practical notes on clincal dermatology

The diagnosis of skin diseases
Dermatology is a morphologically oriented specialty. As in all specialties the medical history is important, however, the ability to interpret the findings is even more important. The diagnosis of skin disease must be approached in an orderly and logical manner, and the temptation to make rapid judgments after hasty aberration must be controlled. The recommended approach to the patient with skin disease is as follows:
Obtain a brief history from the patient especially, noting; duration rate of onset, location, symptoms, previous episodes, family history, history of exposure to allergens, occupation, and previous treatment.
Determine the extent of the eruption by uncovering the patient completely.
Determine the primary lesions with the help of a hand lance.
Determine the nature of any secondary or special lesions.
Formulate a differential diagnosis.
-Obtain a skin biopsy and the following laboratory tests; - 10-20% potassium hydroxide (helps in the diagnosis of fungal infection of the skin) .– Skin scrapings for scabies. – Gram stain -Fungal and bacterial cultures – Cytology (T zanck smear) -Woods light examination -Patch tests -Dark field examination and blood tests studies.

How is a KOH examination performed?
The highest rate of recovery of organisms occurs in specimens taken from the tops of vesicles and the edges of annular lesions. The site should be swabbed with an alcohol pad or water and scraped with a #15 blade. The moist corneocytes are then easily transferred from the blade to a glass slide. One or two drops of KOH (10-20%) are added, and the specimen is cover-slipped. The KOH preparation is gently warmed, but not boiled, and then examined under the microscope. It is important to focus back and forth the material ,so that the refractile hyphae can be found. Fungal hyphae can be recognized by their regular cylindrical shapes with branching and presence of septa. Older lesions may demonstrate numerous rounded spores called arthrospores.

What laboratory tests are useful for diagnosing tinea capitis?
Fluorescing the affected area with a Wood’s light is the quickest technique. If the hair fluoresces a yellow-green, then a fungal infection is likely. However, the lack of fluorescence does not exclude tinea capitis, because Trichophyton tonsurans accounts for 75-90% of scalp ringworm infections, and it dose not fluoresce by wood’s light.
Examination of KOH-treated infected hair is more sensitive and rapidly performed. The best results are obtained when broken-off hairs are examined since these are the ones infected by hyphae and arthrospores. Infection can be immediately apparent on KOH examination if the fungus grows outside the hair shaft (ectothrix). Some dermatophytes, such as T. tonsurans, grow within the hair shaft (endothrix), and a few minutes are required to let the KOH break down the hair shaft so that the arthrospores can be more easily visualized.
The diagnosis can also be proved by fungal cultures (the commonly used culture media are dermatophyte test media and Sabouraud`s dextrose agar with or without antibiotics and cyclohexamide that added to suppress bacterial contaminants and yeasts). The easily broken, infected hairs are embedded in the culture media which needed more than two weeks to grow.