IN-vision

1. Overview
2. Diagnosis
3. Prognosis/impact
4. Incidence
5. Sensory defect infantile nystagmus syndrome
6. Idiopathic infantile nystagmus syndrome
7. Null zone and abnormal head positions
8. Therapies (dedicated page)
9. Effects on visual capability

Overview

Infantile nystagmus syndrome (INS) is a permanent, non-progressive condition causing involuntary oscillations of the eyes. It first appears within the first six months of life and is often associated with a congenital pathology of the visual system. In many cases however, no defect of the visual system can be found, in which case it is classified as ‘idiopathic’ INS. The oscillations in INS are usually conjugate (the same in both eyes) and horizontal in all directions of gaze. There is usually a particular direction of gaze in which the oscillations are at a minimum; this angle of gaze is known as the null zone. Over time, an abnormal head position may develop in order to use the better vision offered by this position.

Other factors are also known to affect the INS oscillations: relaxation, convergence and closing the eyes all tend to reduce the nystagmus intensity. Conversely, stress and tiredness will usually result in an increase in intensity. The inheritance of INS is governed by the type of inheritance shown by the sensory defect that has caused the nystagmus. In the case of idiopathic INS, inheritance can be autosomal dominant, but other forms of inheritance are  known to occur.
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Diagnosis

INS is usually diagnosed within the first six months of life. INS rarely presents at birth. The average age of onset is estimated to be 1.9 months (Gottlob 1997). However, in cases of very low amplitude nystagmus, the condition may go unnoticed for many years.

Since INS often presents in conjunction with a pathology of the visual pathway, a full ophthalmological workup is required in any child presenting with nystagmoid eye movements for the first time. If no comorbid ocular pathology can be found, the INS is described as ‘idiopathic’.

The exact type of INS can be diagnosed through the use of eye tracking equipment. Such technology is rarely found in hospitals. Top of page

Prognosis / impact

INS is a permanent condition, which at present has no known cure. Unlike acquired nystagmus, INS does not usually cause oscillopsia (the illusory perception that the world is moving). The condition is also non-progressive; i.e. vision does not worsen, and the eye movements rarely increase, unless exacerbated by extraneous factors such as stress or ill health. A recent study identified the main aspects of daily life affected by nystagmus (McLean, Windridge and Gottlob 2012). The study identified issues including restricted mobility, difficulties forming relationships, and not wishing to stand out as being different. Top of page

Incidence

Based on a large scale study of the population of Leicestershire, the incidence of INS is believed to be 14 in 10,000 (Sarvananthan et al. 2009). Top of page

Sensory defect infantile nystagmus syndrome

Around 70% of cases of INS are associated with some sort of ocular defect. There are a large range of seemingly unrelated conditions that are associated with INS. In all cases however, the ocular defect in some way impairs vision, usually bilaterally.

The most commonly associated conditions are albinism, cataract, aniridia, coloboma, cone dysfunction, achromatopsia and congenital stationary night-blindness.

The link between visual impairment at birth and the development of INS has led to the theory that nystagmus develops as a response to poor image quality (Harris and Berry 2006). Top of page

Idiopathic infantile nystagmus syndrome

When non-acquired nystagmus presents in the first six months of life with no other detectable abnormality of the eyes, it is referred to as ‘idiopathic infantile nystagmus syndrome’ (IINS). Idiopathic literally means ‘of unknown cause’, and at present it is not known in these cases whether nystagmus is the primary defect, or if an undetectable visual system abnormality exists. In some cases, there is a genetic link between individuals with IINS. Several mutations have been found in the ‘FERM domain containing 7’ (FRMD7) gene in X-linked IINS (Tarpey et al. 2006; Self and Lotery 2007; Xiao et al. 2011; Li et al. 2012; Radhakrishna et al. 2012). Top of page

Null zone and abnormal head positions

The intensity of nystagmus movement varies with direction of gaze. The position(s) of gaze in which nystagmus intensity is at a minimum is known as the ‘null zone’.

In a study of 224 people with INS, 73% were found to have a null zone within 10° of the primary position. Around 44% of nystagmats also exhibit a dampening of eye movements with convergence; therefore near vision may be better than distance in some situations (Abadi and Bjerre 2002).

Since the null zone position of gaze affords the individual better vision, often an abnormal head position (AHP) develops to put the eyes in the optimum position whilst looking straight ahead. In Abadi and Bjerre’s 2002 study, 69% of subjects with an eccentric null zone exhibited an AHP. When the null zone was ≥20° from the primary position, all nystagmats exhibited an AHP.

AHPs are one of the reasons that some individuals feel their nystagmus makes them stand out as being different (McLean, Windridge and Gottlob 2012). Often, the individual will have to choose between using a socially ‘normal’ head position, and adopting their AHP to maximise their ability to interpret social cues. Top of page

Effects on visual capability

Nystagmus can have many adverse effects on visual capability. The exact nature and extent of these effects is determined by the nature of the nystagmus, its waveform, intensity, fixation effort, viewing distance, stress and, if an eccentric null zone is present, head posture.

The presence of oscillating eye movements results in the motion of the visual image across the retina. Contrast sensitivity of the visual system is highly sensitive to even small image movements of a few degrees per second, especially at high spatial frequencies (fine detail). The oscillations in the slow-phase of nystagmus oscillations can reach speeds of 100 degrees per second which will obviously cause a large reduction in contrast sensitivity. Contrast sensitivity is what determines our ability to discriminate objects of different brightnesses apart and in the majority of situations contrast between objects is low. A reduced contrast sensitivity can therefore have a large impact on an individual’s ability to navigate their way around their surroundings. Contrast sensitivity in nystagmus is determined by the speed of the slow-phases which are known as ‘foveation periods’; the longer these foveation periods are, the better the vision will be. The reduction in sensitivity also causes nystagmats to be highly sensitive to visual ‘crowding’. This causes increased difficulty in discriminating two objects that are close together.

Another factor which relates to the level of visual capability in nystagmus is the ability to align the fovea with the object of fixation during the foveation periods. This alignment of the fovea with the target is generally more accurate in adults with nystagmus than children, possibly due to adaptation of the visual system.

Nystagmats have a high incidence of with-the-rule astigmatism in which the eye has a higher power in the vertical meridian than the horizontal meridian. This may be due to a mechanical deformation of the eye or a response by the visual system to the blur caused by the oscillations. This will cause the eye to develop a higher power in one direction than the other in an effort to clarify the vision. Meridional amblyopia is also present, caused by the oscillations which occur along the meridian at 90 degrees to the direction of the oscillations which deprive the visual system of stimuli along that meridian.

Rarely, individuals with INS suffer from oscillopsia (the illusion that the world is moving), although this is much more common in acquired nystagmus. Top of page