LECTURE 1
ANATOMY, HISTOLOGY, PHYSIOLOGY OF THE SKIN. METHODS OF EXAMINATION OF PATIENTS WITH SKIN DISEASES. MORPHOLOGY OF PRIMARY AND SECONDARY SKIN LESIONS.
HUMAN SKIN HISTOLOGY
Human skin is considered the largest organ of the body and form about 16% of the body weight (1) with a surface area somewhat less than 1.5 m2 (2).
-The skin form the external surface of the human body. At the orifices of the mouth, nose and the anal canal, the skin join the mucous membrane at the muco-cutaneous junction (3).
Types of human skin (1):
1. Thick skin (Non-Hairy) which has thick epidermis and found only in the palms and soles as they are the most sites subjected to abrasions and trauma and thick skin shows characteristic parallel ridges and grooves which are called “Finger prints”.
2. Thin skin (Hairy) which has thin epidermis and covers the rest of the body.
Histological structure of the skin: – skin is composed of 3 layers
1. Epidermis
2. Dermis
3. Hypodermis (Subcutaneous fatty layer)
4.
ANATOMY OF THE SKIN
The skin functions as a protective barrier that interfaces with a sometimes-hostile environment. It is also very involved in maintaining the proper temperature for the body to function well. It gathers sensory information from the environment, and plays an active role in the immune system protecting us from disease.
Understanding how the skin can function in these many ways starts with understanding the structure of the 3 layers of skin – the epidermis, dermis, and subcutaneous tissue.
Epidermis
The epidermis is the outer layer of skin. The thickness of the epidermis varies in different types of skin. It is the thinnest on the eyelids at .05 mm and the thickest on the palms and soles at 1.5 mm.
The epidermis contains 5 layers. From bottom to top the layers are named:
stratum basale
stratum spinosum
stratum granulosum
stratum licidum
stratum corneum
The bottom layer, the stratum basale, has cells that are shaped like columns. In this layer the cells divide and push already formed cells into higher layers. As the cells move into the higher layers, they flatten and eventually die.
The top layer of the epidermis, the stratum corneum, is made of dead, flat skin cells that shed about every 2 weeks.
Specialized Epidermal Cells
There are three types of specialized cells in the epidermis.
· The melanocyte produces pigment (melanin)
· The Langerhans’ cell is the frontline defense of the immune system in the skin
· The Merkel’s cell’s function is not clearly known
Dermis
The dermis also varies in thickness depending on the location of the skin. It is .3 mm on the eyelid and 3.0 mm on the back. The dermis is composed of three types of tissue that are present throughout – not in layers. The types of tissue are:
· collagen
· elastic tissue
· reticular fibers
Layers of the Dermis
The two layers of the dermis are the papillary and reticular layers.
· The upper, papillary layer, contains a thin arrangement of collagen fibers.
· The lower, reticular layer, is thicker and made of thick collagen fibers that are arranged parallel to the surface of the skin.
Specialized Dermal Cells
The dermis contains many specialized cells and structures.
· The hair follicles are situated here with the erector pili muscle that attaches to each follicle.
· Sebaceous (oil) glands and apocrine (scent) glands are associated with the follicle.
· This layer also contains eccrine (sweat) glands, but they are not associated with hair follicles.
· Blood vessels and nerves course through this layer. The nerves transmit sensations of pain, itch, and temperature.
· There are also specialized nerve cells called Meissner’s and Vater-Pacini corpuscles that transmit the sensations of touch and pressure.
Subcutaneous Tissue
The subcutaneous tissue is a layer of fat and connective tissue that houses larger blood vessels and nerves. This layer is important is the regulation of temperature of the skin itself and the body. The size of this layer varies throughout the body and from person to person.
The skin is a complicated structure with many functions. If any of the structures in the skin are not working properly, a rash or abnormal sensation is the result. The whole specialty of dermatology is devoted to understanding the skin, what can go wrong, and what to do if something does go wrong.
THE PSYCHOLOGY OF SKIN
And of course, skin is quite important psychologically, as well.
There is much which can be gleaned from skin color, tone and quality.
