Tear dy s funct ion s yndromes such as Sjögren’s, evaporative dry eye or aqueous-deficient dry eye represent a spectrum of ocular disorders with a huge impact on vision, ocular health and quality of life. Efforts to develop new therapies to address these conditions face a daunting gauntlet of intrinsic and extrinsic factors that modulate tear production, tear composition and tear function. While there are a number of tools available for evaluating tears, many seem to fall short in terms of their ability to report reliably and reproducibly on the changing attributes of tear physiology that underlie dryeye diseases. This month we examine the many ways in which tears are measured, and consider the degrees to which these many metrics measure up.
The tear film is an amalgam of ingredients derived from three different sources: goblet cells; meibomian glands; and lacrimal glands.
Lacrimal secretions are mixtures derived from two cell types found within the acini of the glands. Serous cells form acini that secrete electrolytes and mixtures of many different proteins (estimates suggest 200 to 300 different polypeptides in humans). The release of salts such as Na+, K+, Cl- and Ca2+ provides the osmotic force that pulls water from the gland, forming the bulk of the tear volume.
Both basal and reflex tearing occur in response to autonomic inputs, including stimulation of parasympathetic (via transmitters acetylcholine and VIP) and sympathetic (norepinephrine) nerves.
as well as conjunctival sites (including goblet cells) and meibomian glands. Studies in mouse models provide compelling evidence that it is the temperature-sensitive corneal sensory nerves that regulate basal lacrimation, providing a set point of secretory stimulation that is exquisitely sensitive to small changes in corneal surface temperature.
A similar sensory circuit provides input from the upper and lower eyelids, which then feeds back to the orbicularis oculi and levator palpebrae muscles that control blinking.
Layered upon the basal level of tear secretion is a stimulated component that is a response to external and internal factors including diurnal patterns, environmental fluctuations and physiological status. Diurnal changes in tear composition, particularly the variation in the variety and concentration of tear proteins, are well-established.
Environmental effects provide the most significant impact on a patient’s reflex tearing.
Other environmental factors such as light-induced alterations in blink behavior can also have substantial impact on tear turnover and tear-film stability. In addition, there are tear reflex stimuli triggered by either nasal or oral sensory stimuli (one has but to consider the effects of the humble onion). Overall, reflex or stimulated tearing comprises well over half of the total tear volume, and is a key to homeostatic maintenance of tear-film stability and ocular health.
In addition to external factors, systemic physiological factors can also impact the flow of aqueous tears. There is evidence that in older subjects, reduction in whole body hydration can lead to reduced aqueous flow and a concentration of tear fluid components.
Two other key physiological factors that can impact tear flow are the production and secretion of meibum and mucin to complete the triad of components that comprise the tear film. A lack of sufficient meibum, in particular, can alter evaporative properties and result in a reduction of aqueous tears. This highlights the conundrum that while we strive to isolate the specific causes of our patients’ dry eye— aqueous deficiency, evaporative dry eye, Sjögren’s syndrome or MG disease— the interdependence of each facet of the tear film limits our ability to focus treatment on a single underlying defect.
A host of techniques are available for assessing aqueous tear production, for quantifying tear-film properties and for measuring rates of tear turnover.
There is a growing realization that while simple tests such as Schirmer’s or the phenol red thread may provide a measure of tear output suitable for a clinical evaluation, they don’t provide a sufficient level of sensitivity or reproducibility to be applied to drug discovery efforts. Evaluation techniques such as the measurement of tear meniscus height or fluorophotometry appear to be better suited for studies in which a specific metric of tear production is needed.
Meniscus measures can be done with a slit lamp, although they are now more often measured using OCT. Both approaches benefit from being relatively noninvasive. This non-invasiveness is key: Because of the sensitivity of feedback inputs, the issue of reflex tearing is a major hurdle to any successful assessment of aqueous output.
Non-invasive measurement is key: Because of the sensitivity of feedback inputs, the issue of reflex tearing is a major hurdle to any successful assessment of aqueous output.
An evaluation of tear output metrics in the DEWS report states, “For studying the tear film, the greatest opportunity lies in the use of noninvasive techniques involving the sampling of optical radiation reflected from the tear film.”16 One such non-invasive approach is fluorophotometry (FP), a technique that measures the rate at which tears on the ocular surface are replaced.
Fluorophotometry, which is sometimes referred to as tear turnover, uses a fluorescent tracer in the tears and follows the decline of tracer concentration in tears over time. By measuring the kinetics of this process it’s possible to derive values for tear turnover, total tear volume and tear “flow rate.” While the equipment needed makes the process prohibitively
expensive for use in a general practitioner’s office, the reliability and non-invasive nature of the measure suggest that it should be the metric of choice for precise assessment of aqueous production in clinical research.
Like many clinical tools used by the dry-eye diagnostician, FP can be an outstanding evaluation device once the critical parameters are identified and optimized.
In terms of the mechanics of performing FP, a small volume (≈1 μl) of tracer fluorophore is applied to the conjunctival fornix. After a brief delay, the ocular surface is scanned for a luminescence signal at regular intervals for 20 to 30 minutes. The decay in the signal represents the continuous dilution of the fluorophore in the tear volume; by measuring the rate of that decay it’s possible to generate a value for the tear turnover rate, typically in the range of two to four minutes (See Figure 1, at left). Extrapolation to a theoretical zero point can also yield a value for the patient’s total tear volume, but the real value in FP may be in its ability to follow changes in turnover rates before and after test compounds.
Studies conducted at our research firm, Ora Inc., have refined the protocols used for FP in order to improve reproducibility while reducing the variability of the method. These improvements include ergonomic optimization during measurements, as well as adjustments to the volume and concentration of fluorophore that’s used for the measurement. With these refinements, FP can be an invaluable tool in clinical studies of dryeye therapies, either as an inclusion criterion, a clinical endpoint following the clinician’s therapeutic intervention, or both.
A comparison of dry-eye metrics (See Table 1, p. 42) suggests that FP has high sensitivity and specificity, and is superior to the other well-known measures of tear production in terms of its predictive value. Simply stated, FP displays a superior ability to correlate with other signs and symptoms of dry eye such as corneal fluorescein staining and ocular surface disease index survey data. The biggest challenge to the use of FP as a metric going forward is the need for more studies; it’s possible that FP may be of less predictive value with some forms of dry-eye disease, but considering the complexity of aqueous tear-film regulation this challenge is best met by an empirical approach.
It’s likely that a combination of the current standards of ocular surface staining and ocular surface disease index surveys, in combination with objective metrics such as FP, will provide the jump start needed to gain traction in the search for new dry-eye therapies.
Dr. Abelson is a clinical professor of ophthalmology at Harvard Medical School and senior clinical scientist at the Schepens Eye Research Institute. Ms. Kelley and Dr. McLaughlin are medical writers at Ora Inc.