Introduction to Dermatology

The skin is one of the largest organs in the body, encompassing a surface area of approximately 1.8 square meters and constituting around 16% of the total body weight.

Its multifaceted roles include serving as a primary barrier that safeguards the body from harmful external elements and maintaining the structural integrity of internal systems.

The skin is structured into three main layers: the epidermis, the dermis, and the subcutis.

• Acts as a barrier against physical agents
• Offers protection from mechanical injury
• Prevents loss of body fluids
• Reduces penetration of UV radiation
• Assists in regulating body temperature
• Serves as a sensory organ
• Provides a surface for grip
• Plays a role in Vitamin D production
• Acts as an outpost for immune surveillance
• Cosmetic association

Epidermis

The epidermis is composed of stratified squamous epithelium, with a thickness of approximately 0.1 mm, although it is thicker (0.8-1.4 mm) on the palm and sole. Its primary role is to provide a protective barrier. The dominant cell type in the epidermis is the keratinocyte, responsible for producing the protein keratin. The epidermis consists of four layers that represent the progressive maturation of keratin by keratinocytes.

Epidermal Cell Types

In addition to keratinocytes, the epidermis contains various other cell types that contribute to its functions:

• Melanocytes: These pigment-forming cells are responsible for determining skin color.
• Merkel Cells: Nerve-associated epidermal cells that play a role in touch sensation and skin development.
• Langerhans Cells: Dendritic cells derived from the bone marrow, these cells are part of the mononuclear phagocyte system and are involved in immune reactions in the skin. They actively participate in antigen presentation and processing.

Dermis and Subcutis

The skin's dermis and subcutis contribute to its structure and functionality:

• Dermis: The dermis is a strong connective tissue matrix located beneath and closely connected to the epidermis. It is divided into two layers: the superficial papillary dermis and the deeper reticular layer. The thickness of the dermis varies, with regions like the eyelids having a thin layer (0.6 mm) and areas like the back, palms, and soles having a thicker layer (3 mm or more).
• Subcutis: Also known as the hypodermis, the subcutis is composed of loose connective tissue and fat. It can be up to 3 cm thick on the abdomen. This layer serves functions like insulation, cushioning, and energy storage.

Furthermore, the skin's various derivatives include hair follicles, sebaceous glands, apocrine and eccrine sweat glands, and nails.

When evaluating a patient with a skin lesion, consider the following aspects:

• Location: Determine the initial site of the lesion(s) and observe if it has spread to other areas.
• Duration: Determine if the condition has been continuous or intermittent and for how long.
• Progression: Assess whether the condition is improving or worsening over time.
• History: Inquire about previous episodes. Ask about the timing, similarity to current symptoms, and any history of other skin conditions.
• Family and Contacts: Determine if other family members, colleagues, or schoolmates are affected, which could hint at possible sources or transmission.
• Symptoms: Identify accompanying symptoms such as itching, burning, scaling, or presence of blisters.
• Associations: Inquire about any potential associations with medications, other illnesses, or external factors.
• Treatment: Ask about any treatments attempted, whether prescription or over-the-counter.

Examination

During the examination, consider the following:

• Examine Entire Skin Surface: Thoroughly examine the entire skin surface using a hand lens and proper illumination.
• Texture Assessment: Gently palpate lesions to assess their texture.

Distribution

This may give the essential clue, so a full examination is necessary.

For example, there are many possible causes for dry thickened skin on the palms, and finding typical psoriasis on the elbows, knees, and soles may give the diagnosis.

Morphology

Are the lesions dermal or epidermal? Macular (flat) or forming papules? Indurated or forming plaques? With a well-defined edge? Forming crusts, scabs, or vesicles?

Pattern

This is the overall clinical picture of both morphology and distribution. For example, an indeterminate rash may be revealed as pityriasis rosea when the “herald patch” is found.

Assess the nails, hair, and mucous membranes, sometimes in combination with a general examination (e.g. for lymphadenopathy).

