Nystagmus is an eye movement disorder characterised by abnormal, involuntary rhythmic oscillations of one or both eyes, initiated by a slow phase ( Self et al., 2020 ). Jerk nystagmus is nystagmus with a slow phase and a fast phase; pendular nystagmus is nystagmus with only slow phases ( Eggers et al., 2019 ). Nystagmus affects 0.24% of the UK population ( Self et al., 2020 ). Physiological nystagmus can occur with certain types of visual (optokinetic nystagmus) or vestibular stimulation (e.g., by rotating the subject or by introducing warm or cold water into the ear). End point nystagmus can also occur during motility testing, particularly if the child is tired ( Grisham, 1990 ) and if the target is held at the end point position for 15–30 seconds. This chapter will concentrate on nonphysiological nystagmus.
There are several factors, listed here, which cause the investigation of nystagmus to be complicated. The aim of this chapter is to provide an overview of the subject for clinicians who may only encounter nystagmus occasionally, and who need to know when to refer and what optometric management, if any, is appropriate.
Problems in the Evaluation of Nystagmus
Nystagmus is not a condition, but a sign. Many different ocular anomalies can cause nystagmus, or nystagmus can be idiopathic, with no apparent cause.
Attempts to classify the type of nystagmoid eye movement by simply watching the patient’s eye movements often do not agree with the results of objective eye movement analysis ( Dell’Osso & Daroff, 1975 ).
The pattern of nystagmoid eye movements cannot be used with certainty to predict the aetiology of the nystagmus ( Dell’Osso & Daroff, 1975 ). Some general rules exist; for example, infantile nystagmus (INS) is usually horizontal. However, there are exceptions, when INS is not purely horizontal, and there are many cases of horizontal nystagmus which do not have an infantile onset.
The same patient may exhibit different types of nystagmoid eye movements on different occasions ( Abadi & Dickinson, 1986 ). In INS, the eye movements worsen under stressful conditions ( Cham, Anderson, & Abel, 2008b ; Jones et al., 2013 ). In the absence of stress, the eye movements in nystagmus may improve a little with demanding visual tasks ( Wiggins, Woodhouse, Margrain, Harris, & Erichsen, 2007 ). INS is not exacerbated by visual demand per se, rather the need to do something visually demanding of importance to the individual ( Tkalcevic & Abel, 2005 ).
Visual loss in nystagmus is only loosely correlated with the type of nystagmoid eye movements ( Bedell & Loshin, 1991 ). There may be an underlying pathology causing poor vision resulting in nystagmus; a pathology causing, independently, the nystagmus and the poor vision; or a pathology (hypothesised in infantile idiopathic nystagmus) causing the nystagmus which then causes poor vision. Amblyopia may develop secondary to early onset nystagmus ( Abadi & King-Smith, 1979 ; Spierer, 1991 ; Currie, Bedell, & Song, 1993 ).
There are two fundamentally different approaches to classifying nystagmus, based on the aetiology and the eye movement characteristics. This chapter concentrates on the two types of nystagmus that eyecare practitioners will encounter most frequently (infantile nystagmus and latent nystagmus). Eggers et al. (2019) provide a more comprehensive classification of nystagmus from a neurological/vestibular perspective.
Classification Based on Aetiology
Infantile nystagmus (infantile nystagmus syndrome; INS) , occurring in infancy ( Committee for the Classification of Eye Movement Abnormalities and Strabismus, 2001 ), usually the first 6 months of life ( Self et al., 2020 ). The condition was previously called congenital nystagmus or early onset nystagmus .
Infantile nystagmus with a sensory defect , associated with an ocular anomaly causing poor vision, e.g., congenital cataract, optic atrophy, aniridia. A relatively common form of sensory defect nystagmus is albinism , both oculocutaneous (lack of skin and eye pigmentation) and ocular (only lacking eye pigmentation). Many albinos have INS and latent nystagmus (see later), which should be labelled as ‘INS with a latent component’ ( Harris, 2013 ).
Infantile nystagmus without a sensory defect , also known as infantile idiopathic nystagmus . Previous terminology (sensory nystagmus and motor nystagmus) is deprecated because the waveform of the two types is indistinguishable and both result from the same deficit in the smooth pursuit motor subsystem ( Dell’Osso, Hertle, & Daroff, 2007 ).
