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Incomplete Blinking and Its Effects

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Incomplete Blinking and Its Effects Incomplete Blinking and Its Effects Content written by: Osuagwu Levi RO, RCLP Content originally published in the Spring 2010 edition of The Eighth Line Table of Contents • Introduction • Complete Vs. Incomplete Blinking • Inter-Blink Intervals • Blinking and Contact Lens Wear • Patient Education • References • Post Test Introduction Blinking is a protective mechanism for the cornea and conjunctiva, serving to maintain a tear layer over the ocular surface that is necessary for epithelia health and optical performance1. Discussing this topic has not only become necessary due to the effects of industrialisation in our societies which predisposes people to environmental and climatic conditions that could result to dry eye complaints but also due to the increasing number of contact lens wearers in the world. Complete blink among other things, helps to maintain a clean and wet anterior contact lens surface; causes debris to be swept into the inferior marginal tear strip allowing a cleaner tear layer to be distributed as the upper lid ascends; and maximizes the extent of distribution of tarsal goblet cell mucin. Forceful blinking can significantly increase lipid layer thickness provided the meibomian glands have adequate reservoir or secretion, and the gland orifices are not blocked with clusters of keratotic cells2. 1 | Page Complete Vs. Incomplete Blinking The normal apposition of the lids during a complete blink is a means of promoting lipid secretion from meibomian glands3’4. Incomplete blinking may be associated with poor maintenance of the integrity of the lipid layer which is spread across the cornea by the upper lid. For instance, during prolonged reading, when blink rate is significantly reduced4, the lipid layer can disappear and then reappear with conscious blinking5. Incomplete blinking at this period could result in reduction in the integrity of the lipid layer. However, incomplete blink rates may vary widely between individuals. It may also vary for the same individual accordingly, as quantity of blink is normally controlled by visual task, emotional state, mental effort, illumination level, atmospheric conditions etc. Thus, visually and/or intellectually demanding tasks may be associated with higher than normal rates of incomplete blinking. Psychologists have found this tool very helpful in crime investigation & detection. On the other hand, low blink rates are associated with longer inter-blink intervals (time between successive blinks) and therefore increases the risk of significant ocular and/or lens surface drying6. Inter-Blink Intervals When hydrogel lenses are worn, pre-lens non-invasive tear break-up-times (BUTs) of 6- 8secs7 suggests that inter-blink intervals need to be less than 6-8secs to maintain lens surface wetness and to slow precipitation of deposits. So, when blink rates fall below 10 per min, and average inter-blink intervals increase to 6 secs, the risk of surface drying and deposit precipitation is greater. A review of studies that examined blink rates in human subjects, indicated sample- average blink rates that ranged between 10-22 blinks/min8. While this is lower in new born infants (6.2blinks/min), it is longer in pre-school children (8blinks/min) which could be explained by the immature neural system and a more resistant tear film8. Blink rate also varied with activities for example, blink rate during computer use was found to be an average of 4blinks/min9; an average of 8blinks/min while reading; and an average of 21 blinks/min while engaged in a general conversation.9 Blink rate reduces significantly as task difficulty increases from watching a film, to reading, to counting the number of time the letter ‘a’ appeared in reading material9. These results are consistent with the finding that there is a tendency for non-blink periods to be sustained until difficulty target recognition tasks are completed10. 2 | Page However, apart from intra-subject blink rate variation according to various task demands, there is also a large range of inter-subject blink rate variation. For the same experimental conditions of watching an educational film, the blink rate for 20 subjects varied between 6 and 30blinks per minute11 Blinking and Contact Lens Wear Blinking has been emphasized in contact lens practice. Typically the new contact lens (CL) wearer (particularly the rigid lens wearer) blinks too frequently during the 1st few days of lens wear & then enters a period of infrequent blinking. There is also a strong tendency for CL wearers to become incomplete blinkers; the blink begins normally, but once the lid margins make contact with the lens, the blink is completed prematurely. Blinking in rigid lenses serves the important function of continually pumping tears underneath the lens and pumping stale tears containing carbon IV oxide & their waste products out from under the lens. Secondly, adequate blinking keeps the cornea