Short communication
Tear evaporation rates: What does the literature tell us?

https://doi.org/10.1016/j.clae.2017.12.003Get rights and content

Highlights

  • An inaccurate summary of evaporation rates was previously reported.

  • Updated tables of evaporation rates for studies between 1941 and 2016 were created.

  • The tables contain evaporation rates for normal and dry eyes.

  • Most values were converted to one unit of measurement to aid interpretation.

Abstract

Purpose

A previous literature review reported tear evaporation rates (TERs) from studies conducted on rabbits and humans between 1941 and 2003. Closer examination of the presented data revealed inaccuracies in the reporting of some values. This paper presents updated tables of TERs using values from the original papers cited in the review, in addition to incorporating new studies published between 2003 and 2016.

Methods

A copy of each paper cited in the literature review was obtained and checked against the evaporation rate reported in the review. If the expected value could not be found in the cited paper, other papers by the same author were consulted to see if the value had been reported elsewhere. A PubMed and Scopus database search was conducted to find papers published on tear evaporimetry since 2003.

Results

Two new tables of TERs were created, based on the values reported by the original author. To aid in interpretation, the majority of results are expressed in units of x 10−7 g/cm2/sec. Where it was not possible to convert these values, some values are expressed as x 10−7 g/sec, x 10−7 g/sec/eye or W/min.

Conclusions

Two new tables of TERs have been compiled to provide an accurate representation of the values reported in the original papers. These tables can be used as a point of reference for other researchers to compare their results.

Introduction

Dry eye disease is estimated to affect between 5 and 35% of the population [1]. The 2017 report of the Tear Film and Ocular Surface Society (TFOS) International Dry Eye WorkShop (DEWS) II classified dry eye into three main types: aqueous deficient (ADDE), evaporative (EDE), and mixed [2]. Meibomian gland dysfunction (MGD) is the leading cause of EDE, and is commonly encountered in clinical practice. Humans with an absent lipid layer, or an abnormal coloured fringe tear lipid pattern when viewed with specular reflection, have a four-fold increased rate of evaporation [3].

Evaporimetry is used to indirectly measure the rate of evaporation of the aqueous component from the tear film. The rate of water loss from the exposed ocular surface is typically investigated using temperature and humidity sensors incorporated within a goggle, of which there are two main designs − closed-chamber and open-chamber. Closed-chamber devices [[4], [5], [6], [7], [8], [9]] are fully enclosed and are usually housed within a swimming goggle. This prevents the ocular surface from interacting with the external environment. Open-chamber devices [[10], [11]] have a hole within the instrument which exposes the sensor to the ambient surroundings throughout the measurement. Many evaporimeters have also incorporated ventilation so that air of a known relative humidity (RH) and/or air flow can be added to the chamber [[4], [5], [6], [8], [12]].

The most common closed-chamber device reported in the literature [[8], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]] was designed by Mathers [8] and includes a dry air ventilation system (Fig. 1). The majority of published literature available on an open-chamber, unventilated evaporimeter was conducted with the ServoMed EP1 or EP3 (ServoMed, Sweden), which was a dermatological device that was modified for use on the eye [[3], [10], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]]. More recently, spectral interferometry [[44], [45], [46]] and infrared thermography [[47], [48], [49]] have been used to estimate the rate of tear film evaporation. Currently, the only commercially available evaporimeter is an unventilated, closed-chamber device, the Eye-VapoMeter (Delfin Technologies, Finland), which was recently validated for ocular use [9].

Since a standardised and accepted method of measuring and reporting evaporimetry does not yet exist, researchers have found a variety of ways of expressing the human tear evaporation rate (TER) (Fig. 2). Evaporation rates have most frequently been expressed in units of x 10−7 g/cm2/s or g/m2/h. Modified dermatological devices that were originally designed to measure water loss from the skin, such as the Eye-VapoMeter [[9], [50], [51], [52]], the ServoMed EP1 or EP3 [[10], [32]], and the Tewameter [11], calculate rates of evaporation in units of g/m2/h. Other researchers have reported TER in units of μl/min [[8], [13], [14], [15], [42], [43], [44], [53]], μl/cm2/min [[16], [17], [18], [19], [20], [21], [22], [23]], μm/min [[44], [45], [46], [54]], W/m2 [[47], [48]], and W/min [49]. Since there are a number of different ways of reporting TER, this leads to difficulty when comparing and interpreting the values reported between devices and varying ocular conditions or environments.

