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Clinical assessment of the tear film.

The assessment of the tear film is key to aid diagnosis and inform treatment in dry eye patients. This article will discuss the clinical assessment of the tear film by outlining a range of different techniques.
Optometrists               COMMUNICATION   OCULAR        OCULAR
                                           EXAMINATION   DISEASE
Therapeutic optometrists   OPTIONS

Dispensing opticians       OCULAR          CONTACT
                           EXAMINATION     LENSES

Contact lens opticians     COMMUNICATION   OCULAR        OCULAR
                                           EXAMINATION   DISEASE


1 CET POINT

Introduction

The tears are a thin, moist film of fluid covering the exposed ocular surface. Necessary for the health and normal function of the eye and visual system, any abnormality in composition may lead to signs and symptoms of dry eye disease. (1) It is important to assess the tear film to aid diagnosis and inform treatment in dry eye patients. This article will discuss the clinical assessment of the tear film.

Normal anatomy and physiology

A healthy tear film is required for a number of important functions including maintenance of vision, ocular surface defence, lubrication, and transport of nutrients. (2-7) Any dysfunction of the lacrimal functional unit (comprising the lacrimal glands, ocular surface (cornea and conjunctiva), eyelids, meibomian glands, and associated sensory and motor nerves) can cause tear film instability. (8)

The tear film consists of: a superficial outer lipid layer, produced by the meibomian glands; a thicker intermediate aqueous layer, produced by the lacrimal glands (main and accessory); and an inner mucin layer, produced by the goblet cells of the conjunctiva. A number of different tear film structures have been proposed, including most recently the suggestion of an aqueous-mucin continuum, anchored by glycocalyx. (9-11) However, throughout this article and for the purpose of clinical assessment, we will assume the traditional tri-layer structure. The thickness of the tear film has also proven to be subject to great debate. Early estimates varied between 4[micro]m and 8 [micro]m; (12-13) although these figures have been disputed, with values ranging from 3-40 [micro]m, (14-17) it is clinically acceptable to assume a tear film thickness of ~7 [micro]m.

Dry eye aetiology and classification

Published in 2007, the Dry Eye Workshop (DEWS) describes dry eye as "a multi-factorial disease of the tears and ocular surface which results in symptoms of discomfort, visual disturbance and tear film instability, with potential damage to the ocular surface. It is accompanied by increased osmolarity of the tear film and inflammation in the ocular surface functional unit." (8) This workshop classified the condition into two main aetiological groups: aqueous deficient dry eye (ADDE) and evaporative dry eye (EDE).

Aqueous tears are produced by the lacrimal glands (main and accessory). Any deficiency in the function of these glands or a reduced production of aqueous tears causes ADDE. (18) When ADDE occurs as part of a larger systemic involvement it can be further classified into Sjogrens and non-Sjogrens syndrome. (19)

Evaporative dry eye is caused by an increase in tear evaporation. (9,20) There can be both extrinsic and intrinsic causes of EDE. Intrinsic causes may be meibomian gland dysfunction (MGD), disorders of the lid and low blink rate, while extrinsic factors include ocular surface disorders, contact lens wear and ocular surface disease (see Figure 1). (8) Although it is possible to define patients based on aetiological findings using this scheme, these groups are not mutually exclusive and can occur simultaneously in the same patient, making diagnosis (and management) all the more difficult. In a recent clinical study investigating the distribution of ADDE and EDE in a cohort of patients with dry eye disease, Lemp et al reported that 35% of patients had EDE, 10% had ADDE and 55% had a mixed or unknown classification. (21)

Clinical assessment of the tear film

A number of assessments are available to practitioners to help in the appropriate diagnosis of dry eye patients. Generally, these assessments can be divided into five main areas: symptomatology; tear film quality; tear film quantity; ocular surface assessment; and lids and lashes. To ensure a full and appropriate assessment of these patients, it is vital at least one test from each category is utilised, employing a 'least to most invasive' test strategy.

[FIGURE 1 OMITTED]

Symptomatology

Research shows that symptoms of dry eye disease do not correlate well with objective diagnostic tests, except in severe cases. (22,23) However, while many diagnostic tests are unreliable, the response of patients to questioning about dry eye symptoms has been found to be repeatable, (24-26) making this an important step in the assessment of patients. Validated questionnaires can be carried out pre-examination and allow consistency in data collection and a quantitative severity score.

There are a number of dry eye questionnaires available, but the DEQ-5 questionnaire has been shown to work well as a screening tool for routine clinical practice. (27) Consisting of three questions, it rates the frequency and intensity of eye discomfort and dryness, and the frequency of watery eyes.

