Are You Suffering from Dry Eye?

chillibreeze writer — Dr Vinay Agarwal

Dry eye disease can be classified as tear-deficient, in which there is a deficiency of aqueous tear secretion, or evaporative, in which the cause is excessive evaporation.

Dry Eye in Clinical Practice May 2007 90 pages, US$14.99

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Each form of dry eye has certain global features in common, including

• A set of characteristic symptoms
• Ocular surface damage
• Reduced tear film stability
• Tear hyper osmolarity

Increasingly, an inflammatory component has become apparent, which contributes not only to symptoms, but also to the disease process itself. A diagnostic trap, in patients who are highly symptomatic, but who do not appear to have signs of dry eye, is the failure to recognize the presence of superior limbic keratoconjunctivitis (SLK) which can be overlooked if not searched for routinely. Also SLK and dry eye can occur together.

Diagnostic Tests

The global features of dry eye disease can be identified by the following types of diagnostic tests:

• Symptom questionnaires
• Staining to identify ocular surface damage
• Tear break-up time to assess tear instability
• Osmometry for tear hyperosmolarity

1. System Questionnaires:

Various questionnaires have been designed; most of them consist of questions relating to six symptoms:

Do your eyes ever feel dry?
Do you ever have gritty or sandy sensation in your eyes?
Do your eyes ever have a burning sensation?
Are your eyes ever red?
Do you notice much crusting on your lashes?
Do your eyes ever get stuck shut in the morning?

However, like most practicing dry eye specialists, even these questionnaires fail to find a correlation between symptoms, tear deficiency, and ocular surface damage. Perhaps vital staining does not reflect all those events that contribute to symptoms.

2. Ocular surface staining

Epithelial staining to the exposed surface of the eye can be demonstrated with vital and supra-vital stains. Staining of cornea occurs preferentially over its lower part, often more nasally than temporally and frequently in continuity with the bulbar conjunctival stain. Staining of the bulbar conjunctiva occurs over a wedge-shaped zone nasally and temporally, and in advanced dry eye may become confluent. Staining on the conjunctiva may be present in the absence of corneal stain, in milder forms of dry eye.

The following dyes are used for staining:
Fluorescein
Rose Bengal
Lissamine green

Fluorescein staining is the standard method to demonstrate ocular surface damage. This orange dye, which fluoresces green when excited by blue light, is applied to the eyes with a fluorescein-impregnated strip wetted with a sterile drop of saline. Optimal results are obtained by viewing through a yellow barrier filter, such as Kodak Wratten 12 absorption filter, used in combination with the standard blue exciter filter of the slit-lamp.

This technique reveals surface damage on both corneal and conjunctival surface.

Figure 1: Corneal Fluorescein staining in KCS note the staining is in the lower area of the cornea.

Usually staining has a characteristic distribution and is confined to the exposed interpalperbral area of the ocular surface, but in severe dry eye, staining may extend to the unexposed surface of the globe, particularly the upper bulbar conjunctiva. In the absence of the yellow filter staining is poorly seen on the conjunctiva.

In India, rose Bengal is available as impregnated strips similar to the fluorescein dye strips. Instillation is best preceded by topical anesthesia to limit stinging, and care should be taken to remove excess dye from the lid margins on eye closure with a tissue to avoid unsightly staining of the lid skin. The amount of staining seen is dose-dependent. Rose Bengal stain is supposed to demonstrate ocular surface damage by being taken up by dead and degenerate cells. However, it is now believed that the staining of ocular surface due to rose Bengal is due to loss of the normal mucin layer in dry eye disease, allowing the dye to stain live epithelial cells that would normally be protected by mucin in healthy eyes. Recent research has suggested that staining in dry eye is associated with an altered glycosylation of the surface mucin of apical conjunctival cells. Thus, it appears that although stained cells are not necessarily degenerate, they may suffer from impaired expression of membrane mucin.

Figure 2: Rose Bengal staining of the conjunctiva in KCS

Lissamine green is another dye now available in India. This dye when viewed in white light, produces a staining pattern similar to rose Bengal, in that staining is best seen over the white of the sclera and least on the cornea, over a dark iris. Like fluorescein it is well tolerated. This is an advantage over the stinging and pain after rose Bengal.

Tips on using vital dyes for staining the ocular surface   • The time to use one of these dyes is after you have done the external eye exam and refraction. • In the case of rose Bengal, use an anesthetic drop first. • If the patient has mild dry eye, the dye will permeate the nasal bulbar conjunctiva and possibly the superior edge of the lower lid as well. • In more sever cases, the staining may be more intense and involve the cornea as well as the temporal bulber conjunctiva.   Be especially wary if a patient wants refractive surgery because he or she finds contact lenses uncomfortable.

3. Assessment of Tear Film Stability: Non-invasive Tear Break Up Time (NIBUT)
A correlation between ocular discomfort and tear-film breakup time exists.

In hundreds of dry-eye patients, it has been observed that within one second of tear-film breakup time, 73 percent of the patients experience ocular awareness followed by discomfort. This manifestation of ocular discomfort may stimulate the eye to blink, replenishing the tear film and providing protection of the ocular surface. If the patient has a short tear film break-up time due to disease or other factors such as systemic medications known to cause ocular drying or an altered blink rate as a result of staring at a computer screen, symptoms and signs can be exacerbated. Therefore, the relationship between tear-film breakup time and blink rate is critical.

