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Congenital Nystagmus

Nystagmus is defined as involuntary eye movements, commonly called "dancing eyes", "jerking eyes" or "back-and-forth" eyes. Congenital means present at birth or very shortly thereafter. Thus, congenital nystagmus means involuntary eye movements present at birth or within the first month or two of life. However, detection of the nystagmus varies significantly and may not be observed until 5 or 6 months of age. Congenital nystagmus has also been referred to as "congenital motor nystagmus", "idiopathic infantile nystagmus" and "infantile nystagmus" among others.

 

Strictly speaking, congenital nystagmus should be limited to patients who do not have an obvious cause or reason for the nystagmus such as a retinal disease, optic nerve disease or brain tumor. As a consequence, before the diagnosis of congenital nystagmus is made, the patient may need to undergo a series of diagnostic tests to rule-out such diseases or causes. Typically, patients with retinal disease/degeneration will develop nystagmus between 2 and 4 months of age. However, it is often difficult to know exactly when the nystagmus started given that young infants sleep most of the time and when the eyes are open they are open only temporarily.

 

Congenital nystagmus typically involves horizontal eye movements, although vertical and rotary eye movements have also been reported. When the nystagmus is horizontal, it may be pendular (i.e., as if the patient is watching a pendulum swing back-and-forth) or may be more jerk-like (i.e., the eyes appear to move slowly in one direction and than snap back in the opposite direction and start over). In general, as the patient gets older the size or amplitude of the nystagmus decreases and the frequency of the nystagmus increases, particularly when the patient tries to fixate or look directly at an object. The nystagmus will worsen and increase in size and frequency when the patient is tired, sick or fatigued.

 

Many patients with congenital nystagmus will find that their nystagmus may decrease or even stop in certain directions of gaze (e.g., looking right). Also, the nystagmus may decrease in size when the patient looks at close objects (i.e., with convergence). The eye doctor will refer to a "null point" when the nystagmus stops with a certain gaze direction. A "semi null point" occurs when the nystagmus decreases but does not totally stop in the certain direction of gaze.

 

Patients with congenital nystagmus may also experience head nodding or head shaking. Some believe that the head nodding and/or head shaking are compensatory head movements; to compensate for the involuntary eye movements and to improve vision. Although such compensatory head movements may occur in some patients, when investigators simultaneously record eye and head movements in such patients, there is not a high degree of compensation evident. In other words, in many patients that have nystagmus and head nodding or shaking the two may be independent or simply a symptom of the same disorder. Usually, over time, the head nodding and head shaking disappear.

 

Clinical Features and Symptoms

1. There are certain clinical features of patients with congenital nystagmus. These clinical features include:

2. Both eyes have horizontal nystagmus and it is symmetrical ( same in both eyes).

3. The nystagmus typically decreases with convergence.

4. Esotropia (crossed-eyes) is present (about 50% of patients).

5. At some time/age the patient demonstrates head shaking or nodding.

6. No movement of the visual world (oscillopsia) is experienced.

7. The nystagmus is not present during sleep and decreases when the eyes are closed.

8. There is a preferred direction of gaze (null point or semi null point)

9. Demonstrates an opposite OKN effect (2/3 of patients).

10. Life long condition.

11. There are no other significant eye (media, retina, optic nerve) problems.

 

Causes of Congenital Nystagmus

The cause of congenital nystagmus is not known. In terms of genetics, there appears to be different hereditary types of congenital nystagmus including, X-linked, dominant and recessive. In the X-linked type, the unaffected females (mothers) are carriers of the defective gene and they have a 50% probability of passing it along to a male child who will be born with the congenital nystagmus. The affected mothers also have a 50% probability of passing the defective gene to a female child. The female child will be a carrier and not affected. A gene associated with X-linked congenital nystagmus has been reported on the short arm of the X chromosome (Xp11.4 – p11.3).

 

In the dominant form of congenital nystagmus, 50% of all family members have a possibility of receiving the defective gene and having congenital nystagmus. Every generation is affected and, thus, either the mother or father of an affected patient must also have the nystagmus. A patient with the dominant form of congenital nystagmus will have a 50 % probability of having a child with congenital nystagmus. If an offspring does not have the congenital nystagmus then their offspring will not be affected. A dominant congenital nystagmus gene has been linked to chromosome 6p12.

 

In the recessive form of the disease, neither the mother nor father is typically affected and there is typically no family history of such a condition. When a child is born with the recessive form of congenital nystagmus, there is a 25% chance that other children by the same mother and father will have a chance of having the nystagmus. The patient with congenital nystagmus has a very low possibility of having children with the nystagmus.

