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Year : 2022  |  Volume : 67  |  Issue : 1  |  Page : 62-64
Rare case reports: A rarity?

1 Department of Oral Medicine and Radiology, Dr. BR. Ambedkar Institute of Dental Sciences, Patna, Bihar, India
2 Department of Oral Pathology and Microbiology, Govt. Dental College, Dibrugarh, Assam, India
3 Department of Oral Medicine and Radiology, Govt. Dental College, Kozhikode, Kerala, India

Date of Web Publication19-Apr-2022

Correspondence Address:
Simi Thankappan
Department of Oral and Maxillofacial Physician and Radiologist, Kochi, Kerala
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijd.ijd_197_21

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The basic principle of science is to always think of the simplest explanation for a condition. But in post graduate training, distinguishing a rare case from a common case is a dilemma for the naïve clinician. Often the eager and enthusiastic resident trainees are more prone to fall into this “rare case phenomenon.” Innumerable rare case reports in countless scientific journals may give the false impression to the budding trainee that uncommon cases are encountered every now and then. This article stresses on the importance of applying certain rules that can help in making the diagnostic process systematic and simpler. The chance of encountering rare cases are in fact rare and limited, wherreas in some circumstances, can diagnose life threatening conditions too.

Keywords: Case reports, Occam's razor, rare diagnosis, rule of parsimony

How to cite this article:
Thankappan S, Nedumpillil S, Thomas V. Rare case reports: A rarity?. Indian J Dermatol 2022;67:62-4

How to cite this URL:
Thankappan S, Nedumpillil S, Thomas V. Rare case reports: A rarity?. Indian J Dermatol [serial online] 2022 [cited 2023 Jun 4];67:62-4. Available from:

   Introduction Top

A common and often repeated aphorism, constantly hammered at postgraduate students during their training is

“When you hear hoofbeats, think horses; not zebras!!”

This witty statement underlines the basic principle of science; to always think of the simplest explanation for a condition.[1],[2] It is often a conundrum for the trainee to distinguish a rare case from a common case. Add to this the perceived opportunity of treating and/or publishing a rare case and the scales of balance start to tilt away from rational scientific thinking. Innumerable rare case reports in countless scientific journals may give the false impression to the budding trainee that rare cases are the norm. However, it has to be understood by the trainee that the only reason a rare case report is getting published in the first place is because it is rare. And therefore, chances of a resident encountering such a case are limited.

Although no one in the medical field is immune to this phenomenon, it is often the eager and enthusiastic resident trainees who are more prone to fall into this trap of the “rare case phenomenon.” Understanding some common facts can help to avoid the possibility of a simple case being perceived as a rare diagnosis. Diagnosis is a specialized skill the clinician acquires over the years by a systematic and rigorous process of analysis of available data. Some diagnostic rules can help organize and guide the diagnostic decision-making process.

Diagnostic rules to remember

  1. The Rule of Parsimony also known as 'Occam's Razor,'[1],[3],[4],[5] suggests that the diagnostician should prefer a single provisional diagnosis, and if that is impossible, a differential diagnosis is given.
  2. The Rule of Diagnostic Hierarchy[3] suggests that the most severe diagnoses should be prioritized to be ruled out in descending order.
  3. The Rule of Chronology[3] suggests that priority should be given to the disorder that was present for the longest duration.
  4. The Rule of Safety[3] applies when the former rules do not. When a clear diagnosis is not possible, go for the “safest” diagnosis that ranks the highest for treatment possibilities and good prognosis.
  5. The Principle of Percentage[3] suggests that when numerous differential diagnoses are present, give priority to the diagnosis that is more frequently found in that particular population.
  6. The Principle of the Best Data[3] suggests that the decision-making process should be based on the credibility of the data.

Concept of ideal diagnosis and rare diagnosis

A framework of logical steps can help make the complex diagnostic process manageable. The diagnostic process can be compared to that of a funnel that is wide open with different diagnostic conditions at the beginning, but systematically narrows down as most of these conditions are ruled out through observations and logical reasoning.

Experienced clinicians depend relatively less on the present picture (as it can be highly variable) and pay more attention to the clinical picture in relation to history. But this is not the same for resident trainees who have less clinical experience and more experience in theory. Immediately diagnosing a condition because the patient has all the features specified for a particular condition given in the textbook you have read recently, is not the preferred diagnostic approach. We call this “Aunt-Minnie” approach in radiology, which is pattern recognition according to a picture in the radiology book and diagnosing a clinical condition. For all residents, we have to stick to the Socrates method, that is, systematic case taking in diagnosis.

