Chapter 4: Other Types of Nystagmus of Infancy

Nystagmus in infancy and childhood outlines the understanding, evaluation, and treatments of nystagmus in infancy and childhood. Aligning this condition with advanced concepts of developmental brain-eye diseases and summarizing novel treatment paradigms, the authors provide an authoritative resource for both clinicians and scientists in the care of infants and children with nystagmus. The chapters comprised here offer valuable coverage in all relevant areas related to nystagmus: algorithms for examination; descriptions of diagnostic techniques; medical, surgical, and alternative treatments of the visual system in infants and children; methodologies for investigation, including analysis software, models of the ocular motor system, and current hypotheses on the pathophysiology of ocular motor oscillations. Unlike earlier works on this topic, emphasis is placed on the motor mechanisms that cause the various types of nystagmus rather than the diagnosis or treatment of the afferent visual deficits that may accompany them. The study of each type of nystagmus using accurate eye-movement recordings serves as the foundation for differential diagnosis and treatment options. Each chapter summarizes the results of ocular motor research in a narrative manner, identifying the important ideas and observations that point to underlying neurophysiological mechanisms. Based on insights from the authors' combined 75 years of clinical experience, Nystagmus in Infancy and Childhood is a valuable clinical reference for ophthalmologists, neurologists, and other specialists in the treatment of this condition.

OUP UNCORRECTED PROOF – REVISES, 09/06/12, NEWGEN 4 other t ypes of nystagmus of infancy 4.1 NYSTAGMUS BLOCKAGE SYNDROME 122 4.1.1 Characteristics of Nystagmus Blockage Syndrome 123 4.1.1.1 Multiple Types of Nystagmus 123 4.1.1.2 Waveforms and Mechanisms 123 4.1.1.2.1 Target Foveation 123 4.1.1.2.2 Foveation Accuracy 123 4.1.1.3 Purposive Esotropia 123 4.1.1.4 Head Position 124 4.1.1.5 Blockage Syndrome Types I and II 124 4.1.1.6 Foveation, eXpanded Nystagmus Acuity Function, and Acuity 126 4.1.1.7 Efference Copy, Foveation, and Oscillopsia Suppression 127 4.1.2 Treatments of Nystagmus Blockage Syndrome 127 4.1.2.1 Fixation Preference 129 4.1.2.2 Alexander’s Law 129 4.1.2.3 Surgical 129 4.1.2.3.1 Fixating Eye 129 4.2 SPASMUS NUTANS SYNDROME 129 4.2.1 Characteristics of Spasmus Nutans Syndrome 129 4.2.1.1 Waveforms and Mechanisms 130 4.2.1.2 Variable Interocular Phase 131 4.2.1.3 Head Nodding 131 4.2.2 Treatment of Spasmus Nutans Syndrome 132 The greatest obstacle to discovery is not ignorance—it is the illusion of knowledge. —Daniel J. Boorstin I N A DDIT ION to infantile nystagmus syndrome (INS) and fusion maldevelopment nystagmus syndrome (FMNS), there are two rare syndromes found in infants and children: the nystagmus blockage syndrome (NBS) and the spasmus nutans syndrome (SNS).1 4.1 NYSTAGMUS BLOCKAGE SYNDROME The NBS was defi ned clinically as one in which a patient with nystagmus (type undefi ned) damps or changes that nystagmus (again type undefi ned) 122 by willfully deviating his fi xating eye inward (e.g., “convergence”).2–6 The lack of specificity regarding the nystagmus types or waveforms present both before and after “convergence” led to problematic diagnoses and interpretations of the clinical symptoms. Because of this, the NBS remains both a poorly understood and an overdiagnosed phenomenon related to INS. As the name suggests, the nystagmus of these patients diminishes or disappears clinically with the act of willed esotropia while fi xating a distant target. This should not be confused with the damping of IN during true convergence on a near target. • 04_Hertle_Ch04.indd 122 9/6/2012 9:44:45 PM OUP UNCORRECTED PROOF – REVISES, 09/06/12, NEWGEN Based on our research, the NBS can now be more accurately defi ned as a syndrome in which a patient with INS plus a variable esotropia willfully deviates the fi xating eye into adduction to accomplish either a damping of the IN or a switch from IN to a