Understanding Vision Loss in Apparently Healthy Eyes
It is obvious that a person cannot see without healthy eyes. But what is less obvious is that the eyes are only the portals into the visual system of the brain. The eyes focus light, sense changes in illumination, detect colors, and then transmit this information via electric signals to the brain where the signals can be translated into useful information. Each eye sends its data to the brain through a nerve called the optic nerve. This nerve contains over 1 million axons, the biologic wires our nervous system employs, and these axons each originate from cells in the eye called the retinal ganglion cells. These cells make connections to an area deep within the brain that acts as a relay station, and this relay station sends the information further down the line to the very back of the brain. Remarkably, visual signals must therefore travel the whole length of the brain before being perceived. And to be fully analyzed, they actually travel back to the front of the brain. As you can guess, a very large part of the brain, approximately 70-90%, is somehow involved in vision, moving the eyes or controlling structures around the eyes. Therefore, it does not take much imagination to see how almost any type of brain disease can reduce vision, even though the eyes might appear normal.
The Optic Nerve
When an adult patient believes that their vision is worsening, or that they just do not see "right", they should seek the help of an ophthalmologist. While growing children routinely undergo refractive changes correctable by spectacles (and not even notice these changes until screened in school or by their eye doctor!), adults can rarely be so easily treated when they experience changes in their distance vision or color vision.
On the contrary, it is much more likely that the adult is suffering either ocular or brain disease. This does not mean that the disease is life-threatening. For example, a cataract (a cloudy lens) will decrease vision but often never hurt the eye, and cataract extraction remains one of the safest surgeries performed in this country. But what happens when the eye does not have a cataract or some other obvious problem? The next step is to examine the optic nerve, the very front of which is visible at the back of the eye (when viewed with the correct lighting and lenses)
The optic nerve head looks like a pink donut with a white hole in the middle. The optic nerve can look sick in three ways. First, the "donut" could look more like an onion ring, a condition called cupping and very often associated with glaucoma. Second, the optic nerve could look more like a filled donut with no hole, a condition called optic nerve edema, and often associated with dangerous blood pressures or increased fluid pressure within the brain. Third, the optic nerve could look white like a sugar-powdered donut, a condition called optic atrophy. It is important to realize that each of these changes can be terribly subtle, especially in the early stages of an illness (when doctors can offer the most help) and that is why the ophthalmologist spends a great deal of time examining the optic nerve.
In many cases of visual loss with ostensibly normal appearing eyes, the optic nerve is really not normal looking. Once an ophthalmologist suspects that the optic nerve is sick, they will then begin examinations that will help detail the cause of the disease. Hypertension, diabetes, glaucoma, lupus, syphilis, lyme disease, arthritis, AIDS, brain tumors, remote cancers, multiple sclerosis, high cholesterol, heart disease, carotid artery disease, leukemia, lymphoma, congenital metabolic diseases, alcohol abuse, thyroid disease, neurofibromatosis, and others all might first be detected because of their effects on the optic nerve! The ophthalmologist is required to ask the patient very detailed questions and examine the entire patient when performing a consultation. Patients can expect this examination to take longer than the routine eye check-up, but that is because the stakes are so high.
In most cases of optic nerve disease, the eye is an innocent bystander to a more worrisome systemic or cranial process. Unfortunately, the eye is like the canary in the coal mine, and may not be able to return to its former duties. But remember to weigh this loss against the saving of a patient's life!
What to do when the Eye Really Looks Healthy
Vision can be poor when an eye really looks perfectly healthy! When this occurs, it is the responsibility of the ophthalmologist to examine the patient for findings that reveal pathology in cranial portions of the visual pathways. The Ophthalmologist will first speak with the patient to find out what they can and cannot see, when their problems started, and whether they might have any other diseases association with decreased vision. The ophthalmologist will often perform measures of the patient's peripheral vision, called visual field testing. Blind spots are often revealed by visual field testing and can pinpoint the location of damage in the brain when the tests are interpreted correctly. Finally, the ophthalmologist may take advantage of the vast armamentarium of diagnostic imaging studies to help determine the exact nature of the disease process, once the doctor knows where the damage has occurred. The ophthalmologist will often work closely with neurologists and neurosurgeons, because therapy for diseases that decrease vision without damaging the eyes is often beyond their scope. However, the ophthalmologist will remain an active part of the patient's health care team, because they will usually be called upon to help follow the patient's progress.
