Achondroplasia

Achondroplasia is the most common skeletal dysplasia.

• Genetics: Autosomal dominant. FGFR3 (Fibroblast Growth Factor Receptor 3) mutation.
• Incidence: 1 in 20,000.
• New Mutations: 80% are new mutations (unaffected parents).
• Pathophysiology: FGFR3 is a negative regulator of bone growth. Gain-of-function mutation → reduced endochondral ossification.

Endochondral Ossification Defect

• FGFR3 normally inhibits bone growth at the growth plate.
• Gain-of-function → excessive inhibition → short bones.
• Affects endochondral bones (limbs, skull base) more than intramembranous.

Why Spinal Stenosis Occurs

• Short pedicles → narrow spinal canal.
• Progressive symptoms in adulthood.

The kyphosis is named throughout (must-know, mnemonic, complications "90% of infants, avoid early sitting") but never developed — yet its management ladder and the danger of the fixed deformity are classic exam material. Almost all achondroplastic infants develop a flexible thoracolumbar (gibbus) kyphosis at the thoracolumbar junction, driven by trunkal hypotonia, a relatively large head, and unsupported sitting before trunk control matures.

The topic repeatedly invokes foramen magnum stenosis and the Pauli predictors, but the actual decision framework deserves to be set out, because this is the one problem that kills infants and the "when to operate" judgement is examined directly. Deficient endochondral growth of the skull base narrows the foramen magnum, compressing the cervicomedullary junction — producing central apnoea, hypotonia, feeding difficulty, and a small excess of sudden infant death. No single number decides management; the assessment is a composite of three strands:

Putting it together: decompression is driven by the composite picture — symptomatic compression (central apnoea, myelopathic signs) and/or MRI cord effacement with signal change — not by foramen-magnum diameter alone. A radiographically small foramen magnum in a neurologically normal infant with a normal sleep study is monitored, not automatically operated. When indicated, the procedure is a suboccipital decompression (with C1 laminectomy as needed) by the neurosurgical team. Note this is a stenosis/compression problem and is distinct in mechanism from craniocervical-junction instability (developed in that dedicated topic).

Skeletal Dysplasias:

Key Distinguishing Points:

• Achondroplasia: Characteristic face (frontal bossing, midface hypoplasia)
• Pseudoachondroplasia: Normal face (distinguishes from achondroplasia)
• Thanatophoric: Lethal, telephone receiver femur
• Diastrophic: Hitchhiker thumb is pathognomonic

Neurological Complications

Orthopaedic Complications

Anaesthetic and Perioperative Risks

Critical considerations for surgery in achondroplasia:

• Difficult Airway: Short neck, large head, narrow nasopharynx. Plan for difficult intubation.
• Atlantoaxial Instability: Screen with flexion-extension radiographs. Avoid neck hyperextension.
• Restrictive Lung Disease: Small thorax. Optimise respiratory function preoperatively.
• Spinal Anaesthesia: Technically difficult due to narrow canal. Consider GA instead.
• Positioning: Careful positioning to avoid pressure points and cervical injury.

Spinal Decompression Rehabilitation

• Day 1-2: Pain control, mobilisation with supervision
• Week 1-2: Walking program, stair climbing
• Week 2-6: Core strengthening, avoid heavy lifting
• Week 6+: Progressive return to activities

Lower Limb Osteotomy Recovery

• Weight Bearing: Protected weight bearing for 6-8 weeks
• Physiotherapy: Range of motion, strengthening as tolerated
• Hardware Removal: Consider after union confirmed (6-12 months)
• Long-term: Monitor for recurrence during remaining growth

Monitoring and Follow-up

• Neurological: Annual neurological examination
• Spinal: Regular MRI surveillance for stenosis progression
• Lower Limb: Monitor alignment during growth spurts
• Growth: Height velocity charts, orthopaedic review every 6-12 months

Overall Outcomes

• Life Expectancy: Near-normal with appropriate surveillance and intervention
• Functional Independence: Most patients lead independent productive lives
• Cognitive Function: Normal intelligence, no cognitive impairment

Surgical Outcomes

Quality of Life Considerations

• Physical Function: May have limitations but adapt well
• Social Integration: Generally good with supportive environment
• Employment: Wide range of careers possible
• Psychological: Higher rates of depression - screen and support

Global Epidemiology

• Birth prevalence ~1 in 20,000-30,000 live births; the most common non-lethal skeletal dysplasia worldwide.
• ~80% arise as de novo mutations (unaffected average-stature parents); paternal age effect (older fathers).
• Single recurrent FGFR3 Gly380Arg mutation accounts for the vast majority of cases globally.

Side-by-Side Guidance

Practice Variation

• High-resource settings: Neonatal craniocervical MRI, polysomnography, multidisciplinary dysplasia clinics, and access to vosoritide.
• Limited-resource settings: Diagnosis is clinical/radiographic; management is supportive, prioritising recognition of apnoea/cervicomedullary red flags and treatment of symptomatic stenosis and deformity.
• Genetic counselling and peer-support organisations (e.g. national dwarfism/little-people associations) are valuable everywhere for family planning and psychosocial support.

Prophylactic vs symptom-driven foramen magnum decompression There is no consensus threshold for operating on a radiographically small foramen magnum in an asymptomatic infant. Decisions integrate neurological signs, polysomnography (central hypopnoea), and craniocervical imaging rather than canal diameter alone.

Fusion/instrumentation with lumbar decompression in children Wide multilevel laminectomy can destabilise the immature spine. Evidence (Baca/Ain, J Pediatr Orthop 2010) favours adding instrumentation in growing children to reduce revision, but routine fusion in skeletally mature adults remains debated to preserve motion.

Limb lengthening Substantial height gain is achievable but carries high complication rates (pin-site infection, nerve injury, contracture, long treatment time). Whether lengthening is reconstructive or cosmetic is an ethical debate; many advocacy groups oppose routine lengthening for short stature alone.

Vosoritide and other FGFR3-pathway drugs Vosoritide reliably increases growth velocity, but its effect on final adult height, body proportionality, and serious complications (stenosis, foramen magnum compression) is unproven. Newer agents (e.g. CNP variants and FGFR3 inhibitors) are in trials. The aim of pharmacotherapy - height vs reduction of medical complications - is itself contested.