Diagnostic Considerations
The phenotype and radiographic findings of achondroplasia are distinctive; diagnosis of affected children should not be difficult. However, there are other types of short-limb dwarfism associated with mutations within FGFR3. Diagnostic testing by genetic molecular studies is available for confirmation.
Hypochondroplasia
This chondrodysplasia occurs less often than achondroplasia. Hypochondroplasia differs from achondroplasia in its lack of craniofacial involvement and milder phenotypic changes in the spine and hands. Inheritance is autosomal dominant, and the gene has been mapped to 4p16.3, but with a different mutational change within FGFR3. A study by Xue et al suggests that significant overlap exists between the genotypes and phenotypes of achondroplasia and hypochondroplasia, indicating that when either disorder is suspected, molecular genetic testing for both should be performed. [23]
SADDAN (severe achondroplasia with developmental delay and acanthosis nigricans)
Extremely short stature, profound developmental delay, marked tibial bowing, and acanthosis nigricans characterize this rare genetic disorder.
Thanatophoric dysplasia
This is a skeletal dysplasia with significant newborn mortality due to a small thoracic cage and pulmonary insufficiency. Patients who survive beyond the neonatal period have profound developmental delay and severe growth deficiency. It is inherited as an autosomal dominant trait; all cases are due to a de novo mutation.
Differential Diagnoses
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Height for females with achondroplasia (mean/standard deviation [SD]) compared to normal standard curves. The graph is based on information from 214 females. Adapted from Horton WA, Rotter JI, Rimoin DL, et al. Standard growth curves for achondroplasia. J Pediatr. 1978 Sep; 93(3): 435-8.
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Height for males with achondroplasia (mean/2 standard deviations [SDs]) compared to normal standard curves. The graph is based on information from 189 males. Adapted from Horton WA, Rotter JI, Rimoin DL, et al: Standard growth curves for achondroplasia. J Pediatr. 1978 Sep; 93(3): 435-8.
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Mean growth velocities (solid line) for males (top) and females (bottom) with achondroplasia compared to normal growth velocity curves. Dashed lines indicate third percentile, mean, and 97th percentile. Data are from 26 males and 35 females. Adapted from Horton WA, Rotter JI, Rimoin DL, et al. Standard growth curves for achondroplasia. J Pediatr. 1978 Sep; 93(3): 435-8.
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Upper and lower segment lengths for males (top) and (bottom) with achondroplasia (mean/standard deviation [SD]). Data are from 75 males and 95 females. Adapted from Horton WA, Rotter JI, Rimoin DL, et al. Standard growth curves for achondroplasia. J Pediatr. 1978 Sep; 93(3): 435-8.
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Head circumference for females with achondroplasia compared to normal curves (dashed lines). Data are from 145 females. Adapted from Horton WA, Rotter JI, Rimoin DL, et al. Standard growth curves for achondroplasia. J Pediatr. 1978 Sep; 93(3): 435-8.
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Head circumference for males with achondroplasia compared to normal curves (dashed lines). Data are from 114 females. Adapted from Horton WA, Rotter JI, Rimoin DL, et al. Standard growth curves for achondroplasia. J Pediatr. 1978 Sep; 93(3): 435-8.
