Objective: To explore the prevalence of rare diseases in the Continuous Morbidity Registration (CMR) Nijmegen, and to discuss methodological difficulties in the study of rare diseases in general practice.
Methods: We selected all diseases with a prevalence <0.5/1000 patients/year between 1986 and 2006 in the CMR, and we compared the results with rare diseases in the Orphanet database of rare diseases.
Results: We retrieved 71 codes of rare diseases: 15 referred to trauma or intoxications; the remaining 51 could be subdivided into malignancies, specific infections, diseases of short duration, and chronic diseases. Twenty (36%) of these 56 diseases were also listed in the Orphanet database (mostly malignancies and chronic diseases).
Conclusion: Although in the literature emphasis is placed on rare chronic diseases, the general practitioner is also confronted with many rare diseases of shorter duration. The CMR is not yet feasible for studying diseases with an extremely low prevalence.