Blinking Abnormalities

Mechanism of blinking

Eyelid closure during blinking is effected by the orbicularis oculi muscle, which is innervated by the seventh cranial nerve. The act of blinking is accomplished primarily by the upper lid. The lower lid remains virtually stationary. Closure is characterised by a progressive narrowing of the palpebral fissure, in a zipper-like fashion, from the outer canthus to the inner canthus. This moving wave of closure serves to force aqueous in the inter-palpebral fissure toward the lacrimal puncta, thus aiding tear drainage.

Spontaneous blinking occurs in all terrestrial vertebrates possessing eyelids, although the rate of blinking varies considerably among species. Large predatory cats execute less than one blink per minute, whereas some species of small monkeys have blink rates as high as 45 times per minute. Infants have a very low spontaneous blink rate.

Spontaneous blinking occurs in patients who have total congenital blindness, indicating that it is a phenomenon that is not learned or dependent on visual input. The rate of spontaneous blinking may alter in response to changes in the level of visual activity and in different emotional states. General environmental changes, such as the level of dryness or wind flow, may also alter the spontaneous blink rate. The frequency and completeness of blink is reduced during intense concentration, for example, when reading or working on a visual display unit.

Types and patterns of blinking

Researchers must employ covert methods to monitor types and patterns of spontaneous blinking; this is necessary because of the methodological problem that subjects will alter their blinking activity if they are aware that this is being assessed. Typically, subjects under such circumstances will execute an increased proportion of voluntary forced blinks and a greater overall blink frequency. For this reason, hidden observers or video cameras are employed to record blinking activity while the subject, for example, is engaged in discussion or is asked to observe a silent movie.

Zaman and Doughty have highlighted other potential methodological pitfalls in quantifying blinking behaviour. For example, simple averaging of blink rates may not always be appropriate because of the high chance of a non-Gaussian data distribution. These authors concluded that eye-blink monitoring for at least 3 minutes is required for valid data analysis. Fortunately, almost all blink researchers have used observation times in excess of this.

According to Abelson and Holly, blinking can be classified into four basic types:

• Complete blink – the upper eyelid covers greater than 67% of the cornea.

• Incomplete blink – the upper eyelid covers less than 67% of the cornea.

• Twitch blink – a small movement of the upper eyelid.

• Forced blink – the lower lid raises to complete eye closure.

The percentage of all blinks that can be characterised by each of these four blink types, as determined by Abelson and Holly, is illustrated in Figure 4.2 . Subsequent research has confirmed these findings.

Tsubota et al. developed a computer-interfaced ‘blink analyser’ to accurately measure the time course and pattern of blinking in 64 normal volunteers. They found that the average time taken to execute one complete blink (which they defined as the upper lid covering greater than 85% of the cornea) was 0.20 ± 0.04 second and that the average inter-blink period (IBP) was 4.0 ± 2.0 seconds. Taking one complete blink cycle as the sum of the blink time and IBP (0.2 + 4.0 = 4.2 seconds) gives an average blink frequency of 14.3 blinks per minute (i.e. 60/4.2). This result is consistent with previous estimates of the spontaneous blink rate in humans.

The small interruption to visual input during a blink is only thought to be of practical significance in occupations or tasks requiring constant monitoring of rapidly changing visual images. (Paradoxically, blinking is problematic for researchers monitoring blinking activity of experimental subjects, either directly or via video replays; in the latter case, viewing in slow motion solves this problem.) Volkmann et al. proposed that suppression of the visual pathway is associated with blinks, so the momentary interruption to visual input does not produce a conscious interruption to visual perception.

Various authors have suggested that there is a gender difference in blink rate. Hart proposed that males blink more frequently compared with females, whereas Tsubota et al. have suggested the opposite; neither group has statistically validated their claims. Yolton et al. shed light on this issue by measuring the spontaneous blink rate in males, females not using oral contraceptives and females using oral contraceptives. The blink rates observed in these three groups were 14.5, 14.9 and 19.6 blinks/min, respectively. This finding indicates that there is no intrinsic gender difference in the blink rate, but the use of oral contraceptives causes a significantly greater blink rate, for reasons which are unclear.

Purpose of blinking

Spontaneous blinking in non–lens wearers serves the following beneficial functions:

• Maintenance of an intact pre-corneal tear film by constantly spreading the tear film evenly across the corneal surface.

• Removal of intrinsic and extrinsic particulate matter by forcing such debris into the lower lacrimal river.

• Facilitation of tear exchange by constantly swiping tears toward the puncta located at the inner canthus.

