Prenatal Diagnosis of Fetal Skeletal Dysplasia Using 3-Dimensional Computed Tomography: A Case Series

Background: Fetal skeletal dysplasia constitutes a complex group of systemic bone and cartilage disorders. Many fetal skeletal dysplasia phenotypes have indistinct denitions, making denitive prenatal diagnosis dicult. Fetal skeletal dysplasia is typically diagnosed using sonography; however, three-dimensional computed tomography (3D-CT) also aids in making a prenatal diagnosis. This study aimed to examine the ecacy of 3D-CT for the prenatal diagnosis of fetal skeletal dysplasia by comparing the diagnostic accuracy of fetal sonography and 3D-CT. Methods: Based on the suspicion of skeletal dysplasia based on ultrasound examination, we performed 3D-CT prenatally to conrm this diagnosis. In order to minimize exposure of the fetuses to radiation without compromising image quality, we used predetermined 3D-CT settings for volume acquisition. Results: Sixteen fetuses were suspected to have fetal skeletal dysplasia based on ultrasonography ndings. Of these, 15 were diagnosed with fetal skeletal dysplasia using 3D-CT. All 15 fetuses diagnosed with fetal skeletal dysplasia prenatally were conrmed postnatally to have fetal skeletal dysplasia. The denitive postnatal diagnosis (campomelic dysplasia) differed from the prenatal diagnosis (osteogenesis imperfecta) in only one infant. 14 cases (93.3%) were diagnosed by an accyrate classication. Five infants had lethal skeletal dysplasia; one died in utero, and four died as neonates. We determined the appropriate delivery method for each infant based on the prenatal diagnosis. Conclusion: 3D-CT is a valuable tool to augment ultrasound examination for the diagnosis of skeletal dysplasia because it can provide a more denitive diagnosis of the type of dysplasia, enabling better planning for resuscitation of the infant after birth. examination for the diagnosis of skeletal dysplasia. With 3D-CT, the type of dysplasia can be more accurately diagnosed, and this information helps when making plans for the resuscitation of the infant after birth.


Plain English Summary
Fetal skeletal dysplasia is a disease of the bones and cartilage that develops prenatally. It is di cult to ascertain if fetal skeletal dysplasia is present before the baby is born. Fetal skeletal dysplasia is usually diagnosed using ultrasonography; however, three-dimensional computed tomography (3D-CT) also helps in making a diagnosis during pregnancy. Our study investigated whether 3D-CT could be helpful in the prenatal diagnosis of fetal skeletal dysplasia by comparing the diagnoses using fetal ultrasonography and 3D-CT with the diagnosis made at birth. In this study, we performed 3D-CT scans on 16 fetuses that were suspected to have fetal skeletal dysplasia based on ultrasonography ndings. We used predetermined 3D-CT settings in order to minimize the radiation risk to the fetuses while still maintaining image quality. We diagnosed fetal skeletal dysplasia prenatally in 15 cases using 3D-CT. All 15 fetuses diagnosed with fetal skeletal dysplasia via 3D-CT were con rmed at birth to have fetal skeletal dysplasia although the type of dysplasia diagnosed was different in one case. Five infants had lethal skeletal dysplasia; one was stillborn, and four died shortly after birth. We concluded that 3D-CT is a valuable tool to augment an ultrasound examination for the diagnosis of skeletal dysplasia. With 3D-CT, the type of dysplasia can be more accurately diagnosed, and this information helps when making plans for the resuscitation of the infant after birth.
Keywords fetal skeletal dysplasia, fetal diagnosis, 3D-CT, shortening of long bones, gene expression Background Fetal skeletal dysplasia (FSD) is a group of systemic bone and cartilage disorders that develop prenatally and may be detected by fetal ultrasonography. Because most cases of skeletal dysplasia involve the mutation of a single gene, a de nitive diagnosis can be reached if this mutation is identi ed. Although the widespread use of fetal ultrasound imaging has improved the detection of FSD, prenatal diagnosis remains relatively di cult because of the rarity of the condition (approximately one of every 5,000 births) [1]. The diversity of the FSD phenotypes also contributes to the di culty in diagnosing these disorders. FSDs comprise 436 diseases that are classi ed into 42 groups, according to the Nosology & Classi cation of Genetic Skeletal Disorders: 2015 Revision [2]; however, the phenotypic de nition of each condition is often unclear. It is important to make a de nitive diagnosis to differentiate between lethal or nonlethal disorders in order to plan the most effective delivery method and the best postnatal treatment [3].
Three-dimensional computed tomography (3D-CT) is commonly used to diagnose cardiac atherosclerosis, cerebral aneurysms, and cerebral artery stenosis. This imaging technique creates better images of thromboses and vascular wall calci cation in cerebral aneurysms compared to cerebral angiography. In 2004, Ruano et al. reported that fetal 3D-CT was a useful diagnostic method, complementary to ultrasonography, that may improve the accuracy of the prenatal diagnosis of skeletal disorders [4]. Considering both the clinical bene t of this method and the increased risk introduced by radiation, we elected to use low-dose 3D-CT for prenatal diagnosis in this study. The aim of this study was to compare the prenatal diagnosis and de nitive postnatal diagnosis in patients with FSD in order to examine the e cacy of 3D-CT for the prenatal diagnosis of FSD.

