Principles of Diagnosis and Anatomy





Key Words

skin disease, nodules, vesicles, hives, scales, ulcers, milia, comedo, comedones, excoriation, cyst

 




Skin Anatomy


The skin is divided into three layers: the epidermis, the dermis, and the subcutaneous tissue. The skin is thicker on the dorsal and extensor surfaces than on the ventral and flexor surfaces.


Epidermis


The epidermis is the outermost part of the skin; it is stratified squamous epithelium. The thickness of the epidermis ranges from 0.05 mm on the eyelids to 1.5 mm on the palms and soles. The microscopic anatomy of the dermoepidermal junction is complex; it is discussed in detail in Chapter 16 . The innermost layer of the epidermis consists of a single row of columnar cells called basal cells. Basal cells divide to form keratinocytes, which comprise the spinous layer. The cells of the spinous layer are connected to each other by intercellular bridges or spines, which appear histologically as lines between cells. The keratinocytes synthesize insoluble protein, which remains in the cell and eventually becomes a major component of the outer layer (stratum corneum). The cells continue to flatten, and their cytoplasm appears granular (stratum granulosum); they finally die as they reach the surface to form the stratum corneum. There are three types of branched cells in the epidermis: the melanocyte, which synthesizes pigment (melanin); the Langerhans cell, which serves as a frontline element in immune reactions of the skin; and the Merkel cell, which serves as a mechanoreceptor important for light touch.


Dermis


The dermis varies in thickness from 0.3 mm on the eyelid to 3.0 mm on the back; it is composed of three types of connective tissue: collagen, elastic tissue, and reticular fibers. The dermis is divided into two layers: the thin upper layer, called the papillary layer, is composed of thin, haphazardly arranged collagen fibers; the thicker lower layer, called the reticular layer, extends from the base of the papillary layer to the subcutaneous tissue and is composed of thick collagen fibers that are arranged parallel to the surface of the skin. Histiocytes are wandering macrophages that accumulate hemosiderin, melanin, and debris created by inflammation. Mast cells, located primarily around blood vessels, manufacture and release histamine and heparin.


Dermal Nerves and Vasculature


The sensations of touch and pressure are received by Meissner and Vater–Pacini corpuscles. The sensations of pain, itch, and temperature are received by unmyelinated nerve endings in the papillary dermis. A low intensity of stimulation created by inflammation causes itching, whereas a high intensity of stimulation created by inflammation causes pain. Therefore scratching converts the intolerable sensation of itching to the more tolerable sensation of pain and eliminates pruritus.


The autonomic nervous system supplies the motor innervation of the skin. Adrenergic fibers innervate the blood vessels (vasoconstriction), hair erector muscles, and apocrine glands. Autonomic fibers to eccrine sweat glands are cholinergic. The sebaceous gland is regulated by the endocrine system and is not innervated by autonomic fibers. The anatomy of the hair follicle is described in Chapter 24 .




Diagnosis of Skin Disease


What could be easier than the diagnosis of skin disease? The pathology is before your eyes! Why then do non-dermatologists have such difficulty interpreting what they see?


There are three reasons. First, there are literally hundreds of cutaneous diseases. Second, a single entity can vary in its appearance. A common seborrheic keratosis, for example, may have a smooth, rough, or eroded surface and a border that is either uniform or as irregular as a melanoma. Third, skin diseases are dynamic and change in morphology. Many diseases undergo an evolutionary process: herpes simplex may begin as a red papule, evolve into a blister, and then become an erosion that heals with scarring. If hundreds of entities can individually vary in appearance and evolve through several stages, then it is necessary to recognize thousands of permutations to diagnose cutaneous entities confidently. What at first glance appeared to be simple to diagnose may later appear to be simply impossible to identify.


Dermatology is a morphologically oriented specialty. As in other specialties, the medical history is important; however, the ability to interpret what is observed is even more important. The diagnosis of skin disease must be approached in an orderly and logical manner. The temptation to make rapid judgments after hasty observation must be controlled.