Paradoxically, blinking may also be harmful to the already-compromised ocular surface. This has been discussed by Cher, who introduced the concept of ‘blink-related microtrauma’. A prime example is superior limbic keratoconjunctivitis, which is a mechanical result of blinking under prolonged non-physiological conditions. Other ocular surface disorders regarded as primarily derived from blink microtrauma are filamentary keratitis, blepharospasm and severe ptosis, canthal/palpebral froth, conditions resulting from disordered eyelid lining and contact lens–related damage.

The importance of maintenance of an intact pre-corneal tear film has been demonstrated in a number of studies that have examined blinking behaviour in patients suffering from symptoms of dry eye.

Prause and Norn proposed the theory that spontaneous blinking is, in part, a stimulus to rupture of the pre-corneal tear film. They tested this hypothesis by measuring tear break-up. time (TBUT) and the IBP in normal individuals and in patients with dry eye. In both groups, there was a statistically significant positive correlation between these two parameters; that is, the more rapidly the tear film breaks up, the more frequently the patient blinks. That finding was subsequently confirmed by Yap in a group of normal subjects, although two other research groups found no such association.

Prause and Norn also demonstrated that, in general, IBP was slightly less than TBUT, suggesting that patients adopt a blink rate that will prevent tear break-up. Using quantitative videographic analysis, Tsubota et al. found that IBP in patients with dry eye was 1.5 ± 0.9 seconds, compared with 4.0 ± 2.0 seconds in normal subjects.

Indirect evidence of the link between TBUT and IBP comes from the work of Tsubota and Nakamori, who measured the tear evaporation rate from the ocular surface (TEROS) in 17 normal volunteers and found an increase in the blink rate with increasing TEROS. This result is consistent with previous demonstrations of the positive correlation between IBP and TBUT because tear break-up associated with more rapid blinking would be expected to result in higher rates of tear evaporation.

Although the weight of evidence does suggest that the blink rate is, in part, dependent on the integrity of the tear film, other factors must also be involved. This fact was demonstrated by Collins et al., who found that blinking persisted after the instillation of a corneal topical anaesthetic in a group of normal subjects. The blink rate did, however, drop from 24.8 to 17.2 blinks/min. If tear break-up was the sole determinant of the blink rate, blinking would have stopped in the eyes with anaesthetised corneas.

Blink rate

It has long been recognised that contact lens wear can alter blinking activity. Hill and Carney demonstrated, in a group of seven subjects, that blink rate increased from 15.5 blinks/min to 23.2 blinks/min after being fitted with rigid polymethyl methacrylate (PMMA) contact lenses. A similar result was reported by York et al., although Brown et al. did not confirm this result. It appears, however, that PMMA lens–induced alterations to the blink rate may be more related to reflex blinking rather than to spontaneous blinking; that is, the increased blink rate may be a result of continual irritation caused by the lens edge buffeting against the lid margin.

In another group of seven subjects, Carney and Hill demonstrated that the blink rate increased from 12.1 blinks/min to 20.3 blinks/min after the subjects were fitted with soft contact lenses (presumably hydroxyethyl methacrylate (HEMA)). The reason for this is less clear, as soft lenses would be expected to be more comfortable and thus to induce less reflex blinking activity, although it should be noted that the earlier study of Brown et al. found that the blink rate was essentially unaffected by soft lens wear.

Although the blink rate may be altered during contact lens wear, a supplementary consideration is whether or not any alteration to blinking activity is permanent. Yolton et al. reported that the blink rate (16.2 ± 8.9 blinks/min) in a cohort of habitual contact lens wearers (the lens type was not specified) who had stopped lens wear at least 24 hours before the blinking assessment was identical to that of a matched control group of subjects who had never worn contact lenses (16.2 ± 9.5 blinks/min), suggesting that contact lens–induced alterations to the blink rate are only evident during lens wear.

Contact lens wearers experiencing end-of-day dryness have been observed to display an increased blink rate. It is thought that the increased blink rate could act to improve the ocular surface environment and relieve symptoms.

Lopez-de la Rosa et al. reported that the blink rate increased when wearers of hydrogel contact lenses were exposed to simulated adverse environmental conditions (in-flight air cabin environment: 5% relative humidity, localised air flow, 23°C temperature and 750 millibars (mb) atmospheric pressure) for 90 minutes in an environmental chamber, compared with standard environmental conditions (50% relative humidity, 23°C temperature and 930 mb of atmospheric pressure). The authors suggested that contact lens dehydration was the stimulus for the increased blink rate. Kojima et al. reported similar findings.

The type of contact lens care solution used can affect blink rate during contact lens wear. Yang et al. reported that lens care solutions incorporating wetting agents are associated with a significantly reduced blink rate, presumably because of improved pre-lens tear film (PLTF) quality.