Nystagmus is an involuntary ocular oscillation which includes a slow phase eye movement as part of each cycle. Visual impairment can result, either from the oscillation itself, or from an associated visual pathology. Therefore, individuals with nystagmus may benefit from magnifying devices in the same way as other visually impaired individuals. However, nystagmus also has some particular features which can be addressed by unique solutions, and these will be discussed here. Methods of treatment of the nystagmus (e.g. using systemic drugs, or surgery on the extraocular muscles, to reduce the oscillation) are beyond the scope of this book.
Nystagmus can be broadly divided into:
- 1.
Early onset nystagmus (<6 months of age) including infantile nystagmus syndrome (INS) and fusional maldevelopment nystagmus syndrome (FMNS). (Spasmus nutans will not be considered as it disappears within the first 2 years of life and so is unlikely to be seen in a low vision clinic.)
- 2.
Acquired nystagmus, which can occur at any age, but is very rare in young individuals. This is caused by, for example, cerebellar, brainstem or vestibular disorders and needs urgent referral to a neurologist if not previously diagnosed.
The detailed investigation and diagnosis of childhood nystagmus takes place in a few specialist tertiary centres in the UK ( ); and have proposed a specific local clinical pathway managed by orthoptists. Low vision rehabilitation does, however, form an important part of the care pathway.
A comprehensive set of information on nystagmus for parents and carers has been developed by the University of Sheffield and is available for download ( ). In common with parents of other visually impaired children, the most frequent concerns they raise (and for which they would benefit from advice and signposting) are around education and future employment, and ability to drive. In addition, a video ‘The way we see it’, presented by individuals with nystagmus, has been produced by the Nystagmus Network ( ).
Infantile Nystagmus Syndrome
INS was formerly termed congenital nystagmus, but this was not really an appropriate term, because onset is shortly (typically a few weeks) after birth. Infantile nystagmus (IN) is a binocular involuntary ocular oscillation, almost invariably in the horizontal plane (even on vertical gaze). It is conjugate for frequency in the two eyes (typically 3–4 Hz), but may not be exactly matched in amplitude. Amplitude varies considerably between individuals (<1° to >10°), and in the same individual over time. Some individuals have a variant called periodic alternating nystagmus which undergoes a cyclical change (with a period of several minutes) in amplitude and direction,
In very general terms the rhythmic movement of the eyes has been classified as pendular or jerky. In the pendular waveform, the eye makes a slow movement which causes the retinal image to move away from the fovea and then makes a slow movement back. The waveform could be described as approximately sinusoidal with the position of the fovea corresponding to one peak of the oscillation ( Fig. 14.1A ).
In a jerk nystagmus, the eyes move slowly away from the target, and then make a rapid saccadic return (fast phase), before the next slow drift away (slow phase) begins. The nystagmus is described as ‘left-beating’ if the saccadic movements are to the left, and ‘right-beating’ if they are directed the opposite way. Jerk and pendular waveforms can be distinguished by direct observation of the patient’s eyes.
Although the retinal image is constantly in motion, the patient is rarely aware of this and does not report oscillopsia. At most they may notice a horizontal ‘smearing’ or elongation of bright lights against a dark background. It is surprising that when using a magnifying device (such as a telescope), which presumably magnifies the retinal image motion, the majority of patients are still not aware of the motion. In fact, the success rate of nystagmats using telescopes is almost twice that of non-nystagmats ( ): this may be because nystagmats already have perceptual strategies to cope with a moving retinal image, whereas non-nystagmats are disturbed by the apparent movement of the object as they move the telescope. Exceptionally, an individual with IN will find that viewing through the telescope produces oscillopsia and the amount of magnification which can be used may be limited by this: nystagmats should be questioned closely when first using such a device.
Another type of nystagmus which is present from early childhood is latent or manifest latent nystagmus (LN or MLN). This has also been renamed as ‘fusional maldevelopment nystagmus syndrome (FMNS)’. In this condition the nystagmus is much more noticeable when viewing is monocular, and it always takes the form of a jerk nystagmus with the fast phase towards the fixing eye. Classically, in ‘LN’ the nystagmus is not present at all under binocular viewing conditions, but if the patient has a squint, the suppression of one eye even when both are apparently viewing can cause the oscillation to become manifest (MLN). As suggested by the name FMNS, however, the condition is caused by an ocular condition which disrupted binocular vision during early development (e.g. infantile esotropia), so an individual with FMNS who has sufficient ‘binocular’ vision to experience no oscillation under binocular conditions is rare. It can be diagnosed by the change in beat direction as an occluder is moved from one eye to the other: with the left eye covered and the right fixating, it will be right-beating, but will change instantaneously to left-beating as the cover is moved over the right eye. IN can also alter its amplitude or frequency on covering, but rarely alters its direction so consistently. The individual with FMNS is more likely to experience oscillopsia than those with IN, and this may be particularly relevant when using low-vision aids, as viewing with devices is more likely to be monocular, leading to a larger oscillation.
