- Nasolacrimal system is the method that the eye uses to produce, maintain, and eliminate tears from the eye. Tears protect, nourish, and moisturize the eye. Without proper tear function, the cornea and conjunctiva would dry out and develop disorders, as sometimes happens. In the normal eye, the tear film is made up of three layers, which are produced by the lacrimal gland and accessory lacrimal glands and cells. The lacrimal glands are located in the orbit and inner eyelid. The accessory glands and cells are located in the conjunctiva. The top layer of tears is formed by the oily secretion of the meibomian glands. The second layer is composed of watery tears from the lacrimal glands. The third layer, which lies next to the cornea, is of mucous-like consistency and is produced by the accessory glands. The layers are maintained by constant blinking and are all necessary for proper health of the eye. Tears are constantly being produced and drained from the eye. They drain through lacrimal puncta, holes in each inner corner of the upper and lower lids. The tears are gathered in the lacrimal sac, an organ at the junction of the nose and lower lid and then pass under the tissue to drainage ducts in the nasal cavity. The nasolacrimal system is subject to functional disorders, obstructions, and infections that may result in wet eye, dry eye, or dacryocystitis. Inadequate drainage of tears leads to wet eye, which may be congenital or an acquired condition. Obstructions often clear spontaneously within the first twelve months of life. After this time, obstructions are cleared surgically by simple probe and irrigation or silicone intubation procedures or the more serious surgical procedure, dacryocystorhinostomy, which connects the lacrimal sac to the nasal cavity. Acquired obstructions usually occur in middle life and more commonly among women. Tearing becomes severe and annoying. The simple procedures of probe and irrigation or silicone intubation are often ineffective, and a dacryocystorhinostomy is indicated. The dry eye condition occurs as a result of poor tear production (called keratoconjunctivitis sicca), poor tear quality, or inadequate blinking, which leaves the eye open to the drying elements or which does not properly wet the entire surface of the eye. Dry eyes may lead to corneal damage, permanent corneal scarring, and opacification. When the cornea has opacified, vision is lost. Treatment involves treating the underlying causes.
Nearsightedness (myopia) means that objects near are seen more clearly than objects far away. This happens if the eyeball is elongated from front to back rather than round, causing an object to be more sharply focused in the front of the retina instead of on the retina itself. Even with a round eye, nearsightedness can occur if the cornea or lens is too steeply curved. High myopia is a condition in which the eye is extremely long or large, producing extreme nearsightedness. In very large eyes, the structures within them are stretched, causing the retina to become thin and weak. Tears or detachments that impair vision can develop as a result. Myopia affects almost three out of every ten people and is generally first noticed in childhood. The problem quickly progresses during these early years, often requiring new corrective lenses more than once a year. Vision tends to stabilize during the young adult years so that by the twenties or thirties, an individual may not need to change lenses at all or only every few years. Concave lenses, which are thinner in the middle than at the edges, can correct myopia and are available as lenses in eyeglasses or contacts. Refractive surgery is another form of correction and gaining in popularity. The procedure reduces the curvature of the cornea so that the light entering the eye focuses directly onto the retina as it should. Two types of refractive surgery are photorefractive keratectomy and LASIK. According to some experts, nearsightedness may be the result of a lack of both calcium and Vitamin D. Calcium is also important for controlling excessive watering and blinking. Children lacking adequate amounts of protein, often grow up with nearsightedness, so good supplies of protein are essential during the growth years to help prevent this problem. Studies also show that carbonic acid found in soft drinks may also be a contributing factor in the development of many cases of myopia.
Night Blindness is the inability to see clearly at night or in low light. The condition results from damage or a defect in the rods of the retina, which receives reflected light from an object. Light-sensitive cells called rods and cones translate the light into electrical impulses that the retina sends to the brain through the optic nerve. The brain changes the impulses into an image. The cones, packed into the macular center of the retina, discern detail and color and require bright light to function effectively. The rods function in dim light and are responsible for detecting movement and shape. Any dysfunction of the rods may cause damage to night vision. Any peripheral vision loss that restricts the field to five degrees central vision results in night blindness because the usable foveal area contains no rods. Damage to the rods may occur as a result of nutritional deficiency or disease. Vitamin A deficiency contributes significantly to the cause of night blindness since the presence of vitamin A is necessary for proper rod functioning. Vision impairment due to vitamin A deficiency is common in developing countries and (becoming more common in the Western world due to massive processed food consumption). A resulting disease, called Xerophthalmia, is the second major cause of blindness in the world. Xerophthalmia causes changes in the cornea, conjunctiva, and anterior segments of the eye. In acute cases, the cornea can perforate, the iris may adhere to the cornea, and the eyeball may be destroyed. A common and early symptom of this condition is night blindness. Xerophthalmia can be detected with an electroretinogram or a thorough corneal examination and treated with megadoses of oral vitamin A. (It is wise to remember that megadoses of the retinal form of Vitamin A can cause toxicity, but the beta carotene form can be given in an unlimited amount without any toxic side effects.) Night blindness is also a first symptom of retinitis pigmentosa, a degenerative disease of the rods and cones. RP first affects the rods, decreasing vision in the peripheral fields and causing night blindness. It is a hereditary condition for which there is no cure or treatment.
Normal Vision Development
- Birth to 3 months:
- Visual acuity ranges from about 20/400 to 20/1200 and slowly improves to about 20/150 to 20/600. The poor sight is because the retina is still evolving and the fovea (where the eye can see 20/20) has not yet matured. Faces get more attention than anything else at this age, but true depth perception is absent. By the end of the three-month period, the infant will begin to exhibit simple, visually directed reaching towards objects with increasingly better results. Moving objects will also begin to attract attention.
- 4 to 6 months
- Visual acuity continues to improve to about 20/50 to 20/200. If both eyes are seeing about the same, stereopsis (depth perception) kicks in to nearly adult levels. Reaching for a seen object becomes proficient and tracking a moving object is fairly smooth.
- 7 to 12 months
- There is little change in acuity, but other visual skills are continually getting better. The infant can now converge the eyes as close as three inches and focus quite accurately.
- 13 to 24 months
- Visual acuity makes steady gains to 20/30 to 20/80 range and the eyes begin to shift readily to objects of curiosity. Eye-hand coordination becomes almost flawless.
Nystagmus is the term for an involuntary movement of the eyes which may move vertically, horizontally, in circles, or some combination of the three. The condition causes focusing problems and blurred vision. Nystagmus may be congenital or acquired as a result of another disorder. Jerking nystagmus is the most common form of the disorder. The eyes move faster in one direction than in the other. It may be caused by lesions or changes in the brain stem, cerebellum, or vascular system; overstimulation of the systems within the inner ear; hypertension; stroke; multiple sclerosis; Ménière’s disease; labyrinthitis; drug or alcohol toxicity; or brain inflammations, including meningitits and encephalitis. Pendular nystagmus is the lesser common form of the disorder, where the eyes move horizontally and equally quickly in both directions. Pendular nystagmus may be caused congenitally as in a congenital cataract of disorder of the optic disc. It may be aquired after birth as a result of astigmatism, albinism, optic atrophy, or corneal opacification or cataracts. The underlying cause is treated to alleviate the symptoms. If the cause is astigmatism, prescription eyeglasses may be helpful. When the cause is a disease or disorder that can be treated, control of the disease usually results in control of the nystagmus. In unmanageable conditions, the patient may learn to hold the head or body in accommodating positions or learn to focus with one eye only.