In the first chapter, two definitions of the term diagnosis were given. The term can refer to (1) the process of deciding the cause of an illness as well as to (2) the result of that process, which is a label that indicates the putative cause of the problem. The focus of this book is the first definition, that is, the systematic technique of making a diagnosis. This chapter provides a method by which a diagnosis can be made and is the method that is used throughout this book.
Principle of "Economy of Diagnoses"
If a patient has left shoulder pain and a cough, has reported paresthesias in the left hand, and on examination demonstrates a constricted pupil in the left eye, what is the likely cause? One could explain the shoulder pain as due to a rotator cuff tear, the cough due to a common cold, the hand paresthesias due to carpal tunnel syndrome, and the pupillary constriction to a congenital abnormality. Although it is possible that the problems could have been caused by four different diagnoses, health care thinking, and science in general, prefers the simplest hypothesis that can reasonably explain the multiple phenomena—in this case a Pancoast tumor of the upper lung invading into the region of the brachial plexus and the sympathetic ganglia, resulting in Horner's syndrome.
William of Occam, a 14th-century philosopher-theologian,1 is credited with enunciating the principle that when having to choose among competing hypotheses, one should favor the simplest one. Or stated differently, when multiple explanations are available for a phenomenon, the simplest version is preferred. This concept is commonly known as "Occam's razor" (where razor refers to a tool in a logical argument used to cut absurdities from philosophical discourse).
The concept of parsimony of diagnosis has not gone unchallenged. Saint's triad, by Hilliard and Weinberger, emphasizes the importance of considering the possibility of multiple separate diagnoses in a patient whenever his or her history and the results of the physical examination are atypical of any single condition.2 Although in the medical care of children, the probability of several disease processes occurring simultaneously is less likely, be aware that, as the patient ages, there is an increasing likelihood of disease developing in multiple organ systems.
Was Occam wrong? No. Occam did not state that the simpler explanation is always right or that the more complex explanation is always wrong. He emphasized that one should start from the simplest possible explanation and only make it more complex when absolutely necessary. This principle—the economy of diagnoses—has been a cornerstone of differential diagnosis and is still the best approach to making a differential diagnosis.