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At a glance

A form of lethal neonatal dwarfism with boomerang shaped long bones of the extremities resulting in severe skeletal dysplasia.


Unknown. Less than 15 reported cases worldwide have been reported since Kozlowski’s first description of the disorder in 1981.

Genetic inheritance

The precise genetics are unknown, but due to significant male preponderance, X-linked recessive inheritance has been suggested. However, all cases seem to be sporadic. Parental consanguinity has not been reported in any of the cases.


The mutation seems to affect the FLNB (Filamin Beta) gene located on 3p14.3. FLNB acts primarily on stabilization of actin within the cell and is involved in enchondral ossification, vertebral segmentation, and the formation of joints. Filamins are large actin-binding proteins that stabilize the actin cytoskeleton, either as parallel bundles or orthogonal gel networks, and link it with cellular membranes and facilitate interactions between actin, transmembranous receptors and extracellular signals in directing cell proliferation, differentiation, and their migration into their appropriate anatomical locations. The defect is caused by an amino acid substitution in the actin-binding domain of the FLNB molecule.


Based on the characteristic clinical features and radiologic abnormalities consistent with the phenotypical description of the syndrome. Demonstration of histologic changes in the bone and cartilage may be an adjunct to clinical diagnosis, but may be similar to those seen in atelosteogenesis (see “Other conditions to be considered”).

Clinical aspects

Congenital dwarfism with severe micromelia and deformity with absent or deficient ossification of all four limbs is the most striking feature of this condition. The name of this disorder was coined by Kozlowski and refers to the boomerang-like, flat, curved shape of one or more of the long bones. The large joints are not well discernible, the hands and feet are short and broad secondary to brachydactyly due to severe hypoplasia of the metacarpals and metatarsals and phalanges, which is often combined with poly-, oligo- and/or syndactyly. Ossification centers are absent or rudimentary in most vertebral bodies and digits. Ossification itself is delayed or lacking. Talipes equinovarus are common. Defects of the frontal bones in combination with encephalocele have been reported. The facies appears flat with hypertelorism and a nose with a flat and broad root, hypoplastic nasal septum, and slanted nares. Malar hypoplasia, severe micrognathia and short neck are frequent, while cleft palate is rare. Laryngeal narrowing and tracheomalacia have been described. The chest is short and bell-shaped with hypoplastic lungs and the abdomen is protruding significantly and/or may have an omphalocele. Radiologically, the long bones may be absent (either due to agenesis of absent calcification), hypoplastic or characteristically curved and flat, giving the condition its name. Hypoplastic ilia and absent pubic bones are other frequent findings. Boomerang ...

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