[Editor's note: This article is from 2007. Some newer treatments and risk factors are not listed here. See more on specific disorders on BrainWeb]
sections include: common neurological causes of vision loss
The ability to see is one of our greatest gifts. Most of us never have a problem with our sight, other than having to wear glasses. We can see minute objects, experience color, and take in vast panoramas without a thought that something could go wrong. Unfortunately, a number of medical conditions can affect your vision. The eye itself may cause visual disturbances, as in cataracts (clouding of the lens) and glaucoma (rising pressure within the eyeball). This section will not discuss those problems, nor injuries to the eye, but rather the neurological conditions that affect sight—those involving the optic nerve system or the parts of the brain that process visual information.
Your retina acts like the light sensor in a digital camera: its photodetector neurons, the rods and cones and the other retinal neurons to which they connect, convert the light that flows into your eyeball into electrical signals. These neurons and their synapses then feed that information into the eye’s optic nerve, which sends it on to your brain. Some of the optic nerve fibers cross in what is called the optic chiasm, at the base of the brain. The result of the crossing is that the brain can process all visual information about the right half of the world in the left occipital cortex and all information about the left half of the world in the right occipital cortex:
Many conditions occurring along this pathway can affect your ability to see. Diagnosis of a particular problem usually starts with identifying the type of vision loss, how quickly it came on, how long it lasted, and whether any other symptoms appeared at the same time. Often people experiencing a sudden loss of vision assume that they have had a stroke, and that is indeed a possibility. In fact, one out of seven people in the United States will have some type of stroke during his or her lifetime, and most of those strokes will have some effect on the visual system. But other conditions can cause the same symptoms. Recognizing the type of vision loss may help you receive early and effective therapy if you need it, and prevent greater or permanent losses in your ability to see.
Whatever turns out to be the cause of a vision problem, it is a serious problem that demands quick medical attention. Here are some basic rules for responding to this symptom:
- If a person has also suffered loss of consciousness, weakness, immobility, or some form of impaired thinking, the problem is a medical emergency. That holds true even if the symptoms are unilateral, meaning that they affect one eye or one side of the body, and a person feels fine otherwise.
- A person suffering a sudden visual problem and no other symptoms should nonetheless contact an ophthalmologist immediately. If the eye doctor finds nothing physically wrong with the eye, he or she may refer the person to a neurologist for further diagnostic tests.
Common Neurological Causes of Vision Loss
Four types of conditions or accidents are the most common neurological causes of vision loss. They are impaired blood flow to the retina, damage to the optic nerve, pressure on the optic chiasm, and impaired blood flow in the brain. People’s risk for each varies; each has different symptoms, and each is diagnosed and treated differently.
Impaired Blood Flow to the Retina
A number of conditions within the retina may affect vision in only one eye at a time. The most serious is a transient ischemic attack (TIA) that briefly clogs one of the retinal blood vessels. In this case, you would notice an abrupt change in vision in one eye, often a sudden loss of part of the visual field or an effect like a window shade coming down in a few seconds. Fortunately, the unilateral vision loss in TIAs lasts just a few minutes.
Vision loss because of a TIA may be mimicked by migraine headache. However, in contrast to TIAs, migraines usually produce colorful, scintillating patterns in part of the visual field.
The usual cause of TIAs is small particles of cholesterol and other material from arteries in the neck that travel through the blood vessels to the eye area, get stuck, and block blood cells from supplying oxygen to the retina or optic nerve. This causes a temporary or, in rare cases, permanent area of vision loss in that eye. Other symptoms of TIAs, such as numbness or weakness on the other side of the body from the affected eye, would support the diagnosis: something is interfering with the flow of blood to one part of the head. This is not an ischemic stroke of the brain, but it means a person is at higher risk for that life-threatening problem.
Your risk for TIAs of all kinds goes up if you have hypertension or high cholesterol levels, if you smoke, or if you are over 40. For people under 40, retinal TIAs may be due to certain clotting disorders in the blood. Multiple TIAs of the eye could, if untreated, eventually lead to single-eye, or monocular, blindness.