Study design
A prospective study was conducted involving pregnant women whose pregnancy/childbirth was being managed in the Department of Obstetrics and Gynecology at Kyoto Prefectural University of Medicine. Fetuses were included in the study if they had a suspected bone abnormality based on routine ultrasound examinations. We recorded any family history of skeletal dysplasia and complications during pregnancy.

Materials
Pregnant women whose pregnancies/childbirths were managed at our hospital and who met the following selection criteria were included as participants: (1) pregnancy at age 20 years or older at the time of consent, (2) gestational age >26 weeks of pregnancy but had not yet delivered, (3) ultrasonography showing a short fetal femur (≤-2.5 standard deviation of the reference of fetal measurements) or an image of a fracture, curvature, or deformity in a long bone, skull, rib, or spine, (4) the pregnant woman provided written informed consent to undergo 3D-CT.

Sonographic examination
We used Voluson E8 and E10 ultrasound systems (General Electric Healthcare, Tokyo, Japan) to screen for associated anomalies. All operators were experts in fetal sonography. First, we measured the fetus using a standard approach (biparietal diameter, abdominal circumference, and femur length) and identi ed fetuses that displayed femur length measurements below the 10 th percentile. We recommended 3D-CT scanning if the fetal femur or any other long bones were found to be shortened, regardless of whether or not they were abnormally shaped, indicating potential FSD.
Three-dimensional computed tomography 3D-CT (Aquilion RXL; TOSHIBA, Tokyo, Japan) was only performed after thoroughly explaining the effects of radiation exposure to the mother and fetus and receiving informed consent from the mother. In order to minimize exposure of the fetus to radiation without compromising image quality, the following predetermined settings were used for volume acquisition: 1) 1.00-mm thick scans with 1.0-mm intervals for 3.1 s, 2) 120-mA intensity, 3) 120-kV voltage, 4) 0.5-s rotation time, and 5) 1.438 pitch. Keeping these settings constant resulted in a median CT dose index volume of 2.9 mGy (range: 1.9-4.9 mGy). We created a maximum intensity projection image from 3D-CT images for diagnosis, if necessary. Additional tests, such as magnetic resonance imaging (MRI) or amniotic chromosomal testing, were performed if required to determine fetal prognosis or to differentiate skeletal dysplasia from other diseases. Lethality was determined by the presence of a narrow thorax and lung hypoplasia based on ultrasonograms and MRI images.

Postnatal examination
Postnatally, we recorded the neonatal weight and length, and performed an X-ray examination to verify the shape of all bones. After genetic counseling, genetic testing was performed to con rm the diagnosis.
Brie y, genomic DNA was extracted from the umbilical cord or the infant's blood.

Patient characteristics
We identi ed 38 fetuses with a short femur, and based on sonography, 16 of these 38 fetuses had suspected FSD. We performed 3D-CT on these 16 fetuses. The mean maternal age was 32.6 (range: 20-41) years, and the mean gestational age at the rst ndings of FSD was 24.4 (range: 15-36) weeks. The mean 3D-CT imaging period was 30.6 weeks' gestation, which is consistent with the period when CT testing is generally performed for prenatal diagnosis (approximately 30 weeks' gestation). Patient characteristics are presented in Table 1. Of the 16 fetuses that underwent 3D-CT, 15 were prenatally diagnosed with FSD. FSD was con rmed postnatally in all 15 of these fetuses. On the basis of prenatal 3D-CT, we suspected a chromosomal abnormality (trisomy 21) in the remaining fetus, which was con rmed postnatally.