IN can be idiopathic or ‘isolated’ (indicating no known cause or origin, and no association with other conditions) or can be associated with ocular pathology which has been present from birth, such as albinism, aniridia, retinopathy of prematurity or congenital cataract. Less commonly, FMNS is also associated with such conditions. These ocular pathologies have a variety of mechanisms, some inherited, and some idiopathic nystagmus can also be inherited, with X-linked recessive, autosomal recessive and autosomal dominant patterns all being reported. In up to 50% of X-linked idiopathic cases, a defect of the FRMD7 gene has been identified ( ). The associated pathology (if any) does not determine the characteristics of the eye movement, but is likely to affect the visual performance, and some inherited eye disease will have a poor prognosis (e.g. Leber’s congenital amaurosis).
In a pure pendular or jerk nystagmus, the image spends very little time on the fovea before the eye movement takes it away, and this is not compatible with good visual acuity. In most infantile nystagmats, the waveform has been spontaneously adapted to one which affords better vision. ‘Jerk with extended foveation’ or ‘pseudocycloid’ have a long foveation period before the image drifts off the fovea and would support better acuity than a pure jerk oscillation ( Fig. 14.1B ). It is not just the length of the foveation period which is important, however, but its consistent placement: if the retinal image is stationary but does not coincide with the fovea (or only does so on a small percentage of the foveation periods), then again the acuity will be more limited ( Fig. 14.1C ). Thus, the waveform (and the foveation time it allows), foveation accuracy and foveation repeatability are likely to be more important in determining acuity than the intensity (amplitude × frequency) of the oscillation. It is believed that in an individual, the waveform develops from pendular on initial onset, to one with better foveation, during the early years of life ( )
The eye movement characteristics vary considerably under the influence of a number of factors, and one of the most significant is ‘effort-to-see’ or ‘fixation attempt’ ( ). This means that when the patient is relaxed and visual tests are not being conducted, the oscillation is minimal, but as soon as the patient’s attention is directed to the letter chart, the oscillation becomes dramatically increased. Stress, anxiety, fatigue and illness have all been reported to increase the intensity of the oscillation.
The patient may adopt a head turn (to left or right, or chin up or down) when attempting a critical visual task. It is believed that this occurs in order to take advantage of an eye position where the ocular oscillation has a less significant influence on vision: if this ‘null zone’ is in eccentric gaze, and occurs when the eyes are turned to the left, for example, then an equal and opposite head turn to the right will be required to place the eyes in the null zone when viewing a target straight ahead. It may be expected that the head posture would be adopted to take advantage of a minimum intensity of oscillation, but it appears that there are often more subtle factors at work, such as the relative amount of time that the retinal image is maintained on the fovea at each gaze angle ( ). Surgical rotation of the eyes can be used to place them in the required position without an accompanying abnormal head posture. Fusional maldevelopment nystagmus (FMN) also has a gaze direction corresponding to minimum nystagmus intensity, and this is usually determined by Alexander’s Law. This states that the nystagmus is minimal on looking in the direction of the slow phase—for FMN, this involves adduction of the fixing eye. It may be difficult for the patient to use the optimal head posture whilst using a spectacle-mounted telescope or telemicroscope: the housing of the lenses may obscure viewing when the eyes are at the required eccentric gaze angle.
Visual Performance in Infantile Nystagmus Syndrome
Strategies adopted by the patient (often unconsciously) and methods of treatment proposed are intended to reduce the oscillation, and hence the retinal image motion, and thereby improve acuity. It is important to realise that this is only one of the three factors that can impact the visual performance in IN. These are:
- 1.
The current oscillation
- 2.
The deprivation effect of retinal image motion during early development
- 3.
