Achondroplasia

Achondroplasia is a common dysplasia that has abnormal maturation of growth plate chondroblasts. It is characterized by short stature or dwarfism.

Achondroplasia is the commonest type of dwarfism and is associated with short limbs, a large head and often with trident hands. Its incidence is about 1 in 30,000 live births.

There is a disturbance in the division and maturation of growth plate chondroblasts causing a deficiency of chondroid and an inhibition of normal enchondral growth. The limb bones are affected but the trunk is relatively normal.

The degree of severity of each deformity varies and, when it is inherited, the same characteristics appear in the affected offspring.

The typical features of the disease are present at birth which helps to differentiate it from dwarfing associated with muscupolysaccharidosis or dysplasia epiphysialis multiplex.

The cause of the inadequate cartilage development is unknown.

There is no evidence of any endocrine disturbance.

Clinical Features

Dwarfism in which the reduction in height is due almost completely to shortness of the lower limbs is the most striking feature and is obvious in infancy.

The mature adult is usually less than 100 centimeters in height and the mid-point of the body is always above the umbilicus. The limbs are strikingly short in comparison with the trunk and the fingers do not reach below the greater trochanters.

The head is large and the nose is flattened, suggesting hydrocephalus which is sometimes also present. This appearance arises because of lack of growth of the intersphenoid and sphenoid occipital synchondroses which ossify in infancy. The mandible is almost normally developed and appears to be too large.

Trunk growth is less inhibited than that of the limbs, but the spine may show increased lumber lordosis, exaggerated by fixed flexion of the hips and some thoracolumbar kyphosis, though seldom as great as in metatropic dwarfism or mucopolysaccharidosis.

Scoliosis develops in late childhood in 25 percent, and 70 percent of achondroplastics have some spinal deformity.

The pedicles grow inadequately and there is constriction of the spinal canal which, together with deformity, may result in paraplegia in adult life and sometimes in childhood has found that spinal stenosis or narrowing of the foramen magnum results in the development of neurological disturbances in 40 percent of achondroplastics.

The chest is flattened and narrow except distally, where the rib cage is flared.

Humerus is particularly short and there is often limitation of elbow extension and supination of the forearm. The hands are short and broad, with stubby fingers and a particularly short middle finger which is spread away from the ring finger to give the appearance of a trident hand.

The lower limbs are bowed in older children due to overgrowth of the fibula relative to the tibia.

Genu varum develops after walking begins, and tends to increase throughout childhood. At the ankle, valgus deformity occurs as the calcaneus pivots on the distal overlong fibula, but later secondary varus develops.

Walking is delayed because of the large head and small limbs and this, together with hypotonia in the early months of life, may mistakenly suggest that there is mental retardation. There is a rolling gait due to hip flexion deformity and increased lumbar lordosis.

Intelligence, sexual development and muscular development are normal.

Radiological Appearance

The long bones are short, strong and dense, the femora and humeri often being less than two-thirds of their normal length. There is abrupt flaring of the ends of the shafts of the long bones with a characteristic appearance of the lower end of the femur, which is notched centrally to form a V-shaped surface in which the epiphysis lies.

The femoral neck is short with coxa valga, the greater and lesser trochanters are prominent and there is a horizontal growth plate. A the knee, the lateral femoral condyles are smaller than the medial and show a notch laterally to accommodate the upper end of the long fibulae.

The acetabular roof is flat and broad and the ilium is of characteristic almost quadrilateral shape. The sacrum articulates low on the ilia and the sciatic notch is small.

The fifth lumbar vertebra is narrow and lies between the iliac wings.

The length of the spine is approximately normal. The vertebral bodies are usually of normal height and do not show the anterior breaking as seen in mucopolysaccharidosis. There is a steady diminution in interpedicular distances from the first lumbar vertebra distally, the opposite to the normal.

The pedicles are short and thick and the posterior aspects of the vertebral bodies are concave. The lumbosacral angle is increased, a fracture that is present at birth.

Pathology

The growth plate is not as thick as normal but has a greater circumference. There is a shortage of chondroblasts which are irregularly arranged with the failure of columnization.

Adequate bars of calcified chondroid fail to form and ossification is sparse and irregular. Periosteal ossification is normal or excessive so that the cortex of the metaphysic appears to embrace the adjacent part of the epiphysis.

Treatment

Compression of the spinal cord due to spinal stenosis a herniated disc or a posteriorly displaced vertebral body may need to be relieved surgically. Laminectomy alone is sometimes appropriate, but, if the compression is anterior, an anterior approach combined with spine fusion is needed.

In the limbs, disability is likely to arise at the knee or leg because of bow-leg deformity. For established deformity, it is recommended to do correction for appearance, pain or severely abnormal shoe wear by tibial osteotomy at the proximal or distal end.

Improvement in stature can be obtained by lengthening of both lower limbs, provided that the parents are very desirous that it should be done and that they recognize the possible complications that can arise.

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