Accuracy of the clinical diagnosis of Down syndrome.

OBJECTIVES: To determine the accuracy of clinical diagnosis of Down syndrome, identify problems in reaching a diagnosis, to provide recommendations for improvement and estimate a minimum prevalence for all types of Down syndrome.
DESIGN: A retrospective observational study was carried out over a five-year period. Genesis, a database located in the Department of Medical genetics, was used to identify the number of Down syndrome karyotypes including trisomy, translocation, and mosaic sample variants. Age of diagnosis was determined using date of receipt. Karyotyping requests for a clinical diagnosis of Down syndrome were also identified. Patient notes and cytogenetic laboratory reports were used to identify clinical indication for karyotyping.
SETTING: Regional Genetics Centre, covering all cytogenetic analyses for referrals within the entire Northern Ireland population.
RESULTS: 208 postnatal cases of Down syndrome were identified, 197 (94.7%) trisomy, 3 (1.45%) translocation, and 8 (3.85%) mosaic variants. 112 (54.8%) were male and 96 (46.2%) female. 268 samples were taken to confirm or exclude a clinical diagnosis of Down syndrome. 185 of these had Down syndrome, 77 were normal, and 6 had another abnormality. 90% and 100% of trisomy and translocation Down syndrome respectively were diagnosed on the basis of clinical features. This fell to 37.5% of mosaic Down syndrome patients being diagnosed clinically (p < 0.001). Simian crease, sandal gap, epicanthic folds, hypotonia, upslanting palpebral fissures, and protruding tongue are the most frequent characteristic features seen. Similarly epicanthic folds, protruding tongue, simian crease and sandal gap, hypotonia, and upslanting palpebral fissures are also described in a significant proportion of karyotypically normal individuals, thus arousing a suspicion of Down syndrome. 89.4% of patients were diagnosed between day 1 and 7 of life. Of 10.6% patients diagnosed after day 7 of life, 7.6% were adults and 3% children. The minimum prevalence was estimated at 167.9 per 100,000, or 1 in 595 births.
CONCLUSION: In a defined population, with a prevalence of around 1 in 600 births, accurate clinical diagnosis occurred in 90%, 100%, and 37.5% of trisomy, translocation, and mosaic patients. 49.5% of patients had one or more of the following phenotypic findings: Simian crease, sandal gap, epicanthic folds, hypotonia, upslanting palpebral fissures, and protruding tongue. However, the same six features aroused a suspicion of Down syndrome in individuals with normal karyotyping, thus causing undue stress and worry to parents. Mosaic cases may be more common than previously recognised, and often do not have dysmorphic features. It is therefore a diagnosis that should always be considered in those who are educationally subnormal without a definitive diagnosis.