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Chapter: Clinical Dermatology: Diagnosis of skin disorders

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Side-room and office tests - Diagnosis of skin disorders

A number of tests can be performed in the practice office so that their results will be available immediately.

Side-room and office tests

A number of tests can be performed in the practice office so that their results will be available immediately.

Potassium hydroxide preparations for fungal infections

If a fungal infection is suspected, scales or plucked hairs can be dissolved in an aqueous solution of 20% potassium hydroxide (KOH) containing 40% dimethyl sulphoxide (DMSO). The scale from the edge of a scaling lesion is vigorously scraped on to a glass slide with a No. 15 scalpel blade or the edge of a second glass slide. Other samples can include nail clippings, the roofs of blisters, hair pluckings, and the contents of pustules when a candidal infection is suspected. A drop or two of the KOH solution is run under the cover slip (Fig. 3.6). After 5–10 min the mount is examined under a microscope with the condenser lens lowered to increase contrast. Nail clippings take longer to clearaup to a couple of hours. With experience, fungal and candidal hyphae can be readily detected (Fig. 3.7). No heat is required if DMSO is included in the KOH solution.





Detection of a scabies mite

Burrows in an itchy patient are diagnostic of scabies. Retrieving a mite from the skin will confirm the diagnosis and convince a sceptical patient of the infestation. The burrow should be examined under a magnifying glass; the acarus is seen as a tiny black or grey dot at the most recent, least scaly end. It can be removed by a sterile needle and placed on a slide within a marked circle. 

Alternatively, if mites are not seen, possible burrows can be vigorously scraped with a No. 15 scalpel blade, moistened with liquid paraffin or vegetable oil, and the scrapings transferred to a slide. Patients never argue the toss when confronted by a magnified mobile mite. Dermatoscopy  can also be used to detect the scabies mite.

Cytology (Tzanck smear)

Cytology can aid diagnosis of viral infections such as herpes simplex and zoster, and of bullous diseases such as pemphigus. A blister roof is removed and the cells from the base of the blister are scraped off with a No. 10 or 15 surgical blade. These cells are smeared on to a microscope slide, air-dried and fixed with methanol. They are then stained with Giemsa, toluidine blue or Wright’s stain. Acantholytic cells  are seen in pemphigus and multinucleate giant cells are dia-gnostic of herpes simplex or varicella zoster infec-tions . Practice is needed to get good preparations. The technique remains popular in the USA but has fallen out of favour in the UK as his-tology, virological culture and electron microscopy have become more accessible.

Patch tests

Patch tests are invaluable in detecting the allergens responsible for allergic contact dermatitis .


Either suspected individual antigens, or a battery of antigens which are common culprits, can be tested. Standard dilutions of the common antigens in appro-priate bases are available commercially (Fig. 3.8). The test materials are applied to the back under aluminium discs or patches; the occlusion encourages penetration of the allergen. The patches are left in place for 48 h and then, after careful marking, are removed. The sites are inspected 10 min later, again at 96 h and some-times even later if doubtful reactions require further assessment. The test detects type IV delayed hyper-sensitivity reactions . The readings are scored according to the reaction seen. NT Not tested.

0  No reaction.

±  Doubtful reaction (minimal erythema).

+  Weak reaction (erythematous and maybe papular).

++ Strong reaction (erythematous and oedematous or vesicular; Fig. 3.9).

++ + Extreme reaction (erythematous and bullous). IR Irritant reaction (variable, but often sharply cir-cumscribed, with a glazed appearance and increased skin markings).

 

A positive patch test does not prove that the allergen in question has caused the current episode of contact dermatitis; the results must be interpreted in the light of the history and possible previous exposure to the allergen.

Patch testing requires attention to detail in applying the patches properly, and skill and experience in inter-preting the results.


Prick testing

Prick testing is much less helpful in dermatology. It detects immediate (type I) hypersensitivity  and patients should not have taken systemic antihis-tamines for at least 48 h before the test. Commercially prepared diluted antigens and a control are placed as single drops on marked areas of the forearm. The skin is gently pricked through the drops using separate sterile fine (e.g. Size 25 gauge, or smaller) needles. The prick should not cause bleeding. The drops are then removed with a tissue wipe. After 10 min the sites are inspected and the diameter of any wheal measured and recorded. A result is considered positive if the test antigen causes a wheal of 4 mm or greater (Fig. 3.10) and the control elicits negligible reaction. Like patch testing, prick testing should not be undertaken by those without formal training in the procedure. Although the risk of anaphylaxis is small, resuscitation facilities including adrenaline (epinephrine) and oxygen  must be available. The relevance of positive results to the cause of the condition under investigationausually urticaria or atopic dermatitisais often debatable. Positive results should correlate with positive radio-allergosorbent tests (RAST;) used to measure total and specific immunoglobulin E (IgE) levels to


Skin biopsy

Biopsy (from the Greek bios meaning ‘life’ and opsis ‘sight’) of skin lesions is useful to establish or con-firm a clinical diagnosis. A piece of tissue is removed surgically for histological examination and, sometimes, for other tests (e.g. culture for organisms). When used selectively, a skin biopsy can solve the most perplexing problem but, conversely, will be unhelpful in con-ditions without a specific histology (e.g. most drug eruptions, pityriasis rosea, reactive erythemas).

