What is Dry Eye?
Classically, dry eye is described as a disorder of the tear film due to tear deficiency or excessive evaporation. This tear film disorder causes damage to that part of the surface of the eye present between the lid margins, and is associated with symptoms of ocular discomfort.
Dry eye is said to be a disease of the tears and ocular surface, caused by many underlying factors, that results in symptoms of discomfort, visual disturbance, and tear film instability with potential damage to the ocular surface. It is sometimes accompanied by inflammation of the ocular surface.
Normal Ocular Surface
The term ocular surface refers to the cornea (the front, clear ‘window’ of the eye) and the bulbar conjunctiva (the mobile, very thin, clear layer, overlying the sclera or ‘white’ of the eye). In front of this part of the eye is the pre-ocular tear film (POTF).
The POTF has three layers:
- The outermost (lipid) layer, produced by the meibomian glands found in the lid margins. The Zeis and Moll glands of the eyelid margin and lashes also contribute to this layer. Oily secretions in this layer are there to stabilise the aqueous layer underneath and to slow down its evaporation. In the normal healthy eye, the lipid layer's thickness is less than 1/1000 of a millimetre. Meibomian lipids have particular, important, properties for the formation of the tear film and alteration of these properties, in conditions such as blepharitis, can affect the stability of the tear film.
- The aqueous (intermediate) layer makes up about 90 percent of the tear film. This layer contains proteins that have antibacterial activity.
- The innermost layer of the tear film is known as the mucous layer. This mucous serves as an adsorbing interface between the aqueous layer and the hydrophobic (water-repelling) corneal epithelium, i.e. it keeps the aqueous layer “attached” to the corneal epithelium. In addition, it collects cellular debris from the ocular surface. The traditional concept of rapid tear film breakup is based on disruption of the contact between the lipid and mucous layers, or local breakdown of the mucous layer.
There is a normal, continuous resting tear flow which occurs. It is currently thought that waking tear flow is a reflex response to sensory stimuli which arise mainly from the ocular surface. Additional sensory input from the lining of the nasal cavity also makes a contribution and this is why strong smells/odours can make our eyes water. Disease or damage to any component of the tear production system can destabilise the tear film and lead to ocular surface disease that expresses itself as dry eye. Tear film stability, a hallmark of the normal eye, is threatened when the interactions between the various stabilising tear film constituents are compromised by decreased tear secretion, delayed clearance, and altered tear composition. Ocular surface inflammation may then occur as a secondary consequence. Reflex tear secretion in response to ocular irritation is thought to be the initial compensatory mechanism but, with time, inflammation accompanying chronic secretory dysfunction and a decrease in corneal sensation, eventually compromises the reflex response and results in even greater tear film instability.
Importance of the Tear Film
The term “dry eye” refers to ocular surface disorders in which the common aetiology is aqueous deficiency. This can lead to a a number of secondary disorders. Adequate POTF function is necessary for clear and comfortable vision. In addition, the POTF has other important functions:
- It is the initial refracting (light-bending) surface of the eye.
- It serves as the primary source of oxygen to the anterior cornea.
- It supplements the eye's antibacterial defences.
- It provides lubrication for the eyelids and ocular surface.
- It flushes away metabolic waste products and debris.
- It performs a needed anti-inflammatory function following ocular surface injury.
Drugs Which May Cause Dry Eye
Unfortunately many of the common medications we take, for one reason or another, may have side effects. There are lots of different medications which may increase the dryness of your eyes. People respond differently to medications whereby one person might have no side effects and another taking the same medication may have several complications. The following list comprises some of the different medications which have been reported to cause dry eye:
- Antihistamines - These may contribute to a decrease of tear film production
- Antidepressants - These are known to cause ocular drying, or the drying of your eyes
- Sleeping Pills - Side effects commonly stated include dizziness, confusion, and also dry mouth and dry eyes.
Over-the-counter sleep aids, as well as prescription sleep aids, will cause these side effects.