low-amplitude FMN.1 The reported incidence of NBS in esotropic patients is quite variable and may reflect a geographical bias (10.2% in Europe and 4%–5% in America) as well as overdiagnosing in conjunction with a lack of quantitative data. Although Metz and Smith do discuss NBS,7 their recordings clearly show that their patient had FMNS and that the amplitude varied in accordance with Alexander’s law.8–13 Hoyt, in a letter containing no eye-movement recordings, claimed that 8 of 32 patients with congenital esotropia had NBS.14 Without recordings, INS cannot be differentiated from FMNS, and the diagnosis of NBS cannot be verified. 4.1.1 Characteristics of Nystagmus Blockage Syndrome 4 .1.1.1 M U LT I P L E T Y P E S O F N Y S TA G M US The first characteristic revealed by eye-movement data was that there were two mechanisms by which blockage of the ongoing nystagmus can be accomplished; that resulted in two different types of patients with the NBS.15,16 Patients with both types had IN when their eyes were aligned. In Type I, the IN either damped or stopped entirely upon willful esotropia, in much the same way as with true convergence. In Type II of NBS, the INS waveform converts to a low-amplitude FMNS waveform with the onset of the strabismus. Normally, the substitution of the FMNS slow phases for the INS waveforms that allow for better foveation would not be advantageous. However, in these few patients, the low FMNS amplitude results in better acuity than the larger INS amplitude. NBS is often misdiagnosed in FMNS patients with a strong Alexander’s law variation of their nystagmus, which causes them to fi xate with their adducting eye.15Thus, the NBS encompasses two different types of infantile nystagmus and the ability to willfully change the amount of esotropia present to improve the nystagmus waveform and, thereby, visual acuity. The NBS has been reported in an exotropic patient17 and also in a congenitally blind patient.18 4 .1.1. 2 W AV E F O R M S A N D M EC H A N I S M S During the pre-esotropia phase of the NBS, the waveforms of the nystagmus are those of INS (see Chapter 2, Section 2.1.2). However, upon the willful esotropia, they may be either damped INS waveforms or FMNS waveforms (see Chapter 3, Section 3.1.1). As discussed in Chapter 3, the underlying mechanism for the appearance of FMN is hypothesized to be a tonic imbalance related to the failure of fusion development. Although it is possible that the purposive esotropia induces an actual switch from IN to FMN, it is more probable that both types of nystagmus coexist and the FMN is revealed when the IN becomes sufficiently damped by the esotropia. The second possibility is supported by the demonstration that IN and FMN can both be present in some patients.16 4.1.1.2.1 Target Foveation. Target foveation depends on which type of waveform (INS or FMNS) is present and is described in Chapters 2 and 3. In some cases, the nystagmus is totally blocked by the purposive esotropia and foveation is essentially the same as in unaffected individuals. 4.1.1.2.2 Foveation Accuracy. Foveation accuracy also depends on which type of waveform (INS or FMNS) is present and is described in Chapters 2 and 3. In the case of complete nystagmus blockage, the accuracy is essentially the same as in unaffected individuals. 4 .1.1.3 P U R P OS I V E E S OT R O P I A It had been suggested that the nystagmus is actively blocked by convergence innervation, the esotropia thus being caused by sustained convergence and secondary changes in the medial rectus muscles. The differential diagnosis includes infantile esotropia with equal and alternating crossed fi xation, bilateral sixth nerve paralysis, and bilateral Type I Duane syndrome. NBS is characterized Nystagmus in Infancy and Childhood • 123 04_Hertle_Ch04.indd 123 9/6/2012 9:44:45 PM OUP UNCORRECTED PROOF – REVISES, 09/06/12, NEWGEN by a reduction of the nystagmus when esotropia increases. A useful clinical sign in differentiating NBS from other forms of convergence excess esotropia is the absence of pupillary constriction. These patients are not using their accommodative vergence mechanism to block the nystagmus but, instead, are depending upon some other mechanism to bring about damping of the nystagmus. 