Introduction: Congenital upper limb anomalies (CULA) exhibit a wide spectrum of phenotypic manifestations. To describe the full nature of all anomalies, the Oberg, Manske, and Tonkin (OMT) classification was developed. The OMT classification aims to provide a classification which better fits the current concepts of the etiology and allow combined documentation of all anomalies present in a single limb. The introduction of the OMT classification was succeeded by multiple validation studies regarding its clinical application. However, all studies were mainly aimed to test usability of the classification and were not consistent in the classification strategy: In some studies, all anomalies were classified as suggested by the initiators, whereas others only used one diagnoses per limb. Therefore, the aim of this study was to evaluate the various classification strategies used with the OMT classification. Methods: We retrospectively reviewed all patients visiting the Erasmus MC Team for Congenital Hand and Upper Limb Malformations in the Sophia Children’s Hospital between December 2014 and February 2012. All congenital anomalies of both limbs individually were classified according to the OMT scheme as proposed by Tonkin et al; furthermore, all associated anomalies were recorded. In compound hand phenotypes, every hand anomaly present was classified to best describe the clinical features of every patient. To evaluate which hand anomalies commonly required multiple diagnoses used within one patient, the mean number of diagnosis and 95% confidence interval (CI) were calculated and plotted for each OMT class and diagnose specifically. To evaluate whether multiple OMT-term use was effective to discriminate different cohorts of patients, the group characteristics of patients requiring one OMT diagnosis versus patients requiring multiple OMT diagnoses were studied. Results: In total, 736 patients were eligible for inclusion and 953 OMT diagnoses were registered; 449 were unilateral (219 left-handed, 230 right-handed) and 504 were bilateral. The average number of diagnosis for each patient was 1.29 (95% CI, 1.25-1.34). In 20.5% of the patients (n = 155), a combination of OMT diagnoses was used to describe the observed anomalies. The total number of unique combinations of OMT diagnosis was 101, of which 77 combinations were only used once and 24 were used repeatedly. We illustrate that in radial polydactyly, additional OMT diagnosis was often used to convey differentiating characteristics, such as additional ulnar polydactyly, the combination of which was more often observed in patients with a syndromal cause of the anomaly. Conclusion: We conclude that the strategy used in applying the OMT classification is of eminent importance for optimizing the discriminating features of the OMT classification. As suggested by the initiators, all terms necessary to convey the full nature of the anomaly should be used. We illustrate our point using case examples for radial polydactyly.