- Birth Control Pills - Many pills list dry eye as a side effect, mainly because birth control pills alter your hormones. However, whilst stopping the pills should stop the dryness, pregnancy is also known to cause dry eyes.
- Diuretics - These drugs are mostly used to treat high blood pressure.
- ACE Inhibitors - Angiotensin-converting enzyme inhibitors are mostly used to treat high blood pressure.
- Isotretinoin-Type - These drugs are mostly used to treat acne conditions, Drugs e.g. Roaccutane.
- Opiates - Opiate-based medicines such as morphine that help treat extreme pain.
Treatment of Dry Eye – Lubricants
Most of us have heard the term ‘artificial tears’. Many of us know someone who is using these products regularly. However, for most products that identify themselves as artificial tears, it is a misnomer because they do not actually mimic the composition of human tears. Most function simply as lubricants, although some more recent formulations do mimic the salts composition of human tears (e.g. TheraTears® [Advanced V ision Research, Woburn, MA]). Many of the ocular lubricants presently available are not based on clinical efficacy. The description of any given product often specifies permitted active ingredients (eg, emulsifiers, surfactants, and viscosity agents) and concentrations, but may give only limited guidance on inactive additives and solution parameters. Certain inactive ingredients that are used in artificial tears sold over the counter (eg, castor oil in Endura and guar in Systane) are not listed in the product description. It can thus be difficult to prove that any given ingredient in an ocular lubricant acts as the active agent.
Although certain artificial tears have demonstrated more success than others in reducing symptoms of irritation, or in reducing ocular surface dye staining, there have been no large scale, comparative clinical trials to evaluate the wide variety of ocular lubricants.
That said, the main objectives in caring for patients with dry eye disease are to improve the patient’s ocular comfort and quality of life, and to return the surface of the eye and the tear film to the normal state. Whilst symptoms can rarely be eliminated completely, they can often be improved, leading to an improvement in the quality of life. It is more difficult to demonstrate that topical lubricants improve the ocular surface and the tear film abnormalities associated with dry eye. Until agents are developed that can restore the ocular surface and tear film to their normal homeostatic state, the symptoms and signs of dry eye disease will continue.
In theory, the ideal artificial lubricant should be preservative-free, contain potassium, bicarbonate, and other electrolytes and have a polymeric system to increase its retention time.
The single most important advance in the treatment of dry eye has come with the elimination of preservatives, such as benzalkonium chloride, from lubricants. Because of the risk of contamination of multidose products, most either contain a preservative or employ some mechanism for minimising contamination. Preservatives are not required in unit dose vials that are discarded after a single use.
The widespread availability of nonpreserved preparations allows the administration of lubricants much more frequently without concern about the toxic effects of preservatives. For patients with moderate-to-severe dry eye disease, the absence of preservatives is of more critical importance than the particular polymeric agent used in ocular lubricants. The ocular surface inflammation associated with dry eye is exacerbated by preserved lubricants. Preservative-free formulations are crucial for patients with severe dry eye with ocular surface disease and impairment of lacrimal gland secretion, or for patients on multiple, preserved topical medications for chronic eye disease.
Nonpreserved, single unit-dose tear substitutes are more costly for the manufacturer to produce, more costly for the patients to purchase, and less convenient to use than bottled ocular lubricants. For these reasons, reclosable unit dose vials were introduced.
Ocular ointments and gels are also used in treatment of dry eye disease. Ointments are formulated with a specific mixture of mineral oil and petrolatum. Some contain lanolin, which can be irritating to the eye and delay corneal wound healing. In general, ointments tend not to support bacterial growth and, therefore, do not require preservatives.
Gels containing high molecular weight crosslinked polymers of acrylic acid (carbomers) have longer retention times than artificial tear solutions, but have less visual blurring effect than petrolatum ointments.
Ocular lubricants are available that mimic the electrolyte composition of human tears, eg, TheraTears® (Advanced Vision Research, Woburn, MA). These also contain bicarbonate, which is critical for forming and maintaining the protective mucin gel in the stomach. Bicarbonate may play a similar role for gel-forming mucins on the ocular surface.