4 .1.1. 4 H E A D P OS I T I O N Prior to the purposive esotropia (i.e., during the binocular IN phase), there is not likely to be a head turn. However, after esotropia (in both the NBS Types I and II), the fi xating eye is the purposively esotropic eye, necessitating a head turn in the opposite direction. In Type II, because of the Alexander’s law variation of FMN, a large head turn is common. 4 .1.1.5 B L O C K A G E S Y N D R O M E T Y P E S I AND II The characteristics of both the NBS Types I and II and the differences from convergence damping in INS are demonstrated in the following figures. The eye movements of a patient with Type II NBS are shown in Figures 4.1–4.3. The INS waveforms of this NBS patient during binocular fi xation of a target in primary position are shown in Figure 4.1. The jerk and jerk with extended foveation waveforms are predominantly right beating with some cycles that are left beating. Figure 4.2 shows the waveforms of the same NBS patient binocularly fi xating a target in right gaze. As the right eye becomes esotropic and the left eye remains on target, the INS waveform abruptly switches to a jerk-left FMN. When the right eye returns to the target, the nystagmus again becomes IN (dual jerk right with extended foveation). Individuals with FMNS often fi xate stationary targets with their adducting eye, as is illustrated in Figure 3.12. This strategy is also used during smooth pursuit, as is shown in Figure 4.3. Initially, as the target crosses the midline, the right eye takes up pursuit to the left, while the left eye remains esotropic, albeit moving with the same pursuit velocity. As the now rightward moving target again crosses the midline, the left eye takes up smooth pursuit while the right eye remains esotropic. FIGURE 4.1 Horizontal eye position and velocity right- and left-eye data from a patient with nystagmus blockage syndrome showing infantile nystagmus syndrome waveforms when the eyes are aligned and fi xating on a target in primary position. Time markers indicate 1-sec intervals. b, blink; LE, left eye; RE, right eye. 124 • 04_Hertle_Ch04.indd 124 OT H E R T Y PE S OF N Y STAG M US OF I N FA NC Y 9/6/2012 9:44:46 PM OUP UNCORRECTED PROOF – REVISES, 09/06/12, NEWGEN FIGURE 4.2 Horizontal eye position and velocity right- and left-eye data from the same patient shown in Figure 4.1 with nystagmus blockage syndrome showing infantile nystagmus syndrome waveforms when the eyes are aligned (beginning and ending of the interval shown) and conversion to jerk-left fusion maldevelopment nystagmus when the right eye became esotropic. Time markers indicate 1-sec intervals. b, blink; LE, left eye; RE, right eye. FIGURE 4.3 Horizontal eye position and velocity right- and left-eye data from the same patient shown in Figures 4.1 and 4.2 with nystagmus blockage syndrome during smooth pursuit with the adducting eye (i.e., pursuit by the right eye for target positions in left gaze and the left eye in right gaze). Time markers indicate 1-sec intervals. b, blink; LE, left eye; RE, right eye. Nystagmus in Infancy and Childhood • 125 04_Hertle_Ch04.indd 125 9/6/2012 9:44:47 PM OUP UNCORRECTED PROOF – REVISES, 09/06/12, NEWGEN Without accurate, monocularly calibrated eye-movement data, the relative positions of each eye, the determination of the fi xating eye, and differentiating between INS with a latent component, FMNS, or the NBS would not be possible. In Figure 4.4, the eye movements of a patient with INS with a latent component are shown during convergence on a primary position target moving from far to near and back to far. Th roughout the record, the waveforms are those of INS only (initially jerk left with extended foveation, then jerk right with extended foveation); no FMNS were exhibited. The left eye tracked target motion inward (the right eye was esotropic); at near, the IN of both eyes was damped. However, the right eye tracked target motion outward (the left eye