A 35-year-old man with a history of recurrent bone fractures, blue sclerae, and progressive hearing loss is diagnosed with osteogenesis imperfecta. Which is the most common type of osteogenesis imperfecta by prevalence?

Explanation

## Osteogenesis Imperfecta (OI) Classification and Epidemiology **Key Point:** Type I OI is the most common form, accounting for approximately 80–85% of all OI cases. It is caused by mutations in COL1A1 or COL1A2 genes affecting Type I collagen production. ### Sillence Classification of Osteogenesis Imperfecta | Type | Severity | Prevalence | Blue Sclerae | Hearing Loss | Dental Issues | Lifespan | |---|---|---|---|---|---|---| | **Type I** | **Mild** | **80–85%** | **Yes (pathognomonic)** | **Common** | **Yes (dentinogenesis imperfecta)** | **Normal** | | **Type II** | **Perinatal lethal** | 5–10% | Variable | N/A | N/A | **Stillbirth or infancy** | | **Type III** | **Progressive deforming** | 5–10% | Absent/pale | Variable | Variable | **Reduced** | | **Type IV** | **Variable (no blue sclerae)** | 5–10% | **Absent** | Variable | Variable | **Normal to reduced** | ### Clinical Features of Type I OI (Most Common) 1. **Skeletal manifestations:** - Recurrent fractures (often from minor trauma) - Bone deformities develop with age - Osteoporosis on imaging 2. **Ocular findings:** - Blue or blue-gray sclerae (due to thinness allowing choroid pigment to show through) - Pathognomonic for Type I OI 3. **Otologic findings:** - Progressive conductive or sensorineural hearing loss (30–40% of patients) - Caused by otosclerosis and ossicular abnormalities 4. **Dental abnormalities:** - Dentinogenesis imperfecta (opalescent, translucent teeth) - Early tooth wear and loss 5. **Other features:** - Joint hypermobility - Skin hyperextensibility (mild) - Normal intelligence **High-Yield:** Type I OI = autosomal dominant, mild phenotype, blue sclerae, normal lifespan, most common form. Remember: "Type I = Intact collagen quantity, Impaired quality." **Mnemonic:** **BODD** for Type I OI features — **B**lue sclerae, **O**tosclerosis (hearing loss), **D**entinogenesis imperfecta, **D**ecreased bone density. **Clinical Pearl:** The presence of blue sclerae in a child with recurrent fractures is virtually diagnostic of Type I OI. However, sclerae may become less blue with age, so this sign is most useful in younger patients. **Warning:** Do not confuse Type I OI (mild, blue sclerae, normal lifespan) with Type III OI (progressive deforming, no blue sclerae, shortened lifespan). Type III is more severe despite lacking the characteristic blue sclerae.