Radiography
Findings
Genu varum. Image shows rhizomelic shortening of the bilateral femurs with metaphyseal flaring. The bones are wide because of unaffected appositional growth.
Image shows progressive narrowing of the lumbar spinal canal, bullet-nose vertebrae, and marked lumbar lordosis. Note the shortened ribs.
Image shows an enlarged calvaria with a shortened skull base and frontal bossing. Note the midface hypoplasia.
Image shows posterior bowing of the humerus, the principal cause of the loss of elbow extension. Posterior dislocation of the radial head may also contribute.
Trident hands. Image shows widely opposed fingers of equal length.
Shortened ribs.
The radiographic findings in achondroplasia are as follows (see Images above and Images 1-28 in Multimedia):
- Shortening of tubular bones with a normal shaft caliber
- Short extremities and ribs-versus-trunk length
- Short phalanges
- Ball-in-socket epiphyses
- Metaphyseal flaring and cupping
- Circumflex or chevron seat on the metaphysis
- Squared iliac wings and narrow sacroiliac notch (champagne glass)
- Fingers widely opposed and of equal length (trident hands)
- Enlarged skull vault and mandible
- Small foramen magnum
- Narrow anteroposterior (AP) diameter of the spine with a concave posterior surface
- Decreased lumber interpediculate distance and narrow spinal canal
- Hypoplastic (bullet nose) thoracolumbar junction vertebrae
- Narrow vertebral interpediculate distance
Kitoh and associates examined 23 patients (41 elbows) with achondroplasia.13 The extension of the elbow was clinically assessed, and the angle of posterior bowing of the distal humerus was measured on lateral radiographs. Extension of the elbow was limited in 28 elbows(68%), and the mean loss of extension was 13.1°. Posterior bowing of the humerus was seen in all elbows, with a mean angle of 17.0°. The 2 measurements were positively correlated. Posterior bowing greater than 20° caused a loss of full elbow extension. Posterior dislocation of the radial head was seen in 9 elbows (22%).
The mean loss of extension of the elbows was 28.7°; this loss of extension was significantly greater than the loss seen in elbows in which the head was not dislocated (8.7°), though posterior bowing was not significantly different between the groups (19.3° vs 16.3°). Posterior bowing of the distal humerus is a principal cause of loss of elbow extension. Posterior dislocation of the radial head caused further limitation of movement in the more severely affected joints.
Thomeer and van Dijk described surgical treatment of lumbar stenosis in patients with achondroplasia that involved selective widening of the lumbar interapophyseolaminar diameter.8 They found that dynamic lumbar myelography was required for demonstrating the symptomatic level.
Degree of Confidence
Conventional radiography is noninvasive, inexpensive, quick to perform, and fairly reliable. It is generally the first examination performed after birth to confirm the diagnosis of achondroplasia. It is also usually the first examination of choice for patients with a symptomatic spine.
False Positives/Negatives
Conventional radiographs generally provide only bony detail and provide little information about the state of the brain and spinal cord. The cord is better depicted with myelography, though myelography provides little information about anomalies within the cord.
Computed Tomography
Findings
Among infants with achondroplasia and apnea, CT and MRI have repeatedly demonstrated cord compression resulting from direct impingement of the posterior rim of the foramen magnum and C1 arch. Sleep apnea responds well to decompression of the foramen magnum. CT shows that in virtually all children with achondroplasia, there is some degree of compromise of the foramen magnum. In about 96% of such children, the foramen magnum is smaller than 3 standard deviations of the mean. CT and/or MRI can depict this change.
A small spinal canal is present in the cervical region from birth, but symptoms of cervical canal stenosis generally do not occur until middle age or later. If neurologic deficit occurs and does not resolve through conservative measures, laminectomy at multiple levels may be required. Preoperative imaging with CT, CT myelography, and/or MRI is vital for successful surgery. Preoperative and intraoperative myelography, CT, or MRI defines the level of cord and/or root compression caused by dorsolumbar spinal stenosis. Although MRI has largely replaced conventional myelography, CT myelography and intraoperative myelography may still play a role.
Otitis media is a relatively common complication of achondroplasia. To best define the changes affecting the temporal bone that might predispose patients with achondroplastic dwarfism to otitis media, Cobb et al evaluated 9 subjects referred because of hearing loss.21 Patients underwent high-resolution CT of the temporal bone; their results were compared with those of subjects without achondroplasia. A number of morphologic changes were seen: (1) poor development of mastoid air cells, (2) shortening of the carotid canals, (3) narrowing of the skull base, (4) towering petrous ridges, and (5) relative rotation of the cochlea and other structures of the temporal bone.
The most notable change was the rotational effect, which was most pronounced medially and which resulted in abnormal orientations of inner-ear structures relative to middle-ear structures and of middle-ear structures relative to the external auditory canal. The investigators also noted a lack of evidence of otitis media or its sequelae in any of the patients with achondroplasia.
Audiograms were obtained in 2 adults and 4 children. The results showed evidence of mixed hearing loss in the 4 children but only of sensorineural hearing loss in the adults. The authors concluded that the persistent hearing loss in patients with achondroplasia is not a sequela of otitis media, as others have suggested. Intrinsic vestibulocochlear changes below the limits of resolution of high-resolution CT scanning may be responsible.
