Blinking and corneal sensitivity

ACTA 0 P H T H A L M 0 LOG I CA 67 (1989) 525-531

Blinking and corneal sensitivity

Michael Collins, Rhonda Seeto, Louella Campbell and Murray Ross

Centre for Eye Research, Department of Optometry, Queensland University of Technology, Brisbane, Queensland, Australia

Abstract. We have investigated the relationship between corneal sensitivity and blinking patterns. Using a group of 9 young normal subjects, measurements were made of blink patterns, central and peripheral corneal sensitivity and lid margin sensitivity both before and after use of topical corneal anaesthetic, proxymetacaine hydrochloride (Alcaine). Blink patterns were recorded through concealed filming and were later analysed in terms of blink frequency and the type of blinks (proportion of complete, incomplete, twitch and forced blinks). Blink rate was significantly lower following anaesthetic administration, but the relative proportion of blink types remained unchanged. We speculate that one of the factors responsible for normal involuntary blinks is the imminent break-up of the tear film which is sensed by the cornea.

Key words: blink rate - corneal sensitivity - proxymetacaine hydrochloride - corneal anaesthetic.

Blinking is primarily necessary for the formation and maintenance of the precorneal tear film. Nor- mal involuntary blinks have been classified as either; a twitch blink consisting of a small movement (flutter) of the upper lid, an incomplete blink in which the descending upper lid covers less than two thirds of the cornea or a complete blink where the descending upper lid covers more than two thirds of the cornea (Abelson & Holly 1977). Of all involuntary blinks, approximately 80% are complete blinks, 18% are incomplete blinks and 2% are twitch blinks (Abelson & Holly 1977; Carney & Hill 1984; Hill & Carney 1984; Collins et al. 1986).

Blinking can also occur as a defensive mechanism to protect the eyes from potential environmen- tal hazards. These involuntary forced blinks result

from external stimuli such as sudden loud noises, bright flashes of light or fast approaching objects (Records 1979). During a forced blink, the orbicu- laris muscles visibly contract, and the lower eyelid raises to complete lid closure, unlike normal involuntary blinks where the lower eyelid makes only a small nasal movement.

The normal blink rate is about 12 blinkshin (King & Michels 1957), but even under controlled experimental conditions, there are substantial dif- ferences in blink rate between individuals work et al. 1971). There is also evidence to suggest that individuals maintain not only a relatively constant blink rate, but also exhibit a distinctive blinking rhythm with fixed patterns of interblinks periods (Carney & Hill 1982).

A variety of psychological factors may influence the normal involuntary blink rate. York et al. (1971) report an inverse relationship between blink rate and the degree of dficulty of the visual task. Pointer et al. (1985) have noted that soft contact lens wearers adopt a subconscious blinking strategy to suppress blinking during critical tasks. Increased attention generally decreases the blink rate, while anxiety increases blink rate (King & Mi- chels 1957). Increased blink frequency has been reported to occur as a result of fatigue (Luckiesh & Moss 1937). A spontaneous blink usually occurs when shifting gaze from a near to distant object, or when changing the direction of gaze (Records 1979).

Little is known about the local mechanisms involved in normal involuntary blinking. In this study, we have tested the hypothesis that corneal

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sensitivity influences the normal rate of involuntary blinks. We constructed a concealed filming site and recorded the normal blinking patterns of a group of young normal subjects both before and after the instillation of a topical corneal anaesthetic. Our findings suggest that corneal sensitivity may play a partial role in the mechanism mediat- ing normal involuntary blinks.

Material and Methods Subjects A group of 9 subjects were selected from students at the Queensland University of Technology and screened to ensure that they fulfilled the following criteria: - less than 35 years of age, since there is a signif- cant reduction in tear-flow rate in subjects past 40 years of age (Furukawa 8c Polse 1978). - normal tear break-up time (TBUT), greater than 10 seconds (Lemp & Hamill 1973), - no ocular pathology, - no significant history of allergies, - central corneal sensitivity within normal limits, with threshold measuring at least 14 mg/mmn with the Cochet-Bonnet aesthesiometer (Millodot 1984),

y - 3.02 - 0.78x, r - 0.41

" -80 -60 -40 -20 0 2 0

Change In bllnk rate (%)

Fig. 1. Change in blink rate (percentage) versus decrease in central corneal sensitivity (0. The change in corneal sensitivity was calculated by dividing the C'IT (mg/mm2)

after anaesthesia by the C'IT (mg/mm2) before anaesthesia.

- no current or previous history of contact lens wear.

Procedure All experimental testing was conducted in the same laboratory over 3 weeks. During this period the mean daily temperature was 20.5" k 2.5"C and the mean daily humidity 39.9% k 9.8%.

