White coats and fingerprints: diagnostic reasoning in medicine and investigative methods of fictional detectives

Fig A “Obvious is obvious. Look elsewhere.” In a patient presenting with prolonged chest pain, the obvious clue provided by the persistent T wave inversions (top) could automatically lead the clinician into a diagnosis of non-ST elevation myocardial infarction. But why was the mediastinum widened in the chest x ray (middle)? And why was the left radial pulse missing? Type A aortic dissection could be—and actually was—the unifying diagnostic hypothesis, as became apparent at transoesophageal echocardiography (bottom); obstruction of both coronary ostia during diastole by the intimal flap was the main mechanism of myocardial ischaemia in this patient. FL=false lumen; TL=true lumen

Fig B Knowledge as a diagnostic value. A typical echocardiographic image of dilated cardiomyopathy (upper portion) is the overt clue. However, “dilated cardiomyopathy” should be the starting point rather than the final diagnosis. Proceeding along the investigative pathway, medical knowledge combined with capacity of direct observation (left column) allows an aware clinician to reach more sophisticated diagnoses (right column)
 

Fig C Spotting inconsistencies. The echocardiogram (top) clearly shows an abnormally thickened left ventricle in a normotensive patient with congestive heart failure and no valvular disease. The most obvious diagnosis at this point would be “non-obstructive hypertrophic cardiomyopathy.” But is the complete absence of left ventricular hypertrophy in the electrocardiogram (middle) consistent with this diagnosis? Probably not. Spotting this inconsistency should prompt the clinician to reconsider the whole diagnostic edifice and reach the correct final diagnosis of cardiac amyloidosis, as shown by the typical apple-green birefringence on Congo-red staining of the myocardial biopsy (bottom)
 

Fig D Literal adherence to guidelines (non-ST elevation myocardial infarction (NSTEMI), “Inspector Lestrade type”). What is the diagnosis in this 65 year old man with one hour of chest pain, raised troponin, and ST depression in anterior leads on electrocardiography (top left)? Overliteral application of current guidelines could lead to an incorrect diagnosis of NSTEMI. However, ST depression is unusually pronounced in V3-V5 and absent in peripheral leads. This active perception should stimulate the “curiosity” of the clinician and prompt a recording of the V7-V9 leads (top right), revealing the correct diagnosis of true posterior STEMI due to proximal obstruction of the left circumflex artery (bottom)