When bashful the face can turn red, and when frightened the face can turn pale white. The skin acts as an important tool for us to “hail” other humans with important information. A persoeedn’t even have to think about doing this… the skin does it on its own. It is just that smart.
Of course, the skin really ought to be this important, considering just how complicated a structure it is. While it may often be seen as just a thin layer of fleshy material covering the outside of our body, we certainly fail to recognize just how amazingly complex and well-designed it really is.
On average, human skin is only about 2-3 millimeters thick, but crammed into this relatively tiny space is an amazing amount of detail.
Forming the most visible layer of human skin is what is known as the epidermis, which is comprised on the surface by what scientists refer to as dead stratified squamous, keratinized epithelial cells (usually referred to more simply as the epithelium).
These dead top cells flake off the body over a period of weeks, to be replaced by further cells which are constantly growing underneath and are pushed toward the surface by way of mitosis. The epidermis is the site at which a first degree burn may occur, causing pain, but no real lasting damage.
Methods of examination of patients with skin diseases diagnosing
Detailed questions concerning the time of onset and duration of the skin lesion, past and family history and the medication used by the patient are important data that may help in diagnosis of skin diseases.
When dealing with skin problems in infants or children, the mother gives the history or even the diagnosis sometimes. The physician should have great patience, questioning systematically and persistently. It is of prime importance to ask about all the medications that have been given to the patient whether topical or oral.
Most patients consulting dermatology clinics have had their skin problem treated either by a physician, the members of the family, the pharmacist or by the advice of friends. However, when the problem persists or becomes more complicated, they seek for professional help.
Past history of an allergic reaction (asthma, hay fever, urticaria, drug eruption etc.), are very important and should be recorded in the file.
Detailed history concerning the type of feeding and any particular food exacerbating the skin lesions of the child should be taken into consideration. Newborn and infants are not only breast-fed but other supplements may be used as cereals, which may exacerbate their skin problem.
The role of the mother is not to feed or just to fatten her baby. There are other ethical, moral, and educational duties. Supervision of her child from the anatomical, physiological and psychological point is of prime importance. The condition of the skin, whether there is dryness, or the child sweats or not, types of excreta coming out, for change of color, bloody or others all should be taken into consideration and reported to the treating physician. These physical signs may diagnose certain systemic or genetic diseases. The physician may get valuable data from the mother, if he follows a detailed history. He can spend few minutes in questioning and directing the attention of the mother to such details and to be kept in her mind. Some of data in the history save time and effort in reaching an accurate diagnosis. The physician must not feel at all that he spent longer time with his patient.
Certain medications or foodstuffs taken by the mother and pass through breast milk may have an important role in the etiology or exacerbation of dermatitis eczema reaction.
The experienced physician will not often follow a rigid line of questioning as in the type encountered in a typical sheet form. Complete medical history of a patient with a skin disease should trace the development of the disease as accurately as possible; to determine those elements in the patient‘s present or past history which may be related to the skin problem either psychic, personal, emotional status, hereditary and environmental.
In addition to the inquiry concerning familial tendencies to diabetes, cardiovascular disease and tumors, questioning concerning the familial incidences of allergic diseases is of great importance in a wide variety of skin conditions. It is not sufficient to simply ask the question “is there any allergy in the family?”
Hives, eczema, hay fever and asthma should be specifically mentioned. Other common conditions in relation to skin diseases include acne, icthyosis, xeroderma pigmentosa, rosacea, baldness, psoriasis and some other congenital malformations.
As many other diseases, certain skin diseases are seen much more frequently in certain age groups. Despite the smooth and delicate appearance of a baby‘s skin, it seems to be quite similar to the adult skin. Some organs of the newborn are less well developed such as sebaceous glands, hair follicles and dermal connective tissue.
Environment has an important role on the pathogenesis or exacerbation of certain skin diseases in infants and young children.
Hot humid climates predispose to heat rash and fungal skin diseases.
Type of sports and activities, whether indoors or outdoors may have an important role on the skin problem.
Contact with other in the school or in sport clubs, whether using their own clothes or others‘clothes are also some of the important factors.
Standard of living, whether he lives in a flat or in a separate house with swimming pool with surrounding gardens.
Type of pets in the house such as cats dogs, birds and others.
