Laboratory Studies
Commercially available direct DNA analysis for FGFR3 mutations identifies the G1138A (known as point mutation G380R) in patients with achondroplasia and the novel missense mutation (Lys650Met) in tyrosine kinase. Direct DNA analysis of FGFR3 mutations is used for prenatal screening in families at risk (ie, parents who are heterozygous for either the G1138A or the G1138C mutation). Transgenic mice with achondroplastic skeletal dysplasia have been developed, which can potentially speed the research and development of drugs and provide animal models for gene therapy trials.
Epiphyseal growth plate cartilage findings are histologically, immunohistochemically, biochemically, and electron microscopically normal.
Imaging Studies
Diagnosis is assisted with imaging studies and their findings. Confirmation of achondroplasia using these diagnostic tools helps exclude other types of skeletal dysplasias.
Ultrasonography
Fetal ultrasonography is recommended to assist in prenatal diagnosis, if there is concern for achondroplasia. Perform fetal ultrasonography to evaluate skeletal anomalies and to measure the long bones for size, shape, bowing, symmetry, and quality of calcification.
Perform serial ultrasonography starting at the second trimester to plot on growth curves the fetal (intrauterine) femoral length. This can aid in distinguishing a fetus with a skeletal dysplasia from an unaffected fetus; note that the characteristic features of achondroplasia may not be evident before this time. Also use fetal ultrasonography to evaluate the skull for size and shape.
Perform cranial ultrasonography at birth and at age 2 months, 4 months, and 6 months, to monitor ventricular size for hydrocephalus and/or an intracranial bleed. [24] In addition, perform ultrasonography if the following conditions are present:
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If the fontanelle is unusually large and full
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If the occipitofrontal circumference (OFC) increases disproportionately over time
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If clinical symptoms of hydrocephalus are present
Radiography
Plain radiography performed at birth can identify achondroplasia. With the findings of disproportionately short limbs on fetal ultrasonography, plain film radiographs of the neonate may help distinguish achondroplasia from other skeletal dysplasias. [25]
Perform a skeletal survey. Radiographic findings include a contracted skull base, normal trunk length, rhizomelic shortening of the long bones, trident hands, a square-shaped pelvis with small sacrosciatic notch, and proximal femoral radiolucency. In older children, a chevron-shaped (V-shaped) distal femoral epiphyses is seen and growth plates are shortened.
Additional radiographic findings are as follows:
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Progressive interpedicular narrowing in the lumbar spine
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Short pedicles, which can cause spinal stenosis
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Short femoral neck and metaphyseal flaring
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Dysplastic ilium, narrow sacroiliac groove, and flat-roofed acetabula
Pregnancy
Prior to pregnancy for a woman with achondroplasia, radiographic evaluation of the pelvis is recommended to assess for a narrow pelvis. Delivery options need to be discussed, especially with regard to defining difficulties associated with vaginal delivery.
CT scanning
Head CT-scan findings define neuroanatomic abnormalities consistent with arrested hydrocephalus. Enlarged cranial ventricles and changes in the corpus callosum are seen.
MRI
Craniocervical MRI findings include narrowing of the foramen magnum and stenosis in the lumbar spine. [26]
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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.
