Achondroplasia & Skeletal Dysplasia - High-Yield Viva Questions
| Question | Answer / Practical Pearls |
|---|---|
| How do you classify disproportionate short stature, and how is the Upper Segment to Lower Segment (US:LS) ratio interpreted? | • Classification: - Rhizomelic Micromelia: Predominant shortening of proximal segments (humerus/femur) $\to$ Achondroplasia, Hypochondroplasia. - Mesomelic Micromelia: Predominant shortening of middle segments (radius/ulna, tibia/fibula) $\to$ Leri-Weill dyschondrosteosis. - Acromelic Micromelia: Shortening of distal segments (hands/feet). • US:LS Ratio Calculation: Lower Segment (LS) measured from top of pubic symphysis to floor; Upper Segment (US) = Height minus LS. • Interpretation: Normal ratio is $1.7:1$ in neonates, $1.4:1$ at 1 year, $1.2:1$ at 4 years, and $1.0:1$ at 8-10 years. In Achondroplasia, because limbs are short while trunk is relatively normal, the US:LS ratio remains markedly elevated ($1.5-1.7:1$) for age. |
| What are the pathognomonic radiological findings on a skeletal survey in Achondroplasia? | 1. Lumbosacral Spine (AP): Progressive Narrowing of Interpedicular Distance from L1 down to L5 (in normal spines, the canal widens from L1 to L5); short pedicles, posterior vertebral scalloping. 2. Pelvis (AP): 'Champagne Glass' Pelvic Inlet; broad square-shaped ('tombstone') iliac wings with flat horizontal acetabula and narrow sacrosciatic notches. 3. Tubular Long Bones: Short, thick femurs and humeri with flared metaphyses and the 'Chevron Sign' (an inverted-V shaped metaphyseal cup cradling the epiphysis). 4. Skull (Lateral): Small, contracted foramen magnum with shortening of the cranial base, contrasting with a large calvarium. |
| What is the molecular genetic defect in Achondroplasia, and what is the role of advanced paternal age? | • Gene & Inheritance: Autosomal Dominant with $100\%$ penetrance; caused by mutations in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene on chromosome 4p16.3. • The Specific Mutation: $>98\%$ of cases harbor the identical point mutation c.1138G>A (p.Gly380Arg). • Functional Mechanism: Constitutively active Gain-of-Function mutation. Hyperactive FGFR3 continuously inhibits chondrocyte proliferation and terminal differentiation at the growth plate, arresting endochondral ossification. • De Novo Mutations & Paternal Age: Over $80\%$ of cases represent spontaneous de novo mutations, strongly correlated with advanced paternal age ($>35-40\text{ years}$) due to a selective advantage of mutant spermatogonial stem cells during clonal expansion ('selfish spermatogonial selection'). |
| What is the single most lethal complication in infants with Achondroplasia, and how is it monitored? | • Craniocervical Junction Compression (Foramen Magnum Stenosis): - Carries a $2\%\text{ to }5\%$ risk of Sudden Infant Death Syndrome (SIDS) in the first year of life secondary to cervicomedullary compression. - Clinical Red Flags: Central sleep apnea, progressive quadriparesis, asymmetric motor weakness, hyperreflexia, persistent ankle clonus, and developmental regression of head control. - Surveillance: Mandatory non-contrast brain and cervical spine MRI + polysomnography in all infants under 1 year. - Emergency Treatment: Surgical suboccipital craniectomy and C1 laminectomy decompression. |
| Explain the mechanism of action and clinical utility of Vosoritide in Achondroplasia. | • Drug Class: Recombinant C-type Natriuretic Peptide (CNP) analog. • Mechanism of Action: Vosoritide binds to Natriuretic Peptide Receptor-B (NPR-B) on growth plate chondrocytes. This stimulates intracellular cyclic GMP (cGMP), which directly inhibits the hyperactive RAF-MEK-ERK downstream kinase signaling cascade driven by mutated FGFR3. By inhibiting the inhibitor, it restores physiological chondrocyte proliferation and endochondral bone elongation. • Clinical Administration: Administered as a daily subcutaneous injection ($15\text{ mcg/kg}$) in children with open epiphyses (age $\ge 2\text{ years}$). Increases annualized growth velocity by $1.5-2.0\text{ cm/year}$ and improves body proportionality. |