# Achondroplasia and Skeletal Dysplasias

> Achondroplasia and skeletal dysplasias for NEET-PG Paediatrics, covering the FGFR3 mutation, rhizomelic short stature, radiological signs and differentials.

- Canonical URL: https://prepelephant.com/topics/neet-pg/paediatrics/achondroplasia-and-skeletal-dysplasias
- Exam / course: NEET-PG · Subject: Paediatrics
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-15
- Last updated: 2026-10-15
- Reference basis: Nelson Textbook of Pediatrics; Jones Smith's Recognizable Patterns of Human Malformation
- How to cite: "Achondroplasia and Skeletal Dysplasias", PrepElephant, https://prepelephant.com/topics/neet-pg/paediatrics/achondroplasia-and-skeletal-dysplasias

## Direct answer

Achondroplasia is the commonest cause of non-lethal short-limbed dwarfism, caused by a gain-of-function mutation in the FGFR3 gene that impairs endochondral ossification. It produces rhizomelic shortening, frontal bossing and normal intelligence, and most cases arise from new mutations. Skeletal dysplasias form a larger group, and radiological pattern recognition, supported by genetic testing, distinguishes them.

## What you must remember

- **Gene:** FGFR3 gain of function, most often the G380R substitution; inheritance is autosomal dominant, most cases are new mutations, and advanced paternal age is a recognised association.
- **Limbs:** rhizomelic shortening, most marked in the humerus and femur, with a trident hand.
- **Head and face:** macrocephaly, frontal bossing and midface hypoplasia, with normal intelligence.
- **Spine:** thoracolumbar kyphosis in infancy and lumbar lordosis later, with the lumbar interpedicular distance narrowing from L1 down to L5.
- **Pelvis:** a champagne-glass pelvis, with narrow sciatic notches and a flat acetabular roof.
- **Lethal form:** homozygous achondroplasia causes severe thoracic hypoplasia and death in infancy.
- **Complications:** foramen magnum stenosis with hydrocephalus or cord compression, spinal canal stenosis, sleep apnoea and recurrent otitis media.
- **Genetic counselling:** an affected parent has a 50 per cent risk of transmitting the condition to each child.

## Sorting the short-limbed child

A two-year-old boy with short limbs and a large head is referred for skeletal survey. A practical approach asks four questions in sequence: which limb segment is shortest, what the skull and spine look like, which bones are affected on radiograph, and whether the pattern is lethal. The table below compares the main conditions examiners test.

| Condition | Genetic basis | Key feature |
| --- | --- | --- |
| Achondroplasia | FGFR3 gain of function | Rhizomelia, champagne-glass pelvis, lumbar canal narrowing |
| Hypochondroplasia | FGFR3, milder variant | Short limbs with less marked radiological change |
| Thanatophoric dysplasia | FGFR3, severe | Bowed femurs with telephone receiver appearance, cloverleaf skull |
| Osteogenesis imperfecta | Type 1 collagen defects | Blue sclerae, fractures, Wormian bones |
| Diastrophic dysplasia | SLC26A2 sulphate transporter | Hitchhiker thumbs, clubfeet, cleft palate |

Achondroplasia, hypochondroplasia and thanatophoric dysplasia share the same gene, so severity and the pattern of limb change decide between them. Collagen defects and sulphate transport defects look different on both clinical examination and radiograph.

## Where the exam tries to catch you

Skeletal dysplasia questions usually test whether you can link a gene to a radiological sign.

- **Trap:** assuming autosomal recessive inheritance because most parents are of normal stature. **Correct:** the inheritance is autosomal dominant, and most cases arise from new mutations, so normal parents do not exclude it.
- **Trap:** expecting developmental delay in achondroplasia. **Correct:** intelligence is normal, and the main threats are foramen magnum and spinal canal stenosis.
- **Trap:** choosing osteogenesis imperfecta for any child with short limbs. **Correct:** osteogenesis imperfecta is defined by fragile bones and blue sclerae from collagen defects, while rhizomelic shortening with a trident hand points to achondroplasia.
- **Trap:** labelling every short-limbed child as achondroplasia without radiographs. **Correct:** hypochondroplasia and thanatophoric dysplasia share FGFR3 but differ in severity, so radiology and genetic testing separate them.

## Frequently asked questions

### Which FGFR3 mutation is most common in achondroplasia?

The G380R substitution in the FGFR3 gene is the most common mutation in achondroplasia, and it is a gain-of-function change that impairs endochondral bone growth. Genetic testing confirms the diagnosis when radiological features are equivocal.

### Why is the lumbar interpedicular distance narrowed in achondroplasia?

In achondroplasia the lumbar interpedicular distance narrows from upper to lower vertebrae, which is the reverse of the normal pattern. The small canal predisposes to spinal stenosis, so this radiological sign is worth measuring.

### What radiological features distinguish achondroplasia from thanatophoric dysplasia?

Thanatophoric dysplasia shows severely shortened and bowed femurs with a telephone receiver appearance and a cloverleaf skull, and it is lethal in the neonatal period. Achondroplasia has less severe femoral bowing and is compatible with survival into adulthood.

### Why is homozygous achondroplasia lethal?

Homozygous achondroplasia, with two copies of the FGFR3 mutation, causes severe thoracic hypoplasia and respiratory failure in the neonate. Such infants usually do not survive early infancy, which is why the heterozygous form is the one seen in clinical practice.