Achondroplasia baby – symptoms, inheritance, and treatment

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What is achondroplasia?

Achondroplasia is a clinical condition that affects bone development, cartilage growth, and neurological function in humans. It can be grouped under a collective term known as skeletal dysplasia. There are around 400 conditions that can cause skeletal dysplasia. Achondroplasia is the commonest among them, occurring in about 1 in every 40,000 births. 

 

Achondroplasia is a bone growth disorder caused by a gene alteration (mutation) in the FGFR3 gene. This genetic mutation affects certain cell-signaling proteins in the body called the Fibroblast Growth Factor Receptors (FGFR). The abnormal functioning of this protein prevents the conversion of cartilage to bone in the growth plate, particularly in the long bones of arms and legs. This results in shorter bones, abnormally shaped bones, and overall short stature. Individuals with this condition usually attain an average adult height of 42-56 inches. 

 

Achondroplasia causes disproportionate dwarfism. The reason this form of stature is called ‘disproportionate’ is that people with achondroplasia have a normal-sized torso with short limbs. About 20-50% of children born with achondroplasia also experience an array of neurological impairments.

Why does it Occur?

The Genetic Basis

Two specific kinds of mutations in the FGFR3 gene are thought to be responsible for achondroplasia. Although this is a genetic condition, in the majority of the cases, it is not inherited. 80% of the time, a child is born with achondroplasia due to a de novo (new) gene mutation during conception. Hence, a couple with normal stature may have a child with achondroplasia. When the same couple plans to have a child again, the chances of this random gene mutation occurring again is rare. Therefore, there is a high chance that other children born to this couple will not have achondroplasia. 

 

In the remaining 20% of the cases, achondroplasia is passed down as an autosomal dominant inherited disorder. When an individual with achondroplasia plans to have a child with a partner who does not have this condition, there is a 50% chance that the child will have the condition too.

 

In another instance, if both the parents have achondroplasia, there is a:

  • 25% chance of the child having a normal stature,
  • 50% chance of the child being born with heterozygous achondroplasia
  • 25% chance of the child being born with the homozygous form of achondroplasia.

 

In homozygous achondroplasia, the child inherits the condition from both the parents. This is usually fatal.

The Molecular Basis

Whether the mutation has occurred randomly or has been passed down from a parent with achondroplasia, the particular gene affected is the FGFR3 gene. This gene codes for a protein involved in the development and maintenance of bone and brain tissue. 

When the mutation occurs, the FGFR3 protein becomes overly active. This interferes with skeletal development and results in disturbances in bone growth.

Epidemiology

Prevalence

The statistics relating to Achondroplasia may vary with varying sources. According to a study conducted using data from the European Surveillance of Congenital Abnormalities (EUROCAT) Network, the prevalence of achondroplasia is 3.72 per 100,000 live births. 

 

Worldwide, approximately 150,000 individuals have achondroplasia, and an estimated 10,000 of them reside in the US. 

 

Achondroplasia is the most common cause of disproportionate dwarfism as well as dwarfism in general. 

 

Geographic Distribution

Some populations exhibit a higher incidence of achondroplasia. In Denmark, achondroplasia occurs in 1 every 6400 live births. In Latin America 1 in 10,000 live births is affected by this condition.

 

Sex-Related Demographics

Achondroplasia is equally common in both males and females. 

Race Related Demographics

So far there has been no data to suggest that individuals from any particular race are more at risk of developing achondroplasia.

Body Characteristics of Achondroplasia

Each child born with achondroplasia experiences the condition differently. However, some characteristic physical features can be noted:

 

  • Short limbs
  • Large head size
  • Prominent forehead with flattened nasal bridge
  • Crowded or misaligned teeth
  • Trident hand: the presence of extra space between the middle and ring fingers
  • Curved lower spine or lordosis
  • Small vertebral canals
  • Bowed lower legs
  • Short, broad, and flat feet

Other Symptoms

In Infants with Achondroplasia

  • Reduced muscle tone and loose joints may lead to delayed developmental milestones eg. delayed walking. 
  • Neurological complications including hydrocephalus and apnea

In Children and Adults with Achondroplasia

  • Recurrent middle ear infection which if untreated may lead to hearing loss
  • Obesity
  • Difficulty in bending elbows
  • Developing kyphosis or lordosis
  • Neurological complications such as severe spinal stenosis

Neurological Complications

Not everyone with achondroplasia has to go through neurological complications. 

 

However, there is an array of neurological deficits that may arise from arrested bone growth at the base of the skull or the spinal canal, leading to compression of the brain stem and/or the spinal cord.

