Prepared by Troy Marlow, MD and Timothy Han, III, MS from the Department of Radiology at the Medical University of South Carolina, Charleston, SC.

CASE SUMMARY

An 8-year-old boy was seen by an orthopedic surgeon for evaluation of growth and developmental distur-bances. He admits to generalized mus-cle weakness, joint stiffness, and a waddling gait with compromised stability. Physical examination reveals coarse facial features, short trunk dwarfism, short neck, pectus carinatum, protuberant abdomen, and enlarged wrists and hands. His mental capacity was age appropriate. Urine screening for mucopolysaccharides was positive. Fibroblast culture of a skin biopsy showed reduced activity of N-acetyl-galactosamine-6-sulfate-sulfatase.

DIAGNOSIS

Morquio's syndrome

IMAGING FINDINGS

Anteroposterior and lateral views of the lower thoracic spine and lumbar spine reveal flattening and irregularity of the vertebral bodies. Anterior beaking is demonstrated in the thoracic vertebrae on the lateral projection (Figure 1). The pelvic and abdominal films show flared ilia laterally with inferior constriction (wine-glass shape) (Figure 2). Frontal view of the femora reveal enlarged acetabular cavities with rough margins, and there are poorly formed femoral epiphyses and widened femoral necks with coxa valga (Figure 3). Atlantoaxial subluxation at rest can be demon-strated by lateral view of the craniocervical junction (Figure 4). These findings, along with the biochemical laboratory results, help to distinguish Morquio's syndrome from spondyloepiphyseal dysplasia, which shares similar spinal changes, but is more frequently associated with coxa vara.

DISCUSSION

There are at least 12 types of inherited genetic disorders dealing with specific lysosomal enzyme deficiencies. This collective group of mucopoly-saccharidoses (MPS) are associated with an incomplete metabolism of keratan sulfate, dermatan sulfate, heparan sulfate, or chondroitan sulfate within the lysosomes, with the resultant accumulation in the brain, viscera, and joints throughout the body. ²

Morquio's syndrome (MPS type IV) is the most common of these storage disorders and is characterized by the inability to metabolize keratan sulfate. This disease is autosomal recessive and both sexes are affected with the same frequency. The predominant clinical features of Morquio's syndrome are skeletal and neurologic. Both types of Morquio's syndrome are characterized by short-trunk dwarfism, fine corneal deposits, a skeletal dysplasia that is distinct from other mucopolysaccharidoses, and preservation of intelligence.

Children with Morquio's syndrome manifest short-trunk dwarfism within the first 2 years of life. Marked dwarfism, dorsal kyphoscoliosis, muscular hypotonia, and weakness are prominent early childhood presentations. Also seen are coarse faces with short nose, broad mouth, widely spaced teeth with thinned enamel, corneal clouding, ligamentous laxity, and joint stiffness. Adult height rarely exceeds 4 feet. Pectus carinatum (horizontal and protuberant sternum) and a shortened neck with the head appearing sunken into the chest are the norm. Mental capacity is generally normal, but deafness often develops. Most patients survive into their third or fourth decades. ¹

Radiographic features include oval-shaped vertebral bodies with anterior beaking (Figure 1); unossified femoral heads with proximal femoral valgus deformities (Figure 3); and broad, flat ilia (Figure 2). By 2 to 3 years of age, universal platyspondyly with central beaking is almost pathognomonic. The long tubular bones are thickened and shortened as well. ¹

Odontoid hypoplasia is common in children with Morquio's syndrome. Instability of the odontoid process and ligamentous laxity can result in life-threatening atlantoaxial subluxation (Figure 4). ⁵ Atlantoaxial instability with progressive myelopathy is one of the most disabling features of Morquio's syndrome. Neurologic injury and sudden death are likely the result of spinal cord compromise from C1­C2 instability as well as extradural soft-tissue hypertrophy anterior to the spinal cord in the upper cervical spine. ²

The diagnosis is made by urine screening for MPS and confirmed by serum testing for the specific sugar abnormalities or the decreased enzyme activity of N-acetyl-galactosamine-6-sulfate-sulfatase within leukocytes or fibroblast cultures from skin biopsy upon laboratory assay. ²