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At a glance

A multiple malformation syndrome with posterior rib-gaps and Pierre Robin sequence (micrognathia, glossoptosis and cleft palate). It is often lethal in infancy.

Synonyms

Smith-Theiler-Schachenmann Syndrome; Rib-Gap Syndrome; Rib-Gap Defects with Micrognathia.

History

First described in 1966 by the American physician David W. Smith, and the two Swiss physicians Karl Theiler and Gertrud Schachenmann. They reported on a newborn boy with this disease who died within 8 hours from respiratory failure.

Incidence

More than 80 cases have been described to date. Males and females are equally affected.

Genetic inheritance

Familial cases with autosomal dominant (and less frequently autosomal recessive) transmission have been reported, but most cases are sporadic. It is caused by mutations in the SNRPB (Small Nuclear Ribonucleoprotein Polypeptides B and B1) gene, which has been mapped to chromosome 20p13 and encodes components of the major spliceosome.

Diagnosis

Based on the clinical and radiological findings of facial malformations associated with severe costovertebral/costal anomalies (posterior rib-gap defects are required for the diagnosis). Molecular genetic testing will be useful since nonpenetrance occurs in only about 10% of affected families and has significant implications for the recurrence risk.

Clinical aspects

The disease mainly involves the head and neck with brachycephaly (or occasionally microcephaly), microretrognathia, partial facial hypoplasia, and multifocal growth retardation involving vomer, nasoseptal cartilage, and mandibular condyles with limited mobility in the temporomandibular joint, which is indicative of maxillomandibular growth arrest. The hard palate is short with a central cleft (present in the majority of patients), absent soft palate and uvula, glossoptosis, webbed neck (pterygium colli), and posterior nuchal redundant skin folds (which may be associated with a more severe phenotype). Skeletal anomalies may include intrauterine and postnatal growth retardation, (kypho-) scoliosis (in approximately 50% of patients), spina bifida, pectus carinatum, narrow, bell-shaped rib cage, severe costovertebral abnormalities (eg, abnormal costotransverse articulation), and single or multiple posterior rib-gap defects without continuity between the cartilaginous anterolateral portion of the ribs and the posterior, proximal, ossified portions of the ribs (complete absence of ribs has rarely been described). These defects are filled with fibrovascular tissue and often result in flail chest. Tracheomalacia and pulmonary and/or alveolar hypoplasia with restrictive lung disease lead to neonatal respiratory distress, and in more than half of the patients, death occurs in the first year of life and usually is the result of respiratory complications. Cardiac disease (atrial and ventricular septal defects, one case with hypoplastic left heart syndrome) can be associated with this disorder. Additional features have been reported in some patients, including choanal atresia, omphalocele, horseshoe kidney, hypospadias, cystic hygroma, and gastroesophageal reflux disease (GERD). Central nervous system anomalies may lead to mental retardation (in about half the patients; described as mild to moderate) and ...

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