and contact lens in a hydrated condition12. Stewart 13emphasized the importance of blinking habits in contact lens adaptation by defining ‘functional blinking’ as blinking that produces: a) A natural appearance of the wearer b) Optimum lens movement and positioning c) A clean anterior lens surface d) Observable fluid flow beneath the lens. While there has been agreement in the role of adequate blinking in rigid lens wearers, controversies have continued over the need for adequate blinking in soft contact lens wearers. There is no doubt that correct blinking habits are equally important for wearers of soft lens as for wearers of rigid lenses. Even though a number of studies have shown that a certain amount of oxygen can pass through soft lens materials (and the thinner the lens, the more oxygen can pass through), a well-fitting soft lens not only makes slight vertical translational movement over the corneal surface during blink but also flexes, or ‘ripples’ with each blink due to squeezing action of the upper lid. This movement is likened to the tear pumping mechanism of the rigid lenses, the movement allowing tears containing carbon IV oxide to move from the lens with each blink to be replaced by tears containing fresh oxygen. Hayashi & Fatt14 and Hill & Jeppe15 in trying to determine if tear pumping was actually necessary with soft lenses disagreed in the amount of tear pumping necessary but one thing they all agreed on was that some tear pumping was necessary for wearers of soft lenses. With the advantages of adequate blinking for soft lens wearers including: 3 | Page • Removal of sloughed off epithelial cells, mucin, & other debris from underneath the lens by the movement of the lens over the corneal surface. • Constant removal of debris from both the back & front surfaces of the lens, thereby keeping the lens surfaces clean and ‘coating’ of the lenses likely to occur. • Drying of the lenses is also avoided with adequate blinking. • Better oxygen and carbon dioxide exchange increase comfort; better tolerance and longer wearing time • The probability of extending the useful life of the lenses as a result of decreased tendency for lens coating & dehydration. Patient Education Adequate blinking for soft lens wearers can no longer be overemphasized. To this effect, Stewart13 recommends that the CL practitioner teach patients to blink properly. The patient is taught to blink in 3 counts: “One, close; two, pause; three, open wide.” After practicing in slow motion, blinking can be gradually sped up. This exercise results in enhanced tear film thickness and better lubrication of the tear film. It is worth mentioning that the emphasis during blink efficiency exercises is for voluntary blinking that is complete, relaxed and rapid as well as natural in appearance. According to the principles of behaviour modification16 and habit reversal17, patient education is a key element of these forms of therapy. It is necessary to create an awareness of significance of the problems associated with incomplete blinking. It is equally necessary to establish an appreciation of the advantages that are derived from normal complete blink habits. Having clinical photographs, blink instruction guides (BIG) in our consulting room facilitate the assimilation and explanation of these instructions - Statements as, ‘blink exercises may have great benefits, but nobody else can do them for you’, and “blink exercises don’t have any chance of helping you if you don’t do them”18-The goal here is primarily to improve the quality of blinks. 4 | Page References 1. Oyster CW. The human eye: structure and function . Massachusetts: Sinauer Associates, Inc.; 1999. p. 298-310. 2. Korb DR, Herman JP, Exford JM, et al. Tear film lipid layer thickness as a function of blinking. Cornea 1994; 13:354- 9 3. Linton RG, Curnow DH, Riley WJ. The Meibomian glands. An investigation into the secretion and some aspects of the physiology. Br J Ophthalmol 1961; 45:718-23. 4. Ruskell GL. Anatomy and physiology of the cornea and related structures. In: Phillips AJ, Stone J, editors. Contact lenses. 3rd ed. London: Butterworths: 1989. p. 44. 5. Doughty MJ. Consideration of three types of spontaneous eyeblink activity in normal humans: during reading and video display terminal use, in primary gaze, and while in conversation. Optom Vis Sci 2001;78:712-25. 6. McMonnies CW. Exposure keratopathy, lid wiper epitheliopathy, dry eye, refractive surgery, and dry contact lenses. Contact lens & Ant Eye 2007;30:37-51. 7. Young G, Efron N. Characteristics of the pre lens tear film during hydrogel contact lens wear. Ophthalmic Physiol Opt 1991; 11:53-8. 8. Lavesso MM, Schellini SA, Padovani CR, Hirai FE. Eye blink in newborn and preschool-age children. Acta Ophthalmol 2008: 86(3); 275-8. 9. York M, Ong J, Robbins JC. Variation in blink rate associated with contact lens wear and task difficulty. Am J Opthalmol Arch Am Acad Optom 1971;48:461-6. 10. Pointer JS. Eye-blink activity with hydrophilic contact lenses. Acta Ophthalmol 1988;66:498-504.
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