Slender, thin ribs are observed in conditions characterized by generalized bone gracility and slender-ness. Therefore, this section overlaps with others in this book. As anticipated, lack of physiological stress stimuli to the thoracic cage, as a result of either underdeveloped or paralyzed intercostal muscles, interferes with the normal development of the ribs, thus generating a slim rib appearance. Examples of underdeveloped or paralyzed intercostal muscles are provided, respectively, by prematurity (Fig. 2.20) and neuromuscular disorders, including myotonic dystrophy, myotubular myopathy, and spinal muscular atrophy (Osborne et al. 1983). Congenital myotonic dystrophy (Steinert syndrome, OMIM 160900), an autosomal dominant disorder transmitted maternally, presents with severe hypotonia, generalized muscle atrophy, respiratory distress, pleural effusions, multiple hematomas, foot and joint deformities, elevated diaphragm, and thin ribs, while the remainder of the skeleton is not significantly affected (Fried et al. 1975). The classic late-onset disease, which may be either maternally or paternally inherited, is characterized by myotonia, cataract, cardiovascular defects, thick calvaria, generalized muscle atrophy, and gastrointestinal/urinary tract dysfunctions. Thin ribs and elevation of the diaphragm are also seen in neo-nates with myotubular myopathy (OMIM 310400), an X-linked disease mapped to Xq28. Severe hypotonia and generalized muscle weakness, often resulting in early death from respiratory failure, are features in this disorder (Guiraud-Chaumeil et al. 2000). Spinal muscular atrophy type 1 (OMIM 253550), or Werd-nig-Hoffmann disease, an autosomal recessive disorder, consists in progressive degeneration of the anterior (motor) horns of the spinal cord, resulting in
generalized hypotonia and weakness, expressionless face, diminished reflexes, and recurrent pneumonia and/or respiratory failure from diaphragmatic paralysis. Radiographic findings include generalized muscle atrophy, with diffuse replacement of the muscle fibers by intramuscular fat, overtubulation of the long bones,osteoporosis, flexion deformities,hip dislocation, and multiple birth fractures (Burke et al. 1986; Rodiek and Kuther 1985). The thorax is bell-shaped, with thin and bowed ribs (Caro and Borden 1988). The ribs are typically thin, short, and horizontally oriented with splayed ends in achondrogenesis type IA (Houston-Harris type, OMIM 200600) and type IB (Fraccaro type, OMIM 600972),in which multiple rib fractures can be seen. Thinning and shortening of the ribs, as well as rib end splaying, also occur in achondrogenesis type II (OMIM 200610), albeit to a lesser extent. In hypophosphatasia, congenital lethal form (OMIM 241500) the ribs are short and thin,with unossified ends. Thin, gracile bones occur in osteogenesis imperfecta type I (OMIM 166200) and type IV (OMIM 166220). Fractures may be seen in the ribs, often followed by excessive callus formation, which can obscure the delicate, abnormally thin appearance of the rib cortices. Type IIB/III (OMIM 259420) of osteogenesis imperfecta is associated with short and thick limbs. A major radiographic finding in Melnick-Needles syndrome (osteodysplasty, OMIM 309350) is the irregularly thickened, distorted contour of the long tubular bones, clavicles, scapulae, and ribs. The cortical irregularities and multiple constrictions give rise to a ribbon-like appearance of the ribs, which is more prominent in adults. In infancy, there may be slenderness and thinning of the ribs. A narrow chest with gracile ribs is observed in Antley-Bixler syndrome (OMIM 207410), a rare disorder with brachy-cephaly (100%), frontal bossing (100%), craniosyn-ostosis (70%),midface hypoplasia (100%), depressed nasal bridge (100%),proptosis (100%),choanal steno-sis/atresia (80%), dysplastic ears (100%), humeroradial synostosis with fixed flexion of both elbows (100%), bowed femurs (100%), joint contractures (100%), clubfoot, carpal and tarsal synostosis, and long bone fractures (50%) (Antley and Bixler 1975; Jones 1997; Crisponi et al. 1997). Aminopterin/metho-trexate embryopathy is caused by maternal exposure during the first trimester of pregnancy to amino-pterin, a folic acid antagonist used as an abortifacient (Thiersch 1952), or to methotrexate, the methyl derivative of aminopterin used as a chemotherapeutic agent in neoplastic and immune system disorders. Major anomalies in affected individuals are those related to the craniofacial dysplasia (microcephaly, broad nasal bridge, micrognathia, maxillary hypo-plasia, prominent eyes, and low-set ears). Additional findings include growth retardation, mild mesomelic limb shortening, clubfoot, syndactyly, and normal mental development. Occasional features include cleft palate, neural tube closure defects, delayed ossification of the pubic and ischial bones, short thumbs, flaring of the iliac wings with flat acetabuli, dextro-position of the heart, and slender ribs (Bawle et al. 1998; Buckley et al. 1997). Thinning of the ribs, marked hypoplasia of the maxilla, pointed small spinous processes of the cervical vertebrae, brachy-cephaly, thoracic scoliosis, cortical thickening of the tubular bones, and calvarial thinning, are consistent features in Anderson syndrome (OMIM 251450), a condition of probable autosomal recessive inheritance (Anderson 1982). Thin, gracile ribs and tubular bones are characteristic radiographic manifestations in Hallermann-Streiff syndrome (OMIM 234100), a disorder with bilateral microphthalmia, small 'pinched' nose, and hypotrichosis (Christian et al. 1991), and in Pena-Shokeir syndrome type I (OMIM 208150), an early lethal autosomal recessive condition with multiple joint contractures, facial anomalies, and pulmonary hypoplasia (Pena and Shokeir 1974). Similarly, slim tubular bones and ribs, resulting in small thoracic cage, and thin calvarium with fontanels that remain open well beyond the normal closure time, are encountered in progeria (OMIM 176670). Typical of the condition is the progressive thinning and loss of bone in the distal portions of the clavicles, ribs, and distal phalanges (Jansen and Romiti 2000). In oto-palato-digital syndrome, type II (OMIM 304120) a narrow thorax with thin, wavy ribs and clavicles is
seen. Slender and thin tubular bones and ribs are also found in 3-M syndrome (OMIM 273750), a rare disorder of autosomal recessive inheritance, characterized by proportionate dwarfism and a specific facies (frontal bossing, flattened malar region, short nose with upturned nares, long philtrum, prominent mouth with thick lips). The designation 3-M refers to the fact that the last names of the first three authors of the original article begin with 'M' (Miller et al. 1975). Several chromosomal syndromes manifest thin, slender ribs (Fig. 2.21).
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