A Logical Approach TO Dermatologic Differential Diagnosis
THOMAS F. FREDDO, O.D., Ph.D., F.A.A.O.
Professor of Ophthalmology, Pathology and Anatomy
Senior Consultant in Ophthalmic Pathology
Boston Medical Center
THE GOAL OF THIS LECTURE IS TO MOVE BEYOND MERE VISUAL RECOGNITION OF
OBVIOUS DERMATOLOGICAL LESIONS TO AN ABILITY TO ANALYZE AND PROPERLY
DESCRIBE SKIN LESIONS AS A PREREQUISITE TO DEVELOPING A DIFFERENTIAL DIAGNOSTIC
APPROACH
The skin is the largest organ of the body. An understanding of the etiology, description, diagnosis and
management of skin diseases requires a thorough understanding of the basic structure of this organ.
Layers of the Skin: The skin is divided into 3 distinct layers. These are:
1. Subcutaneous tissue, 2. Dermis, and 3. Epidermis
•
Subcutaneous tissue: This layer, just above the muscle usually contains fat, except in a few
locations including the eyelid. Within this layer blood vessels and nerves pass. The deeper hair follicles and
sweat glands are found here.
•
Dermis: This layer is made up of loose connective tissue, The sebaceous glands and shorter hair
follicles are found here. This layer is subdivided into a deeper reticular dermis and a more superficial papillary
dermis.
•
Epidermis: The epidermis in most locations is approximately 1 mm thick. It is thicker at areas of
heavy wear (e.g. palms and soles) and is thinnest on the eyelids. Two categories of cells are found here:
keratinocytes and dendritic cells. The dendritic cells are of 3 major types and include melanocytes (melaninforming cells), Langerhans cells (for antigen recognition and processing) and indeterminate dendritic cells
(function unclear).
The epidermis is divided into 5 layers from bottom to TOP surface.
1. Basal cells - Stratum germinativum
2. Prickle cells - Stratum spinosum
3. Granular layer - Stratum granulosum
4. Lucid layer - Stratum lucidum
5. Horny (keratin) layer - Stratum corneum
The basal surface of the epidermis is uneven. Deeper projections of epidermis into the dermis are called
rete pegs. The dermis present between adjacent rete pegs is the papillary dermis and that below the rete pegs,
but above the subcutaneous tissue, is called the reticular dermis.
Descriptive Terms in Dermatology: Before you can diagnose you must be able to accurately describe
the lesion. Differential diagnostic lists work from these descriptive terms. Lesions are divided into primary and
secondary since the primary lesion may have been altered or obliterated by overtreatment, scratching or
infection.
Primary Lesions
Macules: Flat, circumscribed discolorations of the skin up to 1 cm in size.
Examples: freckles, flat nevi
Patches: Flat, circumscribed discolorations of the skin greater than 1 cm in size.
Examples: vitiligo, cafe-au-lait patches
Papules: Elevated, circumscribed superficial solid lesions up to 1 cm in size.
Examples: Elevated nevi, warts
(Hives and insect bites (wheal) are transitory papules)
Plaques: Elevated, circumscribed superficial solid lesions greater than 1 cm in size.
Examples: localized neurodermatitis
Nodules: Solid lesions with depth not greater than 1 cm in size. These may be above, level with or
beneath the surface. Examples: Small xanthomas, basal and squamous cell CA, melanoma.
Subcategories of nodule or tumor
Papillomatous: Having a cauliflower-like appearance.
Pedunculated: On a stalk
Sessile: Broad-based without a stalk.
Umbilicated: Having a small central crater
Tumors: Solid lesions with depth greater than 1 cm in size. These may be above, level with or beneath
the surface. Examples: Large xanthoma, basal cell and squamous cell CA, melanoma.
Vesicles: Circumscribed elevations less than 1 cm in size containing serous fluid.
Examples: Early zoster or HSV lesions, contact dermat.
Bullae: Circumscribed elevations greater than 1 cm in size containing serous fluid.
Examples: Pemphigus, second degree burns.
** Note that all such lesions on the cornea are referred to as bullae regardless of size.
Pustules: Vary in size and are elevations of the skin containing purulent fluid.
Example: Impetigo
Petechiae: Acquired, circumscribed deposits up to 1 cm in size containing blood or blood products.
Example: drug eruptions
Purpura: Acquired, circumscribed deposit greater than 1 cm in size containing blood or blood products.
Secondary Lesions
Scales: Shedding, dead epidermal cells, either dry or greasy.
Examples: Dandruff, psoriasis.
Crusts: Masses of dried skin exudates.
Examples: Impetigo.
Excoriations: Superficial abrasions. Examples: scratched insect bite.
Fissures: Linear breaks in the skin. Examples: athlete’s foot, extreme dryness.
Ulcers: Irregularly sized and shaped excavations of the skin extending below the epidermis.
Scars: Formations of connective tissue replacing tissue lost through injury or disease. Hypertrophic scars
are called Keloids and these occur more commonly in blacks.
Lichenification: Diffuse area of thickening and scaling with enhancement of skin lines and markings.
Dermatopathologic Terms and Concepts:
Histopathological Terms
Acanthosis: Thickening of the prickle cell layer
Hyperkeratosis: Thickening of the cornified layer of the skin, or development of any
keratinization on a normally non-keratinized surface such as conjunctiva
Parakeratosis: Retention of nuclei in the surface keratin layer. This is a sign of rapid turnover of
the epidermis and is usually accompanied by diminution or total loss of the granular cell layer (the stage
that is sacrificed in the rapid turnover)
Dyskeratosis: Premature keratinization of individual cells below the stratum corneum
Acantholysis: Breakdown of intercellular connections and development of open spaces in the
epithelium.