A Methodical Approach


The recommended approach to the patient with skin disease is as follows:



  • •

    History. Obtain a brief history, noting duration, rate of onset, location, symptoms, family history, medications, allergies, occupation, and previous treatment.


  • •

    Distribution. Determine the extent of the eruption by having the patient disrobe completely.


  • •

    Primary lesion. Determine the primary lesion. Examine the lesions carefully; a handheld magnifier and a dermatoscope are valuable aids for studying skin lesions. Determine the nature of any secondary or special lesions.


  • •

    Differential diagnosis. Formulate a differential diagnosis.


  • •

    Tests. Obtain a biopsy and perform laboratory tests, such as skin biopsy, potassium hydroxide examination for fungi, skin scrapings for scabies, Gram stain, fungal and bacterial cultures, cytology (Tzanck test), Wood’s light examination, patch tests, dark field examination, and blood tests.



Examination Technique


Distribution. The skin should be examined methodically. A visual scan over wide areas is inefficient. It is most productive to mentally divide the skin surface into several sections and carefully study each section. For example, when studying the face, examine the area around each eye, the nose, the mouth, the cheeks, and the temples.


During an examination, patients may show small areas of their skin, tell the physician that the rest of the eruption looks the same, and expect an immediate diagnosis. The remainder of the eruption may or may not look the same. Patients with rashes should receive a complete skin examination to determine the distribution and confirm the diagnosis. Decisions about quantities of medication to dispense require visualization of the big picture. Many dermatologists now advocate a complete skin examination for all of their patients. Some dermatologists advocate a case-by-case approach, since some patients with a focused problem, such as a plantar wart, may be reluctant to undergo a complete skin examination.


Primary Lesions and Surface Characteristics. Lesions should be examined carefully. Standing back and viewing a disease process provides valuable information about the distribution. Close examination with a magnifying device provides much more information. Often the primary lesion is identified and the diagnosis is confirmed at this step. The physician should learn the surface characteristics of all the common entities and gain experience by examining known entities. A skin-colored papule might be a wart, sebaceous hyperplasia, or a basal cell carcinoma. The surface characteristics of many lesions are illustrated throughout this book.


Approach to Treatment


Most skin diseases can be managed successfully with the numerous agents and techniques available. If a diagnosis has not been established, medications should not be prescribed; this applies particularly to prescription of topical steroids. Some physicians are tempted to experiment with various medications and, if the treatment fails, to refer the patient to a specialist. This is not a logical or efficient way to practice medicine.


Primary Lesions


Most skin diseases begin with a basic lesion that is referred to as a primary lesion. Identification of the primary lesion is the key to accurate interpretation and description of cutaneous disease. Its presence provides the initial orientation and allows the formulation of a differential diagnosis. Definitions of the primary lesions and their differential diagnoses are listed and illustrated on pp. 3 to 11 .


Secondary Lesions


Secondary lesions develop during the evolutionary process of skin disease or are created by scratching or infection. They may be the only type of lesion present, in which case the primary disease process must be inferred. The differential diagnoses of secondary lesions are listed and illustrated on pp. 12 to 16 .




Primary Skin Lesions – Macules


Macule







A circumscribed, flat discoloration that may be brown, blue, red, or hypopigmented


Hypopigmented








    • Idiopathic guttate hypomelanosis ( p. 778 )



    • Nevus anemicus ( p. 778 )



    • Piebaldism



    • Postinflammatory psoriasis



    • Radiation dermatitis



    • Tinea versicolor ( p. 520 )



    • Tuberous sclerosis ( p. 1003 )



    • Vitiligo ( p. 771 )




Brown





Blue








    • Ink (tattoo)



    • Maculae ceruleae (lice)



    • Mongolian spot



    • Ochronosis




Red








    • Drug eruptions ( pp. 551 , 560 )



    • Juvenile rheumatoid arthritis (Still disease)



    • Rheumatic fever



    • Secondary syphilis ( p. 388 )



    • Viral exanthems ( p. 539 )





