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Phospholipids: The Glue that Holds the Tear Film Together?

2M CPD in Australia | 1G in New Zealand | 1 April 2019

By Megan Zabell

Dry eye disease has a significant effect on quality of life and comes with a high global economic burden. Managing the tear film is critical to a dry eye treatment plan. While the role of phospholipids in stabilising the tear film has often been debated, studies show that despite their low concentration in the tear film, they do play an important role in stabilising the tears.

LEARNING OBJECTIVES

  1. Understand the structure and function of the normal tear film,
  2. Understand the role that phospholipids play in the tear film,
  3. Recognise the role that ageing and contact lens wear have in destabilising the tear film, and
  4. Be aware of products designed to help stabilise the tear film through the use of phospholipids.

 

Dry eye is a complex multifactorial disease of the tear film and adnexa, known collectively as the ocular surface. It impacts nearly 344 million people worldwide,1 and carries substantial economic load. Various reports have estimated the treatment of dry eye disease costs up to USD 0.15 billion annually for the nation of Singapore2 and up to USD 3.84 billion annually in the USA.3

Diagnosis and development of a treatment plan for dry eye is rarely straightforward because of multiple disease subtypes, and varying degrees of severity in signs that do not always match the symptoms.4 The TFOS DEWS II report4 reminds us there are many unanswered questions regarding the field of ocular surface dryness.

Physiology of Tears

Traditionally it was thought that the structure of the tear film was best described by Wolff in 19465 as a tri-laminar model. This three layered model suggested the posterior mucin layer was adjacent to the corneal epithelium, where it reduced corneal hydrophobicity, the central aqueous layer provided lubricity and nutrients, and the anterior lipid layer retarded tear evaporation. While the three layered model is still referred to, given that we now know concentration of mucin decreases when moving from the epithelium towards the lipid layer,5 a more modern dual phase model has been proposed. This dual model refers to the mucin and aqueous layers as a single entity – the muco-aqueous gel.5 It is currently the preferred model of the tear film in comparison to the older three layered proposal.5

The structure of the tear film, within the ‘dual phase model’ is described as comprising: the muco-aqueous gel layer and the superficial lipid layer. The muco-aqueous gel layer sits adjacent to the corneal epithelium with a gradient concentration of mucins that are strongest posteriorly. This layer loosely interacts with the overlying superficial lipid layer.4 It has been suggested that the tear film is spread over the eye with a two-step process: when blinking, the upper lid pulls a layer of tears over the cornea through capillary action and then the lipid layer drifts upwards, possibly bringing further aqueous tears with it.5 We know oil ordinarily doesn’t stick to water, and so there must be something else going on – some other interactions that allow the layers to work together so our tear film functions to support a normal healthy eye.

The lipid layer is the outermost layer of the tear film that interacts with the air. Proposed to be responsible for preventing tear evaporation, it provides a smooth optical surface6 and reduces the surface tension of the tears, facilitating a high area to volume ratio of the tear film.5 This layer, while possibly variable in thickness across different areas of the tear film, tends to have a mean thickness of about 42nm.7 It is primarily made up of meibum,6 of which wax esters8 and cholesteryl esters9 are major components. These long chain fatty acids are produced by the meibomian glands – sebaceous glands present in the superior and inferior eyelids.5 According to a model of the lipid layer suggested by McCulley and Shine,10 meibum has been observed to form a duplex film at the surface of the tears, due to ≥90% of the lipids being non-polar.5 Non-polar lipids arrange superiorly, at the air interface, and the remaining constituents of the meibum form a thinner layer underneath. Investigations reveal these lipids to be mainly polar in nature, mostly phospholipids and ω-hydroxy fatty acids.10 In fact, phospholipids are the main polar component of the meibomian gland secretions.11

Phospholipids are a group of molecules normally comprised of a polar head and two hydrocarbon hydrophobic chains.12 In this way the molecules are amphipathic, as in the polar head is hydrophilic and the hydrocarbon chains are hydrophobic/lipophilic.

