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Is trichography an underutilised diagnostic aid in dermatology?

What to look out for during a microscopic examination of plucked hairs

15 November 2019, at 9:00am

Trichography is the examination of hair under the microscope in which a hair pluck (about 50 to 100 hairs) is obtained using a haemostat (tips covered with drip set tubing to prevent damage to hair shaft) and mounted onto a slide with liquid paraffin. Laying all the hairs facing in one direction makes examination easier and faster.

The examination of the hair should be performed systematically. Changes to the hair bulb, the hair shaft and the hair tip are all of interest. When combined with the history and clinical examination, they can help make a tentative diagnosis, or give an indication of the relevant differential diagnosis. The procedure is a very good diagnostic aid. Not only is it inexpensive and easy to perform, it also gives instant results and is useful as a diagnostic aid in small mammals too. Like all procedures it does require practice, but with experience the results are easy to assess.

Hair bulb

The hair growth cycle consists of three main phases. Anagen is the active growth phase; catagen is the transitional phase; and telogen is the resting phase. Anagen and telogen hair bulbs can be easily recognised microscopically. Adult animals should have a mixture of anagen and telogen hairs, with the anagen:telogen ratio varying with seasons, breed, age, nutrition and disease status. This ratio can be determined by examining approximately 100 hairs. On its own, the ratio is not particularly useful, but combined with the history and clinical presentation it can give an indication of the possible differentials.

Anagen hair bulbs are smooth and pigmented, and when plucked they tend to bend looking like a club (Figure 1). The presence of mostly anagen hairs in an alopecic dog can indicate anagen defluxion, or that the alopecia is not associated with a defect in hair growth cycle.

Telogen hair bulbs are generally rough, non-pigmented, straight and arrow or spear shaped (Figure 2). If the hair is easily epilated and the hairs mostly in telogen, it suggests that the skin disorder may be associated with a nutritional, endocrine or metabolic disorder, or, if the history is that of sudden hair loss, it could be telogen effluvium. Telogen hairs are predominant findings in alopecia associated with follicular dysplasia, such as cyclical flank alopecia, hypothyroidism and hyperadrenocorticism.

Hair shaft

Normal hair shafts are smooth and of uniform diameter, with well demarcated cortex and medulla. The pigmentation in the hair shaft will depend on the coat colour of the animal. Smooth-haired dogs have straight hair shafts whereas the curly-coated animals have twisted ones. Hair shafts, if misshaped or uneven, can be associated with nutritional or metabolic disease, or with congenital or genetic disorders.

Abnormal melanosomes can result in bulges and fractures in the hair shaft (Figure 3) and are seen in dogs with colour dilution alopecia. This condition is caused by abnormalities in the transfer of melanosomes from melanocytes to surrounding keratinocytes and hair bulbs and by the degradation of melanosomes.

Melanosome clumping, seen in dysplastic hair bulbs (Figure 4), is also seen in colour dilution alopecia. Ectoparasites, such as Cheyletiella, fur mites (Figure 5) and lice, or their eggs (Figure 6), may be seen on hair plucks. Uneven hairs with a fuzzy appearance should be examined for fungal elements. Hyphae may be seen inside and arthrospores on the outside (Figure 7) of the hair shaft. They are best seen under x100 magnification, with increased contrast by partially closing the condenser. Trichoptilosis, a longitudinal split in the hair shaft, indicates trauma. Trichorrhexis nodosa, associated with mechanical or chemical damage, is the loss of cuticle and appears as a nodular swelling in the centre of the hair shaft where the fracture occurs (Figure 8).

Hair tip

Normal hairs taper into a tip. If the tip is fractured or broken (Figure 9) it is generally due to trauma and in most cases associated with licking, biting or scratching.

An onion-shaped hair tip affecting the whiskers and primary hairs of Abyssinian cats has been reported. Affected cats have a poor lustreless hair coat.

Other findings

Follicular casts are accumulation of keratosebaceous substance seen around the hairs. This is an indication of a follicular disorder associated with abnormal follicular keratinisation. They can be seen in diseases such as folliculitis, demodicosis, follicular dysplasia, endocrinopathies and sebaceous adenitis (Figure 10).

Summary

Trichography is a rapid and cost-effective test in which abnormalities of the hair bulb, the hair shaft and the hair tip are all of interest. Findings can be used to support a diagnosis and/or to rule out potential differentials. It requires practice, but once mastered is a very useful tool in either making a list of the most likely differential diagnosis or a tentative diagnosis.