Affected newborns may be small at birth, but otherwise appear normal. With age, other characteristics develop. For instance, chest deformities, feeding difficulties and developmental delay usually are manifest by or before 18 months. Disproportionate short stature, with the arms and legs being disproportionately too long for the torso, typically is present after 18 months. Additional clinical features that may also develop include dolichocephaly (a long skull), Microcephaly (a small head), coarse facial appearance, prognathism (a protruding lower jaw). Intellectual disability ranges from moderate to severe, and worsens with age. Microcephaly occurs in most individuals. Thinning of the corpus callosum has also been reported. The overall health of an affected person is generally good and survival into adulthood is usual.
In addition to the skeletal abnormalities listed above, affected individuals can also develop a short neck and chest, pectus carinatum (protruding breastbone), flaring of the costal margins, kyphosis (excessive backward curvature of the spine), lumbar lordosis (abnormal forward curvature of the spine), scoliosis (side-to-side curvature of the spine), claw-like hands, other joint contractures especially of the elbows and hips, genu valgum and talipes equinovarus (clubbed feet). Further, the metacarpals (bones in the middle of the hand) and phalanges (other bones in the fingers and toes) are shortened. The carpal bones (bones of the wrist) may also be small and irregularly shaped. Rhizomelic shortening of the limbs (disproportionate shortening of the proximal portion of the limbs) may be present also. Histologically, both DMC and SMS exhibit deficient chondrocytic organization and differentiation, and columnar formation that contain populations of degenerating cells with rough endoplasmic reticulum inclusions. On electron microscopic exam, the chondrocytes contain widened cisternae of the rough endoplasmic reticulum, and the vesicles are coated with a smooth single-layered membrane. The above findings suggest that lack of dymeclin may lead to abnormal processing or defective synthesis of cartilage protein.
Other radiographic abnormalities seen in DMC have been extensively reviewed by Spranger et al. (1975) and include a small skull, hypoplastic facial bones, malformed or absent carpal bones, cone-shaped epiphyses of the phalanges, brachydactyly, fifth finger clinodactyly, accessory bones in the hands, odontoid hypoplasia (underdevelopment of the peg-like projection of the second cervical vertebra) with atlantoaxial instability, platyspondyly (flattened vertebral bodies), irregular superior and inferior edges of the vertebral bodies, anterior pointing of the vertebral bodies, hypoplastic ilia (small hipbones), narrow sacrosciatic notch, widen pubic symphysis, dysplastic acetabulum (malformation of the hip socket), small femoral heads (proximal ends of the femurs), and broad metaphyses of the long bones. Bone maturation (bone age) is delayed. Individuals with Smith-McCort syndrome have similar skeletal findings as those associated with DMC.
Secondary problems resulting from the skeletal abnormalities associated with DMC may include spinal compression, dislocated hips and restricted joint mobility. These problems may in turn cause a waddling gait. When it occurs, spinal cord compression in the neck is usually caused by the hypoplasia of the odontoid process and to hyperlaxity of the longitudinal ligament of the upper cervical spine. The pathognomonic radiographic findings for DMC and SMS include constrictions in the middle third of the vertebral bodies (a double-humped appearance), and a lacy appearance of the upper portion of the iliac crest (hipbone). This latter feature because less prominent with time and disappears by adulthood.
MRI findings in DMC include hypoplasia of the odontoid process, posterior disk protrusions in the lumbar vertebrae and the enlargement of the posterior common vertebral ligament. In most individuals with DMC, MRI analyses of the brain have been normal. However, one patient has been reported with cortical atrophy and another with thinning of the corpus callosum.
Both DMC and SMS are progressive disorders. With the exception of reduced length, affected individuals usually are normal at birth. Skeletal findings often are recognized first between 1 and 18 months. The vertebral body constriction abnormalities and lacy pattern of the iliac crests appear by 3-4 years and may persist until adulthood. The vertebral body constrictions are most prominent between ages 8 and 12 years. The Microcephaly in DMC and Short stature in both appear during childhood. Throughout childhood and as adults, thoracic kyphosis, scoliosis, lumbar lordosis, subluxation (partial dislocation) and frank dislocation of the hips, wide-based and waddling gait, genu valgum or varum, and restricted joint mobility appear and may worsen. The treatment of genu varum in this condition has been reported. Adult height is severely reduced with height ranging from 82 cm to 128 cm (32 in to 50 in). Neurologic and behavioral complications in DMC may include hyperactivity, autistic-like behavior, lack of speech and mild to severe Intellectual disability with IQ scores ranging from 25 to 65. Because of the of atlantoaxial instability found in DMC, cord compression is a concern. However, only a few cases have been reported with this complication. Prenatal diagnosis of DMC has been accomplished by finding a pathogenic homozygous mutation in the DYM gene in a fetus with no detectable physical findings.