Hypochondroplasia and achondroplasia

Hypochondroplasia is the milder form of a congenital skeletal disorder called achondroplasia, although the two are sometimes treated as separate diseases.

This article will discuss these two disorders and elaborate on each of them.


What are the symptoms of achondroplasia?

Achondroplasia is a form of congenital bone disorder that is familiar to the public. In this disease, the limbs are significantly shorter, but the length of the body is mostly normal.

People with achondroplasia have a short stature and their maximum height is around 120 centimeters (4 feet). They often also suffer from an enlarged head (macrocephaly), due to unproportionate development of the skull.

Most people with achondroplasia have joint hypermobility (over-flexible joints), and recurrent ear infections. As they age, they often suffer from back pain when walking.

In some cases, patients have bowed lower limbs that can be corrected through surgery. In addition, limb lengthening surgery can help lengthen shorter lower limbs, with a high success rate.


What are the symptoms of hypochondroplasia?

People with hypochondroplasia are often slightly taller than people with achondroplasia, their height can reach 140 centimeters and even 150 centimeters in adulthood (around 4 feet 11 inches). They often have a normal head circumference and suffer from fewer complications during their lives.

 It is important to correctly diagnose these two conditions through genetic testing and x-ray scans (radiography). In most cases, hypochondroplasia causes no orthopedic impairments.


What is the genetic basis of these disorders?

Achondroplasia is caused by mutations to the gene FGFR3. Ninety-seven percent of patients suffer from the same gene mutation. A different mutation to this gene was diagnosed in seventy percent of people with hypochondroplasia.


Since the genetic basis has not yet been discovered in some patients, whenever the common findings for these disorders are detected during pregnancy, genetically ruling out achondroplasia is enough to diagnose hypochondroplasia.

Most cases of hypochondroplasia are sporadic, which means that the first child in the family with this condition is born to two healthy parents who are not carriers of the gene mutation.

When a family history of this condition exists, the form of inheritance is autosomal dominant, which means that one parent passing down a gene mutation is enough for the child to develop this condition.


Prenatal diagnosis

It is currently difficult to diagnose these conditions prenatally since the impaired body proportions only become apparent during thirty to thirty-two weeks of gestation. At this stage, it is already quite late to begin inquiry.

A fetal anomaly scan is performed during week 24 of gestation. In this scan, the difference between average hip length and hip length in achondroplasia is rather small and may be overlooked during the procedure.

Whenever a shorter than normal hip length is diagnosed, the relevant gene mutation can be detected through amniocentesis. In any case, skeletal development must be monitored.

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