Nystagmus blockage syndrome is probably a rare subdivision of INS in which a reduction of the nystagmus during convergence appears to have resulted in an esotropia. The fixating eye is adducted during binocular or monocular vision giving the appearance of a lateral rectus palsy and resulting in an anomalous head posture ( Grisham, 1990 ).
Latent nystagmus (fusion maldevelopment nystagmus syndrome) is a common sequel to infantile esotropia syndrome and is characteristically only present, or greatly increased, on monocular occlusion. However, it is very occasionally found in monocular individuals. The fast phase of the eye movement always beats towards the uncovered eye. Therefore, the direction of the nystagmus reverses when the cover is moved from one eye to the other and this is pathognomonic of latent nystagmus ( Repka, 1999 ). Dell’Osso (1994) stated that both types of latent nystagmus (see below) are always accompanied by strabismus and a cyclotorsional element is usually present, together with dissociated vertical deviation ( Guyton, 2000 ).
Latent latent nystagmus , or true latent nystagmus, which only becomes apparent on monocular occlusion.
Manifest latent nystagmus , which is present without occlusion.
Acquired (neurological) nystagmus , occurring usually after the first few months of life, owing to some pathological lesion or trauma affecting the motor pathways (e.g., multiple sclerosis, closed head trauma). All uninvestigated cases, except voluntary nystagmus, should be referred.
Gaze paretic (evoked) nystagmus , a jerk nystagmus that appears on eccentric gaze and beats in the direction of the gaze. It is associated with cerebellar disorders ( Harris, 1997 ) or some substances (e.g., sedatives, anticonvulsants, alcohol).
Acquired pendular nystagmus , which is associated with brain stem or cerebellar disease, or demyelinating diseases ( Averbuch-Heller and Leigh, 1996 ).
Acquired jerk nystagmus is usually associated with cerebellar or brainstem disease. Down-beating nystagmus is strongly suggestive of Arnold-Chiari malformation when vertical pursuit and the vestibulo-ocular reflex also may be abnormal.
Convergence-retraction nystagmus (induced convergence-retraction) is caused by co-contraction of the extraocular muscles, particularly the medial recti. There is a jerk nystagmus (with discomfort) stimulated by attempted upgaze in which the fast phase brings the two eyes together in a convergence movement with retraction of the globe.
Vestibular nystagmus is usually acquired and sometimes has a ‘saw tooth’ waveform where a slow constant velocity drift takes the eyes off target, followed by a quick corrective saccade ( Grisham, 1990 ).
See-saw nystagmus , one eye elevates and usually intorts as the other depresses and extorts. It is rare, usually associated with parasellar or chiasmal lesions; there may be bitemporal hemianopia.
Dissociated nystagmus , with eye movements that are dissimilar in direction, amplitude, or speed. May occur in internuclear ophthalmoplegia.
Other eye movement phenomena.
Square wave jerks occur in up to 60% of normal subjects and are small horizontal saccades which are quickly corrected by a second saccade ( Worfolk, 1993 ). Square wave jerks and saccadic intrusions are common in Parkinson’s disease.
Ocular flutter is a burst of horizontal back to back saccades with no resting interval between them and can be unidirectional or multidirectional ( opsoclonus ). It can occur transiently in healthy infants, as a side effect of some drugs, or from pathology. About 5% of the population can simulate ocular flutter as voluntary nystagmus .
Spasmus nutans is characterised by the triad of nystagmus, head nodding, and abnormal head posture and usually presents in the first year of life. The nystagmus is a pendular oscillation of variable conjugacy ( Dell’Osso, 1994 ). It is generally benign and only lasts a year or two, but can be associated with pathology ( Grisham, 1990 ).
Microsaccadic opsoclonus are high frequency, small amplitude, back-to-back multivectorial saccadic movements which are visible with slit lamp biomicroscopy and direct ophthalmoscopy ( Foroozan and Brodsky, 2004 ). The condition can cause intermittent blurred vision and oscillopsia. Differential diagnosis includes superior oblique myokymia (p. 285).
Other saccadic disturbances. These include unilateral oculomotor apraxia, Huntington’s chorea, and saccadic dysfunction in dementia and multiple sclerosis.
