► [Absence of a rib pair (most often the twelfth)]
Unilateral aplasia and uni- or bilateral hypoplasia of the 12th pair of ribs are common developmental errors that can occur in otherwise normal subjects. Because these minor anomalies can also occur in random association with virtually every disease their diagnostic value is limited. However, there is a small group of congenital disorders in which this defect consistently occurs.
Eleven pairs of ribs are typically encountered in the chromosome 18 trisomy syndrome (Edwards syndrome) (Ho 1989; Poon et al. 1975), a disorder characterized by developmental and mental retardation, dolichocephaly, short palpebral fissures, overlapping
fingers, and a variety of skeletal defects. These include thin ribs, short sternum, reduced number of sternal ossification centers, and incomplete ossification of clavicles (Fig. 2.15). Eleven pairs of ribs are usually present in campomelic dysplasia (OMIM 114290, 211990). Together with congenital bowing and angulation of the long bones - the radiographic and clinical stigmata of this disorder - slender ribs and clavicles, very small scapulae, and abnormalities of the skull, facial bones, pelvis and spine are features. In addition to some consistent structural rib anomalies, including hypoplasia and fusion, the autosomal dominant spondylocostal dysostosis (OMIM 122600) also manifests eleven pair of ribs (Fig. 2.16). The vertebral anomalies include multiple level fusions, hemivertebrae, butterfly vertebrae, and sagittal clefts. The skull and limbs are not affected. Urinary system anomalies and congenital heart defects are usually present (Rimoin et al. 1968). Eleven pairs of mildly shortened ribs have been reported in the presence of spinal changes associated with limb shortening, severe acrodysplasia, multiple joint dislocations, and severe combined immune deficiency (SCID) (Castriota-Scanderbeg et al. 1997).
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