There are many lines of defense protecting the ocular surface from harm, beginning with reflex-gestures and brow line, and culminating with the eyelashes, lid, tear film and epithelium. Blinks, which last between 300 and 400 milliseconds, are integral to ocular surface health, as they help maintain the tear film.
The spontaneity of the eyeblink has piqued the curiosity of many scientists, and more than 80 years of research has been devoted to unraveling the mechanism behind the human eyeblink. Scientists have proposed various theories for the spontaneous nature of the blink: Some have suggested the presence of an internal blink pacemaker regulated by the brain stem,
A blink can be either a reflex response to corneal stimulation (e.g., irritation by a foreign body) or the result of stimulation of the supraorbital nerve. Blinks are carried out by a series of motor neuron activations. Activation of the orbicularis oculi (supplied by the zygomatic branches of the facial nerve, cranial nerve VII), the abducens (cranial nerve VI) and accessory abducens motor neurons, is responsible for closing the eyelids. The levator palpebrae motor neurons (innervated by the superior branch of the oculomotor nerve: cranial nerve III) lift the upper eyelid.
The action of blinking not only protects the cornea and conjunctiva from external damage, it also enables the removal of debris from the ocular surface. Additionally, blinking allows for the distribution of tarsal goblet cell mucin and works to increase lipid layer thickness.
Blink rate can vary across tasks and states, and can increase in frequency and completeness. Indeed, even the intensity of illumination and glare has been shown to impact the frequency of blinking.
One’s blink rate may also be influenced by internal factors, such as fatigue, stress, medications, ocular surgery and surface conditions. Research, including a paper presented at a meeting for the Society for Psychophysiological Research more than a decade ago, has demonstrated that fatigue and stress induce an increased blink rate in normals.
Patients with dry eye can find their symptoms exacerbated by the additional ocular exposure caused by a decreased blink rate. A decreased blink rate causes an increase in the rate of tear-film breakup, along with a decrease in tear and mucin production and meibomian secretions in both normals and dry-eye patients with the disruption of tear-film components. As such, dry-eye patients are at a particular disadvantage during reduced-blink-rate activities, such as working on the computer, reading, driving, etc. Exacerbation of dry-eye signs and symptoms from a reduced blink rate can lead to visual function disturbances. We must keep in mind that a normal tear film protects the integrity of the ocular surface from keratitis and protects our vision: it is difficult to see through a broken tear film, especially with the presence of keratitis (Ousler GW et al. IOVS 2007;48: ARVO E-Abstract 410). Furthermore, a reduced tear-film breakup time (TFBUT) and the prolonged exposure of the ocular surface produce a disrupted air-fluid interface, which in turn disrupts light transmission to the retina. Hence, visual function is impacted.
Patients with ocular surface dysfunction are at a serious disadvantage when performing tasks that are typically linked to a prolonged interblink interval. This is not to say that patients with a short tear-film breakup time experience great discomfort during regular conversation; rather, ocular surface dryness and irritation are more likely exacerbated during tasks that require greater visual attention and therefore are associated with a lower blink rate. The dry-eye patient with a TFBUT of three seconds is at a specific disadvantage when working on a computer-oriented task, where the interblink interval can slow to as long as 12 seconds. Over the course of an hour, the ocular surface conditions may worsen, and keratitis may increase as a result of the successive, extended interblink intervals.
To date, it has been difficult to quantify the decay of visual acuity within the interblink interval. A new test, however, has been specifically developed and validated by our researchers at ORA Clinical Research and Development here in North Andover to provide an accurate measure of visual function. The interblink interval visual acuity decay (IVAD) test provides the necessary measurement of visual function in real time (Walker PM et al. IOVS 2007;48: ARVO E-Abstract 422). A computer-based paradigm incorporates the presentation of Landolt’s C’s at the patient’s best-corrected visual acuity. The patients are instructed to track the orientation of the C by pressing a button on a keypad. As the patient’s BCVA declines during his interblink interval, the size of the stimuli increases accordingly. Visual acuity decay results are on the order of milliseconds. Unquestionably, visual function includes more than static visual acuity. The ability to measure visual acuity over time, in real-time, is tantamount to obtaining precise information on everyday task performance.
Maintaining one’s best-corrected acuity during the interblink interval is unlikely if the tear-film breakup is early and blink is late, with a significant time between blinks. Such an instance leads to an interval of decreased vision (Walker PM et al. IOVS 2007;48: ARVO E-Abstract 422). A study that investigated the relationship between central corneal staining and visual function in dry-eye patients used IVAD measurements to assess functional visual acuity in real time. Resulting data showed that patients with central corneal staining could not maintain their BCVA for as much time in between blinks as patients without central staining (Ousler III GW et al. IOVS 2007;48: ARVO E-Abstract 410). Consequently, problems can arise over time with rate of information recognition, perceptual processing, fatigue and function.
Clinically, investigating blink rate in relation to patients with ocular surface conditions must be constrained within a specific task and meaningful to the functional outcome of the patient’s everyday life (e.g., reading). If a relevant blink rate is not measured, as is frequently the case,2 no connection may be drawn between the patient’s blink rate and the interference of ocular symptoms with everyday life.
Hence, researchers must make distinctions between blink rates. As no single “normal” blink rate exists, ranges of blink rates on specific tasks are a good starting point. It may be that a patient is an avid reader, a task associated with a long interblink interval. Awareness of the task-dependent nature of blink rates can greatly aid in treating patients with ocular surface conditions, who in turn require more frequent blinking to replenish the ocular surface. Ernest Lowenstein, OD, PhD, of the New England College of Optometry, has proposed a blinking exercise routine for such a purpose. Nevertheless, awareness of such blink patterns themselves can provide insight into the patient’s daily routine and therefore the ability to suggest modifications, such as alternating long interblink interval tasks with shorter ones.
Further environmental influences must also be investigated. The association between scotopic vision and blink rates may also provide further insight into the underlying mechanisms. Blinking during tasks such as night driving may elucidate the role of external lighting influences on blink rate. The impact of air quality (i.e., the amount of irritants and pollutants that are present) must also be considered in relation to the dynamic blink rate.
The ability of drug therapies to treat individuals with ocular surface conditions and normalize their blink rate can have an enormous impact on their quality of life. Therapies that can extend tear-film breakup time can provide a protected ocular surface during the interblink interval, allowing dry-eye patients to go about their daily lives without having to worry about ocular discomfort, irritation or blurring.
Dr. Abelson, an associate clinical professor of ophthalmology at Harvard Medical School and senior clinical scientist at Schepens Eye Research Institute, consults in ophthalmic pharmaceuticals. Dr. Walker is a clinical scientist at Ophthalmic Research Associates in North Andover.