Pseudoachondroplasia

“Pseudo” means “false.” Thereby, this disorder is one that resembles, but is clinically distinct from, achondroplasia.The incidence of pseudoachondroplasia is estimated at 1 in 30,000, however the birth prevalence is not yet known (2).

 
How Pseudoachondroplasia Is Inherited

Pseudoachondroplasia can either be inherited in an autosomal dominant inheritance pattern (1). Parental germinal mosaicism is often found, which explains why many affected children have unaffected parents. Mosaicism causes the condition to behave in an autosomal recessive fashion.

 
Causes of Pseudoachondroplasia

Pseudoachondroplasia results from a mutation in the gene coding for cartilage oligomeric matrix protein (COMP) (1). COMP is a normal constituent of the extra-cellular matrix in cartilage, ligaments, and tendons. Defective COMP results in the accumulation of proteoglycans within cartilage cells.

 
Physical Characteristics

Both the epiphyses and metaphyses are affected in pseudoachondroplasia. Clinically, it is recognized as a form of short-limbed dwarfism, with body proportions similar to those of achondroplasia, yet with normal-sized heads and facial features.

The postnatal onset of short-limbed growth deficiency will not become apparent until between 18 and 24 months of age. Pseudoachondroplasia manifests itself over time. Ultimately, adult stature is between
82 and 130 cm.

Face & Skull
  • Normal head size and facial features
Trunk, Chest, & Spine:
Arms & Legs:
What are the X-ray characteristics?

The radiographic features of pseudoachondroplastic patients include 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 incongruity and exacerbates the effects of hip subluxation.

X-rays of the spine show platyspondyly 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 vertebrae will at first seem deformed, but the 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.

 
Making the Diagnosis

The average length at birth is 49 cm, which is within the normal range. Pseudoachondroplasia is therefore not readily recognized at birth. But, lack of longitudinal growth manifests itself in the first 2 years of life (below 5th percentile on standard growth charts). By this point the abnormal gait is present and measurements suggest pseudoachondroplasia. Diagnosis is typically made between 1 and 4 years of age and is based on clinical examination and characteristic X-ray appearances. Prenatal testing is now available by direct DNA analysis. The test detects the abnormal COMP gene by mutation scanning. Prenatal diagnosis may be appropriate during pregnancy in women with pseudoachondroplasia. It must be stressed that the majority of cases are spontaneous mutations.

 
Musculoskeletal Problems
Spine

The cervical spine should be monitored for the presence of atlantoaxial instability. Lateral flexion-extension x-rays of the cervical spine is recommended, if a pre-existing abnormality such as hypoplastic odontoid is present. Posterior cervical decompression and fusion should be performed if the instability exceeds 8 mm or neurological symptoms (cervical myelopathy) occur. Scoliosis should be looked for and is managed similar to idiopathic curves. Lateral c-spine x-rays should be routinely obtained in all children with pseudoachondroplasia undergoing surgery for any reason.

Lower Limbs

Angular deformities around the knee are corrected using osteotomies. Careful pre-operative planning is essential to restore normal mechanical axes in sagittal and coronal planes (down the middle of the body). Since the epiphyses are distorted, intraoperative arthrography may be necessary to properly visualize the joint surfaces. The effect of ligamentous laxity on alignment should be ascertained 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. Up to 50% of adults will require joint replacement surgery for early onset degenerative arthritis. Hip/ knee replacement surgery in patients with skeletal dysplasia is a technically demanding exercise due to abnormal skeletal size and shape. Subluxation of the hips is a combination of femoral deformity, failure of epiphyseal ossification, acetabular dysplasia (failure of hip socket development), and joint contractures (flexion and adduction). A combination of femoral and pelvic osteotomies may be necessary. Since the femoral head is flattened, a valgus proximal femoral osteotomy is preferred to a varus procedure. If the hip joint is not congruous, acetabular augmentation procedures (Chiari osteotomy or Shelf procedure) are used to salvage the hip.

 
Problems Elsewhere in Body

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

 
What to Watch For

Pseudoachondroplastic patients 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.

Lower extremity pain of gradual onset or changes in walking (waddling/ limping) may also result from altered alignment of the legs. In later life, pain in the hips and knees is usually the result of degenerative arthritis.

Generally all skeletal dysplasias warrant multidisciplinary attention. Regular assessment by an orthopedist, geneticist, pediatrician, dentist, neurologist, and physical therapist will provide the most comprehensive treatment.

 
References
  1. Jones, Kenneth L. Recognizable Patterns of Human Malformation. Philadelphia, PA: Elsevier Saunders. 2006.
  2. Posey, Karen L. Hayes, Elizabeth. Haynes, Richard. Hecht, Jacqueline T. 2004. Role of TSP-5/COMP in Pseudoachondroplasia. The International Journal of Biochemistry and Cell Biology. 36: 1005-1012.
  3. Scott, Charles I. Dwarfism. Clinical Symposium, 1988; 40(1);11-14.
  4. Spranger, Jurgen W. Brill, Paula W. Poznanski, Andrew. Bone Dysplasias: An Atlas of Genetic Disorder of Skeletal Development. Oxford: Oxford University Press. 2002.

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