OI is an autosomal genetic disorder characterized by brittleness of bones and susceptibility to bone fractures from very mild trauma. Fractures may be rare or frequent (more than 200 prior to puberty), and bone fragility may be accompanied by reduced life span and various connective tissue abnormalities: short stature, blue sclerae, joint laxity, dentinogenesis imperfecta, easy bruising, and hearing loss. OI represents extreme variation in severity from lethal perinatal to barely detectable. These clinical features, along with radiological and genetic criteria, were used to separate four major clinical types of OI. The patients affected with the mildest (type I) OI have blue sclerae, osteopenia, slight growth retardation or normal height, and premature deafness; dentinogenesis imperfecta is less common. Fractures are not commonly observed at birth, but begin with am-bulation and decrease following puberty. Type II is the lethal perinatal form; infants exhibit multiple intra-uterine fractures of the ribs and long bones. They have a relatively large and soft cranium, short limbs, and a narrow thoracic cavity; death usually results from pulmonary insufficiency. Type III is the progressively deforming form with multiple fractures, short stature, and severe dentinogenesis imperfecta. Children with type III may die of respiratory problems in infancy; surviving patients require multiple orthopedic rodding procedures and wheelchairs for mobility. Type IV OI is characterized by phenotypical variability, from severe to mild bone deformity, normal sclerae, variable short stature, and possible brittle teeth. The Sillence classification of OI is being expanded to include a greater range of subgroups of patients. Patients with type V, VI, and VII OI exhibit moderate to severe bone fragility. Type V is characterized by three distinctive features: the presence of hypertrophic callus formation at fracture sites, calcification of inter-osseous membranes between the bones of the forearm, and the presence of a radio-opaque metaphyseal band immediately adjacent to the growth plates upon X-ray. The distinctive features of type VI OI are the fishscale-like appearance of the bone lamellae and the presence of excessive osteoids. The characteristic feature of type VII is a rhizomelic shortening of the humerus and femur.
In its various forms, OI probably affects about 1 in 5000 to 1 in 10,000 individuals of all ethnic origins.
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