Race is rarely a crucial determining factor; however this may be of considerable importance in the prognosis of certain skin diseases. Deep mycosis such as coccidomycosis may cause serious prognosis in Negroes.
Certain skin diseases such as leishmaniasis, Yaw, Pinta and Bejel are endemic in tropical areas. Contact dermatitis occurs more in areas having specific plants, trees or exposure to different chemicals.
Certain diseases have seasonal peaks:
Contact dermatitis and hay fever are invariably seen during the season of pollens.
Many diseases including dermatitis and superficial fungal infections, miliaria become much worse during seasons of high environmental temperature, especially with high humidity climates.
Papular urticaria is more common in summer due to insect bites.
Dry skin is often worse in winter because of the low humidity of artificially heated air-conditioned houses and exposure of the skin to long hot baths.
Examination should be under a good light, preferably daylight or a daylight type electrical lamp and using a magnifying lens.
An assessment of a patient‘s normal skin problems can be made at a glance, with experience, but still thorough examinatioot only to the site of the lesion but to other parts of the body should not be neglected.
The physician must get up and do something for his patient. Besides talking to him or adding humor, he should try to give confidence, courage and sympathy to the patient. He can even give “hardened pessimistic cases” an optimistic look to make his patient “cheer up”, especially those who have chronic resistant skin lesion or those with psychosomatic problems. The physician should not be very serious and must not flip, but his light touch with the kindly cheery smile may dispel the gloom.
The physician should not pay all his attention to diagnose and treat the skin lesion only. He has to know that certain skin diseases are manifestations of internal diseases.
Thorough and keen examination should not neglect signs of child abuse either by the parents, or the housemaids, the drivers, or other employees working for the family.
Genital areas should be examined for signs of local infection mainly anogenital warts, contusions, laceration of the anogenital areas.
Morphology of primary and secondary skin lesions.
1. PRIMARY SKIN LESIONS
The primary skin lesions are the original lesions that appear as a result of different stimuli either internal or external. The different primary skin lesions seen on examination are:
Macule – a circumscribed flat area of different color from the surrounding skin. Macules may become raised due to edema, where it is then called maculopapules.
Papule – a raised circumscribed elevation of skin.
Nodule or tubercle – a solid elevation of the skin, larger than a papule.
Plaque – a raised thick portion of the skin, which has well defined edges with a flat or rough surface.
Erythema (redness of the skin surface) -This is the commonest primary skin lesions, which appears in most skin diseases. Erythema is due to dilatation of dermal blood vessels and edema.
Blister – a skin bleb filled with clear fluid.
Vesicle – a small blister.
Bulla – a large vesicle.
Pustule – a skin elevation filled with pus.
Cyst – a cavity filled with fluid.
Secondary skin lesions are modifications or changes of the primary lesions due to infection, trauma or due to other factors. The different secondary skin lesions are :
Scales – a flake of flat horny cells loosened from the horny layer. Fine desquamation of the skin is an ordinary physiological iormal individuals due to the wear and tear. When the formation of epidermal cells are rapid or there is disturbance of normal skin keratinization pathological scaling will result. Skin scaling may be localized or generalized called as exfoliative dermatitis.
Crust – dried serum seen in ruptured vesicles, pustules or bulla.
Excoriation: mechanical abrasion of the skin usually caused by the fingernails in attempt to relive itching.
Fissure – a crack or split in the epidermis.
Erosion – an area of partial loss of epithelium of skin or mucous membrane.
Ulcer – an area of total loss of epithelium of skin or mucous membrane.
Atrophy – loss of thickness of the epidermis or dermis or other tissue.
Lichenifecation – thickness of both prickle cell layer and horny layer with exaggeration of normal skin marks.
Sclerosis – diffuse or circumscribed indurations of the subcutaneous tissues.
Fibrosis – the formation of excessive fibrous tissue.
Abscess – a localized collection of pus in a cavity formed by disintegration or necrosis of tissue.
Cellulitis – an inflammation of cellular tissue, particularly purulent inflammation of the deep dermis and subcutaneous tissue.
Alopecia – localized or generalized loss of hair due to local or systemic factors. Alopecia may be primary or secondary to a local skin disease such as fungal or bacterial infections.
