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Pseudoachondroplasia happens because of a mutation (change) in a gene called COMP.¹ It’s called this, because it is a gene coding for: cartilage oligomeric matrix protein, which is a normal part of the extra-cellular matrix in cartilage, ligaments and tendons. Genes are the instructions for the way the body works, including how it grows. In pseudoachondroplasia, the gene change causes a problem with the process of bone and cartilage growth. The bones grow differently and end up shorter and shaped a little differently than they typically would be.

Although the name is a lot like ‘achondroplasia’, pseudoachondroplasia is a different condition. Pseudoachondroplasia has a different cause and different features.

Pseudoachondroplasia has autosomal dominant inheritance, which means that a child has a 50% chance of inheriting it from a parent who has pseudoachondroplasia. However, there always has to be a first person in the family, and many times a child is the first in their family with pseudoachondroplasia, due to a new mutation.

Talking to a genetic counselor can help families understand how pseudoachondroplasia is inherited.

In general, features of pseudoachondroplasia become more noticeable with time. Usually, a child’s height doesn’t fall below the growth curve until around 2 years of age. Final adult height is extremely variable, even within the same family, with median height reported to be slightly less than four feet tall. Clinically, it is recognized as a form of short-limbed dwarfism, with body proportions similar to those of achondroplasia, yet with typical-sized heads and facial features.

Face and Skull

• Typical head size and facial features

Trunk, Chest and Spine

• Disproportionately long trunk
• Exaggerated lumbar lordosis (sway back)
• Scoliosis (curvature of the spine)
• Atlantoaxial instability (neck bones move more than they should)

Arms and Legs

• Rhizomelic (upper arm shorter than forearm) shortening of limbs 
• Short, wide fingers
• Hip dysplasia and a waddling walk
• Genu varus or valgus (bowlegs, knock-knees) or both (windswept)
• Severe ligamentous laxity (loose joints) with exception of elbows
• Arthritis, joint pain and/or stiffness

X-Ray Characteristics

The radiographic features of pseudoachondroplasia includes short and broad long bones with flaring of the metaphyses. Epiphyseal ossification is delayed. The epiphyses appear irregular and fragmented. The hips and knees are primarily affected. Due to their dysplastic nature, the carpals ossify late.

In the pelvis, the acetabulum (hip socket) is shallow and accentuates hip dysplasia. The triradiate cartilage is also late to mature and ossify. Arthrograms are helpful in identifying joint surfaces and planning surgery for angular deformities. The capital femoral epiphyses are small and irregular in children; in adults, there is marked dysplasia of the femoral head. The femoral head is flattened and fragmented. This leads to hip joint problems.

X-rays of the spine show platyspondyly (flattened vertebral bodies) and flame-shaped anterior projections. The interpedicular distance does not progressively decrease in the lumbar spine. In the neck, lateral X-rays of the cervical spine may reveal odontoid hypoplasia. The vertebral irregularities generally disappear by adolescence. Flexion-extension radiographs should be obtained to rule out atlantoaxial instability. MRI scans of the cervical spine (static, flexion/extension views and CSF flow studies) are helpful in identifying any compression of the spinal cord.

Most kids don’t have every sign and symptom listed here. There is a lot of variety of symptoms among people with pseudoachondroplasia.

A baby with pseudoachondroplasia is typically an average length at birth. This means that without a family history, it's not usually recognized at birth. A child’s height usually falls below the typical growth curve by around 2 years of age. By now, atypical gait (walking) can usually be seen. Diagnosis is typically made between 1 and 4 years of age and is based on clinical examination and characteristic X-ray findings. There are usually complaints of joint pain as well. Genetic testing can be performed for confirmation. 

Spine

The cervical spine should be monitored for the presence of atlantoaxial instability by lateral flexion-extension X-rays of the cervical spine. Sometimes a hypoplastic odontoid is present. Posterior cervical decompression and fusion should be performed if the instability is extreme, or if neurological symptoms occur. Scoliosis should be looked for and managed similar to idiopathic curves. Lateral c-spine X-rays should be routinely obtained in all children with pseudoachondroplasia undergoing surgery for any reason, to clear the neck prior to anesthesia.

Lower Limbs

Angular deformities around the knee such as bow-legs, knock-knees or both (windswept) can be surgically corrected. The effect of ligamentous laxity on alignment should be considered as part of the pre-operative planning. Recurrence of deformity is common and several procedures may be necessary to achieve lower extremity skeletal alignment at maturity. Hip and/or knee joint replacement surgery is not uncommon, due to early onset degenerative arthritis.

Few problems, if any, occur outside the skeleton and general good health can be expected.

Talking about pseudoachondroplasia — both with your child and others — as a difference rather than a problem. Your attitude can help your child develop good self-esteem.

Overall, movement is good, as long as it is the healthy kind for the joints. We recommend joint preservation activities. This includes avoiding repetitive pounding activities like trampolines. And do activities as a family, so that your child will not be singled out by not joining in. Also, taking hot baths (with swirling water if possible) after a long day of activity can help decrease inflammation and joint pain. Over-the-counter medications can also be used as needed for joint pain. School gym class should offer accommodations and activities to tolerance.

Patients with pseudoachondroplasia should look out for neurological symptoms such as weakness of the lower limbs, incontinence, pain in the legs, reduced endurance and tingling/numbness of the legs. These symptoms may indicate compression of the spinal cord in the neck and should be investigated.

Leg and hip pain or changes in walking (such as waddling or limping) may also result from poor alignment of the legs. In later life, pain in the hips and knees is usually the result of degenerative arthritis. An orthopedist can help you decide if lifestyle modifications, medication, or surgical treatment is needed.

1. Briggs MD, Wright MJ. Pseudoachondroplasia. 2004 Aug 20 [Updated 2018 Aug 16]. In: Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2021. Available from: https://www.ncbi.nlm.nih.gov/books/NBK1487/
2. Posey, Karen L. Hayes, Elizabeth. Haynes, Richard. Hecht, Jacqueline T. Role of TSP-5/COMP in Pseudoachondroplasia. The International Journal of Biochemistry and Cell Biology. 2004; 36:1005-1012.
3. Spranger JW, Brill, PW, Hall C, Nishimura G, Superti-Furga, A, Unger, S. Bone Dysplasias: An Atlas of Genetic Disorder of Skeletal Development. Oxford: Oxford University Press. 2018.
4. McKeand J, Rotta J, Hecht JT. Natural history study of pseudoachondroplasia. Am J Med Genet. 1996;63:406–10.
5. Scott, Charles I. Dwarfism. Clinical Symposium.1988; 40 (1):11-14.