Slender Clavicles

This section has significant overlap with other sections in this book dealing with thin bones. The reader may wish to refer to the section headed "Slender, Thin, or Twisted Ribs" (this chapter), and to Chapter 5 (section headed "Slender Tubular Bones") for further information on this topic.

Thin bones are seen in premature infants, in individuals with neuromuscular disorders, including myotubular myopathy (OMIM 310400), myotonic dystrophy (Steinert syndrome, OMIM 160900) and Werdnig-Hoffman syndrome (OMIM 253300), and in those with a malformation syndrome or skeletal dysplasia characterized by gracile bones, such as Cockayne syndrome (OMIM 216400), Hallermann-Streiff syndrome (OMIM 264090), Pena-Shokeir syndrome, type I (OMIM 208150), 3 M syndrome (OMIM 273750), and progeria (OMIM 176670). The rare gracile bone dysplasia (OMIM 602361) is characterized by thin, brittle bones and death in late gestation or early neonatal life (Maroteaux et al. 1988). Affected individuals have facial anomalies, positional abnormalities of hands and feet, short limbs with very thin diaphyses, diaphyseal fractures, and thin ribs and clavicles. Mineralization of the calvarium can be either normal or grossly deficient. In the long bones, the organization of bone into the cortex and medulla is lost, suggesting a failure of bone remodeling. All reported cases have been sporadic. A history of parental consanguinity in two instances raised the possibility of autosomal recessive inheritance (Maroteaux et al. 1988). Costa et al. (1998) described three new cases, an isolated female patient and two brothers. Based on the presence of a dwarfing condi tion of unknown cause in the mother of the affected sibs, a somatic/germline mosaicism of a lethal gene has been suggested. Thomas et al. (1998) reported on male and female sibs. The male infant, stillborn at 35 weeks of gestation, had microphthalmia, anky-loglossia, and abdominal ascites. The thorax was small, with severe pulmonary hypoplasia. The 24-week female fetus had a prominent forehead, flat nasal bridge, upturned nasal tip, and distended abdomen. The limbs showed marked micromelic shortness, redundant skin folds, 5th finger hypoplasia and clinodactyly, and short halluces. Restrictive dermopathy (OMIM 275210) is a rare, lethal syndrome of autosomal recessive inheritance, in which tautness of the skin causes fetal akinesia or hypoki-nesia deformation sequence (Mau et al. 1997). The pregnancy is frequently abnormal, with polyhy-dramnios and decreased fetal activity, and premature delivery is common. Affected individuals are stillborn or die soon after birth as a result of lung hypoplasia. Manifestations include tight, tense, and translucent skin, with spontaneous erosions and fissures in flexion creases, multiple joint contractures, enlarged fontanels,typical face (small,pinched nose, small mouth with contracture of the temporo-mandibular joint, micrognathia, dysplastic ears), thin, dysplastic clavicles, slender and overtubulated long bones of the arms, ribbon-like ribs, and under-mineralized skull (Toriello et al. 1983; Verloes et al. 1992). Another possibly X-linked lethal syndrome of multiple malformations has been described in four male infants (Andre et al. 1981). Its features were an abnormal facies (hypertelorism, down-slanting palpebral fissures, low-set ears, cleft palate, and mi-crognathia), defective ossification of the cranial vault, widened sutures and fontanels, bowed long bones, platyspondyly, and long, thin, sloping clavicles. An association of mental retardation, agenesis of the corpus callosum, and unusual skeletal changes has been reported (Kozlowski and Ouvrier 1993). Bone manifestations include thin ribs; short, straight, and laterally tapering clavicles; multiple wormian bones; small ilia; triangular areas of sclerosis in the iliac bones; striations of some metaphyses; abnormal ossification of the patellas; hypoplastic distal phalanges; and retarded bone age. In Turner syndrome, lateral tapering of the clavicles and thin posterior segments of the ribs are sometimes observed.

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