Space under lids
Precorneal film
Meniscus
The exposed area is quite closely dependent on interpalpebral height, which in turn is determined by the direction of gaze (exposure is considerably greater in upward than in downward gaze, as the upper lid follows the move¬ment of the globe). Thus in downward gaze not all the cornea is exposed, along with a small area of sclera, while in upward gaze we may see all the cornea plus a variable amount of sclera both above and below the limbus as well as larger lateral areas. A rough linear relationship between area and palpebral height is often used: Area (cm2) = 0.28 X (height in mm) — 0.44 . A more precise value can be obtained by computer analysis of images of the eye, allowing sep¬arate estimation of the areas of exposed cornea and sclera . Typical values for the area in normal level gaze are 2-3 cm2, of which 45-55% is cornea. These figures help to strengthen the recommendation to those doing much computer work, to keep the screen as low as possible to minimise ocular exposure and drying because of the reduced blink rate.
The total area of the human conjunctival sac has been estimated as 16 cm2 . If all this is covered by a layer of gelatinous mucus with a water content of about 90%, say averaging 1 |xm thick, 1.44 |xl of fluid would be contained. Where the lids overlie the globe, it is possible that two such layers on the apposed surfaces will be in contact, giving an effective fluid thickness of 2 |xm. However, it seems unlikely that in this case there would be free flow of fluid (e.g. fresh tear fluid entering the upper conjunctival sac through the tear duc¬tules) although fluid transport could occur through a 'squeegee' mechanism in blinking.
Formation of the Film
As the lids close during the blink, the upper and lower menisci are pushed ahead of them and sweep up the fluid forming the preocular film, rather like a windscreen wiper. In the opening phase of the blink, the viscosity of the tears causes fluid to be pulled out of both menisci to create a new film, but opposed to this is the negative pressure due to the concave tear meniscus. As long as the lids are moving, fluid is spread, but when the lids become stationary there is within 0.3-1 s a settling down or rearrangement whereby fluid is pulled back into the meniscus while the bulk of the spread film remains intact. The region closest to the meniscus is however considerably thinned and if fluorescein is instilled, a 'black line' can be seen around the rim of the tear film. This line is so thin that it contains too little dye to fluoresce, and it acts as a barrier to diffusion or flow of fluid into or out of the film in the interblink period. Hence the film is effectively isolated from the rest of the lacrimal system while the eye is open, and is subject to different influences such as evaporative loss at these times. The isolated film has been referred to as 'perched' because it covers the exposed eye but is in a sense independent of the ocular adnexa.
Volume of Various Compartments
We can distinguish three distinct components of the fluid in the lacrimal sac: the film itself, lying between the lid margins; the continuous line of meniscus around the lid margins, joining at the outer canthus and around the caruncle, and the fluid under the lids.
It is still not clear what volume of tears lies under the lids, or whether this should be included as part of the tear film. In the normal eye the lid margins glide in contact with the globe during a blink, and it is thought that there is a slight curvature inwards of the margin of the upper lid to give a 'windscreen wiper' action sweeping the film forward as the lids close. This would suggest that the exposed and the under-lid compartments remain separate; but King- Smith et al. discuss the possibility that the two compartments are connected but that during the blink the upper meniscus changes position, being swept down by the advancing lid margin.
Recent experiments on adding saline to severely dry eyes showed that fluid was absorbed (presumably under the lids) before any lid margin meniscus became visible, implying that the two compartments are connected . The mean under-lid volume was calculated to be 5-6 |xl. The volume of tears in the combined upper and lower menisci can be calculated from their total length (about 50 mm) and cross-sectional area, assuming that their profile is a quad¬rant of a circle; using a mean value of 0.365 mm for the radius of curvature, the normal meniscus volume is about 2.9 |xl [7]. The volume of the preocular film clearly depends on its thickness (see below), but taking commonly-agreed limits of 3 and 10 |xm and an area of 2 cm2, the volume is 0.6-2.0 |xl with a mean probably about 1.0 |xl. Hence the total volume of tear fluid in the external eye is roughly 10 |xl. This does not include additional small amounts such as the fluid over the caruncle.
Clearly there is considerable personal variation in this figure - differences in form of the lid margins, slight inward or outward turning of the lids relative to the globe, positioning of the puncta and height of the palpebral opening can all affect the contained tear volume.