Burrow – a small tunnel in the skin that houses a metazoal parasite, such as the scabies acarus.
Comedo (nes) – a plug of keratin and sebum in a dilated pilosebaceous orifices.
Ecchymoses (bruise) – a macular area of hemorrhage more than 2 cm in diameter.
Petechiae – a punctate hemorrhage spots approximately 1-2 mm in diameter.
Stains or pigments – local hyperpigmentation of the skin following certain skin diseases.
Exfoliation – the splitting off of the epidermal keratin in scales or sheets.
Hemosedroses – in stasis dermatitis.
Fistula – an abnormal passage from a deep structure to the skin surface or between two structures. It is often lined with squamous epithelium.
Keratoderma – a localized hyperplasia and thickening of the stratum corneum.
Striae – linear lesions due to stretch of the skin, either physiological or pathological.
Callus – a horny thickening of the skin.
Milium – a tiny white cyst containing lamellated keratin.
Vegetation – a growth of pathological tissue consisting of multiple closely set papillary masses.
Papilloma – a nipple-like mass projecting from the surface of the skin.
Aphtha – a small ulcer of the mucosa.
3. METHODS OFINVASTIGATION IN DERMATOLOGY
Tzank Test
Cytological examination from the floor of a bulla is used to confirm diagnoses of bullous diseases.
In most bullous eruption the smear will show only inflammatory cells.
In pemphigus numerous acantholytic cells with large nuclei and condensed cytoplasm are found.
Herpes simplex, zoster and varicella lesions: the smear shows large, multi-nucleated and mono-nucleated giant cells and ballooning degeneration of the nuclei.
Diascopy
Diascopy is a simple procedure which will sometimes provide useful additional information for diagnosis of certain skin diseases such as in lupus vulgaris that shows distinctive yellowish, reddish brown apple jelly nodules.
A glass slide or a clear plastic tongue depressor is pressed firmly on the lesion. The temporary exclusion of blood clearly reveals the presence and sometimes the probable nature of dermal changes.
Examination of skin scrapings
This is usually used for the diagnosis of fungal lesions. Scraping is taken of the lesions of the scalp, intertriginous areas, feet or other areas. The skin is cleaned by alcohol swab and left to dry.
Scrape the area with a scalpel or the edge of the slide on a clean slide. Add one drop of 10-20 per cent of KOH or SMS preparation. Hyphae and spores appear as oval bodies and refractile against the background of cells and debris.
Blood picture
Different skin diseases show local or systemic blood changes. Therefore blood picture may be of help to reach a diagnosis and may be indispensable in certain dermatoses. Meanwhile, not every dermatological case is ieed of a list of laboratory tests that may be a burden and might bother the patient leading to loss of confidence in the physician.
Blood picture may show the following in certain skin diseases:
1. Neutrophilia
Neutrophilia may accompany the following skin diseases:
1. Infections, e.g. erysipelas, carbuncle.
2. Inflammatory disorders including pustular or inflammatory psoriasis, erythroderma, and pyoderma gangrenosum.
3. Systemic malignancy (leukemia).
4. Reaction to systemic steroid therapy.
2. Eosinophilia
Eosinophilia is common in the following diseases:
1. Atopic disorders, especially asthma and eczema.
2. Allergy to food or drugs.
3. Parasitic infestations: worms (intestinal or systemic), scabies.
4. Collagen vascular disease, polyarteritis nodosa, dermatomyositis.
5. Bullous disorders: dermatitis herpetiformis, pemphigus, and pemphigoid.
6. Erythema neonatorum.
7. Malignancy, especially Hodgkin‘s disease and eosinophilic leukemia.
3. Lymphocytosis
Lymphocytosis may be present in the following diseases:
1. Viral infections, especially exanthemata and infectious mononucleosis.
2. Bacterial infections: tuberculosis, syphilis, brucellosis, and typhoid.
4. Erythrocyte Sedimentation Rate (ESR)
ESR is usually non-specific test. A raised ESR is usually due to an increased aggregation of red cells due to an abnormality of plasma proteins, notably an increase in plasma fibrinogen associated with the acute or chronic phase reaction.
