Diagnosis of cutaneous disorders can be done using 2 different methods: (1) visual pattern recognition, a rapid, intuitive, nonanalytic method, or (2) an analytic method using algorithms and decision trees. In actual practice most clinicians use both methods.^1,2

Clinicians utilizing pattern recognition rapidly identify the cutaneous findings and compare them with 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 in previous 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 EKG interpretation⁴ and in dermoscopy,⁵ it 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.³ An experienced clinician can usually tell a student what cutaneous findings led to his or her diagnosis, but there may be other subtle, but 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:

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

  
  
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  Location of lesions

  
  
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  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 Section Two of this book.

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 (eg, potassium hydroxide [KOH] examination and skin biopsies).^2,3 These are used as the basis for searches in differential diagnosis lists or decision tree algorithms. An analytic method is helpful in complex cases with atypical or numerous cutaneous findings and systemic complaints. Section Three of this book contains lists of differential diagnoses of skin diseases in various body regions based on the patient’s history, lesion morphology, and laboratory results. It also contains differential diagnosis lists for purpura, pruritus, rash and fever, and leg ulcers.

Both strategies for diagnosis rely heavily on identification and classification of the primary lesion(s). However, there are several pitfalls in the identification of primary lesions. These include:

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  Excoriations may alter or partially destroy the primary lesions (Figure 5-2).

  
  
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  Vesicles, bullae, and pustules may easily break, leaving only erosions or erythema (Figure 5-3). Also, vesicles may develop into pustules as in some case of herpes simplex and zoster.

  
  
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  Postinflammatory hyperpigmentation and hypopigmentation may obscure the primary lesion (Figure 5-4).

  
  
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  The examination may take place too early or too late in an evolving skin disease. For instance, many skin rashes present with pink macules in the first 1 to 2 days and then evolve into their more characteristic findings. As an example, herpes zoster could evolve from a pink macule/patch → pink plaque → plaque with vesicles → bullae → crusts → erosions → scars (Figure 5-5).

  
  
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  Treatment may alter the skin findings, for example, the use of a topical antifungal cream for tinea corporis may remove the characteristic scale or make it difficult to find fungal hyphae on a KOH examination.

  
  
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  Lastly, many common diseases present with multiple types of primary lesions. For example, atopic dermatitis may present with macules, patches, papules, plaques, vesicles, and pustules and with surface changes that may include scale, crust, lichenification, fissures, erosions, and excoriations.