Rare Causes of Nystagmus
There are a number of rare syndromes and pathologies that can cause nystagmus which are uncommonly encountered by community optometrists. These are summarised in Table 18.1 .
|Condition||Classification||Systemic signs||Nature of nystagmus|
|Heimann-Bielschowsky phenomenon (dissociated vertical nystagmus) ( Jeong, Oh, Hwang, & Kim, 2008 )||Vertical oscillation of an eye with impaired vision|
|Vestibular migraine ( Polensek & Tusa, 2009 )||Vestibular||Nystagmus associated with vertigo, nausea, headache. The nystagmus resolves when the headache abates||Positional nystagmus, commonly sustained, low velocity, can be horizontal, vertical, or torsional|
|Vitamin B12 deficiency ( Akdal et al., 2007 )||Brain stem||Bilateral internuclear ophthalmoplegia or downbeat nystagmus|
|Wernicke-Korsakoff syndrome ( Isenberg-Grzeda et al., 2016 )||Neurogenic||Neuropsychiatric deficiency caused by Vitamin B1 deficiency, can occur in alcoholics or cancer patients|
Classification Based on Eye Movement Characteristics
The classification of nystagmus by eye movement characteristics requires apparatus for objectively recording eye movements. Nystagmoid eye movements may be pendular ( Fig. 18.1A ), or jerky, consisting of a fast (saccadic eye movement) phase and a slow (slow eye movement) phase. The direction of jerk nystagmus is defined by the direction of the fast component. In jerk nystagmus, it is important to know whether the slow phase is accelerating ( Fig. 18.1B ), or decelerating ( Fig. 18.1C ) and this requires an eye movement recording of the type shown in Fig. 18.1 . Ideally, a trace of velocity versus time should also be obtained.
The waveform in infantile and many forms of acquired nystagmus can be pendular or jerk. The jerk movement in INS characteristically has an accelerating slow phase ( Dell’Osso & Daroff, 1975 ), suggesting a deficit in the slow eye movement subsystem. Latent nystagmus, on the other hand, has a decelerating slow phase and always beats towards the viewing eye. However, there are occasional patients who have INS with a decelerating slow phase ( Abadi & Dickinson, 1986 ) and Bourron-Madignier (1995) believed that intermediary and mixed forms exist. Dell’Osso (1994) noted that since INS persists in the dark, it is not likely to be a primary deficit of the fixation mechanism. The waveform in INS usually has a torsional component ( Maybodi, 2003 ).
Dell’Osso and Daroff (1975) presented a classification of INS waveforms into 12 different types. The situation is complicated because most people with INS exhibit more than one type of waveform and the waveform cannot be used to determine the type of nystagmus, as classified in the previous section ( Abadi & Dickinson, 1986 ). Indeed, the waveform in a person with INS may evolve with time to develop adaptations which increase the foveation period, described below ( Abadi & Dickinson, 1986 ).
The foveation period is the proportion of time that the object of regard is imaged at or very close to the fovea and during which the image is moving slowly enough for useful information to be assimilated. The precision of foveation is a better of predictor of acuity than the intensity of the nystagmus ( Abadi & Dickinson, 1986 ). Dell’Osso (1994) argued that the ability to use a foveation period explains why patients with infantile and manifest latent nystagmus do not often report oscillopsia. However, Waugh and Bedell (1992) found that people with nystagmus sample visual information continuously, not just during one phase of the nystagmus. Extraretinal signals are likely to play a role in alleviating the perception of motion smear from the eye movements in INS, in the same way as they do during eye movements in normal observers ( Bedell, 2000 ). It is not just the duration of the foveation period that is important in INS, but also the period of temporal integration of the visual system ( Chung & Bedell, 1997 ).
Symptoms and History
Children with a low birth weight (<2000 g) or birth complications are seven times more likely to have nystagmus than other children ( Stayte, Johnson, & Wortham, 1990 ). Approximately 1 in 10 children with cerebral palsy or visual impairment have nystagmus ( Grisham, 1990 ), and it is also more common in children with spina bifida ( Caines, Dahl, & Holmstrom, 2007 ) and vitamin B12 deficiency ( Akdal et al., 2007 ).