Mathers [55] published a literature review in 2004 that discussed the reported values of TER from the ocular surface available at that time. The first table of the review summarised the evaporation rate of 18 studies conducted on rabbits and humans between 1941 and 2003. The table included TER for humans with healthy, normal eyes and diseased eyes, with conditions such as dry eye or MGD.

Closer examination of the table revealed some inaccuracies between the values reported by the review [55] and the original values published by the cited author. These errors could have an impact on the validity of the summarised data, and its use as a point of reference for researchers and clinicians. Since 2003, many other studies have been published on normal and dry eye TER.

The aim of this paper is to present an updated summary of published tear evaporation rates (TERs) for healthy human eyes and dry eye or diseased eyes from 1980 to 2017. The manuscripts cited by Mathers [55] will serve as a synopsis of studies conducted between 1947 and 2002. The intention is to incorporate the original TER as a single unit of measurement (x 10−7 g/cm2/s), to aid in comparison of the reported values.

Section snippets

Methods

A copy of each paper cited by Mathers [55] in his first table, that listed the TER for normal and dry eyes, was obtained via online databases, the university library, or requested through Scholars Portal RACER Inter-library Loan. Papers that were not originally written in English were translated by colleagues that were fluent in German or Japanese. Each cited paper was checked against the values as reported by Mathers [55] for accuracy of transferral. If the specified rate of evaporation could

Results

Table 1 provides an updated summary table of TERs for normal and dry eyes. This includes all four of the animal studies and 12 out of 14 of the human studies that Mathers cited [55], in addition to more recent studies conducted between 2003 and 2016. The revised summary of TER has been expressed as the mean ± standard deviation, with one exception, as the TER was reported as the mean ± standard error [63]. All TERs have been stated in units of x 10−7 g/cm2/s, except for the relatively few

Discussion

In view of the increased number of studies published since the Mathers paper, and the increasing importance of TER measurement as reported by the TFOS DEWS reports, it was important to produce an updated version of the published values of evaporation rate. Moreover, researchers have reported using the data presented in the literature review by Mathers [55] as a reference with which to compare their results [[12], [65]]. Researchers who have used the summarised data from the literature review by

Conclusions

Two new tables have been created to provide an accurate representation of the TER for normal and diseased or dry eyes. The values in the table are based on the original values reported by the author and have been converted to units of x 10−7 g/cm2/s (where possible). The authors cited by Mathers [55] in his literature review have been used to represent studies conducted between 1941 and 2002. Newer data has also been added for studies carried out between 2003 and 2016. These tables can be used

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      Evaporation of human tears in vivo has been extensively studied. It was reported that healthy human tear evaporation rate is 5–10 times slower than water [7,12,31,42,43]. The reduced evaporation from human tears is generally attributed to the ability of tear lipid nanofilms to inhibit aqueous evaporation [1].

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      Other relevant papers were identified from the references cited by the papers revealed from the original search terms. The tear evaporation rate (TER) indirectly quantifies the evaporation of the aqueous layer of the tear film and is an important factor in tear dynamics (Goto et al., 2003; Tomlinson and Khanal, 2005; Wong et al., 2018). This measurement is performed on the exposed ocular surface by employing temperature and humidity sensors integrated within goggles worn over the eyes.

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      Multiple studies of human tear-aqueous evaporation rates in vivo have been described [41–43, 47–52]. The results of these studies were recently summarized in the comprehensive review [15] and it was concluded that, despite a huge variability of the results reported in literature, the current opinion is that in vivo human tears evaporate significantly slower than water, from 3 to 10 times. Nevertheless, numerous in vitro investigations of water evaporation through reconstituted meibum and model-lipid films have shown either very modest [18, 53], or practically no water-evaporation retardation [5,19,20].

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