Since 80s, scientific, programming and public opinion areas have started to consider rare or orphan diseases issue. Rare diseases are morbid conditions characterised by low prevalence, but they are also little known, little studied and they often lack of an adequate therapy. They are often called orphan diseases because of their low desirability for clinical and experimental researches.
The rational to group more clinical features inside the same group, with the common denominator of a population low prevalence rate, consists in their peculiarity of presenting common clinical and welfare problems. One of their main features resides in the specialistic and continuative care they require, which realisation will result impossible without a considerable public support participation. The rareness of the disease results in a low research interest at an etiological and pathogenic level, in a high difficulty of describing the natural history and planning clinical researches, in a low market that can damp the costs of a specific pharmacological research, and finally in an insufficient or very insufficient knowledge spreading, however available in the current practice.
In other words, the low prevalence rate of these diseases causes a lower knowledge progress than theoretically possible and it leads to a non-application of what it is already known in current practice. Therefore, patients could suffer a double harm: being affected by a severe pathology, and being insufficiently recognized,diagnosed and cured. For these reasons, the adjective "rare" has been replaced by the "orphan" one, meaning a lack in attention and resources. And because of these considerations, many of the resources have been addressed to research and welfare systems. This process, started in the United States, has involved all western Countries up till influencing WHO world policies, and recently also Italy. These policies show the necessity to evaluate exactly whole rare diseases impact on population and health services: it constitutes, in fact, the multiplying factor of all available actions and facilitation in rare diseases management.
Since rare diseases do not constitute a recognised nosological group, but just an heterogeneous one with the common factor of a low prevalence rate, it is obvious that they can include a non specified number according to the population frequency cut-off value used to define a pathology as rare.
It does not exist a world-wide consensus definition of rare disease. The USA Congress has stated in 200,000 inhabitants throughout the United States (1/1,200) the maximum number of patients for a pathology to be defined as a rare disease; this is the only existing definition and the most commonly used one. According to the Community Action Program on rare diseases (1999-2003), a prevalence rate lower than 5/10,000 inhabitants of the community population defines a disease as rare. Other Countries have chosen other definitions. For instance, the Japanese law considers as rare a pathology that affects less than 50,000 patients (4/10,000) in Japan. Nowadays, in Italy a stated definition of rare disease does not exist: different parameter rates have been used by various institutes and associations. Rare disease definition considered by the Italian National Health Plan varies from 1 out of 20,000 to 1 out of 200,000 inhabitants resulting in 5,000 pathologies, equal to 10% of the whole existing disorders. The USA Congress definition has been used by the Mario Negri Research Institute for rare diseases in Bergamo. Considering the Italian medical literature several different definitions of rare diseases can be found. For instance, some Italian Authors define as "orphan" those conditions that affect less than 60,000 patients, approximately about 0.1% of the population. The population rates are unhomogeneous due to the different definitions assumed by all these sources.
World Health Organization counts, as rare, at least 5,000 diseases and syndromes. Most of them are caused by a genetic anomaly. Genetic factors, environmental conditions, pathogens propagation, and living habits strongly influence diseases prevalence rates: many disorders are rare in some geographic areas or in some populations and more frequent in others. However, there is a deep lack on rare diseases prevalence data, since only for few of them it exists a case system at a national or international level. Currently a single exhausting classification neither at an Italian nor at a European level does not exist. There have been published various lists of rare diseases related to the assumed definition, or to a specific interest of an association or a working group. Although in the United States a rare disease stated definition already exists, the available American classifications differ from 1,109 diseases counted and described by the National Organization for Rare Disorders (NORD), to 2,117 diseases of the Office of Rare Diseases (ORD) of the National Institute of Health (NIH). The French plan Orphanet suggests a list of approximately 5,000 items, including synonyms, of rare diseases in alphabetical order. The Italian National Institute of Health, acting on commission of the Ministry of Health, has published a rare diseases list divided into categories (for instance, metabolic and genetic disorders, infectious diseases, etc.) for which the Health system will provide the exemption for correlated sanitary performances expenses.
All these classifications include pathologies strongly different for their prevalence, prognosis, care typology and arising age. In the wider rare diseases classifications, there are some very heterogeneous pathologies groups:
According to what written above it is compulsory to underline the lack of an exhaustive and univocal classification necessary to subdivide those pathologies in useful categories to the aim of health and welfare care interventions.