Idiopathic INS is diagnosed by exclusion, and the lengths to which ophthalmologists go to exclude sensory defects varies. As noted later, electrophysiological testing is required.
Many patients with nystagmus adopt an anomalous head position so they are looking in their null position (see later). A patient who reports recent onset oscillopsia (usually accompanied by dizziness) and poor vision is very likely to have acquired nystagmus, requiring referral. Acquired nystagmus may also be associated with diplopia and, in recent cases, past pointing. Most patients with INS do not spontaneously report oscillopsia, but on careful questioning might report this under certain viewing conditions ( Cham, Anderson, & Abel, 2008a ).
Nystagmus is a sign with many different causes and some of these causes are genetically determined, so nystagmus often runs in families. However, in INS many aspects of the waveform are not genetically determined ( Abadi, Dickinson, Lomas, & Ackerley, 1983 ).
Ocular pathology must be excluded in all cases of nystagmus. Particular attention should be paid to the optic discs and visual fields. The degree of ocular pigmentation should be noted: ocular albinos do not have hypopigmentation of the hair and skin but do have reduced iris and fundus pigment, and foveal hypoplasia. Optical coherence topography is useful in differential diagnosis of foveal hypoplasia ( Self et al., 2020 ). An iris transillumination test should be carried out in all cases, as even brown irides can demonstrate the transillumination characteristic of ocular albinism ( Day & Narita, 1997 ). A slit lamp biomicroscope is used with the illumination directed through the centre of the pupil, to create retroillumination. The iris is observed under low magnification and if the red retinal reflex can be seen through the iris, this suggests that there is either iris atrophy or ocular albinism. Ocular albinism usually causes transillumination throughout the iris, but the hypopigmentation can be sectoral on the iris or fundus ( Shiono, Mutoh, Chida, & Tamai, 1994 ). Some normal, nonalbinotic, patients also demonstrate iris transillumination, and this can be seen where there is history of iritis.
Chung and Bedell (1995) found that, in INS, contour interaction (crowding) is greater when stimuli are presented against a black background than against white. This effect can reduce the visual acuity (VA) by two Snellen lines.
Many patients with INS have a high refractive error and early onset nystagmus appears to interfere with normal refractive development ( Sampath & Bedell, 2002 ). With-the-rule astigmatism is especially common ( Jethani, Prakash, Vijayalakshmi, & Parija, 2006 ). This was initially attributed to lid pressure ( Spielmann, 1994 ), but may in fact be due to meridional emmetropisation ( Wang et al., 2010 ). A very careful refraction is required; often the patient will notice a significant improvement with updated spectacles. Some cases of INS have a latent component to the nystagmus (the nystagmus increases when one eye is covered) and monocular refraction is best carried out with a high-power fogging lens over the other eye, rather than an occluder. Similarly, binocular acuities are much more useful in predicting vision in everyday life than monocular acuities ( Norn, 1964 ). The contrast sensitivity function is a useful measure of visual function in nystagmus ( Abadi, 1979 ; Dickinson & Abadi, 1985 ). Accommodative function is often below normal limits in people with INS ( Ong, Ciuffreda, & Tannen, 1993 ).
Latent nystagmus is usually ( Grisham, 1990 ), or always ( Dell’Osso, 1994 ), associated with strabismus and INS is often associated with strabismus. Normal criteria should be applied in deciding whether to treat binocular anomalies. Anecdotal reports suggest that improving sensory and motor fusion can help to stabilise nystagmus in some cases ( Leung, Wick, & Bedell, 1996 ; Scheiman & Wick, 1994 ). Many, if not all, patients with ocular or cutaneous albinism have abnormal visual pathways in the chiasma and no potential for true binocular vision.
Clinical Investigation Of Nystagmus
The eye movements should be observed for a couple of minutes ( Worfolk, 1993 ) and the nystagmus described ( Table 18.2 ). In INS, there is often a gaze null position (a position of gaze in which the nystagmus is reduced), and the null position may change over time ( Abadi & Dickinson, 1986 ). In about 8% of infantile-onset cases, the nystagmus is reduced markedly upon near fixation ( Abadi & Dickinson, 1986 ): a convergent null position.