Diagnosis and Treatment
Anyone who may have suffered a retinal TIA should be evaluated by a neurologist specializing in visual problems, such as a neuro-ophthalmologist. It is very important for doctors to make an early diagnosis in order to prevent future TIAs or strokes.
In addition to cholesterol particles, a possible cause of TIAs is emboli, or blood clots, produced by disease in a carotid artery. Physicians can evaluate this possibility with an ultrasound scan of those arteries and perhaps an arteriogram.
One treatment for TIAs caused by blood clots involves taking anticoagulation medicine to prevent clots from forming so easily. The medications usually prescribed for this condition include aspirin, drugs known to affect platelets (Plavix and Ticlid), and blood thinners such as warfarin (Coumadin).
If you have had a TIA, it is important to treat the accompanying risk factors: hypertension and high cholesterol. People can improve these conditions by dieting, avoiding salt, and, if necessary, taking medications.
Damage to the Optic Nerve
Overall, the most common cause of a vision loss in a single eye in a person under age 50 is optic neuritis, and the most common cause of optic neuritis is multiple sclerosis (MS). The usual first symptom of MS is loss of vision in one eye, accompanied by pain, particularly when the eyes move. Sometimes these eye movements create little flashes of light called photopsias. An individual’s vision usually decreases over a matter of hours, and the loss may last for weeks. Most people have an initial recovery but can suffer later attacks in which they lose their vision totally.
This type of vision loss is brought about by an acute demyelination, or loss of the fatty covering, of the optic nerves. It is therefore called demyelinating optic neuritis and is one of the hallmarks of MS. The process may affect other portions of the brain as well, producing double vision, numbness, weakness, imbalance, or facial pain. Some people also experience loss of bladder control. When a person suffers unilateral visual loss and, either at that time or within a few weeks, unilateral numbness or tingling, MS is the most likely cause.
The usual age when people develop MS is in their 20s or 30s, and the incidence is three times higher in women than men. Early in the twentieth century, epidemiologic studies revealed that people in the northern parts of the United States and of northeastern European ancestry had a higher risk of this condition. Recent studies also indicate that there is a clear genetic tendency toward susceptibility to MS. The disorder affects about 57 people out of every 100,000 people in the United States.
Diagnosis and Treatment
The best test to diagnose multiple sclerosis is a magnetic resonance imaging (MRI) scan. It is extremely important to make a diagnosis early because there are now treatments that may prevent new attacks.
For an acute attack of MS, particularly those involving optic neuritis, doctors prescribe intravenous steroids in the form of methylprednisolone. This hastens recovery and protects the individual from attacks over the next year. Eye discomfort is treated with pain medication so the person can obtain enough rest.
When optic neuritis is the first sign of MS, 90 percent of women and 50 percent of men develop other symptoms of the condition during the next 20 years. These subsequent attacks affect other parts of the nervous system, causing such problems as weakness and numbness on one side, or weakness in both legs when the spinal cord is affected. But at least a third of the people with MS are fortunate in having a very benign course to the disease.
There is no surgical treatment for MS, but new medications called interferons help in reducing, by about a third, the occurrence of new episodes or attacks, both in the optic nerve and elsewhere in the brain. Interferons also reduce areas of demyelination, or MS plaques, over time. The most practical steps that people can take once they contract MS are to lead a healthy life, get regular exercise, and avoid exposure to excessive heat.
Pressure on the Optic Chiasm
The optic chiasm is where your optic nerves cross. The most common cause of visual difficulty in this area is a tumor in the pituitary region pressing on those nerves. This condition produces tunnel vision, making you unable to see anything at the edges of your visual field.
Because the tumor continues to grow, a person’s tunnel vision can become steadily worse. If the tumor grows large enough, it may also affect the temporal lobes of the cerebral cortex on either side, producing seizures, or the hypothalamus, causing disorders in appetite and control of the sodium and potassium in the body. Other symptoms of pituitary dysfunction may appear as well, such as fatigue or, in the rare cases when the tumor secretes growth hormone, increasing hand and foot size.
Pituitary tumors account for approximately 1 percent of all tumors that occur inside the skull, so it is an uncommon condition. There is no known specific cause; neither age, sex, nor ethnicity seems to play a role. Moreover, there seems to be no specific genetic predilection toward developing a pituitary tumor.
