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The Ocular Surface
The Ocular Surface ... is the moist anterior part of the eye
The shining of the Ocular Surface comes from the Tear Film ...
... that maintains the permanent Moisture of the Ocular Surface.
Moisture is necessary for the health of the ocular surface tissue and moisture is also important for perfectly clear vision.
Without sufficient moisture the vision becomes blurry, the tissue is increasingly damaged, and sensations of discomfort occur.
The OCULAR SURFACE performs the first steps of VISION
The OCULAR SURFACE is that part of the eye that permits the entrance of Light.
Only after passing the Ocular Surface
... can this light elicit responses of the Retina in the back of the eye
... that later allow the ´curious´ Brain to construct an image of the outside world on the screen of consciousness.
Without a healthy and functioning Ocular Surface, all other steps of the Vision Process that happen later and further ´behind´ ... are basically meaningless.
The OCULAR SURFACE must be permanently MOIST
Our Ocular Surface has the biological requirement that it must be permanently moist - ´always and everywhere´ - to keep the transparent window of the cornea indeed clear.
Since we are living in a dry environment with an air atmosphere, it is actually quite laborious to preserve this little artificial moist ´ecological niche´ of the Ocular Surface - ´always and everywhere´ !
PS: * except for moistening the Ocular Surface, Tears are also of use to give the emotional signal to our fellow human beings that we are exceptionally sad ... or probably exceptionally happy by shedding ´tears of joy´
The moisture of the tears comes from the Ocular GLANDS
The Tear Fluid is produced by several Ocular Glands.
There are three different types of glands needed, because the tear film on the eye consists of three different types of substances.
- Water ... is the main part of the tears and comes from the lacrimal gland behind the upper eyelid. Several small accessory glands contribute minor amounts of fluid.
- Oil ... from the Meibomian lipid glands, that form elongated strands inside the eyelids, can reduce the evaporation of the water. Oil therefore forms a thin layer on the surface of the tear film.
- Mucus/ Slime ... comes from individual small goblet cells in the mucous membrane of the conjunctival sac. They can be seen as little dots in the microscope. The Slime is needed to connect the water of the tears to the cell surface.
TEARS flow along the Ocular Surface
Tears are produced by the various Glands at the Ocular Surface.
The bulk volume consists of water and comes from the lacrimal gland that is located upstream in the upper lateral side of the bony orbit.
From there the tears enter the conjunctival sac. They are flowing over the visible anterior part of the eye and thus constantly bathe this area.
At the nasal side the ´used tears´ are sucked up by two narrow lacrimal puncta, one at each nasal end of the upper and lower eyelid.
Through the lacrimal drainage system the tears enter the nose. When tears go there in larger quantities, e.g. in emotional crying, they may flow backwards into the pharynx where we can sense their salty taste.
The Meibomian Glands are of particular importance for the Ocular Surface
Meibomian oil protects the tear water from evaporation.
The Meibomian glands produce lipids that are liquid at body temperature and thus form an oil.
The glands are of particular importance for the health of the ocular surface because their oil retards the evaporation of the tear water from the lacrimal glands.
In the dry environment that we live in, even a copious secretion of tear water would rapidly evaporate from the ocular surface if it was not protected by the superficial Meibomian oil layer on the tear film.
The Meibomian glands form individual long bodies that fill the tarsal plates of our eyelids. About 25-30 glands can be found in every healthy eyelid and the little orifice is close onto the posterior lid margin ... as can be seen in an ordinary mirror, by everybody who is interested.
With every blink of the eyelids a little drop of oil is expressed by the lid muscle. It gets from the gland onto the eyelid margin and the tear film. where it protects the aqueous tear film from too rapid evaporation.
Opening the eye lids for the entrance of light ... puts the Ocular Surface in some kind of a DILEMMA
The requirement of Moisture - ´always and everywhere´ - confronts the ocular surface with a problem or, more positively thinking, this gives it the change to deal with a Challenge:
Light can only enter the eye when the eyelids are opened ...
