Metaphyseal Dysplasia – Schmid type is a rare, mild dwarfing disorder. As the name implies, abnormality is demonstrable mostly in the metaphyses of the long bones. Schmid type metaphyseal dysplasia occurs due to a change in collagen type X. This type of collagen is primarily expressed in hypertrophic chondrocytes during endochondral ossification.
Schmid type metaphyseal dysplasia usually presents in the second or third year of life due to a waddling gait and/or slowing growth. Coxa vara is almost universal and is a primary finding in the diagnosis. Other findings include mild to moderate short stature, bowed legs, rhizomelic shortening of the limbs, normal facial appearance, and occasionally mild vertebral abnormalities. Radiologically, there is almost always more involvement in the legs than the arms, and findings are virtually always most severe in the proximal femurs. Radiographic abnormalities tend to improve with age
Medical Issues to be Anticipated
- Expectations: There is no reason to anticipate any shortened lifespan
- Expectations: Mild to moderate small stature is to be expected. There is a quite a bit of variability, with overall, ultimate adult heights in males ranging from 4’5” to 5’2” and in females from 4’1” to 4’10”.
- Monitoring: No growth charts specific for Schmid type Metaphyseal Dysplasia are available. Following growth using regular growth charts can provide a rough estimate of growth rate.
- Intervention: In the more severely affected, adaptive issues may be substantial. Growth hormone may result in some acceleration of growth, but no marked benefit is anticipated due to the intrinsic bone growth abnormality that results in Schmid type metaphyseal dysplasia.
- Expectations: Coxa Vara is almost universal. It contributes to the waddling gait that often leads to initial diagnosis. This is often asymptomatic but sometimes causes substantial hip pain with ambulation.
- Monitoring: Hip radiographs should be obtained in those with hip pain or progressive limitation of hip movement..
- Intervention: Orthopedic surgical correction is indicated in those with severe pain or functional impairments (around 25-50% of those affected.) Correction is usually via subtrochanteric osteotomies.
- Expectations: This is often severe and progressive. Often it includes both femoral and tibial bowing, with the tibial bowing often being more severe. Monitoring: Follow clinically.
- Intervention: Referral to orthopedics if joints are out of plumb, there is knee pain with ambulation, or if decreased walking ability or decreased endurance arises. Around one-half of patients may eventually need leg surgery, especially for tibial bowing. Correction of bowing may help prevent secondary osteoarthritis of the knees, although there are no data specific to this disorder regarding this.
Genetics and Metabolism
Schmid type Metaphyseal Dysplasia is caused by a defect in Col10A1, a gene which encodes the collagen type X alpha 1 chain. Three alpha 1 chains form a homotrimeric protein. The gene abnormality causes shortening of the resultant protein chain and thus malfunction of the collagen. Type X collagen is only expressed in hypertrophic chondrocytes during endochondral ossification; therefore, no non-bony findings should be expected nor have any been reported. Some individuals with similar metaphyseal changes will have no demonstrated abnormality of type X collagen. Whether they should be, nonetheless, considered to have this disorder is unclear.
Schmid type Metaphyseal Dysplasia is always caused by an autosomal dominant gene abnormality. This means that an adult with this disorder will have a 50% chance to pass this poorly functional gene on to each child. Often an individual with this disorder will be born to average statured parents. When this happens it is because of a new chance change (mutation) in only a single germinal cell giving rise to the affected individual. This means that the risk for recurrence in a next pregnancy is near zero.
Diagnosis can be made based on clinical and radiographic features, as well as molecularly with Col10A1 gene testing.