Metatropic dwarfism or metatropic dysplasia I is a rare genetic disorder characterized by extremely small stature, with short arms and legs. Other characteristics of this disorder are a narrow thorax, short ribs, and kyphoscoliosis (backward and sideways curvature of the spinal column) which develops into short trunk dwarfism.
Metatropic dwarfism manifests at birth with short limbs and a relatively longer trunk. They may be a mild thoracolumbar kyphosis. Infants who survive undergo a striking transformation during the first 2 or 3 years of life.
Progressive kyphoscoliosis and lumbar lordosis, rapidly becoming severe, causes further dwarfing and the limb changes become less obvious as they become proportionately longer relative to the trunk.
The inheritance of metatropic dwarfism is autosomal recessive.
Clinical Features of Metatropic Dwarfism
The child is born of parents who show no abnormality. There may be difficulty in establishing respiration because of restricted chest mobility. There are skeletal deformities at birth with short limbs, enlarged knobbly joints, and a mild thoracolumbar scoliosis.
The trunk is of normal length and the head is normal.
The face may be broad with a depressed bridge of nose. Intelligence is normal.
A unique feature is a triangular fold of skin overlying the tip of the coccyx.
The thorax is long and narrow.
Motor milestones are delayed because of joint stiffness of laxity, and, when walking begins, there is a waddling gait, with flexion deformities of the hips and knees and valgus feet. The spine rapidly develops a thoracolumbar scoliosis which soon reaches more than 60 degree, though it may then increase less rapidly in later childhood.
The appearance of a camel hump with secondary lordosis below it is typical. A hypoplastic or absent odontoid process may predispose to quadriplegia weakness or paralysis and should be looked for clinically and radiologically.
The long bones are extremely short with expanded ends. The appearance of ossification centers is delayed. The pelvis shows changes like those of achondroplasia with enlarged acetabula with a horizontal roof, but with the specific feature of a supra-acetabular notch.
The iliac crests are broad, the greater sciatic notches are small and the pubic bones are short. The radius and ulna are short, particularly the ulna. The digits, though clinically long, show radiologically short phalanges. The fibula is longer than the tibia.
The spine at birth shows flattened vertebral bodies with well-developed neural arches, a mild thoracolumbar kyphosis and narrowing of the interpedicular distance in the lumbar spine from above downwards.
By the third year, the vertebral bodies have become irregular with platyspondyly and central anterior projections in the thoracic spine and wedge shaped lumbar vertebrae with a severe kyphoscoliosis. The ribs are short with expanded ends. The skull is normal.
This rare abnormality needs to be distinguished from achondroplasia in early infancy and from mucopolysaccharidoses or spondyloepiphysial dysplasia when spinal deformity increases.
Absence of urinary mucopolysaccharides and the specific clinical and radiological features usually makes the diagnosis clear.
The main reason for presentation to the orthhopaedic surgeon is in relation to the spinal deformity. Altanto-axial instability. If present, atlanto-axial fusion is indicated.
The neck may, apart from this, show limited movements with pain associated with vertebral anomalies and a collar or cervical appliance usually gives relief. Measures to attempt to hold the spine to prevent increasing deformity, using a Milwaukee brace or plaster casts in childhood almost invariably fail.
Early spine fusion as soon as deformity is seen to be increasing unacceptable degree, is advised by Baily using halopelvic traction to maintain position and it may even be possible to perform atlanto-axial fusion at the same time if it is required.