... on the other hand, opening of the eyelids would at the same time immediately deprive the Ocular Surface tissue of its moisture and it would start to dry out.
To solve this Dilemma, the Ocular Surface has to apply a ´Trick´: A very narrow layer of fluid is formed from the tears - this is for obvious reasons termed as the Tear Film.
The Tear Film is thick enough to preserve the moisture for the underlying cells of the tissue but not too thick in order not to harm the transmission of light.
The Ocular Surface in fact manages to be even more ingenious, because it solves the dilemma by even improving the passage of light through the presence of the Tear Film. The Tear Film fills in all subtle inequalities of the ocular surface and thereby provides a perfectly Smooth Surface layer for perfect refraction of the incoming light that allows for perfect Visual Acuity.
The TEAR FILM is the SOLUTION for all requirements of the Ocular Surface ... and for VISION
The BLINK movement of the eyelids distributes the tears into the tear FILM
The Tears are transformed into the thin, homogeneous Tear Film ... through the coordinated Movement of the Eye Lids - the BLINK.
During the blink mainly the upper eye lid wipes over the anterior surface of the eyeball and thus distributes the tears into the very narrow Tear Film.
During the down-phase of the blink the old tear film is removed by the upper eyelid and during the up-phase the upper eyelid draws out a new intact homogeneous tear film.
Dysfunction of the Eyelids and of Blinking is an important factor for Dry Eye Disease
It becomes obvious that any disruption of the EyeLid and of their normal Blink movement and speed is an important factor for the onset on a Dry Eye condition . This is termed as ´Eyelid and Blinking Disfunction´ (abbreviated as LBD).
The TEAR FILM
Only through the coordinated blink movement of the eyelids, together with the very special composition of the Tears , is it possible to form a TEAR FILM that is extremely thin (only about one hundreds of a millimeter).
At the same time this very narrow tear film must still be stable to allow the curious brain to achieve a sufficiently sharply focused image of the outside world.
The Tear Film must be stable for at least 10 seconds on average until it eventually breaks up and triggers a stimulus that induces a new blink that forms a new tear film.
Having said this ... all major things are basically named ... The basic functional pre-condition at the ocular surface for vision is a Stable Tear Film. This is achieved by the Basic Functional Complexes of Tear Secretion/ Production by the ocular glands and of the physical Tear Film Formation by the blink movement of the eyelids.
Different Influence Factors can impair the Tear FILM
The formation and preservation of the Tear FILM is very laborious in the dry environment that we live in, and is thus very vulnerable.
The Tear Film depends on a multitude of very different Influence Factors, that can, in one or the other way, have a positive or negative influence.
All negative influence factors are Risk-Factors - they can, sooner or later, lead to a Dry Eye condition - particularly when they become chronic.
The large number of influence factors that may appear unrelated at first glance can let Dry Eye Disease appear as a ´Tricky Condition´ - even though it basically straightforward.
Deficiency of the Tear Film leads to Cell Destruction, IRRITATION, Inflammation and PAIN
The Tear Film has the function to prevent Drying of the Ocular Surface Tissue. Therefore the most immediate consequence of a Dry Eye condition is typically the Drying and thus Destruction of the Ocular Surface Tissue.
The eye lids can then often be sealed by sticky mucus and encrustations.
This results in a ´downstream´ irritation of nerve fibers They transmit ocular irritation of various degrees and can lead to pain that is often described on and around, occasionally behind, the eye. Inflammatory reactions can reinforce the whole process, eventually leading to severe tissue destruction and to a chronic pain syndrome.
... and the Visual Acuity is diminished, which typically leads to BLURRED VISION
Since the Tear Film also has an important function for the refraction of light and thus for the provision of visual acuity, it is no surprise, that visual disturbance is frequently reported in Dry Eye Disease, typically in terms of unstable visual acuity and blurred vision
A lack of OIL is the main starting point in most patients with Dry Eye Symptoms
According to the present scientific knowledge, the vast majority, i.e. four of five patients with a Dry Eye Condition, does NOT have a primary lack of water but instead a PRIMARY LACK OF OIL in the tear film.
