Correct diagnosis is the sine qua non for the care of any patient with any disorder. If you don’t know the diagnosis you can’t establish the cause, provide the prognosis, or institute appropriate therapy. For patients with mucocutaneous disease a working diagnosis is almost always first established on the basis of clinical symptoms and signs. If necessary, this clinical diagnosis can subsequently be confirmed through procedures such as biopsy, cytology, and microbial culture (see Chapter 3).

There are two major approaches to using clinical symptoms and signs to establish a working diagnosis: the use of visual memory and the use of a morphologically determined diagnostic algorithm. Dermatologists by virtue of repeated exposure to patients with both common and uncommon diseases almost always depend on visual memory. And, because it works for them, they consistently use that same approach when teaching medical students and other clinicians. Thus, in the classroom or at continuing medical education courses, they bombard their audience with clinical photo after clinical photo, leaving the participants with a jumbled mass of impossible to remember images of skin disease.

The use of visual memory works for the dermatologist because of sufficient, repetitive exposure to these diseases. But other clinicians, encountering fewer patients with mucocutaneous disorders, lack the opportunity for sufficient visual reinforcement to dependably recognize uncommon and rare diseases. By analogy, they are put in the position of someone attending a large gathering where they know few of the guests. In spite of many introductions, the names of those they have met quickly disappear from memory. At best, on subsequent encounter, one might remember the face as vaguely familiar but find it impossible to identify the person by name. The same problem occurs for the nondermatologist in a clinical setting. In this situation, when faced with an unrecognized mucocutaneous disorder, one might futilely turn to a standard dermatology textbook but find that the diseases in it are organized on the basis of etiology or pathophysiology. This is, of course, of no help, as one must know the diagnosis first before it is possible to make use of the material contained in such a book. As a last resort one might then end up paging through an atlas hoping to stumble on a disease that matches what was just found on examination of the patient. At best this offers a hit or miss approach and even when it is helpful, it is an inefficient and mostly inaccurate approach to diagnosis.

A better diagnostic approach for the nondermatologist is the use of a diagnostic algorithm in which the diseases are organized on the basis of clinical morphology. This type of diagnostic algorithm directs the clinician to a cluster of diseases that share similarities in appearance. Once having arrived at the correct group of diseases, reading a small amount of text describing the characteristic diagnostic features together with perusal of the associated clinical photos for each disease within that group allows one to arrive at a likely diagnosis that then can be confirmed by a biopsy or other diagnostic test. Importantly, this approach even allows a clinician to make a correct diagnosis of a disorder that he or she has never previously encountered.

Of course, to use a morphologic approach like this, one must be able to describe in dermatologic terms the features that one has seen on examination. This is not very difficult because, as is true for most “foreign” languages, a relatively small number of words can serve one’s basic needs. The dermatologic words that are required for this approach together with the diagnostic algorithm that uses them (Table 2-1) are contained below.

As is true in all fields of medicine, history and physical examination represent the first two steps in diagnosis; they are covered in this chapter. A third step, that of utilizing diagnostic procedures to confirm a clinical diagnosis, is covered in the next chapter.