Bone Dysplasias

Table of Contents

1. Definitions
2. Classification
   • Proportionate Dwarfism
   • Disproportionate Dwarfism
3. Conditions
   • Achondroplasia
   • Pseudoachondroplasia
   • Kniest’s Syndrome
   • Trevor’s Disease
   • Diastrophic Dysplasia
   • Metaphyseal Chondrodysplasias
   • Spondyloepiphyseal Dysplasia (SED)
   • Multiple Epiphyseal Dysplasia (MED)
   • Mucopolysaccharidoses
   • Cleidocranial Dysplasia
4. Key Testing Facts

Definitions

• Dysplasia: Failure to develop, resulting in shortening.
• Dwarfism: A common term for all bone dysplasias.
  • Proportionate Dwarfism: Symmetric involvement of trunk and limb length.
  • Disproportionate Dwarfism: Involves either a short trunk or short limbs.

Classification

Proportionate Dwarfism

• Mucopolysaccharidoses
• Cleidocranial Dysplasia

Disproportionate Dwarfism

Short Limb

1. Achondroplasia/Pseudoachondroplasia
2. Diastrophic Dysplasia
3. Trevor’s Disease
4. Multiple Epiphyseal Dysplasia

Short Trunk

1. Kniest’s Syndrome
2. Spondyloepiphyseal Dysplasia

Conditions

Achondroplasia

Epidemiology

• Most common skeletal dysplasia.

Aetiology

• Mutation in FGFR3 gene.
• Autosomal Dominant (80% are new mutations).
• Affects the proliferative zone of the physis.

Clinical Features (GRUNTS)

• G: Genu Varum, Coxa Valga.
• R: Radial Head Subluxation.
• U: Unusual Faces (frontal bossing, button nose).
• N: Normal Intelligence.
• T: Trident Hand.
• S: Spinal issues (hyperlordosis, kyphosis, stenosis).

Imaging Features (SCIMP)

• S: Spinal (scalloping, short pedicles, thoracolumbar collapse).
• C: Champagne glass pelvis.
• I: Inverted V-shaped distal femur.
• M: Metaphyseal cupping.
• P: Physeal appearance is delayed.

Management

• Symptomatic treatment (e.g., decompression for spinal stenosis).

Pseudoachondroplasia

Differences from Achondroplasia

• Mutation in COMP gene (Chromosome 19).
• Normal facial features.
• Increased cervical instability and early-onset OA.

Kniest’s Syndrome

• Aetiology: Mutation in COL2A1 gene.
• Disproportionate short trunk.
• Characterized by short pelvis, dumbbell-shaped bones, and retinal detachment.

Trevor’s Disease

• Isolated intra-epiphyseal osteochondroma.
• Affects one joint (usually the knee).
• Treatment: Excision (high recurrence).

Diastrophic Dysplasia

Aetiology

• Mutation in SLC26A2 gene.
• Autosomal Recessive.

Clinical Features

• C’s: Cleft Palate, Cauliflower Ears, Club Feet.
• S’s: Skew feet, Spinal stenosis, Kyphoscoliosis.

Metaphyseal Chondrodysplasias

Subtypes

Spondyloepiphyseal Dysplasia (SED)

• Aetiology: Mutation in COL2A1 gene.
• Types: Congenital (severe) and Tarda (milder, X-linked).
• Spinal involvement distinguishes from MED.

Multiple Epiphyseal Dysplasia (MED)

• Aetiology: Mutation in COL9A1 gene.
• No spinal involvement.
• Early-onset OA and epiphyseal abnormalities.

Mucopolysaccharidoses

• Aetiology: Storage of complex sugars in multiple organs.
• Differentiated by complex sugars in urine.
• Most common type: Morquio’s Syndrome.

Cleidocranial Dysplasia

• Aetiology: Mutation in CBFA1 gene.
• Proportional dwarfism.
• Key Feature: Clavicle aplasia (unilateral or bilateral).

Key Testing Facts

• Achondroplasia: Most common skeletal dysplasia; FGFR3 mutation.
• Diastrophic Dysplasia: Cauliflower ears, hitchhiker thumbs.
• Cleidocranial Dysplasia: CBFA1 defect; hallmark is clavicle aplasia.
• Mucopolysaccharidoses: Differentiated by urine sugars (Hunter’s is X-linked).