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SED-congenita typically has anÌýautosomal dominantÌýpattern of inheritance; however, distinct cases ofÌýautosomal recessiveÌýinheritance have also been reported.³ ÌýMost cases of this dysplasia are due to spontaneous mutations.³ ³Ò´Ç²Ô²¹»å²¹±ôÌýmosaicismÌýhelps to explain why affected children are often born to unaffected parents.

SED-congenita is caused by a mutation of the gene coding for Collagen Type II (COL2A1) found on Chromosome 12.^1ÌýÌýType II collagen is a structural protein present in the intervertebral discs, cartilage and eyeballs.

The diagnosis of SED is made on the basis of clinical features and relevant X-rays. Radiographic features that are particularly characteristic are the biconvex appearance of theÌýossificationÌýcenter of the vertebral bodies on lateral radiographs of the spine and the several-year delay in theÌýossificationÌýof the iliopubic ramus andÌýepiphysesÌýof the long bones, particularly theÌýfemoralÌýheads. Moreover, SED-congenita may be suspected in the prenatal period on the basis of ultrasonography. The gene is known, but testing may be difficult considering its size. Certain mutations of the gene have been associated with different forms of SED.

Neck

Individuals with SED haveÌýodontoidÌýhypoplasia. If theÌýodontoidÌýis unstable or forms abnormally, it presses on the spinal cord to causeÌýatlantoaxial instability, which is common to many skeletal dysplasias. It is diagnosed on the basis of neck X-rays and MRI scans. The instability causesÌýcervical myelopathy; it manifests even earlier than in patients withÌýMorquio syndrome. Symptoms, usually of the respiratory type, can be noted in newborns or young infants. Patients will begin to have great difficulty standing independently. Chronic motor weakness will begin to occur in the upper and lower limbs especially, followed by episodes of quadriplegia. Any inability to independently stand and remain balanced does suggest myelopathy. Typically, cord compression is treated by surgical fusion of the vertebrae in the upper part of the neck.

Spine

KyphoscoliosisÌýin theÌýthoracolumbar spineÌýis a common feature in SED. It is present is over 50% of patients. Early diagnosis is by means of regular scheduled physical examinations and X-rays. For small curvatures bracing may be attempted, but this is not always successful. If serial X-rays demonstrate a progressive curve, surgical fusion of the spine may be necessary. In one study, the use of a brace was found to be effective forÌýkyphosisÌýwhen the brace was worn until maturity. ExaggeratedÌýlumbosacral lordosisÌýaffects nearly every SED-congenita patient. It causes an imbalance of the spine in the sagittal plane. TheÌýlordosisÌýis most likely caused by changes in the structure of the vertebral bodies: the pedicles appear abnormally long and the vertical height of the posterior arches appears considerably low. Bracing, around the age of 4 or 5, is a successful attempt to correct theÌýlordosis. However, small children typically do not tolerate the cumbersome brace very well, thereby its practicality is somewhat questionable.

Lower Limbs

Coxa varaÌýis characteristic. The hip is a ball-and-socket joint formed between the pelvis (acetabulum) and the upper part of the femur (head). The head of the femur is connected to the shaft by the neck. Normally the neck makes an angle of 130° with the shaft. In SED, due to abnormal cartilage formation, the neck is unable to withstand the mechanical forces applied to it and the ball gradually bends downwards. Any change in the alignment of the femoral neck weakens the muscles around the hip joint (principally the abductors that stabilize the pelvis during walking) and causes hipÌýjoint contractures. Surgery to realign the femoral neck is recommended if symptomatic or if the neck-shaft angle is less than 100°.ÌýGenu valgusÌýis more common thanÌýgenu varus.

Feet

Though the medical literature indicates an association between SED and clubfeet, this is not our experience. We find flatfeet (planovalgus) to be much more common in children with SED.

Joints

In SED, the part of the bone adjacent to joints is affected. Joint cartilage is also predominantly composed of type II collagen. Premature osteoarthritis is typical. Joint replacement surgery (hips and knees) may be necessary in early adulthood, but this is variable. The presence of associatedÌýjoint contracturesÌýand bony deformities in SED makes such surgery a technically challenging exercise.

Eye

Type II collagen is present in the eye. SED is therefore associated with myopia (short-sightedness) and retinal detachment. Regular review by an ophthalmologist to exclude retinal tears is recommended.

Respiratory Problems

Abnormal chest development in some forms of SED may cause respiratory insufficiency.ÌýSleep apneaÌýand breathing problems can occur due to compression of the spinal cord in the neck.

Ear

Moderate hearing loss may occur, especially for high-pitched sounds. Children with SED are at risk for developing recurrent ear infections due to reduction in the size of the tubes connecting the middle ear cavity to the upper throat (eustachian tube).

In SED, regular assessment by a pediatric orthopedic surgeon, conversant in the management of skeletal dysplasias, is essential. Clinical and radiographic assessment should be conducted every six months, more frequently if closer supervision of an impending problem is necessary.

Additionally, any change in gait pattern should be taken seriously. This may be associated with tiredness, decrease in walking distance, reduced endurance, or muscle pain. Any alterations in sensation (tingling or numbness in arms and legs) or loss of bowel/ bladder control are indicative of spinal cord irritation or compression.

Changes in trunk symmetry, shoulder height differences, prominence of one hip, or rib prominence on bending forwards may indicate a changing curvature in the spine.

Knock-kneesÌýmay also progress over time. The best method of accurately assessing this is to obtain X-rays.

Flatfeet may cause pain, footwear problems, or callosities in the skin.

If central apnea is suspected, a respiratory physician may be sought out to conduct sleep studies.ÌýCentral apneaÌýresults from spinal cord compression from cervical spine instability.

Generally, all skeletal dysplasias warrant multidisciplinary attention. Regular assessment by an orthopedist, geneticist, pediatrician, dentist, neurologist and physical therapist will provide the most comprehensive treatment.

1. Jones, Kenneth L.ÌýRecognizable Patterns of Human Malformation. Philadelphia, PA: Elsevier Saunders. 2006.
2. Kopits, Steven E.ÌýOrthropedic Complications of Dwarfism.ÌýClinical Orthopedics and Related Research. 144: 153-179. 1976.
3. Scott, Charles I.ÌýDwarfism. Clinical Symposium, 1988; 40(1):17-18.
4. Spranger, Jurgen W. Brill, Paula W. Poznanski, Andrew.ÌýBone Dysplasias: An Atlas of Genetic Disorder of Skeletal Development. Oxford: Oxford University Press. 2002.
5. Taybi, Hooshang. Lachman,ÌýRalph S. Radiology of Syndromes, Metabolic Disorders, and Skeletal Dysplasias. St. Louis, MO: Mosby-Year Book, Inc. 1996.