The rare disease burden: a multidimensional challenge.

Although there no single, widely accepted definition of a "rare disease," this group of disorders includes conditions that affect only a small fraction of the population. A large number of rare diseases is caused by defined molecular defects, predominantly the occurrence of pathogenic variant(s) of genes. Thus, they are classified as "genetic diseases," among which there are many neurodegenerative disorders. Despite a low incidence of each such disease, majority of them are severe and no effective treatment is available. This creates a battery of problems for millions of people suffering from these disease as well as to their relatives and caregivers. However, the set of problems is larger; therefore, in this narrative review we summarize and discuss various kinds of problems caused by rare disease, including severe symptoms of patients, everyday problems of patients and caregivers, loneliness and social exclusion, diagnostic difficulties, unavailability of effective therapies and economic difficulties in introducing orphan drugs, and a complexity of studies on rare diseases due to low availability of biological material and complicated pathomechanisms. The global picture of the complex problems caused by rare diseases is presented.