Some causes that raise ESR:
1. Physiological: pregnancy, menstruation, advancing age.
2. Infections.
3. Inflammatory disorders, e.g. vasculitis.
4. Systemic lupus erythematosus (SLE).
5. Tissue destruction.
6. Malignant neoplasms.
7. Paraproteinaemias.
8. Polycythaemia.
Serum Protein Estimation : this test is used in certain diseases as systemic lupus, hypoproteinemia.
Antinuclear Antibody (ANA)
May be present with:
1. Collagen vascular disease, especially SLE.
2. Chronic liver diseases.
3. Hashimoto‘s thyroiditis, thymoma, myasthenia gravis.
4. Pernicious anemia.
5. Tuberculosis.
6. Leprosy.
7. Diffuse pulmonary fibrosis.
8. Lymphoma or other malignancy.
9. Ulcerative colitis.
These tests are indicated in certain diseases such as syphilis and non-venereal treponemas mainly Pinta and Bejel.
Liver function tests
Diseases of liver may manifest with internal and cutaneous manifestations.
Hormonal essay: is an important line in investigating certain skin diseases especially those associated with endocrine dysfunction.
Cryoglobulins tests
This test shows precipitation of proteins when cooled which redisolves again when heated.
Cryoglobulins are not present iormal individuals.
Skin diseases that show positive test:
Purpura Cold sensitivity cyanosis
Raynaud‘s disease, Lupus erythematosus, Lymphgranuloma venerum, Leg ulcers,
Cutis marmorata.
Technique:
In a warm 10 ml. syringe, venous blood is collected and the serum is separated at 37*C, then cooled in a refrigerator to 5*C. A gelatinous precipitate forms that redisolves when rewarmed.
Porphobilinogen Urine Test
This test is specific for acute intermittent porphyria. It is simple and
valuable in screening patients suspected for porphyria.
Technique
5 ml of freshly voided urine is mixed with 5 ml of Ehrlich‘s reagent and mixed with 10 ml aqueous saturated sodium acetate. The solution is then extracted with equal volume of chloroform. Porphobilinogen and urobilinogen form a red aldehyde compound with Ehrlich‘s reagent.
SKIN BIOPSY
Skin biopsy is an important procedure to confirm an accurate diagnosis for a suspected skin lesion.
SKIN TESTS
Skin tests are introduced into the skin by a variety of techniques to study pharmacological and immunological reactions under controlled conditions. Such tests are extremely valuable, but details of the type of test and the time at which it is read must correspond to the pathological process under consideration.
Interpretation of the relevance of tests, either positive or negative, must always be correlated with the clinical picture.
Severe systemic reactions may occur after the use of standard testing solutions, therefore anti-shock measures as oxygen, adrenaline and hydrocortisone injections should always be at hand when skin tests are performed.
Intradermal tests are much more sensitive than percutaneous methods (the tested allergen is 10- to 100-fold more diluted), but they have a lower specificity. Intradermal testing is usually reserved for venom and penicillin allergy testing when percutaneous tests are negative but there is high clinical suspicion of allergy.
Stinging-Insect Hypersensitivity. Adults who present with a history of a systemic reaction to insects (e.g., bee, yellow jacket, hornet, wasp, fire ant) should be evaluated with allergy skin tests. Children who present with only dermatologic manifestations of a systemic reaction are not at substantially increased risk for future anaphylaxis and do not need allergy skin tests. Management of sensitive patients may include education, avoidance measures, self-administered epinephrine, and allergen immunotherapy.
Drug Allergy. Reliable allergy tests for drugs are available only for penicillin and local anesthetics. In many patients with a history of penicillin allergy, the simplest course is to prescribe an antimicrobial agent that does not contain a beta-lactam ring. In patients with a history of penicillin allergy who have a strong indication for use of a beta-lactam antibiotic, penicillin skin tests can be helpful.
These results suggest that penicillin can be given safely to patients with negative intradermal skin tests to penicillin. Patients with positive penicillin skin tests may be at increased risk for drug reaction, but the specificity of intradermal testing is low.