Classi cation of fetal skeletal dysplasia
Prenatal imaging data, postnatal ndings, and pre-and postnatal diagnosis are presented in Table 2. The speci c diagnoses were as follows: osteogenesis imperfecta (OI, n=3), thanatophoric dysplasia (TD, n=2), achondroplasia/hypochondroplasia (ACH/HCH, n=5), hypophosphatasia (HPP, n=2), campomelic dysplasia (CD, n=1), Desbuquois dysplasia (n=1), and Creveld syndrome (n=1). The two infants with HPP were siblings. The pre-and postnatal diagnoses differed in one infant diagnosed with OI prenatally and CD postnatally. In this case, the 3D-CT fetal skeleton ndings (long bone shortening and mild curvature with scapular hypoplasia; Figure 1) were the same as the postnatal ndings. However, we identi ed ambiguous genitalia, one of the characteristic ndings of CD, in the postnatal period.

Lethality
Of the ve fetuses with poor prognoses, one had OI, two had TD, and two had HPP. One of the fetuses with HPP suffered intrauterine fetal death at 27 weeks' gestation, while the remaining four infants died within two days of birth. We identi ed lethality in the prenatal period in all ve fetuses.
Trisomy 21 and achondroplasia Infant 8 was diagnosed prenatally with ACH based on ultrasound ndings of shortened long bones, a narrow thorax, and a trident hand ( Figure 2). After birth, the characteristic trisomy 21 facial features were noted, and the results of chromosomal testing (G-band; 47, XY, +21) con rmed trisomy 21. FGFR3 genetic analysis also indicated a G1193A mutation in G380, indicating that the infant also had ACH.

Postnatal genetic testing
In infant 12, we identi ed dislocation of the knee and elbow, as well as clubfeet, based on sonography and 3D-CT, leading us to suspect Desbuquois dysplasia (Figure 3). Postnatal genetic testing con rmed our diagnosis of Desbuquois dysplasia (type 1) due to a mutation of the 613165 calcium-activated nucleotidase 1 (CANT1) gene.
The two infants with HPP (infants 14 and 15) had the same parents. The rst child exhibited marked long bone shortening and a soft, deformed skull based on ultrasonography at 18 weeks' gestation, which was suggestive of fetal HPP. Therefore, we measured maternal alkaline phosphatase (ALP) levels, which were low. At 27 weeks' gestation, the child suffered from intrauterine fetal death. After the child was stillborn, autopsy imaging was performed. The CT images also indicated a likely diagnosis of HPP. Genetic testing of the placental tissue revealed a compound heterozygote with mutations in two tissue-nonspeci c alkaline phosphatase genes (ALPL; p.K224E, c.1559delT), and con rmed our diagnosis of HPP. The parents' second child exhibited severe shortening of all four limbs at 15 weeks' gestation, with 3D-CT revealing ndings typical of severe perinatal HPP (Figure 4), with hardly any ossi cation. At 38 weeks' gestation, the infant was delivered via Cesarean section but suffered neonatal death at 1 day of age.

