osteogenesis imperfecta type iii
osteogenesis imperfecta (oi) is a clinically and genetically heterogeneous group of inherited disorders of connective tissue, characterized by bone fragility. associated features in some individuals include blue sclerae, dentinogenesis imperfecta, hearing loss, deformity of the spine and long bones and joint hyperextensibility. the disorder is currently categorized into 4 major types according to the phenotypic classification of sillence. there are several sub-types, as well as marked heterogeneity at the clinical, radiological and molecular level, within these groups. there is marked inter- and intra-familial variability, making prenatal diagnosis difficult even in families with an established diagnosis. osteogenesis imperfecta type lll is characterized by progressive deformity of the long bones and spine and is not usually lethal in the perinatal period. about 75% of cases are believed to be the result of new autosomal dominant mutations, while the remaining 25% are autosomal recessive (particularly in africa). empiric recurrence risks are calculated at around 7%.
infants are usually born at or near term with normal birth weight. birth length is usually normal unless there is marked deformity, but with age height falls to well below the 3rd centile. at least half the infants are born with fractures and all have had numerous fractures by the age of 2 years. multiple rib fractures do occur but beading is not seen as in oi type lla. long bone changes are variable and, although fractures and bowing occur, the long bones are generally less deformed than in oi type ll. head size is disproportionately large and there is frontal and temporal bossing which contributes to the triangular appearance of the face. both fontanelles and sutures are wide. wormian bones may be palpable.
other forms of osteogenesis imperfecta, particularly type ll campomelic dysplasia presents with bowing of the tibiae and femora. associated craniofacial anomalies including macrocephaly, cleft palate and micrognathia as well as hypoplastic scapulae are present in over 90% of cases and should help to distinguish the two conditions.
long bone-fractures – bowing – angulation – shortening
skull -decreased ossification
these changes may not be visible until at least 18 weeks gestation
unfortunately evaluation of bone mineralisation by ultrasound is limited
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