The associated visual pathology (if any)
With regard to (1), it has been suggested that visual performance does not increase at all when retinal image motion is prevented ( ) but this conclusion is controversial ( ). Even if decreasing the oscillation does not help, then any increase in oscillation should certainly be avoided, and every attempt made to exploit its minimum habitual state. Therefore, the vision should be tested when the patient is as relaxed as possible: as noted previously, ‘effort to see’ can cause an initial measurement of acuity to be much worse than the habitual level, and it should be measured on several occasions to ensure that an accurate baseline has been reached. If vision is only possible during limited foveation periods, this has led to the concept that nystagmats are ‘slow to see’ and need to be given much longer to establish fixation and extract visual information. They should be given an extended observation time and allowed to use preferred head positions. When the patient with IN is performing visual tasks, head shaking is often observed. This used to be considered to be compensatory, but is now recognized as a component of the syndrome. If the nystagmus increases in monocular viewing, then complete covering of the eye should be avoided. To test visual performance monocularly, a plus lens sufficient to blur the nontested eye (but only by one to three lines) is recommended.
Even in idiopathic IN, with no associated ocular disease to impair vision, point (2) above will limit how much improvement is possible. The visual system has never experienced stable retinal images and identified an ‘orientation amblyopia’ which meant that the detection of vertical contours was poorer than for horizontal contours (which would be less ‘smeared’ by the oscillation). also identified poorer acuity in those children whose nystagmus waveform took longer, in the first few years of life, to transition from pendular to one with longer foveation times. That is, children who spent more of their critical period of development with uncontrolled retinal image motion had a poorer visual outcome.
With regard to (3), the advent of optical coherence tomography (OCT) has made the diagnosis of idiopathic IN less common, as subtle ocular abnormalities (such as foveal hypoplasia) are often identified that would previously have gone undetected. However, many of the conditions associated with IN are stable throughout life (e.g. aniridia, albinism).
Optical Strategies Specific to Infantile Nystagmus Syndrome
Although surgery can be used to remove the need for a head posture in INS, if the gaze angle is not extreme, then bilateral prisms can be used instead to produce the required gaze deviation ( Fig. 14.2A ). The beneficial effect of prisms (or surgery) is often greater than would have been predicted by simply measuring the nystagmat’s acuity whilst adopting their abnormal head position: it appears that the more relaxed head posture with treatment decreases the ‘effort-to-see’ ( ). The amount of prism required means that Fresnel prisms may have to be used to avoid excessive weight and thickness: unfortunately, these create chromatic aberrations, a loss of image contrast and linear reflections from the prism bases.
The nystagmus in INS is often also attenuated with convergence in near vision, and if the effect is clinically significant, the patient’s eyes can almost become stationary when converging to ∼20 to 30 cm. This same reduction in the oscillation whilst the patient fixates a distant acuity target can be stimulated with the use of base-out prisms to produce a converged eye position even for distance viewing ( Fig. 14.2B ). A standard amount of 7Δ base out for each eye is suggested as a starting point, along with modifying the distance refractive correction by adding −0.75 or −1.00 DS to allow for the slight amount of accommodation which may have been stimulated by the convergence ( ). If this amount of prism is sufficient, the use of a small eyesize, and high index lenses, may allow the use of lenses with surfaced prisms (rather than Fresnel prisms). If a larger prism is required, the prism can be used binocularly or monocularly because symmetrical or asymmetrical convergence is equally effective ( ) and this may mean that only one eye has to suffer the poor image quality inherent in the Fresnel prisms.
Even without optical or surgical intervention, the use of a null zone in INS should be encouraged as required: reading may be more comfortable with the book held laterally rather than straight ahead, or a child with a null zone to the left may see much better when viewing the board from a seat on the right of the classroom, compared to one on the left. If the nystagmat has strabismus, then using prism to correct the deviation may be worth trying. If this is able to support any degree of binocular vision, this can sometimes reduce the oscillation.
Individuals with IN invariably have a high refractive error with a marked (usually >3.00 DC) corneal astigmatism with-the-rule ( ). Correction of these significant refractive errors often makes little difference to the recorded visual acuity (for the reasons discussed previously), but full refractive correction should be strongly recommended ( ). A number of writers have suggested that the patient may benefit from contact lenses rather than spectacles ( ). suggested the use of rigid scleral lenses because their bulk would physically damp the oscillation. Rigid corneal lenses may be the best choice because of their suitability for correcting the corneal astigmatism and the possibility of increased proprioceptive feedback from the eyelids leading to modification of the oscillation as the nystagmat becomes more aware of it ( ). Other benefits of contact lenses are reduced aberrations as the lens moves with the eye, which remains looking through the optical centre, and a slightly increased amount of convergence required if the patient is a myope as they will no longer experience base-in prismatic effect from the lenses during near viewing. In fact, a randomized trial of rigid versus soft contact lenses in comparison to spectacles found there was no difference in the nystagmus characteristics between the different conditions. However, visual acuity at distance and near was significantly worse in soft contact lenses compared with rigid lenses or spectacles ( ): it was suggested that the eye movement caused the soft lens to rotate on the eye. Glare control using contact lenses for those with albinism is discussed in Chapter 10 .