Concealed filming of blinking patterns was necessary, since awareness of the observation of blinking patterns is thought to alter the type and frequency of blinking (Doane 1980). A video camera was mounted inside a box positioned approximately 1 m in front of the subject and below eye level. A dark filter obscured the subjects view of the video camera.

Subjects were seated in a chair with an ad- justable head-rest and were asked to keep their heads still in the head-rest during the experimental session. The camera was switched-on and di- rected towards the subject head-rest, prior to the subject entering the laboratory. While filming pro- ceeded, the subjects were engaged in conversation about the experiment they were about to under- take. They were told that the experiment was de- signed to investigate the effect of the corneal an-

y-3.21 -O.llx.r-0.56

-80 -60 -20 0 2 0 -40

Change In bllnk rate (x)

Fig. 2. Change in blink rate (percentage) versus decrease in peripheral corneal sensitivity (CTJJ. Peripheral corneal sensitivity was measured 1 mm in from the lower limbus. The change in corneal sensitivity was calculated by dividing the C'IT (mg/mm') after anaesthesia by the C'IT

(mg/mm2) before anaesthesia.

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2o 1 15

C -

y 4.00 + 0.01~. r I 0.14

-00 -60 -40 -20 0 20

Change in blink rate (K)

Fig. 3. Change in b l i i rate (percentage) versus change in lid margin sensitivity (CTT). Lid margin sensitivity was measured at the centre of the upper lid margin. The change in lid margin sensitivity was calculated by dividing the C'IT (mg/mm2) after anaesthesia by the C'IT

(mg/mm2) before anaesthesia.

aesthetic upon corneal sensitivity, but the concealed filming was not mentioned at t h i s stage.

This period of conversation lasted 3 to 5 min and was followed by measurements of central and periphed corneal sensitivity, lid margin sensitivity and TBUT. Measurements of corneal and lid margin sensitivity were made with the Cochet- Bonnet aesthesiometer (Lunneau) which has a filament diameter of 0.12 111111. The filament was fully extended and applied to the cornea or lid margin and if the subject did not feel the stimulus, the filament was shortened by 5 mm and the procedure repeated until the stimulus was felt by the subject. The length of the filament at the point of subjec- tive recognition of touch was converted to units of mg/mm2, referred to as corneal touch threshold (ClT). Peripheral CTT was measured 1 mm in fi-om the lower limbus, and lid margin sensitivity was measured at the centre of the upper lid margin. The technique of measuring TBUT was consistent with the method suggested by Lemp & Ha- mill (1973), which requires that the eyelids not be held and the minimum amount of excess fluid be introduced into the eye when administring the flu- orescein.

Following these measurements, one drop of the topical anaesthetic 0.590 proxymetacaine hydro-

40

y = 30.10 - 0.24x, r = 0.38

I" ~~

10 20 30 40 50

Blink rate (bllnkWmin)

Fig. 4. Blink rate @links/min) versus TBUT (sec). Both measurements were made prior to instillation of the anaes-

thetic.

chloride (Alcaine, preserved with 0.01 % benzalko- nium chloride) was instilled in each eye. Concealed filming of the subject then continued for a further 12 min while the subject was engaged in conversation. Twelve minutes following the instillation of the anaesthetic, central and peripheral corneal sensitivity, lid margin sensitivity, and TBUT were again measured. Subjects were then informed of the true nature of the experiment.

Analysis of blinking Analysis of the videotape of subjects blinking patterns was accomplished using the slow motion play-back facility of a video cassette recorder. The type of blinks (complete, incomplete or twitch) and the total number of blinks were recorded during the 3 min period before the instillation of anaesthetic and for the period between 9 and 12 min after the instillation of the anaesthetic.

We conducted a control study to investigate the effect of the anaesthetic upon TBUT and found that TBUT was initially shortened (by approximately 27%) following the instillation of the anaesthetic. A decrease in TBUT has previously been noted following the administration of a topical anaesthetic to the eye (Lemp & Hamill 1973). In our control study using one subject, the TBUT had returned to baseline levels within 9 min. Corned sensitivity diminished rapidly after the administration of the anaesthetic and returned to baseline le-

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Proportion of blinks (%) Touch threshold (mg/sq mm) Blink rate (blinks/ Lid

Comp Incom Twitch Central Periph min) margin

vels in approximately 18 min. Based upon these data we analysed the videotape of blinking patterns for the 3 min between 9 and 12 min after the instillation of the anaesthetic. This time delay en- sured that tear film stability was no longer affected by the anaesthetic, but corneal sensitivity remained substantially diminished.