This leads to increased water evaporation and decreased tear film stability .... with a secondary water loss.
Therefore, in most patients, it does not seem to make much sense nowadays to use products that do not contain lipids in one or the other way.
It may be possible to replace the effect of lipids on the tear film by other compounds, or to simply use lipids only, e.g. as a liposomal spray. ... In other words, LIPIDS or respective compounds should nowadays be a component of a typical Tear Supplementation Therapy.
The lack of oil is typically due to Meibomian Gland Dysfunction (MGD) inside the Eyelids
The lack of oil on the ocular surface is typically due to Meibomian Gland Dysfunction (MGD), a mostly obstructive condition of the Oil-producing little glands inside the eye lids.
The glands are blocked by inspissated secretum and excessive keratinization - therefore inspissated oil accumulates inside the glands.
Muscular force during blinking of the eyelids normally helps to express the oil from the Meibomian glands.
Rare Blinking is thought to be an important factor for a lack of oil on the tear film and possibly a co-factor for onset of gland dysfunction.
Intended forceful blinking is known as a simple way to increase the oil layer on the tear film and thus to improve tear film stability in order to prevent ocular surface dryness.
Some Ideas for THERAPY in Dry Eye Disease
How to TREAT Dry Eye Disease ?
In most cases there is a deficiency of the Tear FILM based on a qualitative or quantitative lack of Tear Components.
- Therefore TEAR SUPPLEMENTATION, i.e. the addition of missing tear components - in the form of eye drops or of a spray - is the most frequently used therapy option.
- Sometimes this is termed Tear ´Replacement´ - but, since the (full) Tears with all ingredients can (unluckily) not be replaced at present and in the foreseeable future, the term ´Supplementation´ appears more appropriate.
- All different sorts of eye drops based on aqueous solutions with a lot of different additional ingredients are available, mostly as prescription free over the counter products.
Physical EYELID Therapy
PHYSICAL EYELID THERAPY is reported to restore the function of the Meibomian glands in the Eyelids ... and thus improves Dry Eye Disease
Physical Eyelid Therapy options refer to the fact that the improvement of Eyelid and Meibomian Gland Disease is based on simple but effective physical techniques such as Warming and Moistening, as well as manual Massage and Expression of the Glands with subsequent Scrubbing and Cleaning of the Eyelid Margin.
It is certainly important to note that before any manipulations are done to the eyelids and the eye a clinician should always be consulted for an educated diagnosis and for therapeutic recommendations !
It appears necessary that Physical Therapy is done as a permanent therapy, at best twice a day - because a chronic disease typically requires a chronic therapy. Only a chronic consequent therapy is able to improve the disease condition - still, there is typically only a slow but steady improvement !
PHYSICAL THERAPY consists of 3 Steps
(1) WARMING of the Eyelids
- WARMING, preferably together with moisture
- for at least 10 minutes
- to reach a temperature of 40°C inside the eyelid and thus within the Meibomian Glands
- serves to re-liquefy the inspissated oil inside the Meibomian glands.
In order to achieve 40°C inside the eyelid a slightly warmer (42-45°C) moist warm compress/ cloth must be rewarmed (e.g. from a larger bowl of hot water) every two minutes as shown by studies.
The additional moisture serves to soften up the cornified epithelial squames and lipid encrustations on the eyelid margin.
Commercially available gel masks or even electrically heated and moistened googles may make the potentially laborious effort of physical therapy, twice a day, considerably easier ...
