There are so many competing – dare I say conflicting/overlapping tests - just which one will show if it is indeed the tears? Osmolarity testing seeks to answer this by concentrating solely on the tears – but can we successfully tell tear osmolarity with just our trusty slit lamp alone?
This is what we have set out to explore and present for discussion.
Information about tear osmolarity can be found by clicking on the links below to two short YouTube videos.
Osmolarity Explained in Two minutes
Effect of Hyperosmolarity on the Ocular Surface and Visual Acuity
What happens to the eye
The problem with dry eye as Mrs Smith's previous practitioners will tell you, is that the symptoms just don’t relate very well to the findings. However, there are many reports saying that dry eye syndrome does directly relate to increased osmolarity above 380 m/osmos. Could hyperosmolarity even predict pre-symptomatic dry eye? This is an interesting subject indeed!! We know for sure that osmolarity is a tear thing and if the level is raised then we know the tears are indeed the primary issue.
Please note - we need to examine the tear film in its natural state so.......
If you still place Fluorescein like this
Flooding the eye with lots of saline and also causing reflex tearing is going reduce any
tear hyperosmosis in a trice and prevent you seeing what is actually going on. STOP NOW!
The tear film is the result of tear excretion, constant blinking and evaporation and its innate osmolarity gives a measure of how much comfort this tear film allows. By touching the lids or irritating the eye with fluorescein paper you will totally destroy this delicate, damaged film and give inaccurate, misleading results.
(There are times, of course, when you want to flood the eye with fluorescein – checking for foreign bodies, for example.)
We have to be very, very gentle – reflex tearing or disturbing the current tear film will spoil the result.
So we instil
fluorescein like this for the
first (Meynell-Price) test:
(right image: Before, left image: After)
Video showing side fluorescein instillation:
The only way we can check for hyperosmolarity is by exploiting the fact that the tears will have pools of hyperosmolarity not mixing properly and producing a tear film that is inconsistent. We know that hyperosmolarity will cause localised inflammation and the tear film will be struggling to smoothly flow over the eye.
Our first test is the Meynell-Price test (MP test), based on this principal. Hyperosmolarity over 308 mOsms/L will interfere with the normal spread-ability of the tears. Normal tears show shear strength, consistency and evenness. To put it simply: If you stop blinking the tears continue to be stable, even and protective and just remain there until the tear break up is reached. This is refreshed on every blink reforming this film. In dry eye disease the tears are patchy due to local areas of high osmolarity and this causes areas of increased evaporation and enhanced sensitivity. This is symptomatic of course! However, we must take note that pools of high osmolarity in the tears are masked by the stirring effect of the blink – and blink rate increases with dry eye!
During our first test we ask the patient to keep their eye open and look to the nasal side. A 1 mm spot of fluorescein is very, very lightly placed on the conjunctiva. We find you can do this by simply letting the fluorescein spot transfer over. Dabbing it on will cause reflex tearing. This takes practice but we found it well worth perfecting as the results are so informative!! With practice, the patient won't even feel the fluorescein going in and neither is their eye swimming in yellow. We find it is best viewed with low magnification on the slit lamp using a cobalt blue filter. The person blinks normally and then you say 'look nasally and stop blinking for a moment' – then place the 1mm spot whilst observing through the slit lamp.
A normal tear film will hold the spot and maintain its shape. If the tear film has no pools of high osmolarity - that is its osmolarity is consistent and working normally, then the tiny 1mm fluorescein spot will maintain its shape and slowly spread as it mixes with the tears. This is a normal result.
This is a high osmolarity result and points to tear dysfunction as the primary cause of the symptoms. A hyperosmotic tear film will make the spot spread inconsistently and take on a petalled/ streaking appearance.
Our second test checks the tear meniscus river. This is the movement of the tear meniscus towards the punctum and crosses the whole of the lower lid margin. Powered by blinking it takes ten blinks or less to move the river across. However tear hyperosmosis will take more blinks. The tear meniscus carries a bolus of fluorescein from the tiny spot of fluorescein and you can easily observe its passage across the lower lid. We hoped the number of blinks to move it to the punctum would directly relate to increased tear osmosis.
However, while this was repeatable we found that other barriers such as meibomian damage; conjunctival irregularities and limbal stem cell changes confounded accuracy as they acted as barriers to the steady tear river flow - sometimes even halting it entirely. So while it is useful, we found this test is not gradable. However our third test does enable recordable grading.
Our third test quantifies the inflammation of the conjunctiva. TFOS has recently highlighted inflammation with dry eye syndrome. It is based on the fact that conjunctival inflammation is always accompanied with conjunctival thickening due to inflows of inflammatory bodies and oedema. It means that instead of smoothly sliding over the eye, the eyelid contact is less than perfect. It is a probable cause of lid wiper epitheliopathy. We noticed that the lower lid tear meniscus gets smaller on upward gaze if the conjunctiva is inflamed and the drop is greater and in proportion with tear hyperosmolarity.
If the eye has normal tears we found the lower lid tear meniscus to be of a stable height in any gaze even on looking upwards. This is a very sensitive and valid test, yet extremely simple. We found that tear prism reduction on upward gaze related to dry eye directly. So here is a gradable DED test. To grade we introduced the idea of meniscus instability
Normal tear meniscus stability
Trace (grade 1) meniscus instability means the meniscus drops up to 25% on looking up – this is pre-symptomatic
Mild (grade 2) meniscus instability means the meniscus drops up to 50% on looking up and is symptomatic – usually episodes of moderate DED.
Moderate (grade 3) meniscus instability means the meniscus drops to 25% and is always symptomatic.
Severe (grade 4) is when the tear meniscus disappears under the lid margin on upward gaze.
We find that the meniscus stability test does not require fluorescein to use and grade. It's simply an observation of how the prism changes when our patients look up. We use medium magnification for this.
In conclusion, it is our endeavour to share our techniques to provide a simple yet effective means of successfully telling tear osmolarity with just our slit lamp, which as practitioners we all have to hand. Although we never set out to devise new techniques, what evolved certainly works for us and allow us to monitor and help many patients. We really hope you have found this as interesting a topic as we have and feel compelled to try these methods to further aid and understand the treatment of your dry eye patients – we are sure they (and their eyes) will be very grateful!