Percutaneous testing
Several types of skin testing instruments are available for percutaneous skin testing. Each brand of instrument has its own sensitivities and specificities. Positive-control skin tests (histamine) and negative-control skin tests (diluent) are essential for correct interpretation of skin test reactions. About 15 minutes after the application of allergen to skin, the test site is examined for a wheal and flare reaction. A positive skin test reaction (typically, a wheal 3 mm greater in diameter than the negative control reaction, accompanied by surrounding erythema) reflects the presence of mast cellbound IgE specific to the tested allergen.
Allergy to airborne substances (i.e., allergic rhinitis and asthma) is typically evaluated using a panel of percutaneous skin tests for about 40 allergens. A number of the most commonly used allergenic extracts for skin tests are now ststandardized . Percutaneous skin testing has been used to test for food allergy; however, it is less reliable for evaluating food allergy than for evaluating reaction to airborne allergens.
The specificity for food allergen tests is generally low, partly because of cross reactions between some food groups (e.g., legumes). Negative reactions to suggested food allergens on percutaneous tests make a diagnosis of true food allergy unlikely in most cases; however, the poor specificity of these tests precludes a definitive diagnosis of food allergy based on positive test results alone. A double-blind food challenge should be considered when more clinical certainty is needed in diagnosing a serious food allergy.
There are several types of specific allergy tests.
1- Immediate-type hypersensitivity (IgE) skin tests are typically used to test for airborne allergens, foods, insect stings, and penicillin. Immediate-type hypersensitivity also can be evaluated through serum IgE antibody testing called radioallergosorbent testing (RAST). Immediate-type hypersensitivity skin testing is most commonly used in the diagnosis of allergic rhinitis, allergic asthma, food allergy, penicillin allergy, and stinging-insect hypersensitivity. Skin testing can be performed by the percutaneous route (diluted allergen is pricked or scratched into the skin surface) and by the intradermal route (injection of allergen within the dermal layer).
2- Delayed-type hypersensitivity skin tests (patch-type skin tests) are commonly used in patients with suspected contact dermatitis. Some common allergens for patch testing are rubber, medications, fragrances, vehicles or preservatives, hair dyes, metals, and resins. This review focuses on immediate-type hypersensitivity skin testing and serum IgE antibody testing.
Patch tests are usually used to detect contact sensitizers of the delayed hypersensitivity type. Patch test is easy to apply and more safe than other skin tests.
Different Patch Tests
1. Open patch test – this test is used in testing the plant oleoresins.
Method:
Acetone extracts of the plants, weeds or trees are applied on the skin surface. The area is kept dry and the result is noted after 48 hours.
2. Provocative patch test – this test is used for detection of sensitivity of neomycin, penicillin and benzocaine.
Provocation of the open patch test is maintained by application of 10 per cent of sodium lauryl sulfate to the test area for one hour. A significant reaction may appear when the patch test is done.
3. Vapor patch test – this test is applied for volatile substances such as perfumes.
Apply the vapor or gas to the skin surface under a small glass cup tapped into the skin for 48 hours.
4. Mucous membrane patch test – this test is used for local sensitizing agents for the mouth as mouthwashes, nicotine and toothpaste.
A small suction cup containing the test material is applied to the mucous membrane of the lip and kept for one hour. Control is essential in this type of test.
5. Photo patch test – this test is used for detection of photosensitizing substances such as phenothiazine, sulfa, and photosensitizing plants.
Patch test is done in the ordinary way for 48 hours where the area is exposed to ultraviolet radiation and then read again after another 48 hours. Control test area is necessary for exclusion of false positive or negative reactions.
Technique of Ordinary Patch Test
Patch testing is available nowadays ready, where the antigen can be applied directly on the testing area. The sites for the tests are usually on the back or inner arms. If these are not available the diluted substance can be applied on gauze and covered by elastoblast.
The reaction may be detected after 30 minutes as in contact urticaria. These are usually read at 48-72 h and again up to 1 week.
If pruritus, pain or irritation occurs, patch testing should be removed and mild steroid may be applied to the area.
INTERPRETATION OF PATCH TEST
The result of patch test is interpreted as follows:
1+: Erythema only.
2+: Erythema and papules.
3+: Erythema, papules and small vesicles.
4+: Large vesicles, bullae, and severe local reaction besides erythema.
False Reactions
False positive and negative reactions are common in patch testing. This is due to different factors:
Low concentration or insufficient amount may give false negative.
