OSTEOGENESIS IMPERFECTA (OI)
Synonym: Brittle bone
Definition.
OI literally means imperfectly formed bones. It is a congenital, rarely heritable disorder due to deficiencies in the synthesis and secretion of type I collagen caused by mutations in the genes responsible for the formation of type I collagen. Type I collagen is a major component of the connective tissue in the bone.
Etiology and Pathogenesis.
Disease is due to mutations in genes that encode alpha 1 and alpha 2 chains of the collagen I molecule. The inheritance may be dominant or recessive. A parent with OI has a 50% chance to have a child with OI.
Forms of OI.
There are 4 recognized types of OI, which vary in severity and characteristics.
Type I OI
· Due to autosomal dominant genes inheritance.
· It is the most common and mildest type of OI.
· The structure of collagen is normal, but there is decreased synthesis.
· Clinical signs occur in postnatal life and include:
Ø Fractures even when little force is applied, dentinogenesis imperfecta (imperfectly formed dentine of the teeth), hearing impairment and blue eye sclera.
Type II OI
· Due to autosomal recessive genes inheritance.
· There is abnormal/improperly formed collagen.
· It is the most severe form of OI; lethal to infants in utero or within days of birth
· Death of fetus in utero or shortly after birth
Type III OI
· Largely due autosomal dominant inheritance and less recessive inheritance.
· There is abnormal/improperly formed collagen
· Clinical effects include progressive deformities compared with Type I OI where in addition to features of type I OI, people with type III OI are generally shorter than average and may have spinal deformities (E.g. scoliosis) and respiratory complications.
Type IV OI
· Due autosomal dominant gene inheritance.
· It is moderately severe.
· There is abnormal/improperly formed collagen.
· Clinical signs occurs in postnatal life but are less severe as compared with type I OI.For example people with Type IV OI have normal eye sclera.
Clinical signs and radiographic signs.
Are those of bone fragility due to reduced bone collagen matrix
Clinical signs.
Although OI has prominent skeletal manifestations, other anatomic structures rich in type I collagen such as joints, eyes, skin and teeth are affected as well. Therefore the clinical signs are as mentioned in each type of OI described above.
Radiographic signs.
· Demineralization of the bones.
· Bone cortical thinning
· Multiple fractures.
· Exuberant callus formation caused by fracture healing with poor alignment and weak callus.
Note that a decreased bone density is seen as areas of radiolucency in the radiographic picture.
Diagnosis.
The diagnosis will base on clear evaluation of the patient under the following headings
· History of family member with OI disease
· Symptoms and clinical presentations.
· Histopathology : Decreased bone collagen matrix
· Bone Imaging.
-Bone scan
-Computed tomograph
More summaries about the In and Out of Osteogenesis Imperfecta