Sacral dimple: incidental findings from newborn evaluation (Discussion and Diagnosis)

The Case Presentation can be found on page 768. DISCUSSION Mucopolysaccharidosis type IVA (MPS IVA, Morquio A syndrome, OMIM#253000) is an autosomal recessive disorder caused by the deficiency of the lysosomal enzyme, N-acetylgalactosamine 6-sulfate sulfatase (GALNS, E.C.3.1.6.4). GALNS catalyzes the degradation of glycosaminoglycans (GAGs), keratan sulfate (KS) and chondroitin 6-sulfate (C6S), into soluble products (1). Deficiency of GALNS leads to progressive accumulation of undegraded or partially degraded insoluble products of its substrate within the cell, resulting in dysfunction of the affected organs. MPS IVA patients appear normal at birth and the onset of the disease is observed between 1 and 3 years in approximately 40% of the cases, resulting from characteristic skeletal features including short stature, pectus carinatum, kyphosis (hump back), genu valgum and abnormal gait (2). Generalized skeletal dysplasia without involvement of the central nervous system becomes prominent. Odontoid hypoplasia is the most critical skeletal feature to be found in MPS IVA patients, leading to cervical instability, myelopathy and even sudden death. Other potential complications include pulmonary compromise, valvular heart disease, hearing loss, fine corneal clouding and widely spaced teeth with abnormally thin enamel (1). There is no cure for MPS IVA and patients with severe phenotypes often do not survive beyond the second or third decade of life. Although supportive management and parent education should start early to minimize complications, most patients are diagnosed more than 2 years after their initial symptoms are noticed (2). Early clinical signs and symptoms are not well recognized by physicians because of its rare incidence in population (approximately 1 in 250000 births) and paucity of statistical data (2). In addition, poor test sensitivity often confuses physicians even if they suspect the disease. Generally, total urine GAGs are elevated in other types of MPS while all MPS IVA patients may not have elevations (3). KS accounts for only 0–7% of total GAGs excreted in urine (4,5), and increased KS levels in MPS IVA patients are not always recognized by measuring total urine GAGs, leading to false negative results. Therefore, there is often a delay in diagnosis or misdiagnosis of MPS IVA patients. In this context, GALNS enzyme assay determination and/or direct measurement of KS is required when MPS IVA is suspected clinically. We have recently developed a KS assay method by enzyme-linked immunosorbent assay (ELISA) and liquid chromatography/turbo-ion spray ionization tandem mass spectrometry (TMS, LC/MS/MS), which allows clarification of the difference between controls and MPS IVA patients (6–8). The clinical and biochemical evaluations of this case have demonstrated that: (i) a sacral dimple may indicate a serious abnormality of the spine or spinal cord and its presence may be a feature of MPS IVA. This hypothesis is supported by X-radiographs obtained at birth of this MPS IVA patient showing some changes in the lower vertebral bodies which suggest presence of a skeletal dysplasia, (ii) urine GAG determination was not useful for diagnosis of MPS IVA as described previously and (iii) KS assay and enzyme activity assay were preferred methods for diagnosis once MPS IVA was suspected clinically. To the best of our knowledge, no previous authors have described MPS IVA in which the skeletal dysplasia has been followed clinically and radiologically from birth.