was esotropic). Note that, unlike in the NBS, there was no nystagmus damping during the times when either the right or left eye was maximally esotropic. Th is same lack of esotropia-induced damping is evident in Figure 4.5, where spontaneous reversals of the fi xating eye occurred during fi xation on a stationary, distant, primary position target. In Figure 4.6 we see the eye movements of another patient who had INS plus a variable strabismus during fi xation fi rst on a distant, primary position target and then a target 15° to the right (between the two was a period of conjugate wandering att ributed to inattention). Again, only INS waveforms were exhibited; fi rst was jerk right with extended foveation and then jerk left with extended foveation. No nystagmus damping accompanied the variable esotropia. In Figure 4.7, this patient demonstrated classic INS damping with convergence, despite the esotropia of the left eye. Thus, with eye-movement data one is able to differentiate these clinically similar types of nystagmus characteristics. 4 .1.1.6 F OV E AT I O N , E X PA N D E D N Y S TA G M US A C U I T Y F U N C T I O N , A N D ACUIT Y In Type I, the extended foveation periods will increase the eXpanded nystagmus acuity function (NAFX) over that measured during binocular fi xation. The eye-movement data are indistinguishable from convergence damping in a binocular INS patient with the exception that only one eye is adducted in the NBS. In Type II, the very low amplitude FMN will also yield higher values of the NAFX. During the purposive esotropia, target foveation is accurate, the NAFX is high, and FIGURE 4.4 Horizontal eye position and velocity right- and left-eye data from a patient with infantile nystagmus syndrome (INS) with a latent component and esotropia shown fi xating a distant target with the LE (RE esotropic) as the target is brought inward and then back to its original distant position. During the distant fi xation and convergence, the INS waveform was jerk left while the LE adducts onto the near target. At near, the RE takes on fi xation and the INS waveform reverses to jerk right. As the target moves outward, the formerly fi xating LE remains adducted and the RE maintains fi xation with a jerk-right INS waveform. Time markers indicate 1-sec intervals. LE, left eye; RE, right eye. 126 • 04_Hertle_Ch04.indd 126 OT H E R T Y PE S OF N Y STAG M US OF I N FA NC Y 9/6/2012 9:44:49 PM OUP UNCORRECTED PROOF – REVISES, 09/06/12, NEWGEN FIGURE 4.5 Horizontal eye position and velocity right- and left-eye data from the same patient shown in Figure 4.4 with infantile nystagmus syndrome with a latent component and esotropia during fi xation on a distant target. At the fi rst arrow, the LE became esotropic (RE fi xation); at the second arrow the RE became esotropic (LE fi xation); and at the third arrow, the LE again became esotropic (RE fi xation). Time markers indicate 1-sec intervals. LE, left eye; RE, right eye. visual acuity is at its maximum for the NBS patient regardless of whether it is Type I or II. 4 .1.1.7 E F F E R E N C E CO P Y, F OV E AT I O N , A N D OS C I L L O P S I A S U P P R E SS I O N As was discussed in Chapters 2 and 3, the built-in efference copy of motor commands in the OMS prevents the perception of oscillopsia in either IN or FMN. Thus, patients with the NBS do not normally experience oscillopsia regardless of whether it is Type I or II. 4.1.2 Treatments of Nystagmus Blockage Syndrome The same mechanistic considerations discussed in Chapters 2 and 3 for INS and FMNS, respectively, apply in the NBS. One must fi rst determine whether the patient is Type I or II. The therapeutic choice will depend on the patient’s individual nystagmus type(s) and characteristics. The medical and surgical therapies applied to IN and FMN utilize the respective characteristics of the nystagmus. In much the same manner, INS patients turn their heads to exploit the gaze-angle null of IN, and FMNS patients do the same to exploit the Alexander’s law variation of their FMN. Patients with NBS also apply these same “therapeutic” maneuvers. Head