Acquired Nystagmus
A nystagmus acquired later in life (usually due to some neurological defect) can be distinguished from IN by several characteristics: it will invariably have a pure jerk waveform (without the adaptations found in IN), may be vertical rather than horizontal (and change direction on vertical gaze), will be associated with severe oscillopsia, and have associated systemic neurological symptoms. The eye movement produces an equal and opposite movement of the image across the retina, of which the patient is constantly aware. This oscillopsia often creates feelings of nausea and disorientation and impairs vision because the image of contours perpendicular to the movement is ‘smeared’. As in IN, a gaze direction can usually be found in acquired nystagmus which corresponds to minimum nystagmus intensity, and this is governed by Alexander’s Law: the nystagmus intensity is minimal on looking in the direction of the slow phase. The patient can be encouraged to adopt an appropriate head position to minimise oscillopsia, or prism can be used to shift this null, as discussed earlier for IN. As the nystagmus increases away from the null zone, the sufferer may find it particularly distressing to look towards the fast phase. Whereas it can be helpful to encourage viewing into the null zone, viewing in the opposite direction should be avoided. If symptoms are extreme, it may be beneficial to suggest partial occlusion of the spectacle lenses so that viewing in this direction has to be avoided ( ).
An Image Stabilisation System
A more complete solution would be to use an optical system capable of ‘stabilising’ the retinal image: as the eye moves, the retinal image should move by an equivalent amount so that it falls on the same point on the retina ( ). If the object is initially imaged on the fovea, and the eye rotates, the target should appear to move to the same side so that the observer feels that they are still looking straight at the target. To specify the performance of such a system it is necessary to consider the angular rotation of the eye ( θ ) and the resultant amount of retinal image movement ( θ’ ). In ‘normal’ vision, if the eye rotates about its centre of rotation, and the object stays still, the retinal image would be expected to move across the retina by an equivalent amount and θ’ / θ = 1. Partial stabilisation is represented by 1 > θ’ / θ > 0, and perfect stabilisation by θ’ / θ = 0. Retinal Image Stabilisation (RIS) is often expressed as a factor or percentage, and by definition this is (1 − θ’ / θ ), or (1 − θ’ / θ )100%. Perfect stabilisation also means that if the eye was stationary, and an object moved in the visual field, the retinal image of this object should not move across the retina: although the object subtends a different angle at the centre of rotation of the eye, it should subtend the same angle at the nodal point of the eye so that it does not appear to have changed its position relative to the fovea. In Fig. 14.3 the target moves in the visual field from A to B, and looking at the angle subtended by these two positions at the centre of rotation of the eye, this is an angular movement of θ .
Considering the change in the angle subtended at the nodal point of the eye, this is θ’ : the eye would need to rotate by an angle θ’ in order to put the retinal image back onto the fovea. In ‘normal’ viewing the two angles are the same—if the target moves x degrees, then the eye must rotate x degrees in order to fixate it again. In optical stabilisation, however, θ’ should be zero: no matter where the object is moved to in the visual field, the image should not move and the angle it subtends at the nodal point should stay the same. This occurs when the image is formed at the centre of rotation of the eye because the image position does not change as the eye moves. As this point is approximately 13.5 mm behind the cornea, it requires a very powerful positive spectacle lens in order to focus the image at this point. This obviously then makes the image on the retina very blurred, and a method of diverging the rays onto the retina without changing their direction is needed. This is accomplished by using a high-minus contact lens. The negative power produces the correct focus, and the fact that it moves with the eye means that objects are viewed through its optical centre and so the angle of those rays is not altered. The high plus spectacle lens and the high-minus contact lens are in fact a Galilean telescope, and for this to be afocal, the focal points of the two lenses must be coincident. Thus, the focal point of the contact lens must also be at the centre of rotation of the eye, approximately 13.5 mm behind the cornea ( ) ( Fig. 14.4 ).
FCL=1fCL′=1−fCL=−10.0135=−74.07DS
If the vertex distance v = 0.015 m
FSPEC=1fSPEC′=1r+v=10.0135+0.015=10.0285=+35.09DS
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