Chapter 5: Principles of Diagnosis

INTRODUCTION TO CHAPTER

Diagnosing a cutaneous problem can be challenging compared to other organ systems. Blood studies, imaging, and other standardized testing are usually not that helpful in diagnosing dermatologic diseases as they may be for example in endocrinology, cardiology, or orthopedics.

Fortunately, the skin can be easily examined directly without using special technologies or invasive procedures. Some of the common skin diseases were accurately described and categorized in great detail over 2000 years ago.¹

Diagnosis of cutaneous disorders can be done using two different systems, system 1, a rapid, intuitive, non-analytical method primarily based on visual pattern recognition, or system 2, an analytic method using algorithms and decision trees.^2–4 In general, more experienced clinicians use system 1 and novices use system 2 for diagnosis.²

PATTERN RECOGNITION FOR DIAGNOSIS

Clinicians utilizing pattern recognition rapidly identify the cutaneous findings and compare them to a set of images stored in their long-term memory (sometimes called the "blink" diagnosis).^2,3 These images are typically from clinical findings seen earlier in patients or from pictures in textbooks and other sources. Pattern recognition is more effective in common disorders with typical presentations and in the hands of more experienced clinicians. However, studies have shown that in electrocardiogram (ECG) interpretation⁵ and in dermoscopy,⁶ pattern recognition can be an effective diagnostic strategy even for less-experienced clinicians.

Human beings and all animals are hardwired for visual pattern recognition. Otherwise, we could not easily and quickly identify each other, objects, or predators. We know that pattern recognition can be learned, but can it be taught? One of the problems is that much of visual pattern recognition occurs subconsciously.² An experienced clinician can usually tell a student what cutaneous findings led to their diagnosis, but there may be other subtle, important factors, not easily elucidated that also contributed to the diagnosis.

The most common features of skin disorders used in pattern recognition include the following:

• The morphology of the primary lesion, its surface changes, color, and size.

• Location of lesions.

• Configuration of lesions.

Many common skin disorders have characteristic features. For example, pink plaques with silvery scale on knees and elbows are characteristic for psoriasis (Figure 5-1). These patterns are covered in more detail in individual diseases in Section II.

ANALYTIC METHOD FOR DIAGNOSIS

An analytic method for diagnosis is slower and more methodical. It utilizes a step-by-step evaluation of the patient's history, the physical examination findings, and results of diagnostic tests (e.g., potassium hydroxide (KOH) examination and skin biopsies).^2,3 These are used as the ...