Osmolarity as a Measurement for Dry Eye Diagnosis: Overview of Artificial Tear Supplements
Tears play important roles in protection and maintenance of the eye. It provides protection by preventing infection, allowing for gaseous and nutrient exchange, and lubricating the ocular surface. They also serve to maintain ocular surface tissue metabolism, provide a surface smooth enough to enhance refraction of light, and facilitate the function of blinking by providing a well-lubricated ocular surface. The layers of the tear film are the lipid layer, aqueous layer, and mucin layer. Any form of disruption or abnormality of these may cause negative impacts in the eye. (Donsky, E. 2019)
Dry eye is caused by inadequate formation, retention, and drainage of the tear film layers. (Tomlinson, A. 2006)
It is a disease identified by a tear film that’s unstable which may present with ocular symptoms such as discomfort, redness, foreign body sensation, dryness, excessive tearing, and blurred vision.
In 1903, Schirmer proposed the theory of tear film deficiency and then developed the Schirmer Test- a test used to determine if the eye produces sufficient tears to prevent dryness. (Tsubota, K. 2018)
However, this test is subjective, qualitative, and has poor specificity.
Another test used for the diagnosis and monitoring of dry eye is determination of tear film osmolarity. Osmolarity refers to the concentration of salts dissolved in a volume of solution; therefore tear film osmolarity is the “saltiness” of the tears. As the tear osmolarity increases, surface cells of the eye may get damaged and lead to dry eye. This tear hyperosmolarity may be due to the lacrimal gland’s malfunction in producing normal tear layers. It occurs when there’s evaporation of water from the eye’s surface. The epithelium of the exposed ocular surface then gets damaged and the tear film becomes unstable. Dry eye disease progresses.
The tear film osmlarity test evaluates the osmolarity of a tear film using specific instruments designed for this purpose; such as the TearLab Osmolarity Test instrument.
The TearLab diagnostic test is quantitative, objective, specific and efficient. The patient needs to be explained of the procedure (purpose of the test and method). Then the examiner needs to make certain that the patient did not use any therapeutic eye drops 2 hours prior to the test otherwise a false osmolarity reading will be received. Each test card is packaged in a separate containment package. It should be removed carefully maintaining sterility and then inserted into the TearLab pen. It is secured in place once a beep is heard and a green light is displayed. Whilst holding the plastic wings of the card, remove the protective cover. Then, ensure the patient is positioned correctly (chin slightly upwards, gaze towards the ceiling and away from you) and they’re allowed to blink normally. Stabilize the pen by placing a hand on the patient’s cheek. Do not pull the lower lid down; this will cause the tear lake to disappear. Place the pen above the lower eyelashes near the corner of the eye so it touches the layer of tears on the eyelid, but not on the sclera. The tears are sufficiently collected on the tip of the test card when a beep is heard and the green light goes off. Only 50 nanolitres of tears are required. The osmolarity value is read off the TearLab once the pen is placed back on it. It is given in mOsmol/L. Normal tear film osmolarity values are less than 300, 300-320 is mild, 320-340 is moderate, and above 340 is severe. There should not be a difference of more than 8 between the eyes.
Tear film osmolarity testing is a preferred method of dry eye diagnosis as it is more accurate, specific, quantitative, faster, less painful/less invasive, objective, and more efficient than other tests (such as the Schirmer test which provides frequent discomfort, subjectivity, and inaccurate results- due to reflex tearing among other disadvantages.)
It is important to have an accurate evaluation and possible dry eye diagnosis nowadays, as our technological era advances, frequent work on electronic/digital screens is done increasing the risk for dry eye (just one of the major factors leading to dry eye). If dry eye is left undiagnosed and/or untreated, it could lead to increased eye infection risk, inflammation, corneal-surface abraions/ulcers, and vision problems.
In order to identify and diagnose dry eye disease in patients, dry eye symptoms need to be evaluated. Therefore, a number of questionnaires have been created to aid practitioners in evaluating dry eye symptoms. These are as essential as any other clinical findings. The answered questionnaires should be considered together with reported symptoms and patient signs. (Shiraishi, A. 2018)
A few of the questionnaires will be discussed.