Becker nevus





Erythrasma





Lentigo





Tuberous sclerosis





Phototoxic drug eruption





Idiopathic guttate hypomelanosis






Primary Skin Lesions – Papules


Papule







An elevated solid lesion up to 0.5 cm in diameter; color varies; papules may become confluent and form plaques


Flesh colored, yellow, or white





Brown





Red





Blue or Violaceous






Sebaceous hyperplasia





Basal cell carcinoma





Wart (cylindrical projections)





Wart (mosaic surface)





Nevi (dermal)





Lichen planus





Lichen sclerosus





Seborrheic keratosis





Seborrheic keratosis





Seborrheic keratosis





Melanoma





Granuloma annulare





Dermatofibroma





Flat warts





Molluscum contagiosum





Chondrodermatitis nodularis





Venous lake





Cherry angioma





Pyogenic granuloma






Primary Skin Lesions – Plaques


Plaque







A circumscribed, elevated, superficial, solid lesion more than 0.5 cm in diameter, often formed by the confluence of papules






Pityriasis rosea





Eczema





Seborrheic dermatitis





Pityriasis rosea





Syphilis (secondary)





Psoriasis





Lichen planus





Discoid lupus erythematosus





Cutaneous T-cell lymphoma





Tinea corporis





Tinea pedis





Tinea versicolor





Psoriasis





Paget disease





Sweet syndrome






Primary Skin Lesions – Nodules


Nodule







A circumscribed, elevated, solid lesion more than 0.5 cm in diameter; a large nodule is referred to as a tumor






Basal cell carcinoma





Squamous cell carcinoma





Keratoacanthoma





Melanoma





Hemangioma





Kaposi sarcoma





Cutaneous T-cell lymphoma





Prurigo nodularis





Neurofibromatosis






Primary Skin Lesions – Pustules


Pustule







A circumscribed collection of leukocytes and free fluid that varies in size






Chickenpox





Folliculitis





Gonorrhea (disseminated)





Impetigo





Keratosis pilaris





Herpes simplex





Pseudomonas folliculitis





Dyshidrosis (pompholyx)





Acne






Primary Skin Lesions – Vesicles and Bullae


Vesicle







A circumscribed collection of free fluid up to 0.5 cm in diameter





Bulla







A circumscribed collection of free fluid more than 0.5 cm in diameter






Eczema (acute)





Chickenpox





Dermatitis herpetiformis





Erythema multiforme





Herpes simplex





Herpes zoster






Primary Skin Lesions – Wheals (Hives)


Wheal (Hive)







A firm, edematous plaque resulting from infiltration of the dermis with fluid; wheals are transient and may last only a few hours






Bullous pemphigoid





PUPPP





Angioedema





Angioedema





Dermographism





Hives





Urticaria pigmentosa





Cholinergic urticaria






Secondary Skin Lesions – Scales


Scales







Excess dead epidermal cells that are produced by abnormal keratinization and shedding


Fine to Stratified








    • Eczema craquelé ( p. 109 )



    • Ichthyosis – autosomal dominant (quadrangular) ( p. 160 )



    • Ichthyosis – X-linked (quadrangular) ( p. 160 )



    • Lupus erythematosus (carpet tack) ( p. 676 )



    • Pityriasis rosea (collarette) ( p. 310 )



    • Psoriasis (silvery) ( p. 265 )



    • Scarlet fever (fine, on trunk) ( p. 530 )



    • Seborrheic dermatitis ( p. 304 )



    • Syphilis (secondary) ( p. 388 )



    • Tinea (dermatophytes) ( p. 483 )



    • Tinea versicolor ( p. 520 )



    • Xerosis (dry skin) ( p. 160 )




Scaling in Sheets (Desquamation)








    • Kawasaki disease ( p. 540 )



    • Scarlet fever (hands and feet) ( p. 530 )



    • Staphylococcal scalded skin syndrome ( p. 360 )



    • Toxic shock syndrome ( p. 550 )





Erythema craquelé (dense scale)





X-linked ichthyosis (quadrangular)





Pityriasis rosea (collarette)





Psoriasis (silvery)





Tinea versicolor (fine)





