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Principles of dermatology

Last updated: October 20, 2021

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Dermatology is the branch of medicine concerned with the skin, hair, and nails, as well as the conditions associated with them. Basic knowledge of dermatology is essential for every physician, as approximately 50% of skin-related consultations are initially assessed by non-dermatologists. In the United States, the most common conditions seen by dermatologists include acne, actinic keratoses, non-melanoma skin cancers, benign tumors, and contact dermatitis. Skin lesions may be primary or secondary. Primary lesions (e.g., macules or papules) appear as a direct result of a disease process. Secondary lesions such as scales or ulcers may develop from primary lesions or result from external trauma (e.g., infections, scratching). Dermatological conditions can often be diagnosed based on patient history and physical examination but may require laboratory testing or biopsy to confirm the diagnosis. Dermatological diseases are managed with medication (topical and systemic) and procedures such as surgery, cryotherapy, radiotherapy, or phototherapy. Topical treatments are often the first choice because they cause fewer systemic side effects and are easily administered.

  • Goal: Examine the skin (including of the hands, mouth, and scalp) and nails to help in determining a working diagnosis or differentials and any potential diagnostic/management steps based on observations.
  • Examination techniques
    • Inspection
    • Palpation: evaluation of consistency (e.g., softness, firmness) and depth
    • Typical skin tests as indicated
    • Dermatoscope inspection as indicated

The hands, mouth, scalp, and nails should not be overlooked during a dermatological examination.

Skin examination [1]

  • Determine the type of lesion: See primary skin lesions, secondary skin lesions, and complex skin lesions below.
  • Describe lesion characteristics
    • Location
    • Number (single/multiple)
    • Size
    • Color: e.g., pinkish discoloration
    • Texture: e.g., atrophic, calloused, crusty, verrucous
    • Shape: e.g., round, oval, annular
    • Distribution
      • Symmetric/asymmetric
      • Unilateral/bilateral
      • Diffuse/grouped
    • Secondary changes (e.g., as a result of scratching)

Nail examination

Alterations of the nails
Clinical findings Possible underlying disease
  • Nail clubbing
  • White, grey, or yellowish color with dullness and crumbling of the nail

Examination of the hands

Alterations of the hands
Exam technique Clinical findings Possible underlying disease

Many systemic diseases can manifest with findings on the patient's hands.

Overview of most common primary skin lesions
Primary Lesions Description


Patch (dermatology)
Nodule (dermatology)
  • An elevated lesion, > 1 cm in both diameter and depth
Plaque (dermatology)
Vesicle (dermatology)


Overview of most common secondary skin lesions
Secondary Lesions Description
Scale (dermatology)
Fissure (cleft)
Ulcer (dermatology)
  • Rounded or irregularly shaped deeper lesions that result from loss of the epidermis and some portion of the dermis.
Excoriation (scratch marks)
  • Abrasion produced by mechanical force, usually involving the epidermis (but sometimes reaching the outer layer of the dermis)
Skin atrophy
  • Composed of new connective tissue that has replaced lost substance
  • An overgrowth of scar tissue manifests as keloid (thickened, raised tissue that grows beyond the borders of the scar and shows no regression).


Overview of complex skin lesions [4]
Complex lesions Description



  • Caused by bleeding into subcutaneous tissue, muscle, organ tissue or a cavity
    • Immediately after trauma: red
    • After 24–96 h: dark red; purple; blue/black
      • Cause: The blood coagulates and hemoglobin is degraded into bile pigment.
    • After 4–7 days: dark green
    • After 7 days: yellow; brownish
Subtypes of hematomas = purpura Nonpalpable purpura


  • Flat, red-purple, pinpoint lesions < 3 mm in size

  • Flat, red-purple, larger form of petechiae, > 5 mm in size
Palpable purpura
  • Raised, red-purple lesions
Rashes Exanthem
  • Extended uniform rash (localized or generalized)
  • Reddening of the skin as a result of vasodilation (blanches if pressure is applied)
  • Generalized reddening of the skin
Maculopapular rash
Further lesions Lichenification
  • Thickening of the skin with accentuated skin markings

Histopathologic finding Characteristics Examples

Treatment options

The external nature of the skin allows a variety of treatment options, including:

  • Systemic medications
  • Topical medications
  • Physical procedures
    • Surgery
    • Cryotherapy (treatment that uses liquid nitrogen to expose abnormal tissue to very low temperatures and destroy malignant or premalignant cells; often used to treat skin lesions)
    • Radiotherapy
    • Phototherapy

Types of topical preparations [6]

Drugs must be absorbed into the skin to be effective, so choosing the proper type of topical preparation for the pharmacological agent is important. Examples include:

  • Creams
  • Ointments
  • Lotions, foams, and gels

Topical steroids

Topical steroids are the most frequently used topical treatment in dermatology.

  • Advantages
    • High therapeutic value
    • Relatively safe: few local and systemic adverse effects
  • Most common side effects
  • Common examples
Agent Potency Indication
Hydrocortisone (1%) Low For mild and chronic dermatoses and for use on the face
Triamcinolone (0.1%) Medium For intermediate severity
Clobetasol (0.05%) High For more severe dermatoses
  1. Principles of Dermatological Practice - Examination of the Skin CME. Updated: January 1, 2008. Accessed: September 3, 2017.
  2. Freeman et al.. Corns and Calluses Resulting from Mechanical Hyperkeratosis. American Family Physician. 2002 .
  3. Description of Skin Lesions. Updated: June 1, 2016. Accessed: May 15, 2017.
  4. Primary Care Dermatology Module, Nomenclature of Skin Lesions. Updated: May 15, 2017. Accessed: May 15, 2017.
  5. Leung AK, Chan KW. Evaluating the child with purpura. Am Fam Physician. 2001; 64 (3): p.419-428.
  6. Principles of Topical Dermatologic Therapy. Updated: March 1, 2017. Accessed: September 3, 2017.
  7. Marks JG Jr, Miller JJ . Lookingbill and Marks' Principles of Dermatology. Saunders Elsevier ; 2013
  8. Amirlak B. Skin Anatomy. In: Caputy GG, Skin Anatomy. New York, NY: WebMD. Updated: July 18, 2015. Accessed: May 15, 2017.
  9. Structure of Normal Skin. Updated: May 15, 2017. Accessed: May 15, 2017.
  10. Layers of the Skin. Updated: May 15, 2017. Accessed: May 15, 2017.
  11. Zhang S-X. An Atlas of Histology. Springer Science & Business Media ; 2013