Eyes and Cataracts
by Robert Tallitsch, PhD | May 25, 2023
Video explaining eyes and cataracts using a real patient case study!
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Written by: Robert Tallitsch, PhD
Eyes and Cataracts
Two of the most common complaints of an aging individual are that it becomes harder to see and harder to hear. Cataracts are an age-related condition that starts to form as early as the 40s or 50s but typically does not affect one’s vision until much later in life. In addition, cataracts are the leading cause of blindness worldwide. In this Brain Builder, we will discuss the anatomy of the eye, the causes and symptoms of cataracts, and the two most common treatments for cataracts.
Anatomy of the Eye
The eye consists of three layers: the fibrous tunic (fibrous coat), vascular tunic (vascular coat), and the innermost sensory tunic (also termed the sensory coat, neural coat, or retina).
The fibrous tunic is the outermost layer of the eye, and it has two components — the sclera and the cornea. The sclera, or “white of the eye”, comprises connective tissue and occupies approximately 5/6 of the surface of the eyeball. The sclera serves to maintain the shape of the eye and serves as the insertion for the muscles of the eye. Anteriorly the sclera is continuous with the clear cornea, which allows light to enter the eye. The cornea bulges anteriorly from the sclera, as it has a smaller diameter than the sclera.
The middle layer is termed the vascular tunic, and it is composed of the choroid, ciliary body, and iris. The choroid is a thin, highly vascular membrane that lines the inner surface of the sclera. Posteriorly the choroid is pierced by the optic nerve, and anteriorly the choroid is attached to the ciliary body.
The ciliary body includes the ciliary ring, ciliary process, and ciliary muscles. The ciliary ring connects the ciliary body to the choroid posteriorly and the ciliary process anteriorly. The ciliary process possesses 60-80 radially arranged folds that connect to the ciliary muscles — two layers of smooth muscle that change the shape of the lens of the eye. These muscles are innervated by parasympathetic nerve fibers of the oculomotor cranial nerve (cranial nerve III).
The iris is a contractile disc surrounding the pupil of the eye. The pupil dilates and constricts in response to stimulation of nerve fibers contained within cranial nerve III. Dilation of the iris is accomplished by contraction of the radially arranged smooth muscles following sympathetic stimulation. The iris constricts due to the contraction of the circularly arranged smooth muscles following parasympathetic stimulation.
The sensory tunic, or retina, is composed of an outer pigmented layer and an inner nervous layer. The nervous layer is composed of multiple sublayers. However, the three most important layers from a functional viewpoint are (1) the layer of rods and cones, (2) a layer of bipolar cells, and (3) the layer of ganglion cells, whose axons contribute to the formation of the optic nerve.
Three additional structures are found within the eye: the aqueous humor, the vitreous body, and the lens. The aqueous humor is secreted into the posterior chamber of the eye — the space between the lens and the iris of the eye — by the blood vessels of the ciliary body and the iris. The aqueous humor then passes through the opening of the pupil into the anterior chamber — the space between the cornea and the iris —where it is reabsorbed by the veins of the ciliary process.
The vitreous body occupies the posterior 4/5 of the eye. It is a transparent gel contained within a thin, delicate membrane termed the hyaloid membrane.
The lens of the eye is found just posterior to the iris of the eye, between the aqueous humor and the vitreous body of the eye. The lens is held in place by the suspensory ligament of the lens, which connects the lens to the ciliary body.
Causes and Symptoms of Cataracts
A cataract is formed when proteins within the lens of the eye break down and the lens becomes cloudy. As a result, it is more difficult for light to pass through the lens and land upon the rods and cones of the retina, causing vision problems.
The incidence of cataracts is increased if other family members have developed cataracts, if an individual has diabetes or was (or still is) a smoker or spent a lot of time in bright sunlight. Cataracts may also develop following an injury to the eye.
If an individual is developing cataracts, they will notice some or all of the following symptoms:
- Difficulty seeing at night or in low light
- Being more sensitive to bright lights, such as the headlights of an oncoming car at night
- Seeing halos around streetlights at night
- Needing more light when reading
- Seeing bright colors as faded or yellowish in color
- Blurry vision
Treatment of Cataracts
If cataract symptoms are mild your optometrist or ophthalmologist may simply prescribe glasses or alter the prescription of your current glasses. However, if you are unable to do things that you want or need to do you should consult with an ophthalmologist and discuss cataract surgery.
In cataract surgery, the physician will remove the affected lens and replace it with an artificial lens. Sometimes, years after surgery, the patient’s vision may again become cloudy. This is due to the lens capsule, the part of the eye that holds the artificial lens in place, becoming cloudy. If this new cloudiness affects the patient’s vision the physician may prescribe laser surgery to open the cloudy lens capsule. This typically restores clear vision to the patient.
Cataracts do have a significant impact on one’s quality of life. Scientists continue to study the causes of cataracts and how cataracts may be detected and treated earlier in life. Treatment options that don’t involve surgery are also being studied.
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