Degree of Confidence
The sensitivity of CT myelography is greater than that of conventional myelography. CT depicts bone detail better than MRI. MRI has an obvious advantage of being radiation free, but many clinicians believe that the degree of stenosis is usually best demonstrated with myelography.
False Positives/Negatives
Details of the posterior fossa brain and cord are better depicted on MRI than on CT. Cord edema and changes associated with myelomalacia usually cannot be seen with CT. CT also provides only indirect evidence of associated anomalies, such as syringomyelia, whereas MRI shows such features directly and clearly.
Magnetic Resonance Imaging
Findings
Craniocervical MRI findings may include narrowing of the foramen magnum and C1 canal, effacement of the subarachnoid spaces at the cervicomedullary junction, abnormal intrinsic cord signal intensity, and mild to moderate ventriculomegaly. In the spinal canal, associated anomalies such as syringomyelia and changes of myelomalacia are well depicted on MRI. In addition to depicting spinal canal stenosis, MRI also demonstrates disk protrusions and osteophytes that cause compromise (see Image below and Image 29 in Multimedia).
Achondroplasia: Sagittal section of the cervical spine T2-weighted MRI showing narrowing of the foramen magnum at C1 canal, effacement of the subarachnoid spaces at the cervicomedullary junction, abnormal intrinsic cord signal intensity in this 6-year-old patient who presented with a neurologic deficit.
Brühl et al studied the CSF flow, venous drainage, and spinal cord compression in achondroplastic children and the impact of MRI findings for decompressive surgery at the craniocervical junction (CCJ).22 They examined 25 patients with conventional morphologic imaging and with functional imaging of CSF flow with magnetic resonance angiography (MRA) of the veins and sinuses at the cranial base by using a special protocol. The results were compared with those from age-matched control subjects and were correlated with each other and with the neurologic findings. Distances and angulations at the CCJ on MRIs were similar to those measured on conventional radiographs and CT scans. Therefore, these measures may be used without correction for spatial distortion. Signs of cervical medullary compression, myelomalacia, and intramedullary cyst formation were found in 6, 7, and 3 children, respectively. These alterations were significantly correlated with each other (P <.05).
Semiquantitative evaluation of CSF flow demonstrated interruption of CSF pathways at the CCJ, which was correlated with CCJ narrowing (P <.05). MRA showed significant narrowing of the jugular foramina, with a variable compensatory enlargement of the emissary veins and a significant reduction of the total outflow area (P <.01). These MRI changes were not significantly correlated with neurologic deficits.
The authors concluded that, because of this unexpectedly poor correlation between MRI and clinical findings in children with achondroplasia, the present role of MRI in the clinical setting is limited to the demonstration of spinal-cord compression in individual patients. In 3 patients with prominent neurologic abnormalities, the severe changes demonstrated on MRI strongly supported the indication for surgical decompression.
Yamashita et al evaluated 29 patients with atlantoaxial subluxation using MRI. The atlantoaxial subluxation was related to a variety of pathologies, but 1 patient had achondroplasia. Cord compression was classified into 4 grades according to the degree on MRI. Seven patients had no thecal sac compression (grade 0), 10 had a minimal degree of subarachnoid space compression without cord compression (grade 1), 7 had mild cord compression (grade 2), and 5 had severe cord compression or cord atrophy (grade 3). Although the severity of myelopathy was poorly correlated with the atlantodental interval on conventional radiography, MRI grade and the degree of myelopathy were highly correlated. T2-weighted images showed hyperintense foci in 7 of 12 patients with cord compression (grades 2 and 3).
Kao et al performed MRIs of the craniovertebral junction, cranium, and brain in 10 patients (aged 3 mo to 16 y) with achondroplasia.23 All had narrowing of the subarachnoid space at the level of the foramen magnum, and 5 had compressive deformities of the cervicomedullary junction. Apparent upward displacement of the brainstem and a relatively vertical course of the optic nerve were seen in all patients. Dilated lateral and third ventricles were seen in 5 patients, and bifrontal widening of the subarachnoid space was evident in 4. Skull asymmetry was seen in 2 patients, and an empty sella (confirmed on metrizamide cisternography) was present in 1. In 1 patient, foci of abnormal signal intensity were seen in the cervicomedullary region. The authors indicated that MRI is useful in delineating the many abnormalities of the cranial, cerebral, and cervicomedullary junction present in children with achondroplasia.
Degree of Confidence
MRI is a noninvasive technique and is ideal for children because it does not use ionizing radiation. MRI has an advantage over CT in the degree of detail of the posterior cranial fossa cord that it provides. Early clinical and MRI evaluations are necessary to determine whether infants with achondroplasia have cervicomedullary compression. With early recognition, an immediate decompression can be performed safely to avoid serious complications associated with cervicomedullary compression, including sudden death.
False Positives/Negatives
CT depicts details of bone and the degree of spinal stenosis better than MRI. Claustrophobia may limit the quality of MRI, and motion artifacts may produce false-negative and/or false-positive results.