The range of patient responses is from 'never' to 'constantly' (or 'very intense' depending on the question); this corresponds to a numerical scale. The values are then summed with the calculated score ranging from 0 to 22. A score of >6 is indicative of dry eye disease. A score of >12 is indicative of Sjogren's syndrome (assuming clinical signs are also evident). In this instance, referral should be considered.

Tear film quality

One of the simplest methods to measure the quality of the tear film is to measure tear film break-up time; this can be done both invasively and non invasively. (4,28,32)

Fluorescein tear break-up (FTBUT): This commonly used clinical method involves instilling fluorescein dye into the eye and illuminating the tear film using a slit lamp and a cobalt blue filter. The FTBUT is noted as the time taken between the last blink and the appearance of the first dark, dry spot. Values of less than 10 seconds have traditionally been considered abnormal. However, more recently, cut-offs as low as less than five seconds have been recommended. (33)

Although quick and readily available, this test is not without flaws. The use of impregnated strips causes variability in the volume (often in excess of 17 [micro]l) and concentration of fluorescein delivered to the eye, which has been shown to significantly increase apparent FTBUT. (34,35) The use of laboratory digital micropipettes would remove this variability and reduce the instilled volume, although these devices are not commonly available in a clinical setting.

Non-invasive tear break up (NITBUT): One way to avoid the effect of fluorescein dye is to utilise a non-invasive method of assessing tear break-up. This method generally makes use of patterns projected onto the tear film surface. This can be done in a clinical setting by observing the image of a given pattern reflected on to the tear film surface, for example using keratometer mires or the Keeler TearScope grid pattern (see Figure 2).

The NITBUT is the time between a blink and the first sign of a distortion or disruption of the reflected image. Non-invasive stability values generally tend to be longer than those measured with fluorescein. While the cut off for healthy individuals is >10 seconds with FTBUT, Guillon et al reported a cut off of >20 seconds with a noninvasive test. (36)

Osmoiarity: Tear film osmolarity has been shown to be diagnostic of dry eye disease and the most effective single measure. (37,38) It has long been used in research settings, although the introduction of the TearLab has now enabled this test to be clinically applicable. Using a single use, disposable test chip, 100nl of tear fluid is collected by passive capillary action from the tear meniscus. The pen monitors the collection process and provides an audible and visual signal when the sample of tears has been collected. The pen is then docked into the reader, which calculates and displays the osmolarity result. Osmolarity values greater than 308mOsms/L are a sensitive indicator of mild dry eye disease and values greater than 312mOsms/L are indicative of moderate to severe dry eye. (39)

[FIGURE 2 OMITTED]

Tear quantity

Schirmer I test: Due to its clinical applicability, the Schirmer I test is the most widely used method in determining tear volume. Using a specially designed strip of filter paper, the secretory function of the lacrimal gland can be assessed. Schirmer initially suggested a cut-off of 15mm in five minutes for the normal eye. (40) Shorter cutoffs have since been suggested, indicating values less than 5mm in five minutes are pathological. (25,41,42) The Schirmer I test has flaws, including a lack of sensitivity, specificity and reproducibility, making it difficult to diagnose and monitor borderline dry eye patients most commonly seen in optometrie practice, but due to its clinical applicability, ease and speed of use and low cost, this test is still used as a useful estimate in the measurement of tear volume.

Phenol red thread (PRT): A minimally invasive measure of tear quantity, the phenol red thread has been used as an alternative to Schirmer. (31) Placing a cotton thread impregnated with red pH sensitive dye over the lower eyelid, the colour change (from yellow to red due to the alkaline tears) is measured in mm, with a diagnostic cut off of 10mm in 15 seconds having been suggested. (32) Although the test appears to be a very good non-invasive method of testing tear quantity, it is not known whether the test is an index of tear production or tear volume. It is not widely used by ophthalmologists, although is popular amongst optometrists. The test is reproducible and does not produce corneal staining. It induces less reflex tearing compared to Schirmer testing and is comfortable for patients, making it a preferable option for children. (43)

Tear meniscus height: Another way to measure tear quantity, particularly useful in diagnosing aqueous deficient dry eye is to measure the tear meniscus height (see Figure 3). This can be done by observing the height of the upper and lower tear menisci using a slit lamp and calibrated slit beam. Heights of less than 0.2mm are indicative of reduced tear film quantity. (44)

Ocular surface assessment

Although not strictly a measure of+ tear film assessment, ocular surface assessment is, nonetheless, a vital part of the clinical assessment of a dry eye patient.

Ocular surface hyperaemia: Assessment of bulbar and palpebral conjunctival hyperaemia is important in suspect dry eye patients, to quantify the degree of redness in any inflammatory condition. There are a number of grading scales available, (45-47) and it is vital to note which scale was used for standardised grading.