High concentration and increased amount may cause local irritation.
Improper testing as the substance is not fresh, or presence of impurities in the testing substance or the occlusion was not complete.
The patient is under antihistamine or systemic steroid will give false negative reaction.
INTRADERMAL TESTS
Site used for testing: the injection is made into the superficial layer of the dermis in the flexor surface of the forearm.
Needle used: through a fine bore (26 or 27) needle with its bevel pointing upwards.
Quantity of the solution injected: the quantity, which may be injected, varies from 0.01 to 0.1 ml and routinely 0.05 ml is usually sufficient.
Technique
The test is accomplished by putting the skin under tension with the fingers of one hand: the other hand inserts the needle attached to the tuberculin syringe containing the test material.
Interpretation of the test
Time of reading the test: The optimal time for reading the reactioaturally varies with the pharmacological agent or the type of immunological reaction. Most such tests are read at either 15-20 min or at 48 h, but it may be important to read the tests at other times, e.g. at 4-12 h or after 4 days.
Control solution: The test solution must always be compared with a control solution injected in a comparable site at the same time.
A positive test may be taken as one that is significantly different from the control. Assessment of what is significant is difficult and varies with the enthusiasm of the tester.
The response can be observed at 15 minutes, for example, after an injection of histamine or after immediate-wheal allergy tests, is a wheal with a surrounding flare.
The wheal is a more accurate measure than the flare. When the test is read at 48h, for example, the tuberculin reaction, the sizes of the indurated papule and of the erythematous reaction should be observed.
The measurement of a wheal is usually made by diameter. The size of the weal is not directly proportional to the dose of the active agent but varies also with the total volume of fluid injected. For accurate quantitative observations weal diameters below 4 mm or above 15 mm cannot be relied upon.
This is a modification of the intradermal test and is convenient for much routine allergy testing. The intradermal injections of prick test solutions may be dangerous.
Technique: A small quantity of the test solution is placed on the skin and a prick made through it with a sharp needle. This should be superficial and not sufficient to draw blood.
The size of the weal and flare are measured after 15 min. |
SCRATCH TEST
The scratch test resembles the prick test.
Technique: A linear scratch about 1 cm long, but not sufficient to draw blood, is made through the epidermis. This test gives less reproducible results than the prick test.
Modified prick test
This test is slightly more sensitive than the ordinary prick test, but gives no more reproducible results.
Technique: A drop of the test solution is placed on the skin. A needle is then inserted very superficially and almost horizontally into the skin and lifted to raise a tiny tent of epidermis.
IMMEDIATE WHEAL TESTS
Indications:
These tests are used for detecting IgE antibodies.
The passive transfer test may be used to detect circulating IgE, but is not recommended because of the risk of serum hepatitis or AIDS.
They are principally used in the assessment of hay fever and asthma and have a limited place in the management of atopic dermatitis.
They are disappointing in the diagnosis of urticaria.
False-positive and false-negative reactions are common.
RAST & ELISA TESTS
Rast (Radio-Allergosorbent Test) and ELISA (Enzyme-linked Immunosorbent Assay) are alternative methods to detect and measure circulating antibodies.
Although widely used in the past, serum measurement of the total IgE level is unhelpful in the diagnosis of allergy. Of more clinical use are assays for specific IgE antibodies to suspected allergens.
Assays for IgE antibodies specific to common airborne and food allergens are readily available. IgE antibody tests for venom and drugs have less clinical utility and are not routinely used. RAST was the first widely employed method of detecting IgE antibodies in blood that are specific for a given allergen.
In general, RAST and other laboratory methods for IgE testing are highly specific but somewhat less sensitive than percutaneous tests. Results of laboratory testing for food-specific IgE are generally poor, even less helpful than those for percutaneous skin testing.
RAST or other laboratory testing is typically considered when skin testing is inconvenient or difficult to perform. Most primary care physicians do not have immediate access to a clinical skin testing laboratory, so RAST may be easier to obtain. Some patients cannot undergo skin testing because of skin disease that would obscure wheal and flare results (e.g., extensive atopic dermatitis) or because they cannot stop taking medications that suppress the skin test response. In cases of life-threatening allergy (e.g., anaphylaxis), laboratory testing is sometimes used as a proxy result, keeping in mind its limited sensitivity.
