A heterogeneous inherited chondrodystrophic growth disorder associated with disproportionate short stature and limbs, marked joint deformities, and early-onset osteoarthritis.
Pseudo-Achondroplasia; Pseudo-Achondroplastic Spondyloepiphyseal Dysplasia; Pseudoachondroplasia.
First delineated by French physicians Pierre Maroteaux and Mauric E. J. Lamy in 1959.
4:1,000,000 live births; one of the most frequent of the skeletal dysplasias.
Both, autosomal dominant of gonadal mosaicism and autosomal recessive inheritance has been reported.
Mutations in the COMP gene in chromosome 19p13.1 cause pseudoachondroplasia. COMP is an extracellular calcium binding protein directly involved in chondrocyte migration and proliferation. COMP produces soft cartilage, which deforms markedly under stress. Accumulation of a noncollagenous protein in the rough endoplasmic reticulum of chondrocytes and the absence of a proteoglycan “population” from the cartilage suggest that an abnormally synthesized or processed protein core is not properly transferred to the Golgi system. The structurally abnormal COMP is expressed in tendon explains the loose joints that are a consistent feature of pseudoachondroplasia.
Clinically characterized by marked shortening of the limbs with a normal head and face (which makes the difference with achondroplasia). Radiologically, all long bones are short with widened metaphyses and fragmentation, and irregularities of the developing epiphyses. In childhood, platyspondyly is characteristic, with anterior tonguing as a result of delayed ossification of the annular epiphyses; after puberty, the vertebrae become more normal in appearance. Cytoplasmic metachromasia of fibroblasts is seen under microscopy.
Patients appear normal at birth and growth retardation is seldom recognized until the second year of life when the body proportions may resemble classical achondroplasia. Skeletal lesions occur as a result of stresses on soft epiphyseal cartilage. Thus, at sites such as the anterior vertebral margins, and the wrists, ankles, and knee joints, there may be severe cartilaginous deformity. Sites not subjected to excessive stress, such as the base of skull, face, sacrum, and central portions of the vertebrae, appear relatively normal. Unlike achondroplasia, head size is normal for age and the face appears normal. Intelligence is not affected. Lumbar lordosis, kyphosis, and scoliosis may all occur. Waddling gait. There may be atlantoaxial dislocation and odontoid hypoplasia. The fingers are hyperlax and short, and do not show the trident configuration typical of achondroplasia. Ligamentous laxity contributes to the leg deformities (from genu varum to genu valgum). Chronic compression myelopathy of the cervical cord as a consequence of chronic atlantoaxial dislocation has been reported. Extraskeletal complications are uncommon. Adult height ranges from 80 to 130 cm (31.5-51.2 inches). Debilitating chronic pain affecting weight-bearing joints is observed in the vast majority of patients significantly interfering with quality of life.