ACHONDROPLASIA AND HYPOCHONDROPLASIA - COMMENTS ON FREQUENCY, MUTATION-RATE, AND RADIOLOGICAL FEATURES IN SKULL AND SPINE

An attempt was made to ascertain all the dwarfs in the State of Victoria [Australia]. The incidence of achondroplasia was about 1 in 26,000 live births between 1969-75 when ascertainment was nearly complete. This indicates a mutation rate of 1.93 .times. 10-5/generation in this locus. Paternal age was shown to influence mutation. Ascertainment in earlier years of the study was low despite great effort made to find all cases. Patients with hypochondroplasia were particularly difficult to find, yet 25 cases were found for study. Overlap between hypochondroplasia and achondroplasia was found in all features except facial appearance (which was the basis of definition). Achondroplasia was more severe in all regards, but some individuals with hypochondroplasia were very short and some had extreme degrees of spinal canal stenosis. The classical measurements used to describe the skull changes in achondroplasia failed to distinguish this condition from hypochondroplasia. More efficient indices were devised, but visual assessment of the size of the facial region compared to that of the cranial vault proved more reliable than any index. The clinical distinction based on facial appearance remains the arbitrary basis of definition.