Hydrocephalus

If there is narrowing in the base of the spine, cerebrospinal fluid (CSF) cannot freely flow in and out of the skull. This results in abnormal accumulation of CSF in the spaces inside the brain called the ventricles resulting in hydrocephalus.

 

In hydrocephalus, there will be rapid enlargement of the head circumference and the infant will suffer from:

  • Headache
  • Irritability
  • Lethargy and 
  • Vomiting

 

Children with achondroplasia generally have a large head circumference. Doctors have ways to find out whether the large head is a developmental feature or if the infant is truly suffering from hydrocephalus.  

 

Cervical Myelopathy

Kinking at the brain stem leads to cervical myelopathy. This is associated with:

  • Brisk reflexes
  • Numbness and weakness
  • Poor feeding and irritability
  • Difficulty in walking
  • Sleep apnea

 

If left untreated, brainstem compression may lead to death. 

 

Spinal Cord Myelopathy

As the child grows, the vertebrae may not grow enough to accommodate the nerves going to and from the spinal cord. Nerve root compression leads to spinal cord myelopathy. There may be:

 

  • Pain or numbness and weakness in a specific arm or leg
  • Numbness and weakness in an entire section of the body
  • Loss of bowel and bladder control (in severe cases)

Reaching the Diagnosis

Achondroplasia is diagnosed by its characteristic clinical and X-ray findings. In individuals who are too young to be diagnosed with certainty, genetic testing is carried out in clinical labs to detect FGFR3 gene mutation.

 

Achondroplasia can also be diagnosed in utero, that is when the child is in the mother’s womb. If an ultrasound detects any physical feature consistent with achondroplasia, such as an abnormally large fetal head, the doctor may advise the mother to get genetic testing done. Here amniotic fluid, the fluid present in the mother’s womb is collected and tested for FGFR3 gene mutation.

Treatment Approaches

There is no specific treatment of achondroplasia. Regular monitoring and adopting medical or surgical measures according to the specific need of the individual is the key to treating achondroplasia.

Monitoring a Child with Achondroplasia

 

Children with achondroplasia regularly need to have their height, weight, and head circumference monitored. Doctors compare these findings with growth curves standardized for achondroplasia. 

 

Doctors also take:

  • Periodic X-rays to keep tabs on the position of the spine and the lower extremities
  • MRI of brain and spine to spot if there is any spinal stenosis
  • Occasional CT scans to take pictures of the vertebrae

 

Medical Care

To augment the height of the patients, Somatotropin, a recombinant human growth hormone (GH) is widely used in achondroplasia. The highest acceleration in growth rate is usually seen during the first year of long-term Somatotropin therapy. 

 

For maximum benefits, it is recommended to begin Somatotropin therapy at an early age ( between 1-6 years). 

Surgical Care

In achondroplasia, certain Orthopedic issues may arise, calling for surgical interventions. 

  • In spinal cord stenosis, a decompression and fusion surgery is carried out.
  • In the case of lower extremity malalignment, corrective surgeries are done.
  • If hydrocephalus develops, a ventriculoperitoneal shunt is performed to unclog obstructed CSF drainage.
  • If the brain stem or spinal cord is severely compressed, the surgeon may decide to remove a bone or ligament in the area to make room for the structures of the brain and the spine. 

 

In short, a specific surgical intervention is designed based on the specific need of the patient.

Living with Achondroplasia: The Long Term Outcome

It is worth mentioning that achondroplasia is purely a physical and not a mental condition. Individuals with achondroplasia generally have normal IQ levels. 

 

The only fatal form of the disease is the homozygous form, which is very rare. Although neurological complications of achondroplasia have slightly increased the chance of death during infancy, once somebody is over that, s/he can expect to lead a full lifespan.

 

Certain physical adaptations such as avoiding impact sports to prevent getting hurt in the spine can play a crucial role in maintaining a long healthy life.

Jules Parrot’s Misnomer

The term ‘Achondroplasia’ literally translates to ‘without cartilage formation. This word was first coined by Jules Parrot in 1878 to describe disproportionate growth in people. This is a misnomer though. In achondroplasia, the trouble lies not in forming cartilages, but in converting them to bones. However, medical science later accepted it as a fitting name for the condition. 

 

Another term closely related to Achondroplasia is trident hand. Physician Pierre Marie curiously observed that the hands of a patient population resembled tridents. Soon enough the term ‘trident-shaped hands’ found its way to textbooks and became a key feature in describing attributes of achondroplasia.

EndNote

There may not be a single cure to Achondroplasia but modern medical interventions have come a long way. Prompt diagnosis and utilization of these interventions can play a crucial role in treating achondroplasia. 

 

With proper awareness, necessary physical adaptations, and medical care, individuals with Achondroplasia can live a long fulfilling life. 

 

References

 
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