Ocular Muscles and Optic Nerve
Six different muscles surround each eye and work as a team so that both eyes can focus on the same object. Strabismus is when these muscles do not work together. As a result, one eye looks at one object, while the other eye turns in a different direction and is focused on another object. When this occurs, two different images are sent to the brain — one from each eye. This confuses the brain.
In adults this causes diplopia (double vision) and can be caused by uncontrolled diabetes, thyroid disorders, stroke, brain injury, or eye injury. Adults with mild strabismus that comes and goes may do well with glasses that use a prism to fuse the images together. More severe forms of adult strabismus will need surgery to straighten the eyes. If strabismus has occurred because of vision loss, the vision loss will need to be corrected before strabismus surgery can be successful.
In most children with strabismus, the cause is unknown. In more than half of these cases, the problem is present at or shortly after birth. The brain may learn to ignore the image from the weaker eye. If the strabismus is not treated, the eye that the brain ignores will never see well. This loss of vision is called amblyopia. Another name for amblyopia is “lazy eye.” Sometimes amblyopia is present first, and it causes strabismus. Most of the time, the problem has to do with muscle control, and not with muscle strength. The first step in treating strabismus in children is to prescribe glasses, if needed. Amblyopia or lazy eye must be treated and this is done by placing a patch over the better eye. This forces the weaker eye to work harder. Children may not like wearing a patch or eyeglasses. A patch forces the child to see through the weaker eye at first. However, it is very important to use the patch or eyeglasses as directed. If the eyes still do not move correctly, eye muscle surgery may be needed. Different muscles in the eye will be made stronger or weaker.
The optic nerve is the structure that connects the eye to the brain. The optic nerve consists of nerve tracts (axons) that originate in the retina of each eye. It carries visual information from the retina to the nerve cells in the brain stem, where the information is relayed to the area of the brain that recognizes vision called the occipital cortex.
Optic neuritis – it is inflammation of the optic nerve, it can occur in children or adults, and it may involve one or both optic nerves. The precise cause of optic neuritis is unknown, but it is thought to be a type of autoimmune disorder. The immune system is generally used to fight infection by creating a reaction that combats bacteria, viruses, fungi, and other foreign proteins. In autoimmune diseases, this reaction is mistakenly directed against a normal part of the body, creating inflammation and potential damage. In the case of optic neuritis, the optic nerve becomes swollen and its function is impaired. Inflammation of the optic nerve causes loss of vision because of the swelling and destruction of the protective myelin sheath that covers and insulates the optic nerve. Direct damage to the nerve tracts (axons) may also play a role in nerve destruction.
There are many causes including:
- Multiple sclerosis
- Cranial arteritis
The major symptom of optic neuritis is vision loss, frequently maximal within one or two days and varying from a small area of blurring to complete blindness. Affected individuals may also notice distorted vision, reduced color vision, loss of contrasts, and washed out or less vivid vision than normal. Loss of vision usually develops over the course of a day to two weeks and may be worsened by heat or exercise. Vision loss is usually temporary, but it may be permanent in some cases. Most people who develop optic neuritis experience eye pain that is worsened by eye movement. Pain associated with optic neuritis usually peaks within one week and then goes away within several days. If a definite cause such as infection or other underlying disease is determined, appropriate therapy for that cause can be instituted. When indicated, some patients are treated with steroid or immunosuppressive medications.
Ischemic Optic Neuropathy – it occurs when blood flow from the brain is affected and the oxygen and nutrients fail to reach the optic nerve in sufficient quantity, resulting in nerve fibers being in serious risk. It can happen slowly and without warning, starting with one eye, and then spreading to the other eye. Vision loss is almost sure to happen. In fact, this has become the reason why this optic nerve disorder is also called a stroke of the optic nerve. It is similar to a stroke in the brain because the source is lack of blood supply, but unlike a brain stroke, the optic stroke does not paralyze its victims. Some symptoms are tunnel vision, vision loss, blurred vision, visual field loss, and inability to see contrasts in shades, depth, and lighting. It is possible to recover from this optic nerve disorder if treatment is provided early, and the episodes are very few. About 40% of people who have this condition will improve.