 

Visual acuity in Congenital Nystagmus

Patients with congenital nystagmus typically have some loss of visual acuity, which cannot be corrected with glasses. In general, visual acuity ranges from 20/30 (best) to 20/200 (legal blindness). On average, patients with congenital nystagmus have a visual acuity of 20/60 or better, good enough to drive a car. An important determinant of visual acuity is whether the patient has a null point or semi-null point; that is, if the patient can voluntarily stop or slow down the frequency and size of the nystagmus by looking a certain way. Visual acuity is better in patients with a null point than in patients that do not have a null point.

 

Another important fact for patients that do have a null point is that when the patient gets older s/he may be able to have eye muscle surgery to move the eyes such that the null point is in straight ahead gaze (primary gaze). For example, suppose the patient has a null point when s/he looks to the right. In such patients, in order to look straight ahead s/he would have to turn the head to the left so that s/he is looking straight ahead without the nystagmus. An ophthalmologist could move the eye muscles attached to each eye so that the null point could be in the primary position, when the patient looks straight ahead. Now if the patient looks to the right s/he would have the nystagmus. In a sense, following the eye muscle surgery the brain "thinks" that the eyes are turned to the right but are really straight ahead (and without nystagmus). Such eye muscle surgery could result in some improved vision in straight a-head gaze. In addition, a patient with a head turn could develop skeletal muscle problems from always turning the head to the right, for example. As a consequence, eye muscle surgery may be advised in some patients.

 

Patients with congenital nystagmus can usually read normal print if they’re allowed to hold the text and position their head to maximize vision. Under no circumstances should a teacher or parent force the child to hold the text a certain way or have the head, say, straight ahead. Leave the child determine the best position for reading and school work.

 

Unusual Findings in Congenital Nystagmus

There are several unusual or interesting findings in patients with congenital nystagmus. For example, about 2/3 of patients with congenital nystagmus will demonstrate an opposite OKN effect. Optokinetic Nystagmus (OKN) is elicited by having the patient look at a series of moving vertical black and white stripes, typically on a drum. When the stripes move, say, left to right, the OKN will be observed as a series of slow and fast eye movements with the slow eye movements in the direction of the moving stripes (rightward) and the fast eye movements in the opposite direction (left). A patient with congenital nystagmus may demonstrate just the opposite – when s/he is presented with a series of stripes moving left to right s/he’ll elicit fast movements to the right and slow eye movements to the left.

 

Some patients with congenital nystagmus will demonstrate a condition called "paradoxical pupillary constriction" to darkness. In normal patients, darkness will cause the pupil of the eye to expand and get bigger and lightness will cause the pupil of the eye to constrict or get smaller. In some patients with a diagnosis of congenital nystagmus, darkness causes the pupil to get temporarily smaller and after some time the pupil will slowly get bigger. Strictly speaking, we do not believe that the diagnosis of congenital nystagmus is appropriate for patients who demonstrate the paradoxical pupillary constriction. Such an effect strongly suggests the presence of a retinal disease or degeneration, or possible optic nerve disease as a cause of the nystagmus. As a consequence, the nystagmus is probably secondary to or caused by the retinal or optic nerve disease.

 

Congenital Nystagmus vs Spasmus nutans

Spasmus nutans is defined as a triad of symptoms including asymmetric nystagmus, abnormal head posture and the presence of head shaking or nodding. As such, Spasmus nutans can sometimes be confused with congenital nystagmus. However, close observation can differentiate the two. For example, note the in Spasmus nutans the nystagmus is not symmetrical in the two eyes. In deed, sometimes there is nystagmus only in one eye. In congenital nystagmus the two eyes move in a similar fashion. In Spasmus nutans the head nodding or head shaking occurs BEFORE the nystagmus, usually by 1 or 2 months. Head nodding and head shaking always occurs after the nystagmus occurs in congenital nystagmus. While the nystagmus eventually disappears in patients with Spasmus nutans, usually by 4 years of age, the nystagmus is always present in patients with congenital nystagmus (although it may get so small in amplitude to make it difficult to see). As a cautionary note, remember that patients are individuals and not all symptoms are seen in all patients. As a consequence, sometimes a correct diagnosis is made only after years of observation and when symptoms appear or/and disappear.

 

Misc.

1. Patients with congenital nystagmus may be able to drive a car in most states.

2. Pure congenital nystagmus may be confused with nystagmus in patients with incomplete congenital stationary night blindness.

3. About 10% of patients diagnosed with congenital nystagmus may have congenital periodic alternating nystagmus

4. Patients diagnosed with congenital nystagmus may have ocular albinism (and vise versa)

5. About 10% of blind and visually impaired children have a diagnosis of congenital nystagmus.

6. About 1 in every 1,500 live births have congenital nystagmus

7. The frequency of the eye movements in congenital nystagmus average about 2.8 Hz under binocular viewing and 4.6 Hz under monocular viewing.

8. About a third of patients may have a color vision deficiency (deuteranomaly).

9. 30% of patients with congenital nystagmus may have or develop amblyopia.

 

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