HEREDITARY BONE DISORDERS
Achondroplasia is the most common form of inherited dwarfism. It is caused by an autosomal dominant mutation in fibroblast growth factor receptor 3 (FGFR3). Activation of FGFR3 inhibits cartilage synthesis at the epiphyseal growth plate, resulting in decreased enchondral bone formation and premature ossification of the growth plates.
• Long bones are short and thick, leading to dwarfism with short extremities.
• Cranial and vertebral bones are spared, leading to relatively large head and trunk.
• Intelligence, life span, and reproductive ability are normal.
Osteogenesis imperfecta (OI) (“brittle bone disease”) is a hereditary defect leading to abnormal synthesis of type I collagen.
• Patients have generalized osteopenia (brittle bones), resulting in recurrent fractures and skeletal deformity
• Abnormally thin sclera with blue hue is common
• Laxity of joint ligaments leads to hypermobility
• Involvement of inner and middle ear bones produces deafness
• Occasional dentinogenesis imperfecta, characterized by small, fragile, and discolored teeth due to a deficiency of dentin
• Dermis may be abnormally thin, and skin is susceptible to easy bruising
• Treatment is supportive
Osteopetrosis (or marble bone disease) is a hereditary defect leading to decreased osteoclast function, with resulting decreased resorption and thick sclerotic bones. X-ray shows symmetrical generalized osteosclerosis. Long bones may have broad-ened metaphyses, causing an “Erlenmeyer flask”-shaped deformity. Treatment is hematopoietic stem cell transplantation.
Autosomal recessive type (malignant):
Affects infants and children (causes multiple fractures and early death due to anemia, infection, and hemorrhage)
Autosomal dominant type (benign):
Affects adults (causes fractures, mild ane
Pathology shows increased bone density and thickening of bone cortex. Myelo-phthisic anemia may result from marrow crowding. Cranial nerve compression due to narrowing of cranial foramina may result in blindness, deafness, and facial nerve palsies. Hydrocephalus may develop due to obstruction of CSF.