Presumed homozygous achondroplasia. A review and report of a further case.

Presumptive homozygous achondroplasia (PHA) is a rare disorder. The clinical and detailed histological findings of the fifth case of this chondrodystrophy, studied in detail, are presented here. There is a severe disorder of the normal growth of the long bones, whose nature, however, is not well understood; it shows some resemblance to the pathological changes described in thanatophoric dysplasia which also presents many clinical similarities. The aspect of greatest interest, however, is the relation of PHA to the "classical" form of achondroplasia. Some investigators have maintained that in that disorder there are no significant qualitative, and at most some quantitative, alterations in the growth plate to be demonstrated by histologic examination, but have not explained the apparent abnormalities in the development of the bones and in the external phenotype, which appears to be a milder expression of that seen in PHA. In view of the genetic relationship and external phenotypic similarity one would also have expected in "classical" achondroplasia a milder form of the severe changes seen in PHA. Judging by some of the published reports this is not the case. Attention is drawn to this intriguing discrepancy.