Tear Lipid Layer and Contact Lens Comfort
Tear Lipid Layer and Contact Lens Comfort
The composition of meibum has been reported in animal and human studies with slight variations based on the analysis techniques used. The human meibum includes, predominantly, wax and sterol esters followed in amount by triacylglycerides, diesters, free fatty acids, and a small quantity of phospholipids. Studies that evaluated tear lipids rather than just meibum found a more complex composition with comparatively less wax and sterol esters and more phospholipids, suggesting alternative sources for phospholipids such as corneal or conjunctival epithelial cells. Tiffany stated that a reason for this difference may be because of the ability of lid bacteria to produce lipases that can modify tear film phospholipids so that the meibum, when spread over the tear film to form the lipid layer, might differ in its phospholipid composition to that fresh from the acini of the glands. Phosphatidylcholine (PC) and sphingomyelin are the two major polar lipid species present in meibum. The concentration of phosphatidylcholine to sphingomyelin in meibum is 5:1, whereas, in tears, the proportion is 1:1, which further supports the disparity in phospholipid concentrations between meibum and tear lipidome. Recently, a new class of fatty acids called OAHFA have been identified and quantified in meibum.Table 3 provides a brief comparison of meibum and tear lipid composition based on the current literature.
Contact lens wear might be associated with the compositional instability of the lipid layer. The level of cholesterol in tears decreased immediately with lens wear (1.91±1.9 mg/mL) compared with no lens wear (1.95±1.5 mg/mL) but recovered to habitual levels within 2 weeks of ceasing lens wear. Wear of Etafilcon A lenses (daily disposable lenses) has been reported to reduce the phospholipid concentrations in tears (162±33 μg/mL) compared with tears of the same subjects without contact lens wear (220±35 μg/mL). In the same study, it was proposed that the increased activity of the enzyme secretory phospholipase A2 (sPLA2), which is responsible for cleaving fatty acids from phospholipids and producing free fatty acids, could be a reason for the reduction in phospholipid concentration in tears during lens wear.
Phospholipase enzymes cleave the ester bond of glycerophospholipids at their sn-2 position yielding free fatty acids. Phospholipase enzymes can be broadly classified into two groups, "secretory" and "cytosolic," and are further subcategorized into groups I, II, III, and beyond, based on their amino acid sequence. Nevalainen et al. were the first to detect group II sPLA2 in human tears and lacrimal glands and reported its possible anti-inflammatory functions. In tears, the normal sPLA2 concentration is 54.5±33.9 μg/mL but is higher (79.6±29.6 μg/mL) among the young (20–29 years) and reduced (32.4±27.8 μg/mL) in an older population (>70 years). During contact lens wear (experienced soft contact lens wearers for a minimum of 2 years), the concentration of sPLA2 in tears was decreased (56.3±30.0 μg/mL) compared with its concentration in people who did not wear contact lenses (95.2±48.2 μg/mL).Table 4 provides a comparison of lipid class concentrations in normal tears and during contact lens wear.
The compositional instability of the lipid layer with contact lens wear might have some associations with lens-related discomfort. The in vitro observations by Butovich and coworkers showed increased concentrations of free fatty acids could disrupt or even disintegrate the tear film lipid layer. In addition, reduced tear phospholipids were found associated with shorter tear breakup time among soft contact lens wearers. Because of their amphiphilic and surfactant properties, OAHFAs are suspected to play an important role in maintaining the structural stability of tear film lipid layer along with phospholipids. Lam et al. reported reduced concentrations of OAHFAs in meibum with increased disease severity in a dry eye population. However, the effect of contact lenses on OAHFAs in tears has not yet been studied.
Lipid deposits on contact lens are also a possible cause for lens-related ocular discomfort, and the rate of deposition mainly depends on the lens material used, duration of lens wear, and wearing schedule. Although silicone hydrogel lens materials have high water content and oxygen permeability compared with hydrogel lenses, they are prone to higher lipid deposition, which is further worsened by wearing the lenses for longer hours. Contact lens wear tends to be associated with increasing ocular discomfort over the course of the day, and this end of the day discomfort might be associated with increased lipid deposition. However, an analysis of the amount of lipid extracted from silicone hydrogel lenses worn on a daily wear schedule with measure of comfort only found weak correlations in univariate analysis for comfort on insertion of lenses (r=0.03, P<0.01) and overall comfort during lens wear (r=0.02, P=0.03) (higher amounts of cholesterol extracted lower comfort scores). Cholesterol and wax esters are the principal lipid classes found in lipid deposits from soft contact lenses although lenses can also deposit phospholipids. Hume et al. recovered sPLA2 deposits from both ionic (Etafilcon A) and nonionic (Polymacon) soft contact lens materials worn on daily wear or extended wear schedules. The amount of enzyme deposits on each contact lens material was directly proportional to the enzyme activity on each lens material. There was higher enzyme deposition and higher enzyme activity on Etafilcon A lenses in both daily and extended wear schedules. As mentioned earlier, higher enzyme activity is associated with increased lipid degradation and linked to increased ocular discomfort.