Ultrasonography
Findings
Ultrasonography is generally performed in the antenatal setting and in pregnant women who are at risk for achondroplasia.
Homozygous achondroplasia results in rhizomelic micromelia, normal trunk length, and cloverleaf skull. These cases are lethal. Lung hypoplasia is a major cause of mortality (associated with thoracic narrowing). There is a noticeable disproportion between skull dimensions and/or biparietal diameter (BPD) and limb lengths. The discrepancy between femoral length and BPD is noted as early as 13 weeks' gestation. The femoral length decreases to below the third percentile at 14.0-16.5 weeks' BPD age (mean, 15.6 weeks; 95% confidence interval: 13.4, 17.8). Therefore, femoral growth curves are established in the second trimester. Serial sonography enables prenatal distinction between homozygous and heterozygous disease.
The changes in heterozygous achondroplasia are relatively mild and include short limbs, narrow thorax and abdomen, increased fetal head circumference and BPD, a protuberant forehead, and a diminished interpediculate distance in the spine. Heterozygous disease may not be recognized until late in the second trimester (>24-28 wk); early in the course of disease, sonograms are normal. Rhizomelic limb shortening that predominantly affects the proximal long bones is observed.
Krakow and associates found that 3-dimensional (3D) imaging in the prenatal-onset diagnosis of skeletal dysplasia had advantages over 2-dimensional (2D) imaging in the evaluation of facial dysmorphism, relative proportion of the appendicular skeletal elements, and the hands and feet.24 Of most importance, the patient and referring physician appreciated the 3D images of the abnormal findings more readily than other images; this advantage aided in patient counseling and in managing the pregnancy.25
Degree of Confidence
Patel and Filly retrospectively reviewed serial sonograms of 15 fetuses at 25% risk of homozygous achondroplasia.26 Femoral growth curves were established and were compared with published standards to determine the gestational age. They were calculated according to BPD, at which femoral length crossed below the third percentile. The presence and severity of achondroplasia were clinically determined after birth.
Their results showed that the femoral length crossed the third percentile at 14.0-16.5 weeks BPD age (mean, 15.6 wk) in the 4 homozygous fetuses and at 18.2-26.2 weeks BPD age (mean, 21.5 wk) in the 8 heterozygous fetuses. In the 3 unaffected fetuses, femoral length did not cross percentiles as gestational age increased. The authors concluded that the establishment of a femoral growth curve in the second trimester with serial sonograms enables prenatal distinction between homozygous, heterozygous, and unaffected fetuses, when both parents have heterozygous achondroplasia.
False Positives/Negatives
Parilla and associates reviewed 37 cases of skeletal dysplasia diagnosed antenatally over 8 years.27 Complete follow-up was available in 31 cases. The mean gestational age at diagnosis was 22.7 weeks (range, 14-32.3 wk). In 21 cases, the diagnosis was made before the 24th week. A final diagnosis was obtained in 80% of cases. The antenatal diagnosis was correct in 20 (65%) of 31 cases. Two false-positive diagnoses occurred.
Specific final diagnoses included thanatophoric dysplasia (n = 8), osteogenesis imperfecta (n = 6), Roberts syndrome (n = 2), achondroplasia (n = 3), Ellis-van Creveld syndrome (n = 1), metaphyseal dysplasia (n = 1), spondyloepiphyseal dysplasia (n = 1), distal arthrogryposis (n = 1), caudal regression (n = 1), and glycogen storage disorder (n = 1).
The condition was correctly thought to be lethal in 16 of the fetuses on the basis of early, severe long-bone shortening (n = 13), femur length – abdominal circumference ratio of less than 0.16 (n = 12), hypoplastic thorax (n = 10), marked bowing or fractures (n = 4), short ribs (n = 4), caudal regression (n = 1), and cloverleaf skull (n = 1). The ability to predict lethality was 100%. No false-positive findings with respect to lethality occurred.
The authors concluded that antenatal diagnosis of skeletal dysplasias is problematic. In their series, only 20 of 31 cases were correctly diagnosed. However, the antenatal prediction of lethality was highly accurate. The most common predictors of lethal skeletal dysplasias included early and severe shortening of the long bones, femur length – abdominal circumference ratio of less than 0.16, hypoplastic thorax, and certain distinguishing characteristics.
Nuclear Imaging
Findings
Isotopic cisternography with an intrathecal injection of indium-111 diethylenetriamine pentaacetic acid (DTPA) was commonly used in the past to assess hydrocephalus. It is rarely used now, because it does not increase accuracy in predicting favorable responses to shunting, relative to CT and clinical evaluation.
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Further Reading
Keywords
achondroplasia, osteochondrodysplasia, short-limb dwarfism, dwarfism, inborn genetic disease, bone growth disorder, bone disease, developmental bone disease, chondrodystrophia fetalis, hypoplastic chondrodystrophy, chondrodystrophies, achondroplastic dwarfism, Parrot's disease, rickets fetal, ACH, metatrophic dwarfism II, Kniest syndrome, Kniest's syndrome, pseudoachondroplasia, PSACH, FGFR3 gene