Skin biopsies may be incisional, when just part of a lesion is removed for laboratory examination or excisional, when the whole lesion is cut out. Exci-sional biopsy is preferable for most small lesions (up to 0.5 cm diameter) but incisional biopsy is chosen when the partial removal of a larger lesion is adequate for diagnosis, and complete removal might leave an unnecessary and unsightly scar. Ideally, an incisional biopsy should include a piece of the surrounding normal skin (Fig. 3.11) although this may not be possible if a small punch is used.


The main steps in skin biopsy are:

administration of local anaesthesia; and

removal of all (excision) or part (incision) of the lesion and repair of the defect made by a scalpel or punch.

 

Local anaesthetic

Lignocaine (lidocaine) 1–2% is used. Sometimes adrena-line 1 : 200 000 is added. This causes vasoconstriction, reduced clearance of the local anaesthetic and pro-longation of the local anaesthetic effect. Plain lignocaine should be used on the fingers, toes and penis as the prolonged vasoconstriction produced by adrenaline can be dangerous here. Adrenaline is also best avoided in diabetics with small vessel disease, in those with a history of heart disease (including dysrhythmias), in patients taking non-selective α blockers and tricyclic antidepressants (because of potential interactions) and in uncontrolled hyperthyroidism. There are exceptions to these general rules and, undoubtedly, the total dose of local anaesthetic and/or adrenaline is important. Nevertheless, the rules should not be broken unless the surgeon is quite sure that the procedure that he or she is about to embark on is safe.

It is wise to avoid local anaesthesia during early pregnancy and to delay non-urgent procedures until after the first trimester.

As ‘B’ follows ‘A’ in the alphabet, get into the habit of checking the precise concentration of the lignocaine added adrenaline on the label before withdrawing it into the syringe and then, before injecting it, confirm that the patient has not had any previous allergic reac-tions to local anaesthetic.

Infiltration of the local anaesthetic into the skin around the area to be biopsied is the most widely used method. If the local anaesthetic is injected into the subcutaneous fat, it will be relatively pain-free, will produce a diffuse swelling of the skin and will take several minutes to induce anaesthesia. Intradermal injections are painful and produce a discrete wheal associated with rapid anaesthesia. The application of EMLA cream (eutectic mixture of local anaesthesia) to the operation site 2 h before giving a local anaesthetic to children helps to numb the initial prick.

Scalpel biopsy

This provides more tissue than a punch biopsy. It can be used routinely, but is especially useful for biopsy-ing disorders of the subcutaneous fat, for obtaining specimens with both normal and abnormal skin for comparison (Fig. 3.11) and for removing small lesions in toto (excision biopsy,). After selecting thelesion for biopsy, an elliptical piece of skin is excised.


The specimen should include the subcutaneous fat. Removing the specimen with forceps may cause crush artefact, which can be avoided by lifting the specimen with either a Gillies hook or a syringe needle. The wound is then sutured; firm compression for 5 min stops oozing. Non-absorbable 3/0 sutures are used for biopsies on the legs and back, 5/0 for the face, and 4/0 for elsewhere. Stitches are usually removed from the face in 4 days, from the anterior trunk and arms in 7 days, and from the back and legs in 10 days. Some guidelines for skin biopsies are listed in Table 3.3.

Punch biopsy

The skin is sampled with a small (3 – 4 mm diameter) tissue punch. Lignocaine 1% is injected intradermally first, and a cylinder of skin is incised with the punch by rotating it back and forth (Fig. 3.12). 



Skin is lifted up carefully with a needle or forceps and the base is cut off at the level of subcutaneous fat. The defect is cauterized or repaired with a single suture. The biopsy specimen must not be crushed with the forceps or critical histological patterns may be distorted.

The tissue can be sent to the pathologist with a summary of the history, a differential diagnosis and the patient’s age. Close liaison with the pathologist is essential, because the diagnosis may only become apparent with knowledge of both the clinical and his-tological features.


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