Biochemical Changes
The composition of meibum has been reported in animal and human studies with slight variations based on the analysis techniques used. The human meibum includes, predominantly, wax and sterol esters followed in amount by triacylglycerides, diesters, free fatty acids, and a small quantity of phospholipids. Studies that evaluated tear lipids rather than just meibum found a more complex composition with comparatively less wax and sterol esters and more phospholipids, suggesting alternative sources for phospholipids such as corneal or conjunctival epithelial cells. Tiffany stated that a reason for this difference may be because of the ability of lid bacteria to produce lipases that can modify tear film phospholipids so that the meibum, when spread over the tear film to form the lipid layer, might differ in its phospholipid composition to that fresh from the acini of the glands. Phosphatidylcholine (PC) and sphingomyelin are the two major polar lipid species present in meibum. The concentration of phosphatidylcholine to sphingomyelin in meibum is 5:1, whereas, in tears, the proportion is 1:1, which further supports the disparity in phospholipid concentrations between meibum and tear lipidome. Recently, a new class of fatty acids called OAHFA have been identified and quantified in meibum.Table 3 provides a brief comparison of meibum and tear lipid composition based on the current literature.
Contact lens wear might be associated with the compositional instability of the lipid layer. The level of cholesterol in tears decreased immediately with lens wear (1.91±1.9 mg/mL) compared with no lens wear (1.95±1.5 mg/mL) but recovered to habitual levels within 2 weeks of ceasing lens wear. Wear of Etafilcon A lenses (daily disposable lenses) has been reported to reduce the phospholipid concentrations in tears (162±33 μg/mL) compared with tears of the same subjects without contact lens wear (220±35 μg/mL). In the same study, it was proposed that the increased activity of the enzyme secretory phospholipase A2 (sPLA2), which is responsible for cleaving fatty acids from phospholipids and producing free fatty acids, could be a reason for the reduction in phospholipid concentration in tears during lens wear.
Phospholipase enzymes cleave the ester bond of glycerophospholipids at their sn-2 position yielding free fatty acids. Phospholipase enzymes can be broadly classified into two groups, "secretory" and "cytosolic," and are further subcategorized into groups I, II, III, and beyond, based on their amino acid sequence. Nevalainen et al. were the first to detect group II sPLA2 in human tears and lacrimal glands and reported its possible anti-inflammatory functions. In tears, the normal sPLA2 concentration is 54.5±33.9 μg/mL but is higher (79.6±29.6 μg/mL) among the young (20–29 years) and reduced (32.4±27.8 μg/mL) in an older population (>70 years). During contact lens wear (experienced soft contact lens wearers for a minimum of 2 years), the concentration of sPLA2 in tears was decreased (56.3±30.0 μg/mL) compared with its concentration in people who did not wear contact lenses (95.2±48.2 μg/mL).Table 4 provides a comparison of lipid class concentrations in normal tears and during contact lens wear.
The compositional instability of the lipid layer with contact lens wear might have some associations with lens-related discomfort. The in vitro observations by Butovich and coworkers showed increased concentrations of free fatty acids could disrupt or even disintegrate the tear film lipid layer. In addition, reduced tear phospholipids were found associated with shorter tear breakup time among soft contact lens wearers. Because of their amphiphilic and surfactant properties, OAHFAs are suspected to play an important role in maintaining the structural stability of tear film lipid layer along with phospholipids. Lam et al. reported reduced concentrations of OAHFAs in meibum with increased disease severity in a dry eye population. However, the effect of contact lenses on OAHFAs in tears has not yet been studied.
Lipid deposits on contact lens are also a possible cause for lens-related ocular discomfort, and the rate of deposition mainly depends on the lens material used, duration of lens wear, and wearing schedule. Although silicone hydrogel lens materials have high water content and oxygen permeability compared with hydrogel lenses, they are prone to higher lipid deposition, which is further worsened by wearing the lenses for longer hours. Contact lens wear tends to be associated with increasing ocular discomfort over the course of the day, and this end of the day discomfort might be associated with increased lipid deposition. However, an analysis of the amount of lipid extracted from silicone hydrogel lenses worn on a daily wear schedule with measure of comfort only found weak correlations in univariate analysis for comfort on insertion of lenses (r=0.03, P<0.01) and overall comfort during lens wear (r=0.02, P=0.03) (higher amounts of cholesterol extracted lower comfort scores). Cholesterol and wax esters are the principal lipid classes found in lipid deposits from soft contact lenses although lenses can also deposit phospholipids. Hume et al. recovered sPLA2 deposits from both ionic (Etafilcon A) and nonionic (Polymacon) soft contact lens materials worn on daily wear or extended wear schedules. The amount of enzyme deposits on each contact lens material was directly proportional to the enzyme activity on each lens material. There was higher enzyme deposition and higher enzyme activity on Etafilcon A lenses in both daily and extended wear schedules. As mentioned earlier, higher enzyme activity is associated with increased lipid degradation and linked to increased ocular discomfort.
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