Description, Causes and Risk Factors:

A condition characterized by short stature, delayed closure of the fontanels, and hypoplasia of the terminal phalanges.

The gene for pyknodysostosis has been mapped to the same location as the gene for cathepsin K on chromosome 1q21. Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated "p" and a long arm designated "q". Chromosomes are further sub-divided into many bands that are numbered. For example, "chromosome1q21" refers to band 21 on the long arm of chromosome 1. The numbered bands specify the location of the thousands of genes that are present on each chromosome.

Pyknodysostosis is a rare disorder that is inherited as an autosomal recessive genetic trait. Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother.

Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%. The risk is the same for males and females.

All individuals carry 4-5 abnormal genes. Parents who are close relatives (consanguineous) have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder.

The disease shows equal sex distribution with high parental consanguinity, having an incidence of 1.7 per 1 million births.


Pyknodysostosis is a rare disorder characterized by short stature and increased density of the bones. Individuals with this disorder typically have short arms and legs, an abnormal collarbone, and distinctive facial features including a large head with a small face, small chin, underdeveloped facial bones, a high forehead, high narrow palate and dental abnormalities. Bone fractures are common in individuals with pyknodysostosis. The small jaw may predispose affected individuals to periods of not breathing during sleep (obstructive sleep apnea).


The diagnosis of pyknodysostosis is based on physical features and X-ray findings. Molecular genetic testing is available to identify abnormalities in the cathepsin K gene.MRI findings of pyknodysostosis generally reveal normalcortical thickness in the calvaria, however there is increasedtrabecular bone within the medullary cavity. There is limited literature describing the CT findings, which includeshypoplastic sinuses, poor dentition, and thickening of thecalvaria.


Pyknodysostosis does not have any specific treatment as of date and only supportive measures are needed. Since bone fractures are a primary threat to those affected by pyknodysostosis, it is important that care is taken to prevent or minimize tendencies for a fracture to occur. Such precautions include careful handling of an affected child, along with exercise and activities that are safe and do not require too much impact. Dental hygiene and regular dental checkups are also necessary. Regarding the otosclerosis part treatment options include anti-enzyme, bone resorption moderating drugs, and surgical interventions. Surgery is not recommended in children below age 5 years. Researchers have stated that stapedotomy is a safe and effective treatment in pediatric patients.

Individuals with pyknodysostosis will need orthopedic care when fractures occur. Appropriate dental care should be provided and craniofacial surgery may be warranted. Treatment may be required to relieve the obstructive sleep apnea.

NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.


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