Achondroplasia—Diagnostic Testing

In the teenage period, slowly progressive symptoms such as paresthesias, weakness, pain, and paraplegia develop and may be aggravated by obesity and prolonged standing or walking. Initially, the patient can quickly relieve these symptoms by flexing the spine and hips forward, squatting, or assuming a non–weight-bearing position. As the condition progresses, pain develops and may be localized to the low back or, more commonly, may radiate into the buttocks, posterior thigh, and calf. Muscle weakness and foot drop may also develop. Although these symptoms are more common in the legs, the arms may also be affected.

Patients with symptomatic spinal stenosis require a physical examination with attention to sensory levels, and a careful neurologic history should also be obtained. Specific tests such as somatosensory evoked responses, CT, MRI, and myelography all have a diagnostic function.

Growth rate is normal in the first year of life and then drops to about the third percentile, where it remains for the first decade; it may increase during puberty. Obesity is a common problem. Adult height ranges from 42 to 56 inches.

Children with achondroplasia should not be evaluated against normal developmental milestones but rather against standards developed for children with the condition. Motor skills are often delayed because of the physical difficulties posed by short limbs and hypotonia (which tends to abate by age 2); cognitive skills are usually attained at the expected ages.

Radiographic Findings. The characteristic features are present at birth and change little throughout life. Although virtually all bones of the body are affected, the abnormal configuration of the skull, lumbar spine, and pelvis are hallmarks of the disease. Typical are a shortened skull base, large cranium with prominent frontal and occipital areas, and superimposition of the spheno-occipital synchondrosis over the mastoid. The angle of the base of the skull is 85 to 120 degrees (110 to 145 degrees is normal), and the foramen magnum is small.

The radial heads may be partially or completely dislocated and dysplastic. The phalanges in the hands are short, broad, and conic. The femoral necks are short and the long bones relatively thick and short. Distinctive rectangular or oval radiolucencies in the proximal humerus and femur that are apparent in infancy disappear by age 2. The inverted-V–shaped (chevron) growth plate of the distal femur is characteristic. The fibulas tend to be longer than the tibias, especially at the knees.

A diagnostic feature is a decrease of the interpedicular distance in a caudal direction, primarily in the lumbar spine (in the normal spine, the interpedicular distance increases in the caudal direction). Spinal stenosis, most evident in the lumbosacral region, is more pronounced in adulthood. Lateral radiographs reveal posterior scalloping of the vertebral bodies. Dorsolumbar kyphosis, commonly seen in infancy, disappears with ambulation and is replaced by an exaggerated lumbar lordosis. As the lordosis increases, the plane of the sacrum becomes more horizontal. The pelvis is short and broad with relatively wide, nonflaring iliac wings, small and deep greater sciatic notches, and horizontal superior margins of the acetabulum (champagne glass shape).

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Jul 3, 2016 | Posted by in MUSCULOSKELETAL MEDICINE | Comments Off on Achondroplasia—Diagnostic Testing
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