Percutaneous testing can help establish the correct diagnosis and identify the offending allergens (pollen, mold spores, dust mites, cockroaches, or household pets). Allergen avoidance measures often are difficult to implement and costly. After specific testing, avoidance measures can be targeted to allergens to which the patient is known to be allergic.
Allergen immunotherapy is another option in refractory cases of allergic rhinitis not amenable to the usual control measures. Like allergen avoidance, it can involve a lot of labor and expense. Specific allergy testing can identify patients likely to benefit from immunotherapy and provide guidance about which allergens to include in the therapy regimen. Allergen immunotherapy may be especially beneficial when avoidance and medications no longer control the patient’s symptoms.
Several closely related variants are marketed (e.g., modified RAST, Quidel QuickVue One-Step Allergen screen, Pharmacia Immunocap). Quantitative assays test.
RAST correlates well with skin testing. It is justified in testing very young children, and with allergens associated with risk on prick testing (e.g. drugs).
ORAL PROVOCATION TESTS
These tests must be carried out with care and is only valuable if the test is properly controlled and the patient is co-operative and well motivated. The administration of a drug, food or chemical by mouth may sometimes be called for to confirm the diagnosis of an eruption or to establish its exact cause.
Indications:
Drug eruption. To determine the cause of a drug eruption or to isolate one from a number of drugs or ingredients of a compound drug. It may be a valuable method of proving the cause of a fixed drug eruption but should rarely, if ever, be used if the reaction has been of a generalized or acute nature.
It is applicable only when the drug given and the dose chosen are unlikely to provoke a severe reaction in the patient.
Atopic eczema
Chronic or recurrent urticaria.
Food allergy. The re-introduction of specific foods, or additives such as:
Tartrazine, benzoates and anti-oxidants one at a time, is an established part of exclusion, elimination and challenge diets. It is important that the role of the suspect food is subsequently confirmed by reintroducing it in a disguised form to avoid identification by the patient.
ELIMINATION AND EXCLUSION TESTS
These tests are used to detect the blamed food, additives or beverages that are suspected to cause dermatitis. Exclusion of the suspected material for few days and observing the skin lesion may give an indication of the effect of the eliminated material.
WOOD‘S LIGHT
This is an ultraviolet lamp with Wood‘s filters, which produces a wavelength about 3650 Â. Wood‘s light is an important investigative tool in diagnosis and treatment of specific skin diseases.
Wood‘s lamp may be used to help in the diagnosis of the following lesions:
Fungal infections: Tinea capitis caused by Microsporon species gives bright blue-green fluorescence. It should be noted that Tricophyton tonsurans and Tricophyton violeceum types of ringworm do not give fluorescence.
Erythrasma: gives a coral-red fluorescence.
Pityriasis versicolor lesions when examined in a dark room with Wood‘s light appear as sharply accentuated lesions.
Bacterial infections: Pseudomonas pyocyanea gives a yellowish-green color due to pyocyanin.
The acne bacillus causes a coral fluorescence in the follicles possibly due to porphyrin production. Erythrasma gives coral- red or pink-orange fluorescence.
Detection of pigmentary disorders:
Wood‘s lamp can be used to determine the depth of melanin in the skin, since variations in epidermal pigmentation are more apparent under Wood‘s lamp than under visible light. Wood‘s light accentuates contrast between pigmented and non-pigmented skin and separates hypopigmented from totally non-pigmented areas as in vitilligo and albinism.
Detection of porphyrins:
Porphyrins in urine when examined in dark field by Wood’s light, gives red color or pinkish orange. Porphyrins in feces, blister fluid in porphyria lesions, the teeth in erythropoietic porphyria and blood protoporphyria give also the same fluorescence.
Erythropietic porphyria can be also diagnosed by the fluorescence of red cells.
Tetracycline: deposits in the growing enamel teeth of children that produces a typical yellow color of the teeth under Wood‘s light illumination.
Malignant tumors of the skin especially squamous cell carcinoma gives bright-red fluorescence.
Miscellaneous:
Medications, industrial compounds and other fluorescent materials can be detected specifically by Wood‘s light.