Visual Disorders

Vision is a complicated system. There are a lot of components and processes. Sometimes things go wrong. Here are some of the major examples of visual disorders. Some can be corrected but many can’t. Once vision is gone there is little that can be done.

Nystagmus is a disorder which impairs vision by causing involuntary eye movements. The eyes move rapidly back and forth. Some believe it is an autofocus feature gone wrong. The eyes are searching for focus. Many current models blame the connection between the eyes and the semicircular canals of the inner ear. When looking at a distant object, you can turn your head but maintain your gaze. To accomplish this tasks, signals are exchanged between the eyes and the semicircular canals. When the connection works well, you can maintain a fixed gaze and turn your head without getting dizzy. When something is wrong with the signaling from the semicircular canals, the eyes shift rapidly. In humans, nystagmus is congenital in albinos, can develop in infancy from disease, or comes on in adulthood as a result of drug or alcohol addiction.

Strabismu (strabismic amblyopia) disrupts vision because the eyes don’t align with each other. Sometimes it is called being cross-eyed but misalignment isn’t just horizontal; eyes can be vertically misaligned too. Normal binocular vision uses two eyes looking at the same object from different angles. The brain stitches these images together to create a pseudo-3D image. The problem with strabismus is you are not necessarily looking at the same object.

Amblyopia, sometimes called lazy eye, is a mismatch between the brain and one eye. One eye works well, the other produces fuzzy images. If untreated, the brain gets used to seeing with one eye and doesn’t fix the problem. Treatment frequently involves putting a patch over the stronger eye, forcing the brain to make better connections with the weaker eye. Playing action video games is also used.

Astigmatism is an inherited condition (probably) which produces an irregularly shaped cornea; Symptoms include headaches, eye strain, squinting and vision that is blurred or distorted. in normal vision, light hits the cornea which aims it at the fovea. An irregular cornea causes the light rays to go off target. When you miss the fovea, you get less sharp images. Glasses are ground to counteract the imperfections of the cornea. If the cornea bends light off course, glasses try to redirect it to the fovea.

The vitreous humor is designed to have a continuous flow of fluid. The ciliary body produces and releases this mostly water mixture, and it flows across the back of the cornea and the front of the lens, going out the bottom of the eye through Schlemm’s canal. If there is a blockage at the canal, pressure in the eye builds up, potentially damaging the optic nerve. This build up of pressure is called glaucoma.

The Iris is the colored part of the eye. It is quite stable but can be damaged by injury and infections. In albinos, the pigment is missing, making the iris transparent, and allowing unwanted light to pass through. Light is supposed to only go through the pupil (hole) of the Iris so it can be controlled. Bypassing the pupil gives ghosted images and light sensitivity (photophobia).

The pupil has two sets of muscles: one to open it and another to construct it. As we age, these muscles weaken, impacting night vision. A 20-year old and an 80-year old see about the same in bright daylight. But the 80-year old will have trouble at night or in dim light.

There are two problems with the lens which are associated with age. Sclerosis is the hardening of the lens. It becomes more dense and less flexible. Cataract is when the lens becomes cloudy. Less transparency translates into poorer vision. it is the leading cause of blindness.You can have astigmatism of the lens but it is less common than in the cornea.

Two common problems occur when the cornea and lens are unable to focus light directly on the fovea. Nearsightedness is caused by rays converging too soon. The result is that only close images are in focus. Farsightedness is caused by rays nor converging enough. The focal -pint is behind the retina, so the fovea get a broader beam of light than it should.

As we age the vitreous humor can become less like jelly and more like water. Little dark flakes can break off and get in the way of the light rays. These floaters usually settle to the bottom. When you move or bend over, the floater debris will get stirred up. It is like having a snow globe in your eye. As the vitrious separates, sometimes the retina can become detached. This is posterior vitreous detachmentor (PVD).

The retina is a thin layer of receptors which rests on supporting tissue. Retinal detachment occur when the retina peals off its support tissue. It can start when PVD causes a cut which widens. Injury and cataract surgery can also cause the tearing to occur. Retinal detachment is a medical emergency.

Rods and cones are pretty sturdy. Not much goes wrong with them, except genetics. The X chromosome contains two genes which provide instructions for making the M- and L-cone pigments. OPN1LW specifies the photopsin (OPN1) for long wavelengths (LW). The photopsin medium wavelengths the gene name ends in MW. S-cone photopigment is not controlled by the X chromosome, so S cone color blindness are not sex-linked; it is just as common in women as men.

