12 which describes four different types of OI based on clinical symptoms. In the last two decades, an attempt has been made to base the classification of OI more on its genetic causes rather than on clinical presentation 10–12. However, the genotype-phenotype relation is still insufficiently understood (Table 1). Therefore, this expanded classification with about 20 different types has not proven to be clinically useful, and clinicians and researchers have returned to the phenotyping according to Sillence et al., with the addition of one distinguishable clinical type 8. The phenotype of OI is thus clinically classified into five subtypes (Table 1) in which the severity ranges from barely detectable connective tissue abnormalities to lethality in the perinatal period 8. Type 1 is the relatively mildest and most common form of OI, with a birth prevalence of about 4 per 100,000 live births 8. OI type 1 is characterized by blue sclera, increased fracture frequency without extensive deformities, normal stature, sometimes hearing loss and sometimes dentinogenesis imperfecta. Type 2 is the most severe type and is characterized by such a lack of collagen in the bones that children die in utero during pregnancy, at birth or shortly after. This type is characterized by extensive fractures and bone deformity, micromelic bones and platyspondyly. Lethality occurs due to respiratory failure secondary to pulmonary hypoplasia as a consequence of a small thoracic cage caused by multiple rib fractures, as well as cerebral haemorrhage after vaginal delivery. In persons with type 3, there is small stature, frequent fractures with deformity of the limbs and vertebrae that increases over the years, sometimes causing respiratory insufficiency. Blue sclerae might be present or not, dentinogenesis imperfecta is usually evident. Type 4 is very similar to type 1, a difference being that there are no blue sclerae and some persons have a shorter stature. Type 5 is comparable to type 4, but persons with type 5 have a calcification of the interosseous membrane of the forearm, which restricts hand movements and can lead to secondary dislocation of the radial head. Very typically there is a greater chance of hypercalcification occurring after surgery or a fracture. Life expectancy for persons with all OI types other than type 2 is normal. In individuals with OI type 3, life expectancy may be shorter if there is severe kyphoscoliosis with restrictive pulmonary function 1,13–15. Based on the above-mentioned prevalence data, there are approximately more than 1,100 patients with OI in the Netherlands. Storoni et al. 2 estimated the total number of patients with OI to be 850. The Expertise Center for adults with OI in Isala in Zwolle, founded in 2008, now has a cohort with over 500 adult OI patients in care. Of these 500 patients, 67% have type 1 OI, 12% type 3 and 19% type 4. Of the cohort, 2% have type 5 or other rare (recessive inherited) variants 16.
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