Achondroplasia, also known as chondrodystrophia fetalis or chondrodystrophic dwarfism, is a congenital developmental disorder caused by defective endochondral ossification, primarily affecting the long bones. Clinically, it manifests as a specific type of dwarfism—short-limbed dwarfism. Patients typically exhibit normal intellectual and physical development and are often employed as acrobatic clowns in theatrical troupes or circuses.

bubble_chart Etiology

It is a congenital developmental abnormality, with evident heredity and family history, inherited as an autosomal dominant trait. If one parent is affected, 1/2 of the offspring may develop the disease; if both parents are affected, almost all children will be involved. Due to many patients remaining unmarried or experiencing difficult delivery, they may not have offspring, thereby affecting the inheritance pattern. Therefore, sporadic cases of sexually transmitted diseases account for 90%. Of course, some cases may also result from gene mutations. In twins, one may be affected, or both may be affected, with females slightly outnumbering males.

bubble_chart Pathological Changes

The fundamental pathological changes occur in the process of endochondral ossification, where the longitudinal growth of long bones is impaired, while intramembranous ossification remains unaffected. As a result, the thickness of the bones remains normal, but their shortened length makes them appear relatively thicker. The epiphyseal chondrocytes can proliferate but fail to undergo normal calcification and ossification, leading to enlargement of the bone ends. Microscopically, the chondrocytes do not align in the usual orderly columns but are instead scattered and irregularly clustered. The distinct zones of the ossification process—such as the resting zone, proliferative zone, hypertrophic zone, and zone of provisional calcification—are disrupted. The metaphyseal capillaries fail to penetrate the epiphysis in an orderly manner, hindering normal resorption. Mature chondrocytes cannot calcify, impairing bone growth. Additionally, widespread chondromyxoid degeneration is observed, with swollen cells, enlarged nuclei, and a semifluid matrix structure. Ossification in the affected cartilage is delayed and appears patchy, while calcification between these patches proceeds relatively normally.

1. Dwarfism: This disease is the most common cause of dwarfism. At birth, the fetus exhibits a normal body length but shorter limbs, a difference that becomes more pronounced over time. The proximal limbs, such as the humerus and femur, are shorter than the distal bones, and the child appears overweight. By maturity, the average height is 131±5.6 cm for males and 124±5.9 cm for females. Reports in the literature include heights of 97 cm and 104 cm. The midpoint of the body is above the navel, sometimes even at the lower end of the sternum. The hands can only reach below the lower trochanter of the femur, unlike in normal individuals who can reach the lower third of the thigh. Due to the short limbs, when the legs are extended, the face can touch the toes.
2. Enlarged head: Some patients exhibit grade I hydrocephalus, with a prominent dome-shaped forehead, a saddle-shaped nasal bridge, a flat nose, thick lips, and a protruding tongue (in infants).
3. Kyphosis of the thoracic spine and lordosis of the lumbar spine, with the latter being more pronounced. The sacrum is more horizontal, giving the buttocks a characteristic protrusion.
4. The thorax is flat and small, with abnormally short ribs.
5. Fingers are thick and short, often separated into groups: the fourth and fifth fingers as one group, the second and third as another, and the thumb as a separate group, resembling a "trident." Some patients exhibit grade I limitation in elbow extension.
6. The lower limbs are bowed, resulting in a rolling gait when walking.
7. Intellectual development is normal, teeth are healthy, muscle strength is strong, and sexual function is normal.

bubble_chart Auxiliary Examination

X-ray findings

1. The cranial vault is large, with a prominent forehead, and the parietal and occipital bones are also more protruding. However, the cranial base is short, and the foramen magnum becomes smaller, taking on a funnel-shaped appearance resembling fistula disease, with a diameter possibly only half that of a normal person. If accompanied by hydrocephalus, the lateral ventricles may be dilated.
2. The long bones are shortened, with thickened shafts and narrowed medullary cavities. The epiphyses may appear fragmented or irregular. In the knee joint area, the bone ends often exhibit a "V"-shaped separation, with the ossification centers of the epiphyses fitting precisely into these V-shaped notches. Due to the ossification centers being close to the shafts, the joint spaces appear widened. The lower limbs are bowed, with the fibula longer than the tibia, and the ulna longer than the radius in the upper limbs.
3. The thickness of the vertebral bodies is reduced, but the overall reduction in spinal length is much less pronounced compared to the shortening of the limbs. From the first lumbar vertebra to the fifth lumbar vertebra, the distance between the vertebral arches gradually decreases. Myelography may reveal spinal canal stenosis and multiple posterior disc protrusions.
4. The pelvis is narrow, with flat and rounded iliac bones, all diameters being small. The acetabulum is displaced posteriorly, approaching the sciatic notch, and there is coxa vara, with asymmetry between the acetabulum and femoral head. The ribs are short, and the sternum is wide and thick. The scapular angle is not sharp, and the glenoid cavity is shallow and small.

bubble_chart Prognosis

If the infant does not die prematurely, they can perform various jobs as adults with a good prognosis. A small number of patients may develop hydrocephalus due to a narrowed foramen magnum. The incidence of spinal canal stenosis can reach 40%, mostly in the lumbar spine. Occasionally, it occurs in the cervical or thoracic spine, causing compression of nerve roots or the spinal cord, requiring laminectomy for decompression or foraminotomy. Rarely, osteotomy may be performed due to lower limb deformities. There is no specific treatment for the disease cause.

bubble_chart Differentiation

Generally, it is not difficult, but atypical cases need to be differentiated from dwarfism caused by other reasons.

1. Hypochondroplasia: The manifestations of dwarfism are not very obvious, and the skull is normal.
2. Chondro-ectodermal dysplasia (Ellis-Van Creveld syndrome): This is a short-limbed type of dwarfism, accompanied by chest deformities and cardiac lesions, syndactyly, and poor development of nails and teeth. The shortening of limbs often occurs in the distal bones.
3. Spondylo-epiphyseal dysplasia: Also a short-limbed type of dwarfism, often with destruction of proximal large joints, normal skull, flattened vertebral bodies, and fusion of vertebral ossification centers. The thorax is poorly developed, resembling a bell shape.
4. Rickets and cretinism: Rickets has typical clinical and X-ray manifestations, making it easy to differentiate; cretinism is often accompanied by intellectual developmental disorders.