it might be helpful to add the medical synonym primary macrocephaly to HP:000448
and secondary macrocephaly to HP:0005490
The definition of this term could also list a quantifiable criterium:
head circumference above 97th percentile.
As far as I know the terms "primary" and "secondary" are used to describe the etiology and not the phenotype (see, for instance, http://www.acnr.co.uk/MJ11/16_ACNRMJ11_Mennounou.pdf). The HPO attempts to describe phenotype whilst not making more inferences about mechanism than absolutely needed. Please communicate more details...
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As far as I know the terms "primary" and "secondary" are used to describe the etiology and not the phenotype (see, for instance, http://www.acnr.co.uk/MJ11/16_ACNRMJ11_Mennounou.pdf). The HPO attempts to describe phenotype whilst not making more inferences about mechanism than absolutely needed. Please communicate more details...