Students frequently express doubt on the value of learning about rare diseases [1]. Senior and sometimes specialist clinicians often seem to reinforce this doubt, by advising students they ‘will never see’ a range of rare conditions in general practice; this despite referrals to specialists often being from general practitioners. The perceived lack of relevance of rare conditions to future careers, undermines motivation to learn about anything other than the common.
Nonetheless, cursory examination of any pathology textbook, reveals a plethora of diseases that clinicians may encounter during their careers. A large proportion of conditions described in standard texts, have prevalence rates defining them as being rare; that is a prevalence less than or equal to 1 /1,630 (less than or equal to 200,000 cases in the USA); 1/2,000, or 1/2,500 population, according to the USA, European Union and Japanese definitions respectively [2–4]. While rare diseases may be uncommon, they are many in number, so that from 3.5–5.9% of the population is thought to suffer a rare disease at any point in time [3].
A cohort of patients cared for by any given medical practitioner may be described as a ‘panel’. While the size of the panel varies depending on circumstance, it is reasonable to assume that many medical practitioners, serve panels of approximately 2,000 patients [5]. From this, a general medical practitioner can expect to encounter roughly one case per year, of a spontaneously arising condition with a prevalence of 1/2,000, the threshold for the European Union definition of a rare disease. Should a clinician practice for 30 years, then that single practitioner could expect to encounter one case of a disease during their career, with a prevalence of 1/60,000. Considering a cohort of 200 medical students, all graduating into 30 years of clinical practice, it would be expected that 12 graduates from that group, would encounter one patient each, suffering a condition with a prevalence of 1/1,000,000. Recalling that there are numerous diseases with very low prevalence, it becomes clear that the career numerical probability of encountering a patient with a rare disease is remarkably high. Because the authors serve a dental school, our particular interest lies in dental graduates. As such, we note that the same arguments apply for dental as do for medical graduates, while it is reasonable to estimate that dental panel size is around 1,500 [6].
The historical success of the gambling industry testifies to poor general ability to accurately estimate numerical probabilities [7]. We argue that when students seek to balance the effort of studying rare diseases, against the need to acquire other clinically important knowledge, there is an underestimation of the relevant probabilities, and unhelpful skewing of learning away from otherwise important content. We further argue that carrying this habit post-graduation into clinical practice, may blind clinicians to noticing clinical manifestations of uncommon disorders.
A posture that resists notice of rare diseases seems reinforced by the academic community itself. When confronted with conflicting pressures on teaching time, seemingly superfluous content is readily weeded out. Under-estimation of the numerical significance of low-prevalence disease progressively narrows the scope of pathology courses. One widely accepted approach to selecting which conditions should be studied by dental students, applies numerical scores for ‘commonness’, ‘significance’ and ‘uniqueness’, to select those conditions students are expected to become familiar with [8]. While this seems reasonable, it may also give both the students and faculty, false confidence in the scope of training offered, relative to ultimate clinical need.
It is impossible to know in advance, which of the many rare diseases, the patients of any given graduating clinician will suffer. However, it is entirely possible to use disease prevalence, to calculate the probability of clinicians encountering rare conditions [3]. We here describe our exploration of the career exposure to conditions ranging from the very common to extremely rare, by computer simulation of clinical careers. We also describe a class designed for first year dental students, which is structured around this computer simulation. We further report the effect of this on student perception of the importance of learning about rare diseases, assessed by in-class on-line survey.