Discussion
Prenatal diagnosis of skeletal dysplasia is typically based on sonographic and radiological ndings, and 3D-CT may be a valuable imaging tool complementary to ultrasonography for the diagnosis of skeletal dysplasia. However, it is necessary to select the appropriate settings to minimize the degree of fetal irradiation without compromising image quality [5]. In this study, we showed that when 3D-CT was used along with fetal ultrasonography, the prenatal diagnosis and de nitive postnatal diagnosis differed in only one infant. This infant was diagnosed prenatally with FSD and postnatally with trisomy 21. However, when sonography was used without 3D-CT, prenatal diagnoses differed from de nitive postnatal diagnoses in three infants. In addition, the pre-and postnatal classi cations of the speci c form of FSD differed in one infant. In this infant, the prenatal 3D-CT ndings were the same as the postnatal X-ray ndings. However, we did not identify ambiguous genitalia in the infant (a characteristic nding of CD) [6] on the fetal ultrasound examination. Thus, we diagnosed CD based on the postnatal physical ndings.
Fetal bone 3D-CT (as an auxiliary test for ultrasound examinations) has high diagnostic accuracy when it is performed from 26 weeks' gestation onward. The detection of abnormal ndings outside the skeletal system on sonography can greatly affect the diagnosis [7,8]. For an accurate diagnosis of FSD, both sonography and 3D-CT appear to be useful methods. In some cases, it can be di cult to differentiate skeletal dysplasia from a chromosomal abnormality. Of the 16 infants that underwent 3D-CT in the present study, only one was suspected of having a chromosomal abnormality. In this case, skeletal dysplasia was suspected based on ultrasonography ndings, and the results of 3D-CT suggested trisomy 21. Postnatal chromosomal testing was performed, resulting in a nal diagnosis of trisomy 21 and the exclusion of this infant from the present study. Some cases can be complicated, with concomitant skeletal dysplasia and a chromosome aberration. In particular, combinations of ACH and trisomy 21 have been reported [9,10]. ACH is a mild type of dysplasia in the group of FGFR3 abnormalities, where the parts that grow become hypoplastic due to endochondral ossi cation, which can be di cult to differentiate from trisomy 21 [9,10].
In 2001 [11] and 2014 [12], it was reported that femoral shortening during the second trimester is associated with a high relative risk of chromosomal abnormalities, particularly trisomy 21. If fetal femoral shortening is observed, it is important to consider the possibility of conditions, such as chromosomal abnormalities, fetal growth restriction, and various other anomalies. Attention should also be paid to the presence of abnormal ndings outside of the skeletal system (e.g., abnormalities in cardiac structure, the nervous/gastrointestinal system, and facial features) to reach a differential diagnosis. When making a prenatal diagnosis, it is important to bear in mind that cases of concomitant skeletal dysplasia and chromosomal abnormalities are possible, albeit rare.
Desbuquois dysplasia is a severe autosomal recessive disorder that belongs to the multiple dislocation group [13]. It is characterized by shortened long bones, joint laxity, pre-and postnatal growth retardation, and progressive scoliosis. In the case of infant 12, the mother's rst child also had skeletal dysplasia (congenital dislocation of the knees) that had been identi ed after birth. Although this child had been diagnosed with Larsen syndrome, it had not been identi ed prenatally by sonography/3D-CT nor con rmed via postnatal genetic testing. The mother's other child (infant 12), on the other hand, received a prenatal ultrasound examination and 3D-CT, and was diagnosed with Desbuquois dysplasia, rather than Larsen syndrome. Postnatal genetic testing led to a de nitive diagnosis of Desbuquois dysplasia (type 1) due to a mutation in the CANT1 gene [14,15]. Conversely, Larsen syndrome is due to a mutation in the 603381 Filamin B gene. This example demonstrates that an accurate clinical diagnosis is important to achieve a de nitive diagnosis via postnatal molecular evaluation. Importantly, prenatal 3D-CT imaging is a valid diagnostic method that may be employed in such cases.
HPP is a condition in which tissue non-speci c ALP loss or decrease causes defective bone mineralization and abnormal tooth mineralization [16]. Many cases of HPP are inherited in an autosomal recessive manner, whereas others are autosomal dominant. HPP can be classi ed into the following six clinical forms, which vary based on severity [17]: 1) perinatal severe, 2) perinatal benign, 3) infantile, 4) childhood, 5) adult, and 6) odontohypophosphatasia. If parents have a child with HPP, they should be given genetic counseling with respect to future pregnancies, and the couple can decide whether to undergo carrier detection testing. If both the mother and father are carriers, the child has a 25% probability of being homozygous. Our fatal perinatal case appeared to be a severe case. The symptoms of perinatal cases appear early, resulting in intrauterine fetal death [18], stillbirth, or neonatal death.
Compared to normal fetuses, those with FSD are more likely to be in an abnormal fetal position owing to differences in physique, restricted posture, limited joint range of motion, and fractures [19]. In cases of OI, it can be di cult to prevent intrauterine fractures as they can be caused by fetal movement ( Figure 5).
Studies have shown that, in cases of OI, performing a Cesarean delivery does not increase the frequency of fetal bone fractures compared that with vaginal delivery [20]. Moreover, in cases of the lethal type of OI, fetal survival does not appear to be affected by delivery mode (Cesarean vs. vaginal delivery). For other types of skeletal dysplasias, the mode of delivery is often selected based on whether or not the condition is lethal. When the disease is diagnosed as lethal, a vaginal delivery may be selected even if the fetus is breech. However, in cases with marked head enlargement, a Cesarean section is required regardless of whether or not the case is fatal, owing to the potential di culty in passing through the mother's pelvis. It is important for the parents, obstetrician, and pediatrician to discuss prior to delivery the plan for the resuscitation of the infant after birth. Accurate prenatal diagnosis is required to determine whether or not the case is fatal, and the degree of severity.
When considering the risk of radiation exposure, CT examination has not historically been the preferred method for pregnant women. However, in recent years, high quality images adequate for a diagnosis have been made by low-dose 3D-CT, reducing the level of radiation to which the mother and fetus are exposed. The strength of this study was the accuracy of the prenatal diagnosis by 3D-CT scan in combination with fetal sonography. The limitation of this study is the relatively small sample size. This should be increased in future studies to improve the power of the results. Continuation of this study is important in order to advance the diagnosis of fetal skeletal dysplasias.