The Ocular Surface Disease Index (OSDI) is one of the questionnaires created to evaluate ocular irritation symptoms for dry eyes to assess how the vision is affected by them. The evaluation is based on the patient’s previous week. It consists of 12 items with 3 sub-sections (eye symptoms, visual function and triggers from the environment). The scale used for answering is 0-4 (0:not at all, 4: always). Evaluation of dry eye from the total score is as follows; 0-12: normal, 13-22: mild, 23-32: moderate, and >33:severe. This questionnaire evaluates only the frequency of symptoms and not the severity, making it less accurate than others.
Standard Patient Evaluation of Eye Dryness (SPEED) is another questionnaire used to evaluate how symptoms of dry eye progress over 3 months. It consists of 8 items assessing symptoms such as burning, soreness, scratchiness, watering, grittiness, irritation, eye fatigue, and dryness. It checks if each symptom was tolerable, intolerable, problematic, bothersome, or uncomfortable. This questionnaire evaluates severity as well as frequency of symptoms.
The Dry Eye Questionnaire (DEQ) is a questionnaire used to assess the severity of dry eye. Symptoms are measured based on morning intensity, afternoon intensity, frequency, and extent of irritation. The evaluation is based on the patient’s past week.
The Contact Lens Dry Eye Questionnaire (CLDEQ) assesses the frequency and severity of symptoms of dry eye in contact-lens patients only. Stemming from DEQ, it consists of 36 items with 9 sub-sections (visual changes, dryness, scratchiness and grittiness, discomfort, soreness and irritation, foreign body sensation, photophobia, burning, and itching. These symptoms are evaluated only while patients have their contact lenses on. The frequency and intensity is assessed at different times of day.
The Ocular Comfort Index (OCI) is a questionnaire that assesses eye comfort quickly. It consists of 15 items with 8 identifiable areas (for positive: comfort and for negative: stinging, tiredness, itching, visual stability, dryness, pain, and grittiness). It detects how the symptoms improved post-treatment with lubricants. The frequency and intensity of symptoms are assessed.
The Canadian Dry Eye Epidemiology Study (CANDEES) assesses prevalence and severity of symptoms of dry eye. It consists of 13 items including use of medications, use of contact lens, dryness of mouth, demographics, symptoms wearing contact lens and without wearing contact lens, comparison of how severe symptoms are in the morning and afternoon, red/swollen eyelids, and allergies. (Real, D. 2018)
(See appendices for attached questionnaires)
Artificial tear supplements are drops that are used as lubricants for the eye in treating dry eye or ocular irritation associated with inadequate tear film.
There are 2 types of artificial tear supplements available; those that contain preservatives and those that do not (preservative-free).
The artificial tear supplements that contain preservatives have the advantage of making them last for a longer period of time once they are opened, and can therefore be made available in multi-dosage packaging. The disadvantages are that the patient’s dry eye symptoms can be worsened due to the preservatives disrupting tear film layers, especially if the supplement is being used above 6 times a day. Patients may be allergic to certain preservatives causing an unwanted reaction. Their side effects include stinging, eye irritation, eye pain, burning, or vision changes. Examples of preservatives in these tear supplements include Benzalkonium Chloride (BAK), Stabilized Thimerosal (Thi), Chlorobutanol (Cbl), Sodium Perborate (SP), and Methyl Paraben (MP).
The artificial tear supplements that do not contain preservatives are referred to as “preservative-free”. They have the advantage of not creating a risk for disrupting tear film layers since they do not contain preservatives. The disadvantages are that they have a shorter time in which they can be used due to their lack of preservatives. Due to this, they also come in single-dosage packaging. Their side effects include stinging, eye irritation, eye pain, burning, or vision changes. Examples of preservative-free artificial tear supplements include Systane Ultra, Refresh Optive Advanced, TheraTears, NanoTears TF, and Smooth Lubricant Eye Drops.
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