Phospholipids are common throughout the body for a variety of functions, including being the major component of cell membranes.13 They are also present at biological air-water interfaces – two examples of these are the human lungs, and the tear film.11 At these interfaces, the phospholipids form a layer, so the non polar, hydrophobic chains point towards the air and the polar, hydrophilic heads are submerged in water. In the case of the tear film, the polar heads of the phospholipids orient inferiorly to interact with the muco-aqueous layer, while the long chain hydrocarbons interact with the overlying non polar portion of the tear film lipid layer.11 This interface between the hydrophobic and hydrophilic substances of the tear film allows the tear film to be more stable, while also enabling the lipids to spread evenly and smoothly over the top of the aqueous layer. In this way, phospholipids play a similar interfacial role in synovial fluid.13

As well as forming a layer between substances that wouldn’t otherwise interface well in nature, phospholipids have another very important function – they have been used in pharmacology for decades in the form of liposomes. A liposome is a circular soft-matter vesicle comprised mainly of phospholipids,12 which usually ranges in size from 10nm to 1µm or greater.14 There is normally a central aqueous core contained within either one or multiple phospholipid bilayers, and these can be sourced from either natural or synthetic phospholipids.14 Since 1965, liposomes have been a versatile drug delivery system used for a range of different medical applications including but not limited to cancer therapy, vaccines, ocular delivery, wound healing, and various dermatological uses.12 What makes liposomes so versatile as a vehicle for therapeutic agents is that they can encapsulate both lipophilic drugs within the lipid bilayers, and hydrophilic drugs within the aqueous core.14 While there are not currently any ophthalmological drugs readily available on the Australian market, there have been several promising studies in the areas of liposomal delivery of therapeutic agents for the treatment of infection and glaucoma.12,14

Pathophysiology of Dry Eye

There has been controversy in the past over how much of a role phospholipids actually play in the stabilisation of the tear film. The functionality of phospholipids in the tear film has been questioned due to the relatively low concentrations found in the tear film and meibomian gland secretions in various studies – it has been estimated that the concentration of polar lipids in the tear film lipid profile is anywhere from 0.5% - 13%.13 The wide variation in figures is likely due to different collection methods and various sample analysis techniques.10 A study by Rantamäki and Holopainen in 2016 however, demonstrated polar phospholipids do in fact have an effect on the surface properties of (an artificial) tear film lipid layer at very low concentration, matching the reported physiological concentrations.11 The authors proposed the effectiveness of polar lipids in spreading/stabilising the tear film lipid layer have less to do with the relative concentration of polar to non-polar lipids, as long as there is sufficient polar phospholipid to cover the interface. In other words, there is a crucial amount of phospholipid required to perform the function, and it is independent of the amount of non-polar lipids present in the tear film.11

DEWS II determined that the subcategory ‘evaporative dry eye’ is a bigger contributor to the overall problem of dry eye disease than aqueous deficient dry eye.4 The report also suggests meibomian gland dysfunction, which is a known contributing factor to the development of evaporative dry eye, is the leading cause of dry eye in both clinic and population-based studies.4 A review by Pucker and Haworth in 2015 described the relationship between meibomian gland dysfunction or evaporative dry eye and changes in composition of the polar lipids of the tear film, and found a definite correlation between phospholipids as well as OAHFAs (O-acyl ω-hydroxy fatty acids, another type of polar lipid found in the tear film) and dry eye disease.10 The authors concluded that while they were able to highlight correlations between dry eye disease and the polar tear film lipids, more studies need to be done to investigate evidence of direct causation. However, both phospholipids and OAHFAs are likely important in maintaining homeostasis of the tear film.10

An investigation by Shine and McCulley in 1998 discovered a relationship between two polar lipids, and ocular surface abnormalities consistent with keratoconjuntivitis sicca. Keratoconjunctivitis sicca is traditionally diagnosed with an abnormally low Schirmer’s test result15 indicative of aqueous deficiency. In a previous study, 45% of patients from a sample group with chronic blepharitis were suffering from an associated keratoconjunctivitis sicca.15 This investigation revealed, in a sample of patients with chronic blepharitis, that only subjects suffering keratoconjunctivitis sicca showed lower amounts of polar lipids, in particular two types of phospholipids.15 The authors suggested that a type of ‘evaporative keratoconjunctivitis’15 directly correlated with a reduced quantity of the identified phospholipids, and these patients were in fact suffering from a faster than usual evaporation of the aqueous component, despite the presence of signs and symptoms consistent with keratoconjuntivitis sicca.15

Figure1

 

Treatment Planning

It’s all well and good understanding the normal physiology of tears and the pathophysiology behind dry eye disease, but we must also use our updated knowledge to tailor our treatment plans for patients suffering from dry eye disease.

The mainstay of first line treatment for dry eye has traditionally been ocular lubricants.16 However, as our understanding of dry eye and its causes has evolved, so too has the way we target treatment of this disease.

