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Achondroplasia

Achondroplasia

Achondroplasia is a genetic disorder and the most common form of short-limbed dwarfism. It results from a mutation in the FGFR3 gene (fibroblast growth factor receptor 3), which causes abnormal functioning of this receptor. This abnormality slows the conversion of cartilage to bone, particularly affecting the growth plates of long bones, leading to shortened arms and legs, especially the upper arms and thighs, and a shorter overall stature. Adult height typically ranges between 42 and 56 inches (about 3.5 to 4.6 feet).

Key Features and Symptoms

• Short stature with disproportionate limbs: The upper arms and thighs are more shortened than the forearms and lower legs (rhizomelic shortening).
• Large head (macrocephaly): Prominent forehead (frontal bossing) and a flattened nasal bridge.
• Characteristic hand appearance: Short fingers with a “trident” or three-pronged shape due to spacing between the middle and ring fingers.
• Spinal abnormalities: Curvatures such as kyphosis (forward rounding) and lordosis (inward curvature), and possible spinal stenosis (narrowing of spinal canal) causing nerve compression.
• Delayed motor milestones: Due to hypotonia (low muscle tone) and joint laxity.
• Other features: Bowed legs, prominent abdomen and buttocks, midface hypoplasia (underdeveloped middle facial bones), and sometimes dental crowding.
• Neurological complications: 20–50% of children may develop neurological impairment, including spinal cord compression and foramen magnum stenosis, which can cause breathing problems or hydrocephalus (fluid buildup in the brain).

Causes and Genetics

• Caused by a specific mutation in the FGFR3 gene.
• About 80% of cases arise from spontaneous mutations (no family history), often associated with advanced paternal age.
• The condition is inherited in an autosomal dominant manner; a parent with achondroplasia has a 50% chance of passing the mutated gene to each child.
• The mutation leads to abnormal regulation of bone growth, mainly affecting endochondral ossification (bone formation from cartilage).

Diagnosis

• Clinical features and physical examination.
• Genetic testing for FGFR3 mutation confirms diagnosis.
• Prenatal ultrasound may detect characteristic limb shortening and large head.
• Imaging (X-rays, MRI) to assess bone abnormalities and spinal canal size.

Complications

• Spinal cord compression due to narrow spinal canal or foramen magnum stenosis.
• Hydrocephalus in infancy.
• Obesity is common.
• Recurrent ear infections and hearing loss.
• Respiratory problems in about 10% of affected individuals.
• Normal intelligence is typical.

Management

• Supportive care with multidisciplinary follow-up including orthopedics, neurology, and developmental specialists.
• Monitoring and treatment of spinal stenosis or neurological symptoms.
• Physical and occupational therapy to aid motor development.
• Surgical interventions may be needed for spinal or limb deformities.
• Early intervention for speech delays and other developmental concerns.

Prognosis

• Most individuals have a normal lifespan.
• Quality of life improves with appropriate medical care and management of complications.

Summary Table

Feature	Details
Cause	Mutation in FGFR3 gene causing abnormal bone growth
Inheritance	Autosomal dominant; 80% spontaneous mutations
Key Symptoms	Short limbs (especially upper arms/thighs), large head, trident hands, spinal curvature
Complications	Spinal stenosis, hydrocephalus, obesity, respiratory issues
Intelligence	Normal
Diagnosis	Clinical features, genetic testing
Treatment	Supportive care, monitoring, surgery for complications
Adult Height	Typically 42–56 inches (3.5–4.6 feet)

Sources: Johns Hopkins Medicine1, NORD2, Nationwide Children’s Hospital3, MedlinePlus45, Mayo Clinic6

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References:

1. “International Consensus Statement on the diagnosis and management of achondroplasia”
   • DOI: 10.1038/s41574-021-00595-x
   • Source: Nature Reviews Endocrinology (2021)
   • Relevance: Provides global consensus on clinical care pathways, genetic mechanisms, and multidisciplinary management across all life stages.
2. “Approach to the Patient with Achondroplasia—New Considerations for Optimizing Care”
   • DOI: 10.1210/clinem/dgaf017
   • Source: The Journal of Clinical Endocrinology & Metabolism (2025)
   • Relevance: Discusses updated diagnostic criteria, complications (e.g., spinal stenosis), and emerging therapies targeting FGFR3 signaling.
3. “Achondroplasia: From genotype to phenotype”
   • DOI: 10.1016/j.jbspin.2007.06.004
   • Source: Joint Bone Spine (2008)
   • Relevance: Explores FGFR3 mutation pathophysiology, skeletal dysplasia mechanisms, and genotype-phenotype correlations.