A case report is a descriptive study that documents an unusual clinical phenomenon in a single patient. It describes in details the patient’s history, signs, symptoms, test results, diagnosis, prognosis and treatment. It also contains a short literature review, discusses the importance of the case and how it improves the existing knowledge on the subject.
A similar design involving a group of patients (with the similar problem) is referred to as case series.
Advantages of case reports
Case reports offer, in general a fast, easy and cheap way to report an unusual observation or a rare event in a clinical setting, as these have very small probability of being detected in an experimental study because of limitations on the number of patients that can be included.
These events deserve to be reported since they might provide insights on some exceptions to general rules and theories in the field.
Case reports are great to get first impressions that can generate new hypotheses (e.g. detecting a potential side effect of a drug) or challenge existing ones (e.g. shedding the light on the possibility of a different biological mechanism of a disease).
In many of these cases, additional investigation is needed such as designing large observational studies or randomized experiments or even going back and mining data from previous research looking for evidence for theses hypotheses.
Limitations of case reports
Observing a relationship between an exposure and a disease in a case report does not mean that it is causal in nature.
This is because of:
- The absence of a control group that provides a benchmark or a point of reference against which we compare our results. A control group is important to eliminate the role of external factors which can interfere with the relationship between exposure and disease
- Unmeasured Confounding caused by variables that influence both the exposure and the disease
A case report can have a powerful emotional effect (see examples of case reports below). This can lead to overrate the importance of the evidence provided by such case. In his book Against Empathy: The Case for Rational Compassion, Paul Bloom explains how a powerful story affects our emotions, can distort our judgement and even lead us to make bad moral choices.
When a case report describes a rare event it is important to remember that what we’re reading about is exceptional and most importantly resist generalizations especially because a case report is, by definition, a study where the sample is only 1 patient.
Selection bias is another issue as the cases in case reports are not chosen at random, therefore some members of the population may have a higher probability of being included in the study than others.
So, results from a case report cannot be representative of the entire population.
Because of these limitations, case reports have the lowest level of evidence compared to other study designs as represented in the evidence pyramid below:
Real-world examples of case reports
Example 1: Normal plasma cholesterol in an 88-year-old man who eats 25 eggs a day
This is the case of an old man with Alzheimer’s disease who has been eating 20-30 eggs every day for almost 15 years. [Source]
The man had an LDL-cholesterol level of only 142 mg/dL (3.68 mmol/L) and no significant clinical atherosclerosis (deposition of cholesterol in arterial walls)!
His body adapted by reducing the intestinal absorption of cholesterol, lowering the rate of its synthesis and increasing the rate of its conversion into bile acid.
This is indeed an unusual case of biological adaptation to a major change in dietary intake.
Example 2: Recovery from the passage of an iron bar through the head
This is an interesting case of a construction foreman named Phineas Gage. [Source]
In 1848, due to an explosion at work, an iron bar passed through his head destroying a large portion of his brain’s frontal lobe. He survived the event and the injury only affected 1 thing: His personality!
After the accident, Gage became profane, rough and disrespectful to the extent that he was no longer tolerable to people around him. So he lost his job and his family.
His case inspired further research that focused on the relationship between specific parts of the brain and personality.
- Sayre JW, Toklu HZ, Ye F, Mazza J, Yale S. Case Reports, Case Series – From Clinical Practice to Evidence-Based Medicine in Graduate Medical Education. Cureus. 2017;9(8):e1546. Published 2017 Aug 7. doi:10.7759/cureus.1546.
- Nissen T, Wynn R. The clinical case report: a review of its merits and limitations. BMC Res Notes. 2014;7:264. Published 2014 Apr 23. doi:10.1186/1756-0500-7-264.