Alongside our increased understanding of meibomian gland dysfunction and evaporative dry eye, has come the development of a category of lipid containing eye drops.16 Currently in Australia there is a range of lipid based products available to help relieve the symptoms of evaporative dry eye, and a much smaller sub category of those containing phospholipids. The only two drop based formulations containing phospholipids are Alcon’s Systane Balance and Systane Complete lubricating eye drops.

Systane Balance was specifically designed by Alcon for patients with dry eye symptoms associated with meibomian gland dysfunction.17 It contains the patented LipiTech system, a micro-emulsion of mineral oils and a polar phospholipid (phosphatidylcholine) surfactant to minimise the evaporative loss of tears from the ocular surface.17 As well as the LipiTech system, the drop also contains propylene glycol, hydroxypropyl-guar, borate, and sorbitol. Studies on the chemical makeup of Systane Balance have shown it to be a stable emulsion, with the unique combination of ingredients making it adhesive on the ocular surface.17 Prior to instillation in the eye, the drop has low viscosity but after instillation, the higher pH of the ocular surface causes the hydroxypropyl-guar viscoelastic networks to form and adhere to the ocular surface, releasing the oil micelles over time.17 In various clinical studies, Systane Balance was shown to be associated with substantial improvements in tear film lipid layer thickness after topical instillation in patients suffering from dry eye disease secondary to meibomian gland dysfunction.17 Results two hours after instillation showed that Systane Balance lubricating drops increased lipid layer thickness compared to a competitor lubricating drop containing mineral oil.17 It was also shown to induce less blur and aid the function of the meibomian gland over the competitor lipid-containing eye drop.17 Additionally, Systane Balance was found to be more effective than habitually prescribed treatments in reducing the signs and symptoms of dry eye associated with meibomian gland dysfunction.17

Systane Complete, the most recent release from Alcon, is referred to as “the all-in-one drop” (Figure 1). It contains a nano-emulsion of mineral oil and phosphatidylcholine as well as propylene glycol, hydroxypropyl-guar, borate, and sorbitol. Systane Complete contains more hydroxypropyl-guar than Systane Balance, and therefore provides greater protection for the ocular surface via moisture retention. A comparison (in vitro study)performed between Systane Balance and Systane Complete lubricating eye drops showed the use of Systane Complete had three times greater cell protection via moisture retention when compared to Systane Balance or a nano-emulsion vehicle alone.18 Tissue on tissue lubrication has also been shown to be significantly sustained with Systane Complete in comparison to Systane Balance, with an improvement of 33%.19

Tear phospholipid levels have been shown to be affected by both short term and long term contact lens wear.20 While more studies are needed to properly investigate the changes to the tear film lipodome during contact lens wear,21 it has been hypothesised that contact lens wearers who are symptomatic for contact lens discomfort have increased activity of phospholipases in the tear film. This leads to increased hydrolysis of phospholipids compared to that of non-symptomatic wearers.21

Figure 2

 

Dailies Total 1 by Alcon are daily disposable contact lenses that incorporate phosphatidylcholine into the lens material using patented SmarTears technology – a phospholipid is embedded into these water-gradient contact lenses (Figure 2). As the natural phospholipids are lost from the wearer’s tear film, the embedded phospholipid is drawn out of the contact lens to supplement their tear film lipid layer.21 This technology can help address contact lens related dryness through lipid layer stabilisation.22 In a comparison study looking at the clinical performance of three silicone hydrogel daily contact lenses, Dailies Total 1 were found to have a significantly longer non-invasive tear break-up time measured on eye compared to two competitors.23

The SmarTears technology is also incorporated into Dailies Total 1 Multifocal contact lenses. This is significant, given that we know the profile of polar lipids in meibum, as well as the amount of meibum produced, significantly changes with age.24,25 The amount of meibum produced is thought to reduce with age due to the accepted fact that meibomian gland acini are lost with increasing age.25 Aging is also accompanied by an increase in the opacity of meibomian gland secretions and eyelid structural changes.24 As such, it seems even more important to provide extra support to the lipid layer of presbyopic wearers by embedding multifocal lenses with SmarTears technology. A study by Kern et al, measuring the effect of switching pre-existing soft multifocal contact lens wearers who were experiencing some end of day discomfort to Dailies Total 1 Multifocal contact lenses, showed that not only did switching increase comfortable wearing time, but it also reduced the percentage of subjects reporting dryness symptoms in comparison to their habitual multifocal soft contact lenses.26