Retinitis pigmentosa is a genetic disorder which impacts the rods. The disorder can be caused by genes that are autosomal or X-linked. It is a type of Rod-Cone Dystrophies, which impacts the rods first, followed by damage to the cones. As rods die, night vision decreases. This is followed by loss of peripheral vision. With the rods are substantially gone, only cones remain which results in tunnel vision.

Tunnel vision

There are many types of Cone-Rod Dystrophies, disorders that impact the cones first, followed by the rods. They are rare genetic disruptions of normal vision. When the cones die or don’t work, the result is tunnel blindness.

Monochromatism is rod vision only, with no functioning cones. It is very rare (10 out of a million). You see everything in shades of gray, and poor acuity. This is the true definition of color-blind but not what people usually think of. Another result of having only rods is the eyes are very sensitive to light. You have to wear sunglasses, sometimes even indoors, to protect their eyes.

Dichromatism is a group of disorders where only two cones function properly. One of the three cone photopigments is missing. Protanopia is missing the L pigment (long wavelength). It is rare in men (1%) but 10x more common in women. Deuteranopia is missing the M pigment (middle wavelength). Its incidence is about 1% of men, and .01% of women. Tritanopia is the third type of Achromatopsia. It is missing the S pigment (short wavelength). It is rare in both in men and women.

Anomalous Trichromats is a group of disorders where all three cone types are present but one has a photopigment with an abnormal absorption spectrum. Protanomalous has an abnormal L pigment produces “red weakness.” It occurs is 1% of men, but only.021% of women. Deuteranomalous is more common, particularly in men. It is the abnormal of the M pigment, also called “green weakness.” It affects 5% of men, but only .04% of women. Tritanomalous occurs very rarely in both men and women. It is the result of an abnormal S pigment.

Visual dead spots can be annoying or unnoticed. Permanent or long lasting visual dead spots (scotomas) are handled by the brain quite well. it simply ignores it. The optic disc is, of course, a visual dead spot. It is the hole in lower back of the eye where the ganglion neurons exit the eye and become the optic nerve.

Newly formed dead spots are another matter. They can severely disrupt vision, depending on their type, size, shape and location. There are two types of scotomas: positive and negative. The optic disc is a negative scotomas. It is a loss of vision in a particular spot. Loss of vision in the middle of your visual field is called central scotomas. Slightly off to the side, it would be called paracentral scotoma. An additional problem, of course, is you can have more than one dead spot.

Positive scotomas are additions. They are spots of color. They can be straight ahead or off-axis. And they can be jagged or wavy. Scintillating scotoma can fade in and out, and are associated with the auras that accompany some migraines. They seem to be disruption of electrical signals that don’t last long.

Some scotomas are wiring problems outside the eye. Junctional scotoma are the result of damage to the connection between the optic nerve and the optic chiasm. Optic nerve damage can occur, and it rapidly leads to blindness. It is characterized by the loss of vision in one eye over an hour or so. There may also be pain when you move the eye. Cerebral achromatiopsia occurs in the brain. Other brain functions are normal but brain damage causes vision problems, including the loss of color vision.

Aside from receptor and neural damage, blood vessels can also cause vision loss. First, vision problems can arise as a byproduct of diabetes. Diabetic retinopathy is more than numbness in the feet or pain in the legs. It causes capillaries to easily burst. When this happens in the choroid blood system which feeds the photoreceptors, it cause leakage of blood and fluid into the eye.

The other problem with the choroid is macular degeneration. The macula is the depression at the back of the eye where the fovea sits. It is near center, off to the side a bit. The choroid is behind it. In dry macular degeneration, yellow deposits of cellular debris (drusen) build up. Depending on the size and number of deposits, the macula can get pushed away from the choroid. Unable to get nourishment, the photoreceptors in the affected area die.

Drusen

Wet (exudative) macular degeneration occurs when blood vessels from the choroid invade the macula, detaching it. This is an even more serious condition because of the difficulty stopping leakage while maintaining blood flow to the photoreceptors.

Treatments include laser coagulation and medication, with varying amounts of success. This is the major cause of blindness in older adults. It is particularly troubling because it results in losing image detail. It often means not being able to read or recognize faces.