Head growth in Rett syndrome.

The longitudinal development of head growth was investigated in girls with Rett syndrome (RS). Growth retardation was expressed in standard deviation (SD) scores. In classic types, the mean head circumference fell successively to 2 SD scores below the norm at the age of 4 years. After the age of 8 it stabilized close to -3 SD scores. In forme fruste variants, the mean head circumference was within normal limits; however, it was significantly below the norm, -0.8 SD scores. In girls with classic RS, head growth had decelerated by less than 1 SD score in 20% of the girls at the age of 6 years and in 10% at the age of 12 years. In forme fruste variants only a small decline in head growth occurred. Head growth decline may thus be very small in classic RS and is usually not present at all in forme fruste variants. In the future, it should, therefore, neither be regarded as a necessary diagnostic criterion for classic RS, nor as a valid one for forme fruste variants.