The study of disease is the study of patterns. Making a diagnosis is a process of matching a patient’s presentation to a set of signs, symptoms, and findings that represent a defined biologic process. This matching process allows you to offer interventions and make prognoses on the basis of your observations.
Students beginning their study of disease would do well to create a mental template –an outline– for each disease that makes recognizing its pattern easier. You begin to understand a given disease once you are able to define it and describe its prevalence, pathology, presentation, differential diagnosis, and associated methods of treatment and prevention. With this information in hand, the novice will be appropriately knowledgeable and ideally receptive to new information as it becomes known.
Definition For each disease, construct a one-paragraph description of the problem using terms and concepts understandable to an educated layperson. If you must use medical terms (ischemia, for example) be certain that you clearly understand the definition; don’t define a buzzword with another buzzword. Also, remember that the precise medical meaning of some words differs from the vernacular. Inflammation, for example, connotes a specific biologic process. Do not use inflammation as a synonym for irritation or swelling.
Prevalence It may seem that the prevalence of a disease is interesting only to public health officials, but that’s not so. Prevalence is germane to clinical decision making for individual patients. That’s because most diagnostic tests do not establish or exclude the existence of a disease; rather, they simply refine the estimate of its likelihood. The chance that a disease is present after a test result is known depends not only on the power of the test but also on the probability of disease before the test. Prevalence of disease is often the best first estimate of that pretest probability. For example, you may wonder whether a mass that appears on an MRI scan is a ganglion cyst or a synovial cell sarcoma. Even without scrutinizing the images, you will soon know that a ganglion is much more likely because it is much more common. The pearl to recall is “When you hear hoof-beats in Texas think horses, not zebras.”
Pathology From the student’s perspective, pathology is the most essential information: a description of processes gone wrong. The key to understanding how things can go wrong, of course, is to have a strong sense of how things can go right. For example, to understand that a parathyroid adenoma may cause hypercalcemia, you must first understand the basics of calcium homeostasis. When learning pathology, try to correlate the disturbances in normal anatomy and physiology with the clinical manifestation of disease. That, unfortunately, may not always be possible. For instance, do not simply assume that arthritis is painful because two rough joint surfaces rub together. Although that may make intuitive sense, we know that many patients with rough joint surfaces are asymptomatic.
Presentation. Try to create a mental image of the presentation of each condition. Naturally, it will be easier to do this once you have actually seen patients with the disease. To summarize the presentation of musculoskeletal conditions, I use the mnemonic SOIL: Subjective data (the patient’s symptoms); Objective physical findings of the disease; Imaging (most often x-rays); and Laboratory data. Of course making diagnoses can be more complicated than simply recognizing a specific SOIL pattern. You must have the wisdom to order the proper tests or perform the right maneuver on physical examination. Obviously, you will not diagnose Paget’s disease by detecting an elevated serum alkaline phosphatase level if you fail to order that test.
Differential Diagnosis. To make a correct diagnosis, you must first contemplate all of the possibilities. If you do not consider a condition, it is highly unlikely that you will recognize it. To do so, you can of course refer to preprinted lists—all conditions that cause knee pain in middle-aged people, for example—but it is more effective (not to mention educationally useful) to use lists of your own creation. Therefore, when you study a disease, learn its differential diagnosis—that is, the list of conditions that may occur with similar symptoms. This will not only help you when you are forced to make a diagnosis, but will allow you as you learn about a disease to concentrate on those features that make it unique.
Treatment and Prevention. As a medical student, your studies do not focus specifically on treatment and prevention. Nonetheless, the study of therapeutic modalities may help reinforce concepts of pathology. For example, if you see that sciatic pain is relieved by removal of the herniated disks that impinge on the nerve root, you can infer that the disk material causes pain. Likewise, if you see that surgery does not always eliminate pain, it is hoped that you will be motivated to question that dogma too. Studying therapeutics will help you to think like a physician, but don’t get bogged down with the details of treatment at this point. There is plenty of time for that later.
Miscellaneous facts Collecting seemingly random facts about diseases can help you remember important information. For example, you may know that English sailors have been called “limeys.” The reason for this is because all English sailors since 1795 were issued a daily ration of lime juice (or other citrus), which is rich in vitamin C. This seemingly random fact is helpful in recalling essential data about collagen biology. It turns out that Vitamin C is needed to convert proline and lysine into hydroxyproline and hydroxylysine. Without this hydroxylation, collagen cross-links break down, weakening connective tissue. This bad collagen causes many problems, including gum disease. The mental image of a toothless sailor sucking on a lime, thus, serves as a hook into a trove of biologic information.
The outlines suggested in this chapter are necessarily simple, but the subject matter of each is complex. Entire courses of study are devoted to each of them. Our challenge, as Einstein put it, was to “make things as simple as possiblebut no simpler”, a tricky balance we did not always maintain. Still, by carefully reading these chapters and constructing your own disease outlines, it is hoped that you will not only be able to acquire information rapidly, but that you will also develop a more sophisticated understanding of disease patterns as you gain additional experience.