Skeletal dysplasias — branching viva

Opening branch — the disproportionate child

A two-year-old has rhizomelic shortening, a large head with frontal bossing and midface hypoplasia, trident hands, and genu varum, with normal development. The candidate must give the diagnosis of achondroplasia, name the FGFR3 gain-of-function mechanism that brakes chondrocyte proliferation at the growth plate, and explain why around eighty per cent of cases arise as new variants in average-statured parents. The investigation is a skeletal survey and molecular confirmation of the recurrent FGFR3 variant, and the management is syndrome-specific surveillance plus the consideration of vosoritide. [1] [2]

The examiner probes the surveillance. The candidate names the foramen magnum stenosis and sleep-disordered breathing that threaten the achondroplastic infant, the recurrent otitis media and conductive hearing loss, the risk of hydrocephalus, and the need for syndrome-specific growth charts. The teaching point is that the complications of achondroplasia are life-threatening even when the child looks well, so structured surveillance rather than reassurance is the standard of care. [1] [2]

Second branch — the fracturing child

The case shifts to a four-year-old with blue sclerae, bowing of the lower limbs, and a history of three fractures after minor falls. The candidate diagnoses osteogenesis imperfecta, names the type I collagen (COL1A1 or COL1A2) defect, and explains the Sillence classification from mild type I to lethal type II. The examiner tests the active distinction from non-accidental injury: the candidate names the discriminating features — the connective tissue phenotype of blue sclerae and joint laxity, the generalised osteopenia and wormian skull bones, and the family history — and the role of molecular testing of COL1A1 and COL1A2 in resolving genuine uncertainty. [3]

The management discussion covers bisphosphonate therapy to reduce bone resorption and fracture rate, intramedullary rodding of deformed or repeatedly fracturing bones, physiotherapy and mobility support, and lifelong hearing and dental surveillance. The candidate explains that these therapies have transformed the outlook for the moderate and severe forms, and that the diagnosis and its distinction from abuse must be secured before any irreversible child-protection or therapeutic decision. [3]

Third branch — the lethal short-limbed newborn

The case moves to the delivery room, where a term newborn has respiratory distress, a narrow chest, very short limbs, and a large head. The candidate gives the lethal skeletal dysplasia differential — thanatophoric dysplasia, achondrogenesis, osteogenesis imperfecta type II, and the short-rib polydactyly syndromes — and identifies the small thorax with short horizontal ribs as the predictor of lethality. The management is a shared decision to palliate rather than escalate once the lethal dysplasia is confirmed by the bedside radiograph, followed by molecular confirmation for recurrence counselling. [2] [4]

Closing synthesis

The examiner asks for the single unifying principle. The candidate states that every skeletal dysplasia reduces to four questions — is it lethal, which limb segment is shortened, which molecular pathway is disrupted, and what surveillance and therapy does that pathway demand — and that the FGFR3 gain-of-function mechanism unifies the commonest non-lethal dysplasia (achondroplasia) with the commonest lethal dysplasia (thanatophoric), with vosoritide now overriding that brake to restore linear growth. [1] [4]