ACTA 0 P H T H A L M 0 LOG I CA 69 (1991) 92-94

Tear break-up time is related to blink frequency

Maurice Yap

Optometry Section, Hong Kong Polytechnic, Hong Kong

Abstract. The relationship between tear break-up time and blink frequency was investigated in a group of 41 subjects. A significant relationship between tear break- up time and blinking was found. This suggests that tear stability plays an important role in stimulating normal involuntary blinking.

Key words: tear break-up time - blink frequency - blink in- terval - involuntary blinking.

The integrity of the normal tear film breaks down unless its 3-layered structure is reconstituted peri- odically by blinking (Holly 1973). This suscepti- bility to disruption is used clinically as a measure of the stability of the tear layer. The tear break-up time (TBUT) is the time interval between the last blink and the appearance of a break (observed as a dark spot) in the fluorescein-stained tear layer. Prause & Norn (1987) hypothesized that there might be a relationship between the TBUT and blink rate. Their investigation revealed a signifi- cant positive correlation between TBUT and blink interval (r = 0.33, i.e. a negative correlation be- tween TBUT and blink frequency). This confiied their original hypothesis although the correlation coefficient was relatively low. Three factors could have adversely affected the strength of the correla- tion. First, it is unclear whether the study was per- formed under controlled and constant environ- mental conditions. Second, blink frequency was es- timated whilst their subjects’ visual acuities were recorded. It is known that blink rates are affected by the level of difficulty of the visual task per- formed work et al. 1971). Third, elderly subjects were used and it is known that corneal sensitivity decreases with age (Millodot 1977). Collins et al.

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(1989) videotaped the blinking patterns of younger subjects whilst engaging them in conversation and found a correlation between blink rate and TBUT of 0.38. However, the relationship was not signifi- cant (P< 0.10). Whilst Prause & Norn (1987) used 32 subjects, Collins et al. (1989) used only 9 sub- jects.

The aim of this study was to investigate the rela- tionship between blink frequency and TBUT in a well-controlled test environment using a relatively large number of subjects.

Material and Methods

Forty-one healthy Chinese subjects (20 males, 21 females) aged between 10 to 45 years (mean: 22.4 years) participated in this study. Contact lens wearers, past and present, were excluded as con- tact lens wear affects blink patterns (Carney & Hill 1984; Hill & Carney 1984). None were using medi- cation.

The air-conditioned examination room tem- perature varied from 20-25°C and humidity 40- 43% during the study period. There was no ob- vious air current from the ventilation system of the room. Room illuminance was set at 350 lux throughout.

Upon arrival for the test, subjects were kept in a partitioned cubicle within the examination room for a period of 5 min. The subjects were told only that they were participating in a routine test for tear quality which involved observation using a slit-lamp and they were asked to wait whilst the test was being set up. A hidden video camera was

framed and focused on the subject’s face during this 5 min period. Had the subjects known that their blinking patterns were being observed, this may have had some effect on their blinking pat- terns (Doane 1980). The tape was later replayed and the blink frequency calculated for the last 2 min of the observation period. The average blink interval over this period was then computed. The first 3 min were regarded as the adaptation period to the test environment. In the analysis of blinks, the method described by Abelson & Holly (1977) and Collins et al. (1989) for the measurement of in- voluntary blink rates was used. TBUT was measured using the standard clinical

technique (Lemp & Hamill 1973). A fluorescein strip was moistened with 1 drop of a commercial unpreserved saline. The strip was applied flat to the upper bulbar conjunctiva once. The subject was positioned on the slit-lamp and observed under cobalt blue light. The subject was instructed to blink 5 times and then stare straight ahead with- out blinking. The eyelids were not held. A stop- watch was used to record the time taken for the first dry spot to appear. This procedure was re- peated 3 times for each eye and an average found for each eye. The lower value of the 2 eyes was taken as the TBUT for that subject.

Results

TBUT and blink intervals varied greatly from indi- vidual to individual. The mean TBUT value for the group was 14.9 sec (SD: 14.8) and the average blink interval was 5.2 sec (SD: 5.7). 16 subjects (39%) had TBUTs less than 10 sec.

The plot of blink interval and TBUT is shown in Fig. 1. A correlation coefkient of 0.69 was ob- tained which was highly significant ( P < 0.001). The best fitting line was Blink Interval =0.266 (TBUT) + 1.227. The regression line (line B) for normal subjects from Prause & Norn’s study is in- cluded in Fig. 1 for comparison.

