Introduction

Dry eye syndrome (DES), also known as dry eye disease (DED), is a condition that affects your eyes when they don’t produce enough tears or the tears dry up too quickly. This can lead to, causing dryness, irritation, and vision problems.¹ DES can also cause inflammation of a part of the eye called the cornea. This is known as keratitis.^1,2 Tear film dysfunction is the connecting cause of DES and keratitis; it refers to the tear film (the liquid in your eyes) being unstable or imbalanced, in terms of composition, so it is not able to carry out its function of lubrication and protection properly.

Anatomy and function of the tear film

If your eyes are healthy, they will produce a liquid that forms a layer over the eyes called the tear film. The tear film helps to lubricate the eyes, protects against infections, prevents debris from entering the eye, helps focus light for clear vision and supplies oxygen and nutrients to some cells in the eye.³ 

Layers of the tear film

The tear film is mainly made up of three different layers, but these layers usually mix and overlap. The layers include an inner mucus layer, a final lipid layer and a layer made up of water in between.^1,3

The middle layer is water with important metabolites, proteins, electrolytes and oxygen, which are all necessary for optimum eye health. It also contains many antimicrobial factors that protect the eye from pathogens trying to enter from the external environment.^1,3

The inner mucus layer is mainly composed of proteins called mucins that are produced by cells, such as goblet cells, in the conjunctiva. This layer helps to stabilise and anchor the middle aqueous layer, spreading it across the whole surface of the eye.^1,3

The lipid layer is the top layer and contains many different fatty acids and lipids, including cholesterol and phospholipids. It stabilises the tear film by delaying evaporation of tears from the eye, preventing them from drying up quickly. Many of the lipids are produced at the edge of the eyelid by the meibomian glands.^1,3

Factors that influence the composition of the tear film 

The composition of the tear film can change based on factors such as age, whether you wear contact lenses, if you’ve had ocular surgery, external environmental conditions such as temperature and air pollution (including smoke and other irritants), daily activities and emotions.^4,5

There are three main types of tears that have different compositions: basal tears, emotional tears and reflex tears.^3,5

• Basal tears are always in your eyes and carry out the function of the tear film described above. They can contain high amounts of salts, protein and lipids³
• Reflex tears are formed in excess to help wash away harmful irritants such as chemicals and foreign material/debris entering the eye (e.g., when cutting an onion). Therefore, these tears are more dilute and can contain higher amounts of antimicrobial compounds and antibodies (proteins produced by the immune system) to protect against pathogens^3,5
• Emotional tears are produced when you are sad, happy or experiencing other emotions and are influenced by psychological, biological and social factors. Emotional tears may be thicker and stickier than the other tear types because they carry more stress hormones and proteins^3,5

Tear film instability

A stable tear film is essential for optimum eye health and clear vision. Tear film stability refers to the ability of the tear film to maintain its thickness and structure across the surface of the eye over time between periods of blinking.⁶ Tear film instability can be determined based on how easily and quickly the tear film breaks apart prematurely before blinking. (American Academy of Ophthalmology) Tear film instability can cause deterioration of vision, including blurry vision, impacting your ability to complete daily tasks such as driving, reading, and using a computer, which may also cause you to feel stressed.^7,8,9

How is tear film dysfunction responsible for dry eye syndrome?

Tear film dysfunction, commonly referred to as dry eye syndrome (DES) or dry eye disease (DED), occurs when the tear film is unstable or the composition is not balanced, causing reduced moisture on the eye’s surface, leaving the eyes dry, irritated and unprotected.¹

Subtypes of DES: Quantitative vs. qualitative tear film deficiency

There are 2 main types of DES,¹⁰ and they are based on either quantitative or qualitative tear film deficiency.¹¹

• Quantitative tear film deficiency refers to the decreased quantity and production of tears due to an issue with the aqueous layer. This type of DES is known as aqueous-deficient dry eye (ADDE)^1,12
• Qualitative tear film deficiency refers to an issue with the composition and quality of the tear film and is related to problems with the innermost mucus or outer lipid layers. This can potentially result in increased tear evaporation even if there is no issue with tear production. This type of DES is known as evaporative dry eye (EDE)^1,12

Meibomian gland dysfunction can cause EDE. In this condition, the meibomian glands that produce the lipids for the outer layer don’t work properly, therefore producing fewer lipids and causing increased tear evaporation.^1,12,13 Goblet cell deficiency can cause DES as well. In this condition, the goblet cells that produce mucin proteins for the inner layer don’t work properly, therefore causing issues with tear anchoring, stabilisation and spreading over the surface of the eye.¹²

