Achondroplasia is a bone growth disorder that causes disproportionate dwarfism. It is an autosomal dominant disease. This means that only one abnormal gene inherited from one parent leads to the condition. People with achondroplasia are short in stature with a normal sized torso and short limbs. It’s the most common type of disproportionate dwarfism.
The word achondroplasia literally means “without cartilage formation.” Cartilage is a tough but flexible tissue that makes up much of the skeleton during early development. However, in achondroplasia the problem is not in forming cartilage but in converting it to bone (a process called ossification), particularly in the long bones of the arms and legs. Those affected have an average adult height of 131 centimetres (4 ft 4 inches) for males and 123 centimetres (4 ft 1inch) for females.
• Disproportionately large head-to-body size difference with shortened arms and legs (especially the upper arm and thigh)
• Prominent forehead (frontal bossing) and depressed nasal bridge
• Underdeveloped midface and relative jawbone prominence
• Underdeveloped cheekbone resulting in tooth crowding
• Short appearing fingers with the ring and middle fingers pointing in opposite directions giving the hand a three-pronged (trident) appearance
• Limited elbow extension and rotation as well as limited hip extension
• Decreased muscle tone (hypotonicity)
• Often prominence of the mid-to-lower back with a small hump (gibbus)
About 20 percent of cases are inherited. The mutation follows an autosomal dominant inheritance pattern. This means that only one parent needs to pass down a defective FGFR3 gene for a child to have achondroplasia.
If one parent has the condition, the child has a 50 percent chance of getting it. If both parents have the condition, the child has:
• a 25 percent chance of normal stature
• a 50 percent chance of having one defective gene that causes achondroplasia
• a 25 percent chance of inheriting two defective genes, which would result in a fatal form of achondroplasia called homozygous achondroplasia
Infants born with homozygous achondroplasia are usually stillborn or die within a few months of being born.
Difference between achondroplasia and skeletal dysplasia (dwarfism)
Achondroplasia is the most common form of skeletal dysplasia, which is the umbrella term used to identify hundreds of conditions that affect the growth of bones and cartilage. Achondroplasia specifically targets bone growth in your arms and legs.
Achondroplasia is caused by a mutation in fibroblast growth factor receptor 3 (FGFR3) gene. This gene is mainly responsible for making the protein, fibroblast growth factor receptor 3. This protein contributes to the production of collagen and other structural components in tissues and bones. When the FGFR3 gene is mutated it interferes with how this protein interacts with growth factors leading to complications with bone production. Cartilage is not able to fully develop into bone, causing the individual to be disproportionately shorter in height.
In normal development FGFR3 has a negative regulatory effect on bone growth. In achondroplasia, the mutated form of the receptor is constitutively active and this leads to severely shortened bones. The effect is genetically dominant, with one mutant copy of the FGFR3 gene being sufficient to cause achondroplasia, while two copies of the mutant gene are invariably fatal (recessive lethal) before or shortly after birth (known as a lethal allele).
Risk Factors –
The genetic defect can be passed from parent to child. However, in about 80 percent of cases, achondroplasia results from a spontaneous mutation (a sudden genetic defect) that occurs in the developing embryo.
Children with achondroplasia often have –
• less muscle tone, which becomes a reason of delayed walking and motor skills
• bowed legs
• issues with joint flexibility
• breathing problems
• ear infections
• crowded teeth
• hydrocephalus, or “water on the brain”
• spinal stenosis, which is a narrowing of the spinal canal that can compress the spinal cord
• sleep apnea
• numbness or tingling in legs because of nerve compression
• have difficulty bending their elbows
• develop new or more severe spinal stenosis
▪︎ During Pregnancy
Doctors can use ultrasounds to detect achondroplasia before birth if your baby’s arms and legs appear shorter than average and if their head is large. Most cases of achondroplasia aren’t confirmed until after birth.
▪︎ After Birth
• Physical examination.
• Genetic testing (FGFR3 gene).
• MRI or CT scan to identify muscle weakness or spinal cord compression
There’s no cure for achondroplasia, but almost everyone who’s diagnosed is able to live a full and healthy life. Monitoring height, weight and head circumference is recommended during early diagnosis in infants to track growth progress.
Management of achondroplasia is focused on taking care of potential complications, which may include –
• Weight management and encouraging healthy eating habits to prevent obesity.
• Surgery (ventriculoperitoneal shunt) to decrease fluid pressure on your brain or to correct a life-threatening complication called craniocervical junction compression.
• Surgery to remove adenoids and tonsils.
• Growth hormones.
• Use of continuous positive airway pressure (CPAP) nasal mask for apnea.
• Ear tubes or antibiotics to prevent ear infections.
• Support for socialization.
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