Discussion

The group average TBUT of 14.9 sec is just at the lower limit of the range (15-34 sec) considered nor- mal by Lemp & Hamill (1973). It is also well below the normal average TBUT reported by Norn (1969)

40 1 ‘i;i 30 U - C 0 0 Q) cn - 20 - Q > Q)

C

b

c

- 10 x C .- - rn

B

P O ! I I I 1

0 20 40 60 80

TBUT (seconds) Fig. 1.

Scatterplot of blink interval against TJ3UT. Regression l i e A is derived from the present data. Regression line B

is from the normal subjects of Prause & Norn’s study.

of between 25-30 sec. 16 out of the 41 subjects (39%) in this present study had a TBUT of less than 10 sec and would be considered abnormal using the crite- rion suggested by Lemp & Hamill (1973). The rea- son for these low TBUT values is not known, al- though this finding is consistent with the clinical experience of contact lens practitioners working amongst the Chinese community in Hong Kong.

A strong correlation (r = 0.69) between TBUT and blink interval was found in this study. This supports the hypothesis by Prause & Norn (1987) that the break-up of the tear layer may be a stimu- lus for normal involuntarv blinking. The weaker correlation (r = 0.33) between these 2 factors re- ported by Prause & Norn (1987) is likely to be due to the methodology used to record blink rates, a point which they acknowledge. In the present study, the subjects were unaware that their blink- ing patterns were being observed and they were not asked to perform any visual task during the period of observation. Collins et al. (1989) video- taped their subjects to record blink frequency but engaged their subjects in conversation about the experiment during the recording period. They found a slightly stronger correlation (r = 0.38)

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which was not statistically significant. Their sub- ject numbers were small and the fact that the sub- jects were concentrating on what was being said may have affected their blinking patterns (Records 1979). The slopes of the regression lines from Prause & Norn’s study and the present study were similar (Fig. 1). However, Prause & Norn’s line was displaced higher about the y-axis (blink interval). This lends further support to the observations that concentration (in reading the letter chart) and dif- ficult or critical tasks decrease blink frequency (York et al. 1971; Pointer et al. 1985).

The results of this study establish that there is a relationship between TBUT and inter-blink inter- val. Instability in the tear layer is probably de- tected by the cornea prior to tear break-up and the blink sequence initiated to reconstitute the tear film.

Acknowledgments

Doane M G (1980): Interaction of eyelids and tears in cor- neal wetting and the dynamics of the normal human eyeblink. Am J Ophthalmol89: 507-516.

Hill R M & Carney L G (1984): The effect of hard lens wear on blinking behaviour. Int C1 Clin 11: 242-248.

Holy F J (1973): Formation and rupture of the tear film. Exp Eye Res 15: 515-525.

Lemp MA & Hamill J R (1973): Factors affecting tear film break-up in normal eyes. Arch Ophthalmol 89: 103- 105.

Millodot M (1977): Influence of age on the sensitivity of the cornea. Invest Ophthalmol Vis Sci 16: 240-242.

Norn M S (1969): Dessication of the precorneal tear film. I. Corneal wetting time. Acta Ophthalmol (Copenh) 47 865-880.

Pointer J S, Gilmartin B & Larke J R (1985): Visual perfor- mance with soft hydrophilic contact lenses. Am J Optom Physiol62: 694-701.

Prause J U & Norn M (1987): Relation between blink fre- quency and break-up time? Acta Ophthalmol (Co- penh) 65: 19-22.

Records R E (1979): Physiology of the Human Eye and Visual System, pp 1-24. Harper & Row Publishers, Hagerstown.

york M, ong J & Robbins J c (1971): Variation in blink rate associated with contact lens wear and task dif& culty.

I thank w. K. Tam, K. K. Ling, s. L- Chan, D. Cheng, c. w. Look and S. M. Chan for their assistance in this study. I also thank Prof. B. Brown and Mr. M. Collins for their helpful comments on the manuscript.

Optom & Am Acad Optom 48: 461-466.

Received on September 3rd, 1990.

Author’s address:

Optometry Section, Hong Kong Polytechnic, Kowloon, Hong Kong.

References

Abelson M B & Holly F J (1977): A tentative mechanism for inferior punctate keratopathy. Am J Ophthalmol 83: 866-869. Maurice Yap,

Carney L G &Hill R M (1984): Variation in blinking beha- viour during soft lens wear. Int C1 Clin 11: 250-253.

Collins M, Seeto R, Campbell L & Ross M (1989): Blinking and corneal sensitivity. Acta Ophthalmol (Copenh) 67: 525-531.

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