[Free Full Text] Prediction of Adult Height for children with achondroplasia

In average, the growth rate of a  healthy child during the first  year of life will be between 21 and 25 cm (8,27   and 9,84 in).

This growth rate will decrease after the first year:

	weight	length
ages 1-2	2.5 kg/yr	12 cm/yr
ages 2-3	2.0 kg/yr	7 cm/yr
ages 3-4	2.0 kg/yr	7 cm/yr

	weight	length
Boys 5-12	3 kg/yr	6 cm/yr
Girls 5-10	3 kg/yr	6 cm/yr

A way to roughly guess the child's adult height is to double the height at two years.

And the height during puberty increases again. Boys' growth spurt is longer and faster than the girls', so they eventually overcome them and become taller than girls.

	weight	length
Girls	3.4 kg/yr	7.6 cm/yr
Boys	4.8 kg/yr	12.5 cm/yr

Source from kids growth

But in achondroplasia, all this changes.

In 2005, Dr. Dror Paley et al., published the article: Multiplier Method for Prediction of Adult Height in Patients with Achondroplasia

The multiplier method predicts the height at skeletal maturity (adult age) by using an age and gender-specific coefficients multiplied by the current length or height at that age.  

In this case, a coefficient is a specific number that when multiplied by the height of the child, will give a predicted height in adult age.

Unlike average persons, who achieve three-quarters of their total height by age 9 years, achondroplastic persons achieve three-quarters of their total height by age 7 years. This shows that not only the amount of growth but also the rate of growth is different.

Data tables taken from "Multiplier Method for Prediction of Adult Height in Patients with Achondroplasia".

In another paper by Hoover-Fong et al., 2008, Age-appropriate body mass index in children with achondroplasia: interpretation in relation to indexes of height,  

"the median height velocity in infants of average stature shortly after birth was 3.7 cm/mo (44 cm/y), that in our achondroplastic infants (boys and girls combined) was 1.7 cm/mo (20 cm/y).  

By 1 year of life, median height velocity decreased to 1.2 cm/mo (14.4 cm/y) in children of average stature, whereas that in our achondroplastic cohort at 1 y of life is 0.8 cm/mo (10 cm/y).

The median height velocity in children of average stature older than 10 years ranges from 5.5 cm/y to just under 7 cm/y, whereas the median height velocity of our achondroplastic cohort remained steady at 5 cm/y from 2 to 10 y of age. However, unlike males and females of average stature, there is no evidence of a pubertal linear growth spurt in children with achondroplasia. Rather, median height velocities throughout the pubertal years remain at 5 cm/y in boys and girls through the age of 16 y.

This is in stark contrast with the median height velocity curves for children of average stature with a median peak height velocity of 9.3 cm/y in boys at 13.5 y of life and of 8.3 cm/y in girls at 12 y of life.

Thus, gains in height are particularly limited in achondroplastic subjects during what would be expected to be periods of greatest linear growth: both infancy and puberty.

Meanwhile, the BMN-111 drug is expected to increase the growth rate in children with achondroplasia up to 50%, reaching the goal of our children achieve average height. All the results from BMN-111 clinical drug trial must be well evaluated to admit if this expected growth rate will become real.