The Pharmacological Treatment of Nystagmus: a Review
Total Page：16
File Type：pdf, Size：1020Kb
Review The pharmacological treatment of nystagmus: a review Rebecca Jane McLean & Irene Gottlob† Ophthalmology Group, University of Leicester, UK 1. Introduction 2. Acquired nystagmus Nystagmus is an involuntary, to-and-fro movement of the eyes that can result in a reduction in visual acuity and oscillopsia. Mechanisms that cause 3. Infantile nystagmus nystagmus are better understood in some forms, such as acquired periodic 4. Other treatments used in alternating nystagmus, than in others, for example acquired pendular nystagmus nystagmus, for which there is limited knowledge. Effective pharmacological 5. Conclusion treatment exists to reduce nystagmus, particularly in acquired nystagmus 6. Expert opinion and, more recently, infantile nystagmus. However, as there are very few randomized controlled trials in the area, most pharmacological treatment options in nystagmus remain empirical. Keywords: 3,4-diaminopyridine, acquired nystagmus, acquired pendular nystagmus, baclofen, downbeat nystagmus, gabapentin, infantile nystagmus, memantine, multiple sclerosis, periodic alternating nystagmus, upbeat nystagmus Expert Opin. Pharmacother. (2009) 10(11):1805-1816 1. Introduction The involuntary, to-and-fro oscillation of the eyes in pathological nystagmus can occur in the horizontal, vertical and/or torsional plane and be further classified into a jerk or pendular waveform [1]. Nystagmus leads to reduced visual acuity due to the excessive motion of images on the retina, and also the movement of images away from the fovea [2]. As the desired target falls further from the centre of the fovea, receptor density decreases and therefore the ability to perceive detail is reduced [3]. Visual acuity also declines the faster the target moves across the fovea [4]. Three main mechanisms stabilize the line of sight (to static targets) so that the image we see is fixed and clear. The first is fixation, which has two components: i) the ability of the visual system to detect retinal drift and initiate corrective eye movements; and ii) the suppression of unwanted eye movements away from the viewed target. The second mechanism is the vestibulo-ocular reflex. Motion detectors in the inner ear generate eye movements to compensate for perturbations and provide clear vision during head movement. The final mechanism is the gaze-holding system, enabling the eyes to hold an eccentric position. A disruption in any of the three mechanisms can result in the eyes drifting away from the target and corrective fast eye movements being made, leading to nystagmus [5]. Nystagmus can be infantile or acquired in later life and it is important to differentiate between the two types. This can be achieved by considering not only the time of onset of the nystagmus, but also the waveform characteristics of the nystagmus. For those who have infantile nystagmus (IN), this can be idiopathic or secondary/associated to another eye disease, such as retinal disease, albinism, low vision or visual deprivation in early life [1]. Nystagmus can be acquired through vestibular or neurological disease. The most common causes of acquired neurological nystagmus are stroke or multiple sclerosis [6]. It is possible to differentiate between different types of nystagmus by examining the waveform. IN usually occurs in the horizontal plane and has a jerk waveform – a slow phase drift and a fast corrective phase repositioning the eye. The direction of a jerk nystagmus is described using the fast phase. In IN, the slow phase usually 10.1517/14656560902978446 © 2009 Informa UK Ltd ISSN 1465-6566 1805 All rights reserved: reproduction in whole or in part not permitted The pharmacological treatment of nystagmus: a review has an increasing velocity [7]. Often, IN consists of pendular pharmacological treatment for AN and IN. We do not cover waveforms (sinusoidal movements) interrupted by regularly peripheral vestibular nystagmus because this is usually associated occurring foveating saccades. Acquired nystagmus (AN) can with vertigo and postural imbalance. The symptoms associated appear in the horizontal, vertical or torsional plane and can be with this type of nystagmus are usually transitory and therefore of a jerk or pendular waveform [5]. AN jerk waveform presents it is not necessary to treat peripheral vestibular nystagmus as very frequently with a decreasing slow phase velocity (Figure 1). an isolated symptom [15]. Central vestibular nystagmus is Nystagmus waveforms can be viewed in detail via eye- addressed under acquired neurological nystagmus forms. movement recordings. Eye movements can be documented using different methods. Electroculography, the scleral search 2. Acquired nystagmus coil and video eye-tracking devices are the most