Thickness of Precorneal Film
Estimates of tear volume involve knowing the thickness of the film. This is not easy to measure, although several methods have been used over the years. Simple methods include isolating an area of tear film by pressing the end of a wide-mouthed syringe onto the eye and measuring the volume of fluid sucked off , absorbing fluid over a known area by placing a disc of absorbent paper on the eye , or measuring fluorescence intensity after adding a known amount of fluorescein to the film. More recently the variation of intensity of light reflection has been analysed in three ways (varying angle, frequency or wavelength). Ocular coherence tomography can measure corneal thickness with and without a contact lens and estimate the film thickness by difference. All these methods are summarised by King-Smith et al.. Some estimates of tear film
Precorneal film
Meniscus
The exposed area is quite closely dependent on interpalpebral height, which in turn is determined by the direction of gaze (exposure is considerably greater in upward than in downward gaze, as the upper lid follows the move¬ment of the globe). Thus in downward gaze not all the cornea is exposed, along with a small area of sclera, while in upward gaze we may see all the cornea plus a variable amount of sclera both above and below the limbus as well as larger lateral areas. A rough linear relationship between area and palpebral height is often used: Area (cm2) = 0.28 X (height in mm) — 0.44 . A more precise value can be obtained by computer analysis of images of the eye, allowing sep¬arate estimation of the areas of exposed cornea and sclera . Typical values for the area in normal level gaze are 2-3 cm2, of which 45-55% is cornea. These figures help to strengthen the recommendation to those doing much computer work, to keep the screen as low as possible to minimise ocular exposure and drying because of the reduced blink rate.
The total area of the human conjunctival sac has been estimated as 16 cm2 . If all this is covered by a layer of gelatinous mucus with a water content of about 90%, say averaging 1 |xm thick, 1.44 |xl of fluid would be contained. Where the lids overlie the globe, it is possible that two such layers on the apposed surfaces will be in contact, giving an effective fluid thickness of 2 |xm. However, it seems unlikely that in this case there would be free flow of fluid (e.g. fresh tear fluid entering the upper conjunctival sac through the tear duc¬tules) although fluid transport could occur through a 'squeegee' mechanism in blinking.
Formation of the Film
As the lids close during the blink, the upper and lower menisci are pushed ahead of them and sweep up the fluid forming the preocular film, rather like a windscreen wiper. In the opening phase of the blink, the viscosity of the tears causes fluid to be pulled out of both menisci to create a new film, but opposed to this is the negative pressure due to the concave tear meniscus. As long as the lids are moving, fluid is spread, but when the lids become stationary there is within 0.3-1 s a settling down or rearrangement whereby fluid is pulled back into the meniscus while the bulk of the spread film remains intact. The region closest to the meniscus is however considerably thinned and if fluorescein is instilled, a 'black line' can be seen around the rim of the tear film. This line is so thin that it contains too little dye to fluoresce, and it acts as a barrier to diffusion or flow of fluid into or out of the film in the interblink period. Hence the film is effectively isolated from the rest of the lacrimal system while the eye is open, and is subject to different influences such as evaporative loss at these times. The isolated film has been referred to as 'perched' because it covers the exposed eye but is in a sense independent of the ocular adnexa.
Volume of Various Compartments
We can distinguish three distinct components of the fluid in the lacrimal sac: the film itself, lying between the lid margins; the continuous line of meniscus around the lid margins, joining at the outer canthus and around the caruncle, and the fluid under the lids.
It is still not clear what volume of tears lies under the lids, or whether this should be included as part of the tear film. In the normal eye the lid margins glide in contact with the globe during a blink, and it is thought that there is a slight curvature inwards of the margin of the upper lid to give a 'windscreen wiper' action sweeping the film forward as the lids close. This would suggest that the exposed and the under-lid compartments remain separate; but King- Smith et al. discuss the possibility that the two compartments are connected but that during the blink the upper meniscus changes position, being swept down by the advancing lid margin.
Recent experiments on adding saline to severely dry eyes showed that fluid was absorbed (presumably under the lids) before any lid margin meniscus became visible, implying that the two compartments are connected . The mean under-lid volume was calculated to be 5-6 |xl. The volume of tears in the combined upper and lower menisci can be calculated from their total length (about 50 mm) and cross-sectional area, assuming that their profile is a quad¬rant of a circle; using a mean value of 0.365 mm for the radius of curvature, the normal meniscus volume is about 2.9 |xl [7]. The volume of the preocular film clearly depends on its thickness (see below), but taking commonly-agreed limits of 3 and 10 |xm and an area of 2 cm2, the volume is 0.6-2.0 |xl with a mean probably about 1.0 |xl. Hence the total volume of tear fluid in the external eye is roughly 10 |xl. This does not include additional small amounts such as the fluid over the caruncle.
Clearly there is considerable personal variation in this figure - differences in form of the lid margins, slight inward or outward turning of the lids relative to the globe, positioning of the puncta and height of the palpebral opening can all affect the contained tear volume.
Thickness of Precorneal Film
Estimates of tear volume involve knowing the thickness of the film. This is not easy to measure, although several methods have been used over the years. Simple methods include isolating an area of tear film by pressing the end of a wide-mouthed syringe onto the eye and measuring the volume of fluid sucked off , absorbing fluid over a known area by placing a disc of absorbent paper on the eye , or measuring fluorescence intensity after adding a known amount of fluorescein to the film. More recently the variation of intensity of light reflection has been analysed in three ways (varying angle, frequency or wavelength). Ocular coherence tomography can measure corneal thickness with and without a contact lens and estimate the film thickness by difference. All these methods are summarised by King-Smith et al.. Some estimates of tear film
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