EYELIDS AND TEAR CHANNELS

Eyelids protect the eyes from drying and from getting flying particles into them.

Fig.4. Cross section of the eye lid and the eye. Sebaceous glands have their openings at the root of the eye lashes. If such a gland is inflammed it is called "hordeolum" or sty. Meibomian glands are imbedded in the cartilage of the eye lid.

Meibomian Glands secret oil. If the long, thin opening of the gland becomes occluded, the gland continues to secret and a small nodule is formed in the eyelid. It may become inflammed. Usually it disappeares in a few weeks. If not, it is removed through a small wound in the skin or in the conjunctiva, the mucous membrane.

Since there are only 30 or so Meibomian glands in each eye lid, it is important to prevent inflammations in them. If most of them are destroyed by inflammations, too little oil is secreted by the remaining Meibomian glands. The oil normally covers the tear layer and decreases evaporation. If too little oil is secreted, the tear layer evaporates quickly and the cornea may get dry spots.

Tear glands secret tears. The glands are located near the eyes behind the outer upper rim of the bony orbit. Tears flow down over the eye and into the tear channels in the inner corner of the eye lids. The tear ducts are hair thin and therefore easily blocked. If there is swelling of the mucous membrane of the nose, eyes usually become watery and irritated.

Fig.5. Tear gland and tear passage into the nose. Tear gland is in the upper outer corner near the eye. Tear ducts and tear sac are in the inner corner of the eye.

Tears are sucked into the tear ducts because of the low pressure in the tear sac during movements of the eyelids. They are pumped out of the tear sac into the nose by the same movements. If a person has facial nerve palsy the eye lids move less than normally, the eye is watering. If the lower lid is hanging badly, the corneal surface may become too dry despite the watering.

Figure 5a . 'Butterfly' tape is an effective way of keeping the lower lid in a better position in the acute phase of facial nerve palsy.

The hanging lower lid can be supported by tape as the first aid to decrease drying of the cornea (Figure 5a). If the nerve palsy is permanent, an operative correction of the lower lid may be necessary.

It is possible to measure the movement of the tears by using a special colour. A small amount of the colour is placed on the surface of the eye. After that it is measured how soon the colour arrives into the nose. The tear passages can be examined using x-ray techniques, too.

The surface of the eyes often becomes irritated in cold weather. The eyes start watering. Tear passages do not need to be occluded. There are just too many tears. The best thing to do is to protect sensitive eyes from the cold wind. Side shields (Fig.6) help a lot. A hood is the most effective protecting device on a windy day. It prevents the wind from getting between the spectacles and the eye.

Fig.6. Side shields are fixated on the spectacles.

Figure 6a. You may make the side shield yourself. Cut a piece of paper like the one in Figure 6a. Cut two slits in it and thread it on the side part of your glasses. If the form of the side shield fits your face, cut it in clear plastic, otherwise modify the form for better fit. Note that the edge of the side shield is at the level of the front surface of the glasses to prevent the wind from getting between the lens and your eye.

Tear film on the cornea has three layers: oil, water and mucus.

Fig. 7. Structure of the tear film. There is a thin layer of mucus on the surface of the cornea, on it there is a layer of water that contains salts. The outermost layer is a thin layer of oil that decreases evaporation of water, and thus drying of the corneal surface. (A better picture you get by clicking here.)

The surface cells of the cornea are hydrophobic, which means that waterdrops will not spread on such surface but roll aside. The thin layer of mucus is needed for making the surface hydrophilic, the layer of water can then spread evenly on the corneal surface.

If the mucus layer is uneven, some patches on the cornea have no mucus on the surface. The water layer rolls aside, the surface dries and wrinkles a little. This stretches the nerve endings and causes a sticking pain in the eye.

Changes in the structure of the tear film are common. Eyes are somewhat dry and irritated, now and then there is a sensation of sticking pain. The therapy is very simple: the corneal surface is protected with artificial tears or a very thin layer of ointment. It is always advisable to use a glas rod to apply ointments and all thicker drops into the eye. Otherwise, if one applies directly from the tube or the dropper, the amount of thick fluid or ointment is too big, the layer will be too thick and vision will be blurred.

Fig.8. Apply the ointment like this: apply a very small amount, 1 mm or 1/25 of an inch on the rod. Place the rod on the mucous membrane of the lower eyelid. Look upward and close the eye. Draw the rod out. The ointment stays on the eye.