The stability of the tear film depends on the chemical and physical characteristics of that film interacting with the conjunctival and corneal epithelium via the membrane spanning mucins. The mucin layer is usually thought of as a surfactant or wetting agent, acting to lower the surface tension of the relatively “water-hating” ocular surface, thus making the corneal and conjunctival cells wettable.
The tear film is probably best described as a hydrated, mucin gel whose mucin concentration decreases with distance from the epithelial cell surface. This may explain why most of the available water-containing lubricants are only minimally effective in restoring the normal homeostasis of the ocular surface. In addition to washing away and diluting out irritating or toxic substances in the tear film, artificial lubricants hydrate gel-forming mucin. While some patients with dry eye have decreased aqueous lacrimal gland secretion, alterations or deficiencies involving mucin will also cause dry eye.
Although many topical lubricants, with various viscosity agents, may improve symptoms and objective findings, there is little evidence that any one agent is superior to another. Most clinical trials involving topical lubricant preparations will document some improvement (but not resolution) of the patient’s symptoms and improvement in some objective measures. However, the improvements noted are not necessarily any better than those seen with the vehicle or with other nonpreserved artificial lubricants. The elimination of preservatives and the development of newer, less toxic preservatives have made ocular lubricants better tolerated by dry eye patients. However, ocular lubricants, which have been shown to provide some protection of the ocular surface epithelium and some improvement in patient symptoms and objective findings, have not been demonstrated in controlled clinical trials to be sufficient to resolve the ocular surface disorder and inflammation seen in most dry eye sufferers.
Reducing Tear Outflow.
In addition to supplementing the natural ocular tear film, one can also reduce the rate at which tears drain away from the surface of the eye. The main method of achieving this is by use of punctal plugs. There are 3 main types of punctal plug:
- Silicon plugs which sit at the opening of the punctum, flush with its surface and extending down into the vertical part of the canaliculus. These plugs come in various sizes, are easily placed under local anaesthetic, cause little or no irritation, can be left in place for many months and can be easily removed.
- Collagen or other absorbable punctal plugs. These are typically small cylindrical rods of dissolvable material which are easily inserted into the first part of the canaliculus. They typically last for 6 - 10 weeks and tend to dissolve away of their own accord. They do not typically cause any irritation.
- Flow-controller punctal plugs. Similar to the silicon plugs, the flow-controllers sit in the punctal opening and limit, but do not completely stop, the drainage of tears from the ocular surface.
In severe dry eye, the punctal openings can be more permanently closed by cauterisation, resulting in scarring of the punctal opening.
Who does dry eye affect?
Any age group can be affected but dry eye is much more common over the age of 60 and tends to affect women more than men. Some drugs can cause or worsen dry eye symptoms and certain conditions (e.g. Rheumatoid Arthritis) are also associated with dry eye.
Is there anything I can do about it?
Various measures may help, such as:
- Dealing with associated conditions such as blepharitis and generally keeping your eyelids clean.
- Removing eye makeup carefully at the end of the day.
- Using a humidifier, especially if in an airconditioned environment.
- Taking regular short breaks when working on a computer (to avoid prolonged staring)
- Eating a healthy diet which includes Omega-3 (such as in oily fish).
Will it ever get better?
This depends upon the underlying process. If the condition has developed gradually due to increasing age, then it is unlikely to disappear completely, but your eye surgeon should be able to advise a treatment regime which helps control your symptoms.
If your dry eye has developed after an acute event, e.g. viral infection, then it may well resolve over time, but can take months to do so.
What if drops don’t work?
There are currently many different types of lubricant drops, gels and ointments. Most people will find a combination that suits them.
If symptoms persist, then other measures such as punctal plugs (to reduce tear drainage from the surface of the eye), an EyeBag to provide heat treatment, or Blephasteam goggles, may be considered.