Conclusion

As described earlier, our understanding of the tear film has increased from the original three layer model originally proposed by Wolff.5 We now consider the tear film as biphasic, with a muco-aqueous gel layer and overlying lipid layer. The lipid layer itself can be broken down into a thicker non-polar lipid layer and thinner underlying polar lipid layer.10 The polar lipid layer is very complicated and bears further investigation, due to the differences found so far between studies. However, we do know that it contains predominantly phospholipids and ω-hydroxy fatty acids.11 While phospholipids are in very low concentration in the tear film, they have been shown to play an important role in stabilising the tears, despite this low concentration.11

As more becomes known about the normal structure of the tear film, and issues that can arise, so should our treatments for the complex issue of dry eye disease. Alcon is one company with a range of products, including Systane Complete and Systane Balance lubricating eye drops, that are designed to support the tear film as a whole. These contain different emulsions of mineral oil and phospholipids, as well as hydroxypropyl-guar and propylene glycol in varying concentrations to adhere to the ocular surface and help stabilise the tear film lipid layer. The use of SmarTears technology, which slowly releases phospholipids from the Dailies Total 1 contact lens material as natural phospholipids are lost from the tear film, can also combat tear film instability and discomfort related to contact lens wear.

Tear Film Stability: A Case Study

Currently there is no readily available test to assess the phospholipid content of a patient’s tear film in a clinical setting. However, there are multiple ways to assess tear film stability, which we know can be influenced by the phospholipid content of the tears.

One method of assessing the stability of the tears is to measure the tear film break up times (TFBUT) non-invasively. This can be done on various devices, such as the Medmont E300 corneal topographer. This device uses Placido disc videokeratography to assess the quality of reflected mires. It applies a topographical analysis algorithm to determine when the tear film is breaking up.1 The tear film can be represented either through images of the mires, or a topographical map of tear film stability, with warmer colours representing unstable areas of the tear film.1

In order to experiment with the tear stabilising properties of the SmarTears technology and Water Gradient material in Dailies Total 1, an Alcon male staff member in his 30s was recruited (Mr A).

Mr A is not a habitual contact lens wearer but was asked to wear a Dailies Total 1 contact lens in his left eye and a daily disposable silicone hydrogel lens without SmarTears in the right. He was asked to go about his normal business for four hours before returning for tear film stability measurements.

Figure 3:  Right Eye – SiHy DD lens
Left: DD SiHy Lens mires at 0s  Middle: DD SiHy lens mires at 5s  Right: DD SiHy lens mires at 12s

 

Figure 4:  Left Eye – Dailies Total 1
Left: DT1 mires 0s  Middle: DT1 mires 5s  Right: DT1 mires 12s

 

As per standard procedure for this equipment, Mr A was initially asked to blink twice before holding his eyes open for as long as possible. As shown in Figure 3, the mires of the right eye began distorting initially, indicating that the overlying tear film was already unstable. As time passed, the mires became more distorted and began to blur into one another, particular infero-centrally.

In comparison, the mires of the left eye have no apparent distortion and by the 12 second mark, there is only some minor wavering of the mires in the infero-temporal periphery (Figure 4). Overall, this indicates that the tear film overlying the Dailies Total 1 contact lens on Mr A is much more stable than the one over the top of the standard daily disposable silicone hydrogel lens.

It is highly likely that the tears on top of the SiHy lens were unstable immediately after the blink because, in between inserting the lenses and taking the measurement, Mr A spent time using a computer in an air conditioned environment with low humidity. Both the environment and computer use are associated with a decrease in tear film stability.2

Interestingly, these adverse conditions had less of an effect on the eye with the Dailies Total 1 contact lens. This is likely due to the combination of the Water Gradient material and SmarTears technologies.

Figures provided by Alcon Professional Affairs. NP4: A21902968811
References
1. Downie, LE Automated Tear Film Surface Breakup Time as a Novel Clinical Marker for Tear Hyperosmolarity in Dry Eye Disease. Invest Ophthalmol Vis Sci 2015; 56: 7260-7268
2. Wilcox et al TFOS DEWSII Tear Film Report The Ocular Surface 2017: 369-406

   

Megan Zabell BOptom graduated from the University of Melbourne as a therapeutically endorsed optometrist in 2011. She worked in a private optometry practice for seven years and also taught preclinical optometry at the University of Melbourne for four years. Ms Zabell has a particular interest in the treatment of dry eye, as well as fitting contact lenses, which is what drew her to join Alcon Vision Care’s Professional Affairs Team late in 2018. Ms Zabell also enjoys volunteering her optometric skill, having done so in Vanuatu and Nepal.

