Visual disturbances are abnormalities of sight. Visual disturbances associated with neurological disorders often include double vision (diplopia), moving or blurred vision due to nystagmus (involuntary rapid movements of the eyes), reduced visual acuity, reduced visual field, and partial or total loss of vision as in papilledema, a swelling of the optic disc, or in blindness. Visual disturbances are often symptoms of other disorders, in particular neurological disorders, but can also occur due to muscular disorders, vascular diseases, cancer, or trauma. Additionally, diseases such as diabetes and hyperthyroidism can contribute to the visual abnormalities. Some visual disturbances arise from congenital conditions that are often hereditary.
Diplopia
Diplopia, or double vision, causes a person to see two objects instead of one. There are two main reasons for diplopia: one is a physical change in the lens, conjuctiva, or retinal surface; the second reason involves an inability of the brain to overlay the images seen with both eyes, which happens in a person with normal vision. The first type usually involves only one eye and is not corrected by covering of the eye. Scars or other physical defects in the eye cause the split of a single image, thus resulting in double vision. In contrast, the second type usually involves both eyes (binocular) and is corrected when one eye is covered. Binocular diplopia arises when the eye movement in one direction is prevented, and is often a congenital (present at birth) condition. Binocular diplopia is usually caused by misalignment of the eyes, which can be nerve or muscle related.
The nerves involved in diplopia include three cranial nerves: the oculomotor nerve (third cranial nerve), the abducens nerve (sixth cranial nerve), and the trochlear nerve (fourth cranial nerve). These three nerves direct the movements of six extraocular muscles. Four muscles are innervated by the third cranial nerve, and the other two are innervated exclusively by either the fourth or the sixth cranial nerve. This arrangement allows the physician to determine the cause of visual disturbances observed in a patient. Misalignment of the eyes can be in any direction: inward, outward, upward, downward, or a combination. Damage to the third cranial nerve can cause outward and downward turning of the affected eye and the inability to pass midline in either of the two directions. Fourth cranial nerve damage will result in vertical diplopia, which is compensated by head tilting. Head turning is used to compensate for sixth cranial nerve damage that prevents outward movement of the eye.
Nystagmus
A different type of visual disturbance, nystagmus, is caused by abnormal eye movements and often results in blurred vision. Normal control of the eye movements depends on the neuronal connections between the eyes, brain stem, and the cerebellum. Changes in the central nervous system or peripheral labyrinthine apparatus can cause the uncontrolled, repetitive eye movements known as nystagmus. There are many types and subtypes of nystagmus depending on the underlying cause and movement involved. The most common form involves a jerking motion from side to side (horizontal nystagmus). The rapid eye movements can also appear in a vertical direction, usually indicating a problem with the central nervous system. Rotary movements are also sometimes observed in nystagmus.
Although nystagmus by itself does not cause loss of vision, it is often associated with poor vision. Nystagmus can develop in early childhood or in adulthood. Childhood nystagmus can be associated with eye defects (cataract orretinal disorders) or result from unknown causes (congenital idiopathic nystagmus). Most cases of congenital nystagmus are not caused by a disease process and are familial.
If nystagmus develops later in life, it can be a sign of a serious underlying problem such as stroke, multiple sclerosis, or complication from head trauma. The direction of the eye movement can help the physician to diagnose the underlying neurological problem. For example, in an unconscious person, vertical nystagmus can indicate brain stem damage. This illustrates that eye movements not only cause visual disturbances, but are also an important diagnostic tool to determine if the brain is still alive.
The presence of the occulocephalic reflex (doll's eye movements) in people with coma shows that the brain stem is intact. The physician turns the patient's head from side to side or left to right to elicit the reflex. When the reflex is present, the eyes appear to move freely in the opposite direction from the direction the head was turned, thus moving in relation to the head. When the eyes remain fixated, this suggests lack of cerebral activity. Another important diagnostic test is the cold caloric test. The cold caloric test traces the direction of nystagmus to assess the oculovestibular reflex. An unconscious person's ear is injected with cold water, causing slow horizontal movement of the eyes towards the stimulation, which is followed by a fast return of the eyes to the midline.