The criterion in assessing the type of blink con- formed to the method proposed by Abelson & Holly (1977) and is described in the introduction. Only the blinking pattern of the right eye (chosen arbitarily) was analysed for each subject; Records (1979) notes that blinking is bilateral, isochronous and of equal amplitude in each eye.

TBUT (set)

Results

Following the administration of the anaesthetic, there was a significant reduction in the total number of blinks (Wilcoxon’s sign rank sum test, P < 0.05; t-test, 0.05 < P < 0.10). The mean blink rate before the instillation of the anaesthetic was 24.8 blinks/min (SD = 10.2) and after the anaesthetic was 17.2 blinks/min (SD = 6.6). However, not all subjects showed a decrease in blink rate after the instillation of the anaesthetic. One subject showed a slight increase in blink rate, and another subject showed no change in blink rate after the administration of the anaesthetic.

Central corneal sensitivity showed a substantial reduction from a mean CTT of 11.2 mg/mm2 before anaesthesia to a mean CTT of 56.8 mg/mmz after anaesthesia (approximately 12 min after the instillation of Alcaine). Peripheral corneal sensitivity was similarly reduced from a mean CTT at 11.4 mg/mm2 before anaesthesia to a mean of 72.8 mg/ mm2 after anaesthesia. Lid margin sensitivity was 18.4 mglmm‘ before anaesthesia and 63.2 mg/mm2 following anaesthesia.

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The change in blink rate after the instillation of the anaesthetic shows a moderate correlation with the change in central corneal Sensitivity. The correlation was not statistically significant (r = 0.41, P > 0.10); the data are presented graphically in Fig. 1. The moderate association between the change in blink rate versus the change peripheral corneal sensitivity was also not statistically significant (r = 0.56, P > 0.10) (Fig. 2). There was little correlation between the change in blink rate and the change in lid margin sensitivity (r = 0.14, P > 0.10) following the instillation of the anaesthetic (Fig. 3).

The mean TBUT before the instillation of the anaesthetic was 21.8 sec (SD = 4.4) and 12 min after the instillation of the anaesthetic was 23.3 sec (SD = 5.8) (Table 1). This change in TBUT is not sig nifcant (t-test). There was no significant change in the percentage of complete, incomplete or twitch blinks after the instillation of the anaesthetic (t- test) (Table 1). The relationship between blink rate and TBUT before the instillation of the anaesthetic shows a moderate correlation which is not statistically significant (r = 0.38, P > 0.10) (Fig. 4).

Discussion

The significant decrease in blink rate following the instillation of a corneal anaesthetic supports the hypothesis that corneal sensitivity is at least partly involved in the mechanism controlling normal involuntary blinks. Lyle & Page (1975) anecdotally report that the ‘blink reflex is inhibited’ during corneal anaesthesia.

The relative proportion of blink types before and after the anaesthetic was largely unchanged even though the rate of blinking diminished. This finding suggests the mechanism which relates corneal sensitivity to normal involuntary blinking is

the same for all 3 blink types: complete, incomplete and twitch.

To investigate the potential for the experimental protocol to influence the subjects blinking pattern, the procedure was repeated with one subject on another day using preserved, buffered saline solu- tion (placebo) substituted for the Alcaine. Follow- ing the first experimental trial using Alcaine, this subject was not informed of the true nature of the experiment (ie. concealed filming of blink patterns). The saline was disguised in a bottle similar to that of Alcaine, and the contents of the bottle were not discussed with the subject. The experimental procedure was then conducted in an idential manner to the first testing. The blink rate remained virtually unchanged after the saline instillation, compared with a substantial reduction in blink rate in the same subject following the instillation of the anaesthetic. This result suggests that the experimental protocol had little influence upon the reduction in blink rate observed after the use of the corneal anaesthetic.

There was a large intersubject variation in the change of blink rate following instillation of the anaesthetic ranging from a 67% reduction in one subject to a 13% increase in another subject. This variation is not accounted for by variations in the reduction in corneal sensitivity following the instillation of the anaesthetic, since the correlation of these factors was only moderate and did not reach statistical significance. There are a number of possible explanations for the intersubject va- riance noted in the change of blink rate following the administration of the anaesthetic in this study. The corneal epithelium is known to be affected in a variety of ways by topical anaesthetics and preservatives (Ramselaar et al. 1988; Josephson & Caffery 1988; Burnstein 1980; Wilson & Fullard 1988). It may be that intersubject variations in sus- ceptibility to these corneal changes has con- founded the association between blink frequency and corneal sensitivity in this study by increasing the ‘noise’ in the data. Variations in blink rate associated with psychological factors such as attention, emotional factors such as apprehension or anxiety, and involvement in the conversation taking place during filming, were difficult to control in this study and probably also contributed to the va- riance in blink rate between subjects.

