Visual Disturbances Health Article

Definition

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.

Abnormalities in eye movement can result from conditions such as cranial nerve paralysis (paresis), neuromuscular disease (e.g., myasthenia gravis), multiple sclerosis, infection, stroke, overactive thyroid (Grave's disease), or direct trauma to the eye. Diplopia can also be a result of a growing tumor, which presses on the nerves involved in eye movements.

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 or retinal 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.