Diagnosis and symptoms of DES

There is not just one single sign/symptom or test that can be carried out for diagnosing DES. The doctor or medical examiner may ask you many questions regarding all your symptoms and past medical history, including allergies, surgeries or medications you take. (Cleveland Clinic) He may also conduct a few tests and take a few images of the eye to diagnose DES, looking at tear stability, tear volume and the condition of the surface of the eye and the eyelid.¹ Symptoms of DES can include the following:

• Dryness
• Irritation
• Sensitivity to light 
• Burning sensations
• Watery eyes
• Mucus and crust forming in your eyes
• Redness
• Blurred vision¹

(Cleveland clinic, Guy’s and St Thomas’ NHS Foundation Trust)

Some examples of these tests and imaging techniques include:

• Tear film breakup time (TBUT), including interferometry^1,11
• Schirmer’s test^1,11
• Eye surface staining¹
• Eyelid examinations – e.g., meibography^1,11

Since DES can cause inflammation in the eye, there may be specific chemical markers that usually appear in inflammation, and so tests can be carried out to detect the levels of these chemical markers.¹⁴ The final evaluation is usually made by considering both what you have said to your doctor and what has appeared from your test results. 

Keratitis: overview and clinical relevance

Keratitis is a condition where irritation and inflammation occur in the cornea of the eye, which is the clear, curved tissue on the surface of the eye that covers the pupil and iris. There are two forms of keratitis. One is infectious, caused by microbes, and is also known as microbial keratitis. The other is non-infectious, caused by other factors such as injury, specific eye conditions, wearing contact lenses, autoimmune disorders, nerve damage, exposure to irritants and DES.² (Cleveland Clinic)

Dryness and irritation from DES can cause inflammation of the eye's surface, including the cornea, leading to keratitis. DES can also leave the cornea unprotected, allowing for microbes to take advantage and enter, causing microbial keratitis.¹⁵

Some forms of non-infectious keratitis that are associated with DES include exposure keratitis and neurotrophic keratitis. Exposure keratitis is a form of keratitis where the eyelids do not completely cover the eye and close, leading to continued exposure of the surface of the eye to the external environment, causing dryness, irritation and inflammation.¹⁶ Neurotrophic keratitis refers to inflammation and degradation of the cornea due to nerve damage. The cornea relies on nerves for nutrients, healing and sensation,³ and so if those nerves are damaged, you may feel a loss of feeling in your eye, causing tear film dysfunction and DES. It may also impact the healing ability of your eyes, causing blurry vision or vision loss in severe cases.¹⁷ 

How to manage tear film dysfunction/dry eye syndrome

The main aims of treatment and management strategies are to restore and protect the function of the tear film, soothe your eyes by easing/preventing symptoms of DE, and restore clear vision. (Cleveland Clinic) If possible, treatments should address the underlying causes, especially if DES is caused by infections or secondary conditions.

The main method of managing tear film dysfunction and DES include using artificial tears to replace lost moisture for lubrication of the eye, and the different types include eyedrops, eye gels or eye ointments. (Guy’s and St Thomas’ NHS Foundation Trust) Other methods of managing and preventing DES include keeping the eyes clean to prevent blocked glands in the eye. Using warm compresses, using humidifiers, giving your eyes a rest, and avoiding environmental triggers all help manage and prevent DES.¹ Surgery may also help in cases of exposure keratitis to allow your eyelids to close properly.¹⁶

In severe cases, therapeutic contact lenses may be used as these contact lenses can help to heal and protect your eyes or lubricate them while correcting your vision.¹ It’s best to check with your doctor or medical examiner about the suitability of this option for you specifically, since wearing contact lenses can also cause DES itself.

Summary

The tear film is a central element regarding your eye health as it helps to keep your eyes lubricated and protected from germs and irritants, to give you clear vision. Tear film dysfunction is an underlying cause of DES and keratitis and can be influenced by factors including age, wearing contact lenses and external environmental conditions. To ensure a correct diagnosis, you should explain your symptoms and past medical history to your doctor. Various tests, such as Schirmer’s test or TBUT, and imaging techniques can be used to diagnose DES and keratitis. Through this, the best treatments that are targeted specifically for the patient and their symptoms can be recommended.