4 years, usually in the first or second year of life.   The duration of the disorder is lifelong, and the loss of skills is generally persistent and progressive.   In most instances, recovery is quite limited, although some very modest developmental gains may be made and interest in social interaction may be observed as individuals enter later childhood or adolescence.   The communicative and behavioral difficulties usually remain relatively constant throughout life.

      Periods of developmental regression may be observed in normal development, but these are neither as severe or as prolonged as in Rett's Disorder.   For the differential diagnosis between Rett's Disorder and Autistic Disorder see Autistic Differential Diagnosis.  Rett's Disorder differs from Childhood Disintegrative Disorder and Asperger's Disorder in it's characteristic sex ratio, onset, and pattern of deficits.   Rett's Disorder has been diagnosed only in females, whereas Childhood Disintegrative Disorder and Asperger's disorder appear to be more common in males.   The onset of symptoms in Rett's Disorder can begin as early as age 5 months, whereas in Childhood Disintegrative Disorder the period of normal development is typically more prolonged (i.e., at least until age 2 years).   In Rett's Disorder, there is a characteristic pattern of head growth deceleration, loss of previously acquired purposeful hand skills, and the appearance of poorly coordinated gait or trunk movements.   In contrast to Asperger's Disorder, Rett's Disorder is characterized by a severe impairment in expressive and receptive language development.