Essence of avoiding the 'rare disease' mindset

Understanding these common facts can help to avoid the possibility of a rare disease diagnosis being given wrongly.

Rule 1: “Common diseases occur commonly and rare ones are rarely seen”

It does not need to be stated that common diseases are common because they are seen more frequently while rare diseases are rare in the first place due to the fact that they are seen rarely. So, applying the law of probability, most of the time, what you may be seeing is a common disease. The concept of rarity can also be geographic; with some diseases affecting people in a particular area only and very rarely occurring in other parts of the world.

Rule 2: “Never let past, particularly recent past experiences guide you”

The mind is a very flexible thing easily influenced by past experiences. If a clinician has seen a patient with non-specific cutaneous features, and a clinical diagnosis of a rare skin disease/syndrome has been given by that clinician, which comes out as a confirmatory diagnosis, it gives confidence to that clinician. This past event can preclude his judgment in the future when he diagnoses diseases with even straightforward clinical features.

When faced with a rare clinical condition, the clinician can ask three simple questions to himself:

  1. Has past experience guided my judgment?
  2. Did I lead or influence the patients' answers?
  3. Is my experience with such cases limited?

If the answer to any of these questions is in the affirmative then it is best that the diagnostic process be started all over again with a fresh approach or the patient be referred to someone having a better experience with such cases.

Rule 3: “Think in terms of 'rare presentation of common disease' rather than 'common presentation of rare disease”

The first step in diagnosis is to look for the common presentation of a disease. If that is not evident at first, then look for an uncommon clinical presentation of a common disease. For example, common skin conditions like lichen planus or pemphigus should be the first thing a clinician should think of rather than something like Darier's disease. A rare diagnosis, if correct, can even be life-saving but it should, however, be the exception, rather than the rule.

Rule 4: “Not everything we are taught is correct”

The word of a professor or senior staff is powerful. Superiors hold great power in the eyes of trainees because they control evaluation. Healthy skepticism can be a great asset in a trainee and can encourage the faculty to seek evidence for their statements.

Sometimes, we need to think rare…

This happens when one comes across a case which defies all possible common explanations. Diligently sticking to the simple approach can only be detrimental to the patient's interests. The delay in providing the correct diagnosis in such cases can even lead to life-threatening complications.

Another thing to consider is the growing incidence of health tourism as well as the migration of doctors to foreign lands. This increases the chance of a doctor encountering a disease in some foreign land which may be rare in his/her own country or foreign patients coming to India with a disease which may be rare here. Thus, the concepts of common and rarity no longer apply in the same old way in a fast-shrinking world.

   Conclusion Top

The practice of medicine demands that the patient be given the speediest, timely, and more importantly, appropriate treatment. A rare diagnosis, if correct, can even be life-saving in such a scenario. This should, however, be the exception, rather than the rule. The golden rule in diagnosis should be to “keep it simple.” A clear and unbiased approach toward diagnosis for each patient coupled with due diligence to follow procedure would help the students to stick to this golden rule and avoid the pitfalls of a rare diagnosis.

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Conflicts of interest

There are no conflicts of interest.

   References Top

Boylan K. Diagnosis made easier: Principles and techniques for mental health clinicians. J Can Acad Child Adolesc Psychiatry 2007;16:136-7.  Back to cited text no. 1
Kessel R. How doctors think: Clinical judgement and the practice of medicine. J R Soc Med 2006;99:205.  Back to cited text no. 2
Welfel ER, Ingersoll RE. The mental health desk reference: A practice-based guide to diagnosis, treatment, and professional ethics. Hoboken, NJ: John Wiley, 2001.  Back to cited text no. 3
Talley NJ, O'Connor S. Clinical Examination: A Systematic Guide to Physical Diagnosis. United Kingdom, Elsevier Health Sciences, 2013.  Back to cited text no. 4
Montgomery K. Aphorisms, maxims, and old saws: Some rules of clinical reasoning. How Doctors Think: Clinical Judgment and the Practice of Medicine. Oxford University Press, 2006. p.103-20.  Back to cited text no. 5


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