The blink rates of subjects in this study is higher than blink rates reported in other studies (King &

34 Acta Ophthal. 67.5

Michels 1957), although this probably reflects the fact that blink rates were measured while the subjects were engaged in conversation. Conversation is reported to increase blink frequency (Records 1979), which highlights the difficulty in controlling psychological variables in any study of blinking patterns, such as this study.

The mechanism which accounts for the association between blink frequency and corneal sensitivity is not clarified by this study. It may be that changes in tear film stability prior to tear film break-up, are detected by the sensory nerve endings in the cornea which thereby triggers an involuntary blink. Support for this theory comes from the moderate association between blink rate and TBUT in this study and a similar finding previously reported by Prause & Norn (1987). Our findings showing substantially higher correlations between the change in central and peripheral corneal sensitivity and the change in blink rate following instillation of the anaesthetic, compared with the change in lid margin sensitivity and the change in blink rate, adds further weight to this argument. Holly (1973) has anecdotally suggested that dry spot formation triggers an involuntary blink be- cause it ‘irritates the nerve endings’ and Prause & Norn (1987) have similarly suggested the pos- sibility that ‘seconds before the slit-lamp evidence of tear break-up ... changes have taken place in the precorneal film sufficient to stimulate a new blink’.

The mechanism which initates an involuntary blink may result from localized changes in tear film stability prior to tear break-up. The interblink period must be less than tear film break-up time to maintain the integrity of the tear film and stop the formation of dry spots between blinks. Therefore some change in the tear Wcornea, just prior to tear film break-up would be a convenient mechanism to trigger the normal involuntary blink and maintain tear film stability. Holly (1973) has suggested a mechanism for tear film break-up based upon gradual lipid contamination of the mucin layer. Sharma & Ruckenstein (1985) have suggested an alternative mec.hanism of tear film break-up based upon dispersion forces acting in the much layer which eventually lead to tear film rupture as the aqueous layer of the tears comes into contact with the hydrophobic epithelium. Localized changes at the epithelial surface via either of these tear break-up mechanisms could theoretically be

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‘sensed’ by the rich underlying plexus of corneal nerves. Corneal nerves are known to be sensitivity to touch and pain (Millodot 1968), while some con- jecture exists regarding their ability to sense changes in temperature (Lele & Weddell 1956; Beuerman & Tanelian 1979). If the cornea does sense the imminent break-up of the tear film, then the sensation might involve touch, ionic changes or temperature changes at the epithelial surface related to this phenomenon.

There are a number of difficulties with the theory that normal involuntary blinks are initiated primarily by imminent tear film break-up. For example following enucleation of both eyes, blink rate is reported to remain constant (Records 1979). The wearing of soft contact lenses creates an artifi- cial surface over the cornea and it becomes im- possible for the cornea to sense imminent tear break-up on the front surface of the lens. Similar difficulties arise in the case of hard contact lens wearers. After approximately 3 weeks of wear, both soft contact lens wearers (Carney & Hill 1984) and hard contact lens wearers (Hill & Carney 1984) have been reported to show increased blink frequency. On the other hand, Alexander-Katz (1971) and Sarver et al. (1969) report a reduction in blink rate following long term soft and hard contact lens wear, respectively.

Corneal sensitivity is significantly reduced following surgical procedures such as cataract extrac- tion and keratoplasty. After cataract surgery, central corneal sensitivity is reported to be reduced by up to 85% (Holden et al. 1982), and there is little recovery of sensitivity up to 8 years following the surgery (Draeger 1979). Following keratoplasty, there is initially a profound loss of central corneal sensitivity and only minimal recovery of sensitivity after 3-4 years post-surgery (Ruben & Colebrook 1979; Draeger 1979). Ageing also causes a substantial loss of corneal sensitivity, particularly past about 50 years of age (Millodot 1977). We antici- pate that the blink rate of older patients and patients who have undergone these surgical procedures will be diminished. This could result in dry eye problems or cause difficulties during contact lens wear for these patients.

We have demonstrated a significant reduction in blink rate following the administration of a topical corneal anaesthetic (0.5% proxymetacaine hydrochloride) and confirmed previous research sugges- ting a weak association between blink rate and

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TBUT. These findings are consistent with the hypothesis that changes in tear film stability prior to tear break-up, are sensed by the cornea and subse- quently initiate a normal involuntary blink.

Acknowledgments We thank Bruce Johnson for his assistance in the experiment and Brian Brown and Christine Wildsoet for their helpful advice.

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Received on May lst, 1989.

Author's address:

Centre for Eye Research, Department of Optometry, Queensland University of Technology, 2 George Street, Brisbane, Queensland, Australia, 4000.

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Blinking and corneal sensitivity