(2) Subsequent Lid MASSAGE to EXPRESS and relieve the obstructed glands
- when the inspissated oil that is stuck inside the Meibomian glands is sufficiently warmed it will become more liquid again
- a careful eyelid massage can then express the pathologic content from the Meibomian glands onto the lid margin
- massage must always be directed towards the gland orifice on the lid margin, i.e. towards the palpebral fissure (as shown in the animation to the right)
- the upper eyelid is thus massaged downwards
- the lower lid is massaged upwards
- anyway, as mentioned earlier, in a chronic disease, such as Dry Eye Disease, a chronic therapy is necessary to improve the condition. A single treatment can typically not result in any major progress. Therefore even a correct careful eyelid massage does not necessarily result in a noticeable expression of inspissated Meibomian lipids. But consequent therapy will result in steady improvement of the disease condition.
- when the oil is very hard and insufficiently re-liquefied it may be necessary to perform a more vigorous expression of the eyelid , e.g. between two fingers.
- during any procedures applied to the eye by a layman it must therefore alway be safeguarded that no harm and wounding is caused to the eye, eyelids and glands !
(3) Eyelid/ Lid Margin HYGIENE concludes physical therapy
- finally, the eyelid margin together with the base of the eyelashes could be cleaned
- from the expressed pathological oil
- from bacteria and bacterial products that occur on every normal lid margin but increase in disease
- from cornified skin squames that deposit on the lid margin
- Lid Margin Hygiene can be done by a soap-free wiping or ´scrubbing´ of the lid with a cotton-tipped rod or a cosmetic pad in order to remove any debris, encrustations and foam from the lid margin.
- it appears important to avoid any soap, shampoo, or any other detergents, as has previously occasionally been recommend., because that would conceivably harm the tear film lipid layer.
- Oily solutions of any kind appear more suitable for eyelid cleaning.
- Commercial eyelid cleaning kits are available that contain everything necessary in a pre-prepared set and may make eyelid hygiene much easier.
The advantage of such physical therapy options is certainly, that this can mostly be done at home by the patients themselves. It may appear as a disadvantage that physical therapy and in particular lid hygiene has to be done routinely once or twice a day, similar e.g. to tooth brushing, but requires slightly more time.
Please Remember: A Happy Eye Lid typically makes a Happy Dry Eye Patient !
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Various other THERAPY OPTIONS are discussed in other Chapters of the full length English Language Pages of the OSCB Information WebSite.
CONTACT LENSES are a fantastic Optical Tool ... customized to the Ocular Surface
Contact Lenses are a fantastic Optical Tool with some clear optical advantages compared with spectacles.
At the same time they provide greater ´freedom´ for outdoor activities, sports and society events.
Contact Lenses are, as their name indicates, in direct ´contact´ with the Ocular Surface.
Contact Lenses are in fact sitting in the middle of the Ocular Surface Functional Unit - with potential influence on all tissues and on the tear film.
Contact Lenses thus have a somewhat ambiguous role for the Ocular Surface - with Pros and Cons.
Contact Lenses are swimming on the Eye ... and thus have higher requirements for the Tear Film
Contact Lenses can be used for medical purposes such as the optical correction of an irregular corneal shape or for the protection of the eye e.g. in severe cases of Dry Eye Disease.
On the other hand, Contact Lenses have higher ´requirements´ for the amount and/or quality of the tear film.
It may well be, that an individual with a borderline normal tear film, who is still without symptoms, may develop Dry Eye symptoms upon insertion of an ordinary soft hydrogel contact lens.
A typical side effect of Contact Lenses is therefore the onset of Dry Eye symptoms and eventually probably of Dry Eye Disease.
On the other hand an individual with a severe Dry Eye Disease may profit from the application of a special so-called ´Scleral´ Contact Lens that is able to preserve the tear solution underneath the contact lens and in front of the cornea.
HYGIENE is a CRITICAL ISSUE in Contact Lens Wear
Contact Lenses have seen great improvements in the past decades.
This has lead to a reduction in the frequency and severity of side effects.
Insufficient Hygiene can lead to serious and sight-threatening infections in contact lens wear.
This is still an important issue, particularly for inexperienced wearers.
Due to the increasing use of disposable contact lenses for short term wear, particularly daily disposables, the risk of infection can be reduced.
Please find more information below on => Contact Lenses and the Ocular Surface