Diagnostic Tests: Tips from the Dermatologist #2

This is the second in a series of short articles with the aim to encourage colleagues in practice to become familiar with and use more often this diagnostic test. A trichogram will give a rapid, convenient and inexpensive indication of whether a patient has a certain ectoparasite infestation, a structural defect of the hair shaft and will identify the stage of the hair cycle.

To perform trichography there is no need for specialist equipment apart from a good microscope, forceps/hemostat or rubber covered clamp, mineral oil, slide and a cover slip.

Trichography involves microscopic examination of plucked hairs. It is commonly done by dermatologists to -

  • Investigate causes of alopecia
  • Check for arthrospores and hyphae if dermatophytosis is suspected
  • Rule out ectoparasites
  • Look for broken hair tips when self-induced alopecia is suspected in e.g. “hidden groomers” cats. If the tips are tapered the hairs may fall out for other reasons
  • Determine hair stage: anagen (growing stage) vs. telogen (resting stage)
  • Identify structural defects of the hair shaft

Technique

  • Pluck a good number of hairs in the direction of hair growth using a pair of haemostat forceps, the jaws of which should be protected by drip tubing to avoid damaging the shaftsTrichogram
  • Hold the forceps/clamp close to the skin surface and grasp all hair shafts which emerge
  • Mount the hairs onto a microscope slide in liquid paraffin
  • If multiple samples are on the same slide place the hairs in parallel order on the slide, separate them to evaluate roots and tips adequately
  • Cover hairs with a cover slip
  • Examine hair tips, shafts and roots under a low power light microscope

Findings

Ectoparasites

Microscopic examination of hair plucks can be useful to detect:

  • Demodex mites. Hair plucks are particularly helpful when skin scraping examination cannot be easily done from some areas (e.g. close to the eye, pododermatitis). Skin scraping is a more sensitive technique than trichography for the diagnosis of demodicosis and therefore the disease should not be ruled out on the basis of not finding mites on hair plucks.
  • Lice, Cheyletiella mites and their eggs (Fig 1).

Structural defects

  • The presence of abnormal melanin clumps in the hair shaft suggests a genetic condition like colour dilution alopecia or black hair follicular dysplasia (Fig 2). Please note that there are some breeds like Weimaraner which may normally have melanin clumps along the hair shaft.
  • The presence of abnormality of the hair shaft is uncommon but defects like medullary trichomalacia, trichoptilosis, trichorrexis nodosa and pili torti have been reported in cats and dogs.

Fractured hairs

Normal hair tips taper to a fine point (Fig 3). Fractured hair shafts indicate self-trauma, which is often due to pruritus (Fig 4). This is a useful test to demonstrate self-trauma in cases of feline symmetrical alopecia, particularly when the owner may not be aware of over grooming behaviour. In the case of dermatophytosis, affected hairs may be fractured and covered with spores and penetrated by hyphae.

Examination of hair bulbs

  • Hairs in anagen (growing) phase have a rounded, curled, bent and often smooth and pigmented root
  • Hairs in telogen (resting) phase are spear-shaped and lack pigmentation, although the base of the hair may show a roughened or brush-like edge

In general, both telogen and anagen hairs will be seen on trichography in healthy cats and dogs. Assessment of anagen/telogen ratios can be helpful in ascertaining the cause of alopecia although care must be taken in interpreting these ratios. Breeds such as the Nordic breeds retain large numbers of telogen hairs for a long period of time and are said to have ‘telogen dominated hair cycles’. Conversely, breeds including the poodle and bichon frise have ‘anagen dominated hair cycles’ where the hair continues to grow. Despite this breed variation, the absence of anagen hair roots in a trichogram would be suggestive of a hair growth cycle disorder (eg endocrinopathy).

Follicular casts

The presence of follicular casts (accumulations of keratosebaceous material around the hair shaft) indicates a follicular cornification disorder of the hair follicle. Follicular casts are usually seen in dogs with sebaceous adenitis.

 

Fig 1. Fig 2.

fig 1 egg fig 2 cda

 

Fig 3. Fig 4.

fig 3 hair tip fig 4 fract hair