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What is achondroplasia?

Achondroplasia is a bone disorder affecting about one in every 10,000 infants. It is caused by a mutation in the FGFR3 gene that impairs the growth of bone in the limbs and causes abnormal growth in the spine and skull.

Approximately 20-50% of all children with achondroplasia will experience a neurological impairment. This is caused by compression created as they literally grow faster than their bones. The stunted bone growth at the base of the skull and the spine can cause the spinal cord and brain stem to become compressed. This can lead to key nervous system structures — like the brain stem, spinal cord, spinal nerve roots and cerebrospinal fluid (CSF) spaces — to also compress. Eventually, this may lead to neurological deficits like:

Cervico-medullary Myelopathy: Compression at the foramen magnum – the bony hole at the base of the skull through which the brainstem and spinal cord exit the skull – can cause a child’s brainstem to “kink.” This can cause a child to have:

  • very brisk reflexes
  • numbness
  • weakness
  • difficulty walking
  • loss of bowel and bladder control
  • sleep apnea – periods during sleep when the child stops breathing.

Brainstem compression can ultimately lead to death if it is left untreated, so parents and physicians of achondroplastic children should watch for the symptoms outlined above.

Hydrocephalus: When the narrowing near the base of the spine prevents cerebrospinal fluid (CSF) from flowing freely around the brainstem or in and out of the skull, the CSF collects in ventricles (spaces in the child’s brain). The resulting condition is hydrocephalus. In babies, the most evident symptom of hydrocephalus is a quickly enlarging head circumference. Additional symptoms include:

  • headaches
  • irritability
  • lethargy
  • vomiting

Because an enlarged head is normal in achondroplastic children, pediatricians can use a special head circumference growth chart to distinguish between normal achondroplastic growth and possible hydrocephalus.

Spinal Cord Myelopathy: Sometimes, the vertebrae of achondroplastic children do not grow enough to allow sufficient space for nerves exiting and entering the spinal cord to pass in and out of the bony spinal column. If only a single nerve root is compressed, a child may experience pain, numbness or weakness in a specific arm or leg. They may seem to prefer using one hand over another very early as babies, or complain of pain in their back or affected arm. In more severe cases, the entire spinal cord can be compressed, causing weakness and numbness in the entire body below the spinal cord pinch as well as loss of bowel and bladder control.

Symptoms of achondroplasia

The following are the most common symptoms of achondroplasia. However, each child may experience symptoms differently. Symptoms may include:

  • shortened arms and legs, with the upper arms and thighs more shortened than the forearms and lower legs
  • large head size with prominent forehead and a flattened nasal bridge
  • crowded or misaligned teeth
  • curved lower spine – a condition also called lordosis (or "sway-back") which may lead to kyphosis, or the development of a small hump near the shoulders that usually goes away after the child begins walking.
  • small vertebral canals (back bones) – may lead to spinal cord compression in adolescence. Occasionally children with achondroplasia may die suddenly in infancy or early childhood in their sleep due to compression of the upper end of the spinal cord, which interferes with breathing.
  • bowed lower legs
  • flat feet that are short and broad
  • extra space between the middle and ring fingers (Also called a trident hand.)
  • poor muscle tone and loose joints
  • frequent middle ear infections which may lead to hearing loss
  • normal intelligence
  • delayed developmental milestones such as walking (which may occur between 18 to 24 months instead of around one year of age)

The symptoms of achondroplasia may resemble other problems or medical conditions. Always consult your child's physician for a diagnosis.

transabdominal fetal ultrasound
Achondroplasia can be diagnosed before
birth by fetal ultrasound.

How is achondroplasia diagnosed?

Achondroplasia can be diagnosed before birth by fetal ultrasound or after birth by complete medical history and physical examination. DNA testing is now available before birth to confirm fetal ultrasound findings for parents who are at increased risk for having a child with achondroplasia.

If your child has been diagnosed with achondroplasia, you should be looking carefully for signs of muscle weakness, changes in bowel or bladder function, asymmetrical reflexes or respiratory problems like sleep apnea. If any of these symptoms occur, you’ll want to obtain a thorough work-up of your child by consulting a pediatric neurologist, pediatric neurosurgeon or pediatric orthopedist, along with magnetic resonance imaging (MRI) of the child’s head, neck or spine.

Treatment for achondroplasia

If a dangerously compressed region of the foramen magnum or spinal column is found, a surgeon can remove bone and ligaments from the problem area to make more room for neural structures. The structural integrity of the neck and spinal column are not compromised in these procedures and patients tend to do very well. Because children are continuing to grow, they may require additional decompressions. Surgical decompression is most successful when performed quickly because symptoms allowed to progress for months or years can become permanent.

An achondroplastic child who has developed hydrocephalus may require a ventriculo-peritoneal shunt. In this case, a pediatric neurosurgeon can relieve the accumulation of CSF in the child’s ventricles by placing a long, thin tube under the skin. One end is placed in the child’s ventricle and the other in his or her abdomen. CSF is allowed to flow at a controlled pace out of the child’s head into his or her abdomen, where it can be quickly and safely absorbed into the bloodstream.

When symptoms are identified and acted upon in a timely manner, modern medical and surgical care allows achondroplastic children to grow up with intellectual and social capacities equal to that of other children.

To request a consultation or make an appointment, please contact Johns Hopkins Pediatric Neurosurgery at 410-955-7337.

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