Special Techniques for Verifying Skin Conditions

Clinical diagnoses of many skin conditions can be verified by a number of special techniques:

• Ultraviolet Radiation: The use of Wood’s lamp, which emits ultraviolet radiation, can cause hair and skin to fluoresce in certain fungal infections. It may also reveal vitiligo or hypopigmented macules in tuberous sclerosis.
• Microscopy: Viewing skin scrapings (treated with a potassium hydroxide solution) under a light microscope can help confirm the presence of fungal hyphae. In scabies, the mite can be extracted using a needle and viewed under the microscope.
• Smears and Swabs for Bacteriology: Collecting smears and swabs from lesions can aid in bacterial diagnosis.
• Virological Techniques: Smears from broken lesions of herpes simplex or herpes zoster can be examined microscopically using tzanck smear.
• Surgical Biopsy: Surgery can be used for both treatment (excision of lesions) and to confirm diagnoses.
• Patch/Prick Test: Used for diagnostic purposes.

Primary Skin Lesions:

• Macule: A flat, color-changing skin lesion, ≤ 1cm in diameter, not raised above the surrounding skin surface. Examples: pityriasis versicolor, vitiligo.
• Patch: A larger (>1 cm), flat lesion with a different color from the surrounding skin. It differs from a macule only in size.

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• Erythema: Vascular dilation and inflammation.

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• Papule: A small, elevated solid lesion ≤ 1cm in diameter, raised above the surrounding skin. Example: small pimple.

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• Nodule: A larger (>1 cm), firm lesion raised above the surrounding skin. It differs from a papule only in size (e.g., dermal nevus).

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• Tumor: A solid, raised growth > 5cm in diameter.

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• Hemangioma: A vascular papule or nodule known as a hemangioma.

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• Plaque: A large (>1 cm), flat-topped, raised lesion. Example: psoriasis or keloids.

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• Vesicle: A small, raised, clear fluid-filled lesion, ≤1cm in diameter, raised above the surrounding skin. Fluid is often visible, and the lesions are often translucent.

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• Bulla: A clear fluid-filled, raised, often translucent lesion > 1cm in diameter.

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• Pustule: A vesicle filled with purulent fluid. Note: The presence of pustules does not necessarily signify the existence of an infection.

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• Wheal: A raised, erythematous papule or plaque, usually representing short-lived dermal edema.

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Telangiectasia: Dilated, superficial blood vessels.

Purpura: A blue-brown discoloration of the skin due to extravasation of erythrocytes.

Petechia: Small dot-like purpura measuring 1-2mm, whereas ecchymoses are more extensive.

Lichenification: A distinctive thickening of the skin with accentuated skin-fold markings.

Crust: Dried exudate of body fluids that may be either yellow (serous exudate) or red (hemorrhagic exudate).

Milia: Small, firm, white papules filled with keratin (may resemble pustules).

Cyst: A soft, raised, encapsulated lesion filled with semi-solid or liquid contents.

Erosion: Loss of epidermis without associated loss of dermis.

Ulcer: Loss of epidermis and at least a portion of the underlying dermis.

Excoriations: Linear, angular erosions often covered by crust due to scratching.

Atrophy: An acquired loss of substance. In the skin, this may appear as a depression with intact epidermis (i.e., loss of dermal or subcutaneous tissue) or as sites of shiny, delicate, wrinkled lesions (i.e., epidermal atrophy).

Scales/Squames: Flakes on the skin which are easily detached pieces of keratin. On the scalp, it's called dandruff.

Scar: A change in the skin secondary to trauma or inflammation. Sites may be erythematous, hypopigmented, or hypertrophic depending on their age or character.

Pruritus: A sensation that elicits the desire to scratch. Pruritus is often the predominant symptom of inflammatory skin diseases (e.g., atopic dermatitis, allergic contact dermatitis).

Hyperkeratosis: Increased keratin proliferation causing skin thickening (e.g., soles of feet).

Hyperpigmentation: Increase in the color of the skin.

Hypopigmentation: Decreased skin pigmentation.

Depigmentation: Total loss of skin color due to complete absence of melanocytes.