common techniques, although there are advantages and disadvantages 2.1 Acquired pendular nystagmus for all methods [8]. The quality of electroculography tends to Acquired pendular nystagmus (APN) is a sinusoidal movement be affected by artefact but, as it is relatively non-invasive, that can occur in the horizontal or vertical plane and that often the technique can be used in children. The scleral search coil has both horizontal and vertical components. The oscillations is not usually well tolerated for more than 30 min but has the may be conjugate or disconjugate and, in some cases, mon- advantage of recording three-dimensional eye movements ocular. This form of nystagmus occurs in many conditions, the (horizontal, vertical and torsional). Most video eye-trackers most common being disorders of central myelin (often multiple record in only two dimensions; however, this head-mounted sclerosis) and vascular disease (syndrome of oculopalatal piece of equipment has no contact with the eyes and is well myoclonus) [16]. These two main causes result in APN that tolerated, even by children. Another advantage of the video differs in clinical features. In association with demyelinating eye-tracker is high-quality recordings. disease typically the amplitude has been described as being Nystagmus can be quantified in terms of amplitude (size) and greater in the eye with the poorer vision, frequency is 2 – 8 Hz frequency (cycles per second). The intensity of the nystagmus and intranuclear ophthalmoplegia is commonly associated. can be measured by multiplying the amplitude by the fre- For APN related to the syndrome of oculopalatal tremor, the quency [9]. Nystagmus intensity can vary with eye position oscillation may be vertical or disconjugate vertical/torsional, and often there is a position of gaze in which the oscillations frequency is 1 – 3 Hz and may be synchronized with are minimal. This is usually referred to as the null point. If movements of the palate and other branchial muscles [5]. the null point does not coincide with the primary gaze posi- It is likely that different pathophysiologies are involved in tion then a head position may be adopted to reduce the each form of APN. One hypothesis for APN in demyelinating nystagmus [10]. This can result in a face turn when the nys- disease suggests that in patients with APN the oscillations arise tagmus is in the horizontal plane or a chin up or down in the neural integrator for eye movement – the gaze-holding position when nystagmus occurs in the vertical plane. mechanism that allows the eye to be held steady at an eccen- Nystagmus can be distressing for both those with IN and tric position [17]. The nucleus prepositus hypoglossi and medial AN. Although patients with IN and AN both have reduced vestibular nucleus region are known to play a part in the vision, patients with AN tend to suffer from oscillopsia (the neural integrator function, which may be affected by a motor illusion of constant movement of the surroundings) and hence feedback signal from cell groups of the paramedian tracts to may be more troubled by the condition. The constant percep- the cerebellar flocculus [18]. On magnetic resonance imaging, tion of motion is extremely distressing to the patient. In fact patients with multiple sclerosis often show lesions that may the impact of nystagmus is significant, with visual functioning affect the paramedian cell groups. It is therefore proposed scores worse in nystagmus than that of other visual impair- that APN arises from an unstable neural integrator which ments such as age-related macular degeneration [11]. The causes the oscillations [19]. Microinjections of agents of agonist occurrence of nystagmus is also higher than early estimations. or antagonist action at receptors for gamma-aminobutyric Previously it was thought that the prevalence of nystagmus acid (GABA), glutamate and kainite in the region of the was 1/1000 [12]; however, more recent data has estimated the nucleus prepositus hypoglossi and medial vestibular nucleus prevalence to be that of 2.4/1000, more than double the in monkeys caused gaze-evoked nystagmus with centripetal previous figures [13]. eye drifts. Injections of the GABA-A agonist, muscimol, Some forms of nystagmus are better understood than others. close to the centre of the medial vestibular nucleus caused In forms that are understood, the range of knowledge drift away from the central position with increasing velocity includes animal models, the neurotransmitter involved and waveforms, suggesting an unstable neural integrator [20]. pharmacological treatment that can be effective. In other Similar effects were also observed in cats [21]. It has been forms there is limited knowledge [14]. There are very few postulated that the use of GABAergic agents may suppress randomized controlled