Ocular surface staining: Indicating the effects of a defective tear film, ocular surface staining can be measured using a number of dyes; fluorescein to highlight epithelial cell loss and either rose bengal (cornea) or lissamine green (conjunctiva) to indicate dead and devitalised cells. The extent of staining can be quantified clinically using a grading scale such as the Oxford system, (48) which is useful in a clinical setting as it is more sensitive to smaller changes and has a wider range of scores in comparison to other scales available. (49) Using this scale, staining is represented by punctate dots on a series of panels (A-E). Staining ranges from 0-5 for each panel and 0-15 for the total exposed interpalpebral conjunctiva and cornea.

Lid parallel conjunctival folds (LIPCOF): The measurement of LIPCOF has been shown to be useful in the diagnosis of dry eye disease. (44) The presence of LIPCOF is assessed by viewing the area perpendicular to the nasal and temporal limbus on the bulbar conjunctiva above the lower lid. A combined score can then be calculated by adding nasal and temporal LIPCOF grades. A cut off of [greater than or equal to] 2 is indicative of dry eye. (44)

[FIGURE 3 OMITTED]

Lids and lashes

Assessment of lids and lashes: Blepharitis is a common problem, with underlying conditions. It may be seborrhoeic (associated with oily skin and scalp dandruff), staphylococcal (bacterial over-colonisation) or a combination of the two. Signs of blepharitis include oily secretions, collarettes, missing/misdirected lashes as well as symptoms including photophobia, tearing and blurred vision. A full discussion of the characteristics of blepharitis is beyond the scope of the current article.

Meibomian gland expression: It is important to assess the lids and lashes thoroughly, including expression of the meibomian glands to determine patency. Often, in meibomian gland dysfunction (MOD), the lids are inflamed, glands obstructed or reduced in number and the expression inspissated. The assessment of the quality and quantity of expressed meibomian oil is important to monitor for MGD. Under slit lamp observation, and using digital expression, pressure is applied to the central eight glands of the lower lid until meibum is expressed and graded. (50) The appearance of the lipid should be recorded in terms of clarity viscosity and colour, where viscosity is described using a 4-point scale (see Table 1).

Blinking: The stability of the pre-corneal tear film is important in the study of dry eye. Tear film distribution and stability are blink dependent. (52) Eyelid closure during a blink proceeds from the temporal to nasal side of the eye spreading the tears across the ocular surface and facilitating drainage through the lacrimal punctum. A normal inter blink period is 5.97 seconds. (53) In dry eye, this is significantly reduced (2.56 seconds) (53) and in concentrated tasks such as reading a VDU use, blink rate reduces by half (from 22.4 [+ or -] 8.9/min to 10.5+6.5/min). (54)

Conclusion

The tear film is a complex, dynamic structure and appropriate assessment is essential. The aim to use fewer/ non-invasive methods of assessment is vital to ensure as little disruption of the natural stasis of the tear film as possible. No single test is 'gold-standard' in the assessment and diagnosis of tear film disorders. However, by applying a combination of tests, practitioners will be better equipped to select the most appropriate management.

Exam questions and references

Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk. Please complete online by midnight on 7 October 2016. You will be unable to submit exams after this date.

For references, visit www.optometry.co.uk, and click on the 'Related CET article' title to view the article and accompanying 'references' in full.

Course code: C-51802 Deadline: 7 October 2016

Learning objectives

[Optometrists]

* Be able to elicit relevant symptoms from patients with dry eye (Group 1.1.2)

* Understand the appropriate methods for tear film assessment (Group 3.1.7)

* Be able to manage patients presenting with dry eye (Group 6.1.7)

[Therapeutic optometrists]

* Understand the appropriate methods for assessing patients with dry eye (Group 2.1.2)

[Dispensing opticians]

* Understand the appropriate methods for tear film assessment (Group 3.1.2)

* Understand the clinical characteristics of dry eye (Group 5.2.2)

[Contact lens opticians]

* Be able to elicit relevant symptoms from patients with dry eye (Group 1.1.2)

* Understand the appropriate methods for tear film assessment (Group 3.2.4)

* Be able to manage patients presenting with dry eye (Group 5.3.1)

Dr Louise Madden MCOptom, PhD, Dip Tp (IP)

Dr Louise Madden graduated from Glasgow Caledonian University (GCU) in 2005 and subsequently completed a PhD there. She has worked as a community optometrist in independent and multiple practices around Central Scotland and currently holds the post of lecturer in Vision Science at GCU. She specialises in ocular therapeutics, anterior eye disease and contact lenses and gained the Diploma in Therapeutics for Independent Prescribing in 2013.
Table 1

Grade    Description of expression

1        Free flowing liquid
2        Slight increase in viscosity, still free flowing
3        Noticeably thickened
4        Toothpaste consistency
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Title Annotation:Dry eye
Author:Madden, Louise
Publication:Optometry Today
Date:Sep 1, 2016
Words:2612
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