Diagnosis and Treatment
Over time, we have learned the growth patterns of pituitary tumors. Physicians can diagnose them early by testing a person’s levels of hormones that may be affected, such as growth hormone and prolactin. Further tests then determine what sort of tumor is causing the problem, and how large it has grown.
The majority of these tumors secrete no substances and are therefore known as nonsecreting pituitary tumors. They manifest themselves mainly by pressure on the chiasm, causing the tunnel vision. It is extremely important to find these tumors early and treat them, either medically or surgically, to prevent permanent loss of vision.
Some pituitary tumors secrete prolactin, a hormone that causes loss of a woman’s menstrual cycle and lactation, or discharge from the breasts. In this case, the drug bromocriptine may be useful in stopping the symptoms and reducing the tumor size. Doctors must watch the tumor carefully with repeated MRI examinations to make sure it is shrinking.
If the tumors are large, surgeons should remove them, usually by performing an operation through the nose. Afterward, a person will need hormone replacement therapy because the pituitary gland will no longer be available to control the production of hormones in the body.
The defect in the outside portions of the visual field caused by a pituitary tumor is severe and irreversible. If you have tunnel vision, you must always be aware that you do not see to the sides as you once did. You must move your eyes and head to scan your environment on either side to make up for your loss of vision.
Impaired Blood Flow in the Brain
After information from either of your optic nerves reaches the lateral geniculate nucleus inside your brain, it is relayed to the many appropriate neurons in the visual cortex. Any problem in this system can produce seeing problems, usually affecting one half of a person’s field of view. This symptom is different from being able to see only out of one eye. No matter which eye you use, you would see only half of the world, or half of an object. The problem is not in your eyes, but in the part of the brain processing the eyes’ information.
The most common cause of these same-side visual defects is a stroke. (Strokes can also affect the retina and optic nerve fibers, in which cases they affect all vision through one eye rather than vision to one side.) The usual first sign of a stroke affecting the visual pathway is a sudden loss of vision on one side. You may also feel numbness of the face, arm, or leg, or weakness on one side of your body. If you have difficulty seeing to the left and have left-sided facial, arm, and leg numbness or weakness, the problem is probably on the right side of the brain—specifically, in the cerebral cortex.
The risk factors for strokes include hypertension, smoking, high cholesterol levels, diabetes, and a variety of clotting disorders. Increasing age also brings a higher incidence of stroke. When the damage is due to lack of blood supplying part of the brain, usually because of a clot blocking a blood vessel, it is called an ischemic stroke. When a blood vessel bursts in the brain, the problem is called a hemorrhagic stroke. These types of stroke are treated differently, so swift and accurate diagnosis is vital.
A person may also experience a brief, temporary loss of vision in one side of space. This is often caused by a TIA, or period of impaired blood flow, affecting the part of the brain that processes visual information. Though not a stroke, a TIA is a warning that you are at much higher risk for a stroke. You should immediately consult a neurologist who is expert in diagnosing visual symptoms and prescribing preventive measures.
Diagnosis and Treatment
Along with a physical examination, the most important test for strokes is an MRI scan. It is important that doctors make their diagnosis early because treating many strokes immediately can either eliminate or reduce a permanent loss of vision.
If the cause of stroke is a clot of blood coming from the heart or elsewhere in the body (an embolism), doctors may try to prevent further attacks with anticoagulant medications: Heparin right away, and Coumadin for long-term prevention. People with irregular heart rates due to atrial fibrillation have a 70 percent reduction of stroke risk after they start taking Coumadin. The most modern treatment includes dissolving clots with such medications as TPA (tissue plasminogen activator) if physicians can diagnose an impending stroke within three hours.
The visual symptoms of a stroke do not become more severe as time passes; in fact, if the initial damage was not too severe, a person’s vision tends to improve gradually. Loss of vision due to a TIA that is identified quickly and treated effectively is even more likely to improve to normal. As with other visual defects, if you have suffered vision loss from stroke, you must learn to compensate by moving your eyes more frequently and always remaining aware that you cannot see in one area of space.
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