Autosomal dominant ichthyosis (quadrangular)





Kawasaki disease (desquamation)





Scarlet fever (desquamation)





Staphylococcal scalded skin syndrome (desquamation)






Secondary Skin Lesions – Crusts


Crust











    • A collection of dried serum and cellular debris; a scab



    • Acute eczematous inflammation ( p. 90 )



    • Atopic dermatitis (face) ( p. 152 )



    • Impetigo (honey colored) ( p. 331 )



    • Pemphigus foliaceus ( p. 641 )



    • Tinea capitis ( p. 500 )





Atopic dermatitis (lips)





Impetigo (honey colored)





Pemphigus foliaceus





Tinea capitis






Secondary Skin Lesions – Erosions and Ulcers


Erosion







A focal loss of epidermis; erosions do not penetrate below the dermoepidermal junction and therefore heal without scarring






Tinea pedis





Candidiasis





Neurotic excoriations




Ulcer







A focal loss of epidermis and dermis; ulcers heal with scarring






Ulcer





Chancroid





Pyoderma gangrenosum






Secondary Skin Lesions – Fissures and Atrophy


Fissure







A linear loss of epidermis and dermis with sharply defined, nearly vertical walls






Eczema





Intertrigo





Perlèche




Atrophy







A depression in the skin resulting from thinning of the epidermis or dermis






Lichen sclerosus





Morphea





Topical and intralesional steroids






Secondary Skin Lesions – Scars


Scar







An abnormal formation of connective tissue implying dermal damage; after injury or surgery, scars are initially thick and pink but with time become white and atrophic






Keloid





Herpes zoster





Porphyria





Cystic acne





Hidradenitis suppurativa






Special Skin Lesions


Excoriation


An erosion caused by scratching; excoriations are often linear





Excoriation




Comedo


A plug of sebaceous and keratinous material lodged in the opening of a hair follicle; the follicular orifice may be dilated (blackhead) or narrowed (whitehead or closed comedo)





Comedones




Milia


A small, superficial keratin cyst with no visible opening





Milia




Cyst


A circumscribed lesion with a wall and a lumen; the lumen may contain fluid or solid matter





Acne cyst





Epidermal cyst





Pilar cyst




Petechia


A circumscribed deposit of blood less than 0.5 cm in diameter





Henoch–Schönlein purpura




Purpura


A circumscribed deposit of blood greater than 0.5 cm in diameter





Sun-damaged skin




Burrow


A narrow, elevated, tortuous channel produced by a parasite





Scabies burrow




Lichenification


An area of thickened epidermis induced by scratching; skin lines are accentuated so the surface looks like a washboard





Lichenification




Telangiectasia


Dilated superficial blood vessels





Telangiectasia – rosacea





Spider angioma






Regional Differential Diagnosis Atlas


Most skin diseases have preferential areas of involvement. Disease locations are illustrated below; diseases are listed alphabetically by location on pp. 20–74 . Common diseases that are obvious to most practitioners are not included.







Diseases such as contact dermatitis and herpes zoster that can be found on any skin surface have also been omitted from most of the lists.


Anus






Warts





Eczema





Lichen planus





Inverse psoriasis





Streptococcal cellulitis





Baboon syndrome





Allergic contact dermatitis





Herpes simplex





Secondary syphilis





Anal excoriation





Candidiasis




Areola (Breast)






Acanthosis nigricans





Eczema, subacute





Eczema, subacute





Paget disease, nipple





Paget disease, areola





Seborrheic keratosis




Arms and Forearms






Atopic dermatitis





Bullous pemphigoid





Lupus erythematosus





Pityriasis alba





Eczema, subacute





Erythema infectiosum





Keratosis pilaris





Nummular eczema





Herpes zoster





Polymorphous light eruption





Neurotic excoriations





Sun-damaged skin




Axillae






Neurofibromatosis





Pustular psoriasis





Hidradenitis suppurativa



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Mar 9, 2020 | Posted by in Dermatology | Comments Off on Principles of Diagnosis and Anatomy

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