Blindness
Blindness is the partial or complete loss of vision. The leading causes of blindness are glaucoma, cataracts, and diabetic retinopathy. Blindness can also result from eye diseases, optic nerve disorders, or brain diseases involving visual pathways or the occipital lobe of the brain. The patterns of visual field reduction depend on the area that is being affected by disease. Damage to visual pathways as a result of macular degeneration, retinal detachment, or optic nerve atrophy can affect one or both eyes. In contrast, damage to the optic nerve chiasm or the pathway beyond it affects both eyes. There are many eye diseases that can cause visual abnormalities or/and blindness, including retinal detachment, cataracts, retinal disorders (often inherited), and macular degeneration.
Macular degeneration is the leading cause of blindness for those over age 55 in the United States. The macula is the central portion of the retina that records images and sends them from the eye to the brain via the optic nerve. If the macula deteriorates, the eye loses the ability to see in fine detail. The cause of macular degeneration is not fully understood, but risks for the disorder increase with age. Other abnormalities in the central retina can lead to blurry vision or can affect color perception. Color blindness can also originate from the lack of one or more type of cones, a type of light receptor on the eye. Total color blindness (monochromatic vision) is very rare; most commonly, varying levels of single color deficits are found among people with color blindness. Central vision can also be destroyed by small hemorrhages in the retina as a result of the aging process or diabetic retinopathy.
The neuronal diseases affecting the optic nerve and causing blindness can result from developmental abnormalities (hereditary or sporadic), abnormalities in the blood vessels causing an insufficient blood supply to the eyes or optic nerve, glaucoma, and demyelinating and inflammatory diseases such as multiple sclerosis, tumors, toxic agents, and trauma.
Optic nerve damage
Papilledema, the swelling of the optic nerve, can result from increased intracranial pressure or optic nerve deterioration (optic neuropathy). Inflammation, lack of adequate blood supply to the optic nerve, and certain diseases such as multiple sclerosis can cause the optic nerve to deteriorate. A brain tumor, bleeding or blood clots in the brain, brain swelling due to encephalitis or trauma, or a blockage in cerebrospinal fluid circulation can cause an increase in pressure inside the skull (intracranial pressure). The condition is often life threatening, and correct diagnosis of papilledema is important.
Papilledema arising from increased intracranial pressure is often accompanied by other symptoms, including diplopia, nausea, headache, and reduction of the visual field. When diagnosing papilledema, the physician looks for swelling of the optic disc (the area where the optic nerve enters the eye). The early signs include slight changes in appearance of the edge of neural tissue. Later, the disc rises from the retinal surface and can appear pale or can show signs of hemorrhages in severe cases. Persistent, chronic papilledema can cause atrophy of the optic nerve head and result in blindness.
The optic nerve can also be damaged by increased intraocular pressure (IOP) as in glaucoma. The pressure develops in aqueous area of the eye and is transmitted to the back of the eye, causing an initial reduction in peripheral vision and leading eventually to blindness. Glaucoma is often a complication arising from diabetes.
Additionally, optic neuritis, or inflammation of the optic nerve, can cause permanent loss of vision. Demyelinating diseases such as multiple sclerosis, systemic infections, diabetes, and hereditary factors can cause optic neuritis. Optic neuritis can also be a secondary complication of diseases such as meningitis, sinusitis, or tuberculosis, or reactions to toxins or trauma.
Other important causes of blindness are tumors affecting the optic chiasm (the area in the brain where the optic nerves cross) such as gliomas, cerebral tumors, and pituitary adenomas. In these cases, the transfer of visual stimuli through the optic nerve and visual pathways is directly affected and results in blindness.
BOOKS
Acheson, James, and Paul Riordan-Eva. Fundamentals of Clinical Ophthalmology: Neuro-Ophthalmology. London: BMJ Books, 1999.
Glaser, J. D. (ed). Neuro-ophthalmology, 3rd ed. Philadelphia: Lippincott Williams and Wilkins, 1999.
OTHER
"Double Vision (Diplopia)." InteliHealth Inc. February 28, 2004 (June 3, 2004). <http://www.intelihealth.com/IH/ihtIH/WSIHW000/9339/23796.html>.
