Defects in Segmentation

► [Fusion or bifurcation of the ribs]

Segmentation defects are the third most common (20 %) congenital anomalies affecting the ribs, after supernumerary ribs (30%) and agenesis/aplasia of the ribs (26%). Under the designation 'segmentation defects' a number of different anomalies are grouped together, including bifidity, fusion, and bridging, each of which can be found in isolation or in variable combinations with one or more of the others (Fig. 2.26). Forked ribs are more common than rib fusion (Guttentag and Salwen 1999). Bifurcation consists in duplication of the anterior portion of the rib, most often the fourth. A variant of this pattern involves duplication of a cartilaginous segment. The elements of the neurovascular bundle, i.e., the interthoracic artery and nerve, either follow the main rib or branch (Osawa et al. 1998). Fusion can be partial (posterior, middle, or anterior portions of the ribs are involved) or complete, involving the entire arch. Partial fusion of the posterior segments of the first two ribs is common. Bone bridging is a focal joining of two or more adjacent ribs by bone outgrowths. It can be a congenital anomaly or occur after a trauma. The two components of the bridge may be fused, or they can articulate with each other, resulting in pseudarthrosis (Fig. 2.27).

All these anomalies are often isolated in otherwise normal individuals and devoid of any clinical significance. However, they also occur in combination with vertebral segmentation anomalies, either as an isolated association (e.g., spondylocostal dysostosis) or in the context of syndromic disorders, such as femorofacial syndrome (OMIM 134780), Alagille syndrome (OMIM 118450), multiple pterygium syndrome (OMIM 265000), and several others. Segmentation anomalies of the vertebrae and ribs

Child Bifid Rib

Fig. 2.26. Bifid ribs. Note synostosis involving two contiguous upper thoracic ribs on both sides. On the right, the first and second ribs are separate structures posteriorly, are fused with each other in their middle segments, and are bifurcated anteriorly. On the left, a vertically oriented first rib articulates with a forked second rib

Fig. 2.26. Bifid ribs. Note synostosis involving two contiguous upper thoracic ribs on both sides. On the right, the first and second ribs are separate structures posteriorly, are fused with each other in their middle segments, and are bifurcated anteriorly. On the left, a vertically oriented first rib articulates with a forked second rib

Right Side Rib Fusion

Fig. 2.27. Bone bridging in a female newborn. Observe bone bridging resulting in rib fusion (fifth and sixth ribs) and pseudarthrosis (seventh and eighth).Also note multiple rib fusion on both sides and hypoplastic/absent twelfth rib pair. There were no other skeletal or visceral defects in this child

Fig. 2.27. Bone bridging in a female newborn. Observe bone bridging resulting in rib fusion (fifth and sixth ribs) and pseudarthrosis (seventh and eighth).Also note multiple rib fusion on both sides and hypoplastic/absent twelfth rib pair. There were no other skeletal or visceral defects in this child are so tightly intertwined that they have been regarded as a 'developmental field defect' (Martinez-Frias and Urioste 1994), the 'field' being a group of embryonic structures that are controlled and organized in a 'spatially ordered, temporally synchronized, and epi-morphically hierarchical manner' and therefore respond as a single developmental unit (Opitz 1982). As such, they can be observed in different clinical and eti ological patterns. For example, segmentation anomalies of the vertebrae and ribs can occur in sporadic association with other 'errors in septation complex' (axial mesodermal dysplasia), including severe myelomeningocele and diastematomyelia (OMIM 222500) (Kozlowski 1984). The reported prevalence for vertebral and rib deformities in patients with myelo-meningocele has ranged from about 14.5% (Carstens and Wiederspohn 1989) to about 50% (Naik et al. 1978) in different series. Possible vertebral anomalies include hemivertebrae, fused vertebral bodies, fused vertebral arches, fused ribs, absence of ribs, and absence of vertebral bodies (Naik et al. 1978). Occasionally fused ribs alone, in the absence of vertebral malformations at the same level, have been implicated in the development of progressive scoliosis (Damsin et al. 1997).

Multiple anomalies of ribs (fused, bifid ribs, eleven pairs of ribs) and vertebrae (hemivertebrae, fused, hypoplastic, missing, and 'butterfly' vertebrae) occur as the only detectable defects in 'spondylocostal dysostosis' (OMIM 277300), a broad designation for a group of different but related disorders, or for a single entity with striking clinical variability (Fig. 2.28). Analyzing the radiographic and clinical findings of a total of 141 patients with multiple vertebral segmentation defects, Mortier et al. (1996) identified three subtypes: (1) Jarcho-Levin syndrome, an early lethal autosomal recessive form characterized by a symmetrical 'crab-like' appearance of the chest and, occasionally, by additional anomalies including congenital heart defects and urogenital anomalies; (2) an autosomal dominant form of spondylocostal dysostosis (polydysspondy-ly, OMIM 122600), manifesting only short stature, the shortening being mainly in the trunk (Rimoin et al. 1968); and (3) a mild autosomal recessive form of spondylothoracic dysostosis, characterized by clinical and radiographic features similar to, but usually more severe than, those of the autosomal dominant form. Autosomal recessive spondylocostal dysostosis can be caused by mutation in the human DLL3 gene, which is homologous to the mouse delta-like 3 gene and to the Notch-ligand delta in Drosophila and which is localized to 19q13 (Turnpenny et al. 1999; Bulman et al. 2000). The dominant and mild recessive forms are phenotypically similar and cannot be distinguished on the basis of the radiologic features alone (Ayme and Preus 1986). It is conceivable that the early lethal form and that with longer survival are produced by homozygosity for alleles at the same locus. Costovertebral segmentation defects can also occur sporadically, probably representing a hetero-

Hypoplastic Ribs
Fig. 2.28. Spondylocostal dysostosis, dominant type in a male newborn. Note multiple dysplastic, fused, and hypoplastic ribs. Multiple vertebral anomalies (hemivertebrae, butterfly vertebrae, sagittal clefts) are also evident

geneous group. Casamassima-Morton-Nance syndrome (OMIM 271520) is an association of spondylocostal segmentation defects, anal atresia, and urogenital anomalies. Vertebral and rib anomalies, and also the overall thoracic appearance, are similar to those of spondylothoracic dysplasia (Casamassima et al. 1981). The demonstration in a fetus with similar anomalies of a balanced translocation t(6;9) (p12;q12) inherited from the mother has raised the possibility that this region is the locus for this autosomal recessive thoracic dysplasia (Daikha-Dah-mane et al. 1998). In nevoid basal cell carcinoma syndrome (Gorlin syndrome, OMIM 109400), several rib abnormalities are evident, including bifid, fused, splayed, agenesic, and hypoplastic ribs (Gorlin 1987). Based on a large series of patients, the following frequencies for each radiographic feature have been calculated: calcification of the falx cerebri 65 %, calcification of the tentorium 20%, bridged sella 68 %, flame-shaped lucencies of phalanges, metacarpals, and carpal bones of the hands 30 %, bifid ribs 26%, hemivertebrae 15%,and vertebral fusions 10%

(Kimonis et al. 1997). The occasional finding of bifid ribs on routine chest radiograms performed in two preterm infants has enabled the identification of several members of a family as persons affected by Gorlin syndrome (Evans et al. 1991). In COVESDEM syndrome (OMIM 268310) radiographic evidence of fusion anomalies of the ribs and spine, hemiverte-brae, and mesomelic shortening of the extremities, are of diagnostic value. Bifidity of the distal phalanges of fingers and toes is a specific but inconstant radiological sign (Giedion et al. 1976).

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