Dry Eye Dx and Tx
The Impact of Demodex on Eyelashes and Their Follicles
BY KATHERINE M. MASTROTA, MS, OD, FAAO
There is renewed interest in the role of Demodex, a skin mite, in blepharitis and ocular surface disease. It is currently accepted that "cylindrical cuffing" along the shaft of the eyelashes is pathognomonic of Demodex mites harbored within the eyelash follicles and associated sebaceous glands. Eyelash loss (madarosis) and lash fragility may also herald the presence of Demodex, among other pathologies.
We could infer, therefore, that the presence of abnormal numbers of Demodex mites within the eyelash follicle is disruptive to the normal process of eyelash growth and loss. But what is normal?
Examining the Eyelashes
Much less is known about eyelash physiology as compared to that of human hair. The next three paragraphs outline salient points of a 2010 article by Thibaut et al that characterizes the human eyelash.
Eyelashes grow in imperfect rows of five to six in the upper lid and three to four in the lower lid. Their mean number is 90 to 160 in the upper lid and 75 to 80 in the lower lid; their length is 8mm to 12mm in the upper lid, 6mm to 8mm in the lower lid. Eyelash follicles are free of arrector pili muscle and are served by the glands of Zeis (sebaceous glands that empty into the middle portion of the eyelash follicle) and Moll (modified sweat glands).
The eyelash fiber has a structure very close to hair, with three compartments from the outside to the inside: the cuticle, the cortex, and the medulla. Both hair and eyelash follicles go through a cycle of growth (anagen), transition (catagen), and resting (telogen). Eyelashes have a shorter cycle compared to hair (which is why eyelashes are short), with a growth rate of 0.12mm/day. The anagen phase lasts about 34 days, and the telogen phase is about 90 days. Lashes last five to six months before falling out.
Could Demodex overpopulation damage the follicle stem cell niche?
Eyelash follicles have the same overall structure as scalp hair follicles and are rooted approximately 2mm deep into the dermis through the epidermis. Stem cells housed within the hair follicle renew the follicle at each lash cycle. Stem cells in eyelash follicles are found all along the outer root sheath (the outer-most layer of the hair follicle). Human hair follicles appear to have two distinct niches of stem cells along the outer root sheath.
Currently, it is thought that pathological changes secondary to demodicidosis of the eyelids/ eyelashes result from:
• The mites blocking follicles and tubules of sebaceous glands.
• Reactive hyperkeratinization.
• Epithelial hyperplasia from micro-abrasions caused by the mites' claws.
• The mites acting as bacterial vectors.
• The hosts' inflammatory reaction to the presence of parasite chitin as a foreign body.
• Stimulation of the hosts' humoral responses and cell-mediated immunological reactions in response to the mites and their waste products (Czepita et al, 2007).
Could madarosis and eyelash compromise in patients who have Demodex overpopulation be due to damage to the follicle stem cell niche? I, like you, await the answer to this question. CLS
For references, please visit www. clspectrum.com/references.asp and click on document #217.
Dr. Mastrota is Secretary of the Anterior Segment Section of the American Academy of Optometry. She is a consultant or advisor to Allergan, B+L, BioTissue, Nicox, and OcuSoft and is a stock shareholder of TearLab Corporation. Contact her at firstname.lastname@example.org.