 

 

Contact: Australia, Alcon Laboratories (AUS) 1800 224 153. New Zealand, c\o PharmaCo (NZ) 0800 101 106.
NP4 # :A21901945630
 
References
1. Market Scope 2016 Dry Eye Products Report: A Global Market Analysis for 2015 to 2021
2. Waduthantri S et al, Cost of Dry Eye Treatment in an Asian Clinic Setting, PLoS One 2012; 7(6): e37711
3. Yu et al, The Economic Burden of Dry Eye Disease in the United States: A Decision Tree Analysis. Cornea 2012; 30: 379-387
4. Craig et al, TFOS DEWS II Definition and Classification Report. The Ocular Surface 2017; 15: 276-283
5. Wilcox et al. TFOS DEWSII Tear Film Report, The Ocular Surface 2017; 369-406.
6. Bron et al. Functional aspects of the tear film lipid layer. Experimental Eye Research 2004; 78(3): 347-360
7. King-Smith et al. Application of a Novel Interferometric Method to Investigate the Relation between Lipid Layer Thickness and Tear Film Thinning. Inves Ophthal & Vis Sci 2010; 51(5): 2418-2423.
8. Butovich et al. Evaluation and Quantitation of Intact Wax Esters of Human Meibum by Gas-Liquid Chromatography-Ion Trap Mass Spectrometry. Inves Ophthal & Vis Sci 2012; 53(7): 3766-2781.
9. Butovich I. Fatty acid composition of cholesteryl esters of human meibomian gland secretions. Steroids 2010; 75(10): 726-733
10. Pucker et al. The Presence and Significance of Polar Meibum and Tear Lipids. The Ocular Surface 2015; 13(1): 26-42.
11. Rantamäki A and Holopainen J. The Effect of Phospholipids on Tear Film Lipid Layer Surface Activity. Invest Ophthal & Vis Sci 2017; 58(1): 149-154
12. Ahmed et al. Liposome: composition, characterisation, preparation, and recent innovation in clinical application. Journal of Drug Targeting 2018; 21: 1029-2330
13. Campbell et al. Tear Analysis and Lens-tear Interactions: Part II. Ocular Lipids – Nature and fate of Meibomian Gland Phospholipids. Cornea 2011; 30(30): 325-332.
14. Mishra et al. Recent Applications of Liposomes in Ophthalmic Drug Delivery. Journal of Drug Delivery 2011; 2011: 14 pages.
15. Shine WE and MucCully JP. Keratoconjunctiitis Sicca Associated With Meibomiam Secretion Polar Lipid Abnormality. Arch Ophthalmol 1998; 116: 849-854
16. Jones et al. TFOS DEWS II Management and Therapy Report The Ocular Surface 2017: 580-634
17. Benelli, U. Systane lubricant eye drops in the management of ocular dryness. Clinical Ophthalmology 2011; 5: 783-790
18. Alcon data on file, 2013
19. Alcon data on file, 2013
20. Saville, J. Phospholipids in human tears, meibum and deposited onto contact lenses. Doctor of Philosophy thesis, School of Chmistry, University of Woollongong, 2013.
21. Craig et al. The TFOS International Workshop on Contact Lens Discomfort: Report of the Contact Lens Interactions With the Tear Film Subcommittee. Invest Ophthalmol Vis Sci 2013; 54: TFOS123-TFOS156
22. Pitt et al. Loading and Release of a Phospholipid From Contact Lenses. Optometry and Vision Science 2011; 88(4): 502-506.
23. Varikooty et al. Clinical Performance of Three Silicone Hydrogel Daily Disposable Lenses. Optometry and Vision Science 2015; 92(3): 301-311
24. Sullivan et al. influence of Aging on the Polar and Neutral Lipid Profiles in human Meibomian Gland Secretions. Arch Ophthalmol 2006; 124(9): 1286-1292.
25. Bron et al. TFOS DEWS II pathophysiology report. The Ocular Surface 2017; 15: 438-510
26. Kern et al. Clinical Outcomes for DailiesTotal1 Multifocal Lens in Symptomatic Patient. July 2011.
 

' phospholipids are the main polar component of the meibomian gland secretions '