Achondroplasia is an inherited bone disorder that results in dwarfism characterized by short limbs, an average-sized trunk, and a large head. At Columbia University Irving Medical Center/NewYork-Presbyterian Hospital, we specialize in surgically treating brain- and spine-related conditions that may arise with achondroplasia, such as hydrocephalus, kyphosis, spinal stenosis, or an abnormal craniocervical junction.
Achondroplasia is the most common form of dwarfism, accounting for approximately 70% of cases. It occurs in approximately one in 25,000 live births. Average height for men with achondroplasia is 4 feet, 4 inches (131 cm), and average height for women is 4 feet, 1 inch (124 cm).
Most symptoms of achondroplasia are linked to improper bone growth in the spine, skull, and extremities. Classic symptoms may include the following:
- Short stature
- Shortened arms and legs, particularly the upper arms and thighs; short, broad hands, and feet
- Facial features such as macrocephaly, frontal bossing, flattened nasal bridge, and midface hypoplasia
- Difficulty straightening or rotating the elbows
- Trident hand
- Bowed legs
- Curvature in the spine—kyphosis and lordosis
- Hypotonia—infants with achondroplasia often have weak muscles. They tend to take longer to walk and perform other motor skills than infants without the condition.
- Recurrent ear infections, which can lead to hearing loss
- Misaligned or overcrowded teeth
The following symptoms can involve serious health problems related to the brain and spine:
- Hydrocephalus: This rare condition occurs when fluid builds up in the brain, and it may be serious. Symptoms of hydrocephalus include a rapidly enlarging head, headaches, lethargy, vomiting, and irritability. If untreated, the pressure on the brain may lead to brain abnormalities.
- Spinal stenosis: Spinal stenosis is the narrowing of the bony passageway that protects the spinal cord. This potentially serious condition may put pressure on the spinal cord and surrounding structures. Symptoms vary depending on the location of the pressure. Compression of the lower spinal cord may produce symptoms such as pain and numbness in the legs and trouble walking. Compression of the upper spinal cord may cause weakness or numbness in the arms and legs or interfere with breathing, which may increase the likelihood of death during infancy if untreated.
- Abnormal craniocervical junction: The craniocervical junction is the joint between the base of the skull and the spine. The foramen magnum may be too narrow or may fuse with vertebrae in the neck. Either condition may cause brainstem compression. Symptoms of brainstem compression include neck pain, headache, weakness in arms and legs, and sleep apnea. These conditions can be life-threatening if untreated.
Intelligence and lifespan are usually unaffected by achondroplasia. Obesity is a common health problem for patients with the condition.
Achondroplasia can be diagnosed before or after birth.
Before birth, ultrasound imaging may detect certain physical characteristics like macrocephaly. A sample of the amniotic fluid, the fluid surrounding the baby in the womb, can be taken for genetic testing to determine whether the baby has a mutated FGFR3 gene.
Once a child is born, a physical examination and X-ray can provide a diagnosis. When symptoms are atypical, genetic testing can identify a mutation in FGFR3 and confirm a diagnosis.
Macrocephaly is common in achondroplasia and can be confused with hydrocephalus. Doctors measure the head and track the measurements on a head circumference growth chart to differentiate between the growth patterns of macrocephaly and hydrocephalus.
A complete history and physical examination can provide a diagnosis for spinal stenosis and abnormal craniocervical junction. Imaging tools, including X-ray, computed tomography scan (CT scan), magnetic resonance imaging (MRI), and myelogram, can confirm a diagnosis. These imaging scans provide pictures of the abnormal bone structures in the spine and base of the skull and of the compressed spinal cord and brainstem.
For bones in the arms and legs to lengthen, cartilage must convert to bone. In achondroplasia, cartilage is not converted to bone because a protein that plays a role in this process is mutated. The protein is called fibroblast growth factor receptor 3 (FGFR3). It is produced by the gene FGFR3.
FGFR3 is passed down from parent to child in what is called an autosomal dominant manner. Autosomal inheritance means that the gene is located on one of the autosomes (chromosome pairs 1 through 22), which do not differ between the sexes. Therefore, males and females are equally affected by autosomal genes. Dominant means that only one gene is necessary to express the trait. Therefore, a person needs to receive only one copy–rom either parent–of the altered gene to develop achondroplasia.
About 20% of people with achondroplasia receive the altered FGFR3 gene from a parent with the condition. If one parent has achondroplasia and the other doesn’t, there’s a 50% chance with each pregnancy of having a child with achondroplasia. If both parents have achondroplasia, possible outcomes for each pregnancy are the following:
- 25% chance of having a child without achondroplasia (normal stature).
- 50% chance of having a child with achondroplasia (short stature).
- 25% chance of having a child who inherits two altered FGFR3 genes and has a severe form of achondroplasia. These individuals are often stillborn or die shortly after birth.
The remaining 80% of people with achondroplasia acquire a de novo FGFR3 mutation at conception—neither parent has the condition. Paternal age of 36 years or more at the time of conception is associated with increased risk of de novo FGFR3 mutation.
Currently, no “cure” is available for achondroplasia. Instead, most treatments aim to relieve complications caused by the conditions. Our neurosurgeons specialize in surgically treating the complications related to the brain and spine.
Kyphosis, a hunched upper back or neck, presents during infancy. Observation is the first-line approach because the condition often goes away by the age of 3. If kyphosis persists, then surgery is considered to correct the forward-curving spine.
A narrowing spinal canal can be surgically treated to relieve the pressure on the spinal cord. Similarly, for brainstem compression, surgery can relieve pressure on the brainstem and upper part of the spinal cord. Several surgical procedures exist, and our neurosurgeons’ choice of procedure depends on a patient’s needs.
Hydrocephalus can be surgically treated to reroute the fluid and decrease pressure inside the skull.