How does one perform the NIBUT?

If you have your patient stare straight ahead and monitor the time from his last complete blink and the moment he reports ocular awareness, this time will be within approximately one second of his tear-film breakup time.

Non-invasive Breakup Test

• Obtain a stop watch or clock • Blink twice, then stare straight ahead • Record the time between the last complete blink • and the first sensation of ocular awareness This time (in seconds) is the non-invasive breakup time.

Interblink Interval (IBI) and Ocular Protection Index (OPI)
This can be calculated in the clinic by observing the patient as he reads the Vision Chart without his being aware that the blink rate is being measured. The number of blinks per minute divided by 60 will give the inter blink interval.

How to calculate Ocular Protection Index (OPI) in the Clinic

Calculate the OPI by:

Visual count of blinks/min. while patient reads Vision Chart; Inter-blink Interval (IBI) is determined by dividing 60 by blinks per minute Measure tear-film breakup time using the slit lamp or the non-invasive test;   If NIBUT is > IBI Patient is PROTECTED If NIBUT is < IBI Patient is at RISK   Severity may be determined by the degree of discrepancy between IBI and NIBUT

The understanding of the OPI brings us to the concept of interaction of IBI and NIBUT or TFBUT. It also explains the concept of discomfort in dry eye patients (Figure 3). When the inter blink interval is longer than the ability of the tear film to protect the intact ocular surface the surface will begin to show signs of drying and the severity will depend on the degree of mismatch between the spread of the tear film at the completion of the blink and the onset of the next blink.

Figure 3: Interaction between the blink rate and ocular discomfort.

4. Tear Meniscus Height

Evaluation of the tear meniscus height is a rough estimate of tear quantity, and assessing the amount of debris in the tear prism also demonstrates tear quality. Several authors have suggested that a decreased tear meniscus height indicates decreased aqueous; however, there has been minimal work to systematically classify clinical ranges in normals and dry eye patients.

A tear meniscus height for dry eye is approximately 0.25 mm in height, while normals demonstrate a height of around 0.50 mm. Therefore, a dry eye cut-point value where values ¾ 0.3 mm are considered indicative of dry eye can be utilized clinically.

5. Fluorescein Tear Break-Up Test (FTBUT)

It is a provocative test in the sense that the intillation of fluorescein shortens the normal break-up time. Break-up is best observed with the use of blue exciter filter and yellow barrier filter, while the patient refrains form blinking. The yellow filter is, however, not essential.

The break-up time is the time that elapses from the last blink to the first appearance of a dark spot in the fluorescein stained film. In general, a break-up time of less than 10 seconds suggests an unstable tear film. Tear break-up time is reduced in all forms of dry eye.

6. Characterizing Dry Eye Disease

Schirmer Test

The Schirmer I test without anesthetic is a test of reflex secretion in response to conjunctival stimulation. It is a useful test for the evaluation of dry eye, although the diagnosis or exclusion of dye eye cannot be made on the basis of this test alone. Less than 6mm of wetting after 5 minutes indicates a diagnosis of tear deficiency. The reliability of the test may be affected by environmental conditions such as temperature or humidity.

The Schirmer I test can be performed after instillation of a topical anesthetic, and it has been assumed to measure the basal secretion rate in the absence of a reflex component. Nasal anesthesia reduces the Schirmer value obtained from the test.

The Schirmer II test assesses reflex secretion of tears in response to nasal stimulation in addition to the conjunctival stimulation. This test is very uncomfortable for the patient, as it involves vigorous stimulation of the nasal mucosa. Wetting of the strip in response to this test has been shown to be reduced in the more severe forms of dry eye, such as in Sjogren’s syndrome.

7. Assessment of oil glands

Clinical assessment of extent of meibomian gland dysfunction is the clinical method used to assess extent of evaporative dry eye. This simplest method of assessing meibomian glands involves quantification of occluded gland orifices and grading the quality of expressed oil secretion.

There are several other tests which are not mentioned in the above discussion because of their lack of standardization or availability in India. Some of these are the Phenol Red Thread Test, Tear meniscometry, Meibometry, Lysozme assays.

Suggested sequence of tests for the diagnosis of Dry Eye Disease   1. Noninvasive Tests a. Noninvasive tear break-up test (to assess tear stability) b. Tear meniscus height and quality (to assess tear volume) 2. Minimally Invasive Tests a. Fluroscein Tear break-up time (to assess tear stability) b. Staining of bulbar conjunctiva and cornea (to assess ocular surface damage) A five minute gap is recommended before the next test 3. Tests of Tear volume or secretion a. Schirmer I test ( to assess reflex tear flow) b. Schirmer II test in response to nasal stimulation (to assess enhanced reflex tear flow) 4. Additional dye tests a. Rose Bengal Staining (to assess ocular surface damage) b. Lissamine green staining (to assess ocular surface damage) 5. Oil gland assessement

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Out of 5 “chilies”, our editorial team gave this article...

 

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—About our writer: Dr Vinay Agarwal is a Mumbai based cornea surgery specialist.

 

 

 

 

 

 

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