turns, for whichever type of nystagmus they are used, may be cosmetically unatt ractive or even may lead to neck problems when severe. However, head turns are not defects associated with the INS, FMNS, or NBS, but rather, constitute purposive and therapeutic patient-administered “therapy.” Successful amelioration of a head turn can only occur if its advantages, vis-à-vis better visual function, are otherwise achieved (e.g., surgically moving the IN null to primary position or inducing convergence that both damps and broadens the IN null). Nystagmus in Infancy and Childhood • 127 04_Hertle_Ch04.indd 127 9/6/2012 9:44:50 PM OUP UNCORRECTED PROOF – REVISES, 09/06/12, NEWGEN FIGURE 4.6 Horizontal eye position and velocity right- and left-eye data from a patient with infantile nystagmus syndrome (INS) and variable strabismus. Initially, the patient was fi xating a primary position target with a jerk right with extended foveation INS waveform. Then there was a conjugate wandering (presumably due to inattention) until the instruction to look at a target at 15° (right gaze). After the refi xating saccade, the INS waveform was jerk left with extended foveation. Time markers indicate 1-sec intervals. LE, left eye; RE, right eye. FIGURE 4.7 Horizontal eye position and velocity right- and left-eye data from same patient shown in Figure 4.6 with infantile nystagmus syndrome (INS) and variable strabismus while fi xating a distant target with the RE (LE esotropic) as the target is brought inward. The jerk right with extended foveation INS waveform at distance damped completely at near. Time markers indicate 1-sec intervals. LE, left eye; RE, right eye. 128 • 04_Hertle_Ch04.indd 128 OT H E R T Y PE S OF N Y STAG M US OF I N FA NC Y 9/6/2012 9:44:50 PM OUP UNCORRECTED PROOF – REVISES, 09/06/12, NEWGEN 4 .1. 2.1 F I X AT I O N P R E F E R E N C E If the patient fi xates with the straight eye in Type I NBS, there will be no head turn. If the newly esotropic eye is used, a head turn will be necessary and will depend on the amount of esotropia employed by the patient. different from ordinary strabismus; the latter is not under conscious control. Although it has been suggested that bimedial recession may help those with NBS, published studies are lacking. Similarly, the Faden operation may benefit those with NBS. Simple surgical correction in NBS has not proven successful.19 4 .1. 2. 2 A L E X A N D E R ’ S L AW 4.2 SPASMUS NUTANS The amount of Alexander’s law variation with SYNDROME gaze angle is idiosyncratic. Its effects on the amplitude change in FMN were discussed in Chapter 3, Section 3.1.2 and demonstrated in Figure 3.11. Therefore, the presence or amount of head turn will also be idiosyncratic and could affect the optimal therapy. 4 .1. 2.3 S U R G I C A L The surgery for NBS is aimed at reducing the head turn required after the patient utilizes esotropia to damp (Type I) or damp and change (Type II) the nystagmus. Because the patients are able to fuse and only manifest esotropia when they desire, standard strabismus-surgery approaches do not apply. If they have strong fusion reflexes, perhaps a bimedial recession procedure would produce the required damping for Type I NBS patients in the same manner as in binocular INS patients. For Type II NBS patients, if the INS damping produced by realigning the eyes is sufficient, perhaps the switch to FMN would not occur; if the underlying mechanism in Type II is not a switch to FMN but a manifestation of a low-amplitude FMN that coexisted with a more prominent INS, the FMN would then also appear. 4.1.2.3.1 Fixating Eye. As stated earlier, if fi xation remains with the stationary eye as the other is deviated inward (as in Fig. 4.2), no head turn would be necessary and surgery would be problematic. If, however, fi xation is taken up by the deviated eye (the most common scenario), a head turn results and surgery can be tailored to reduce that turn. In the NBS, it is possible that the bimedial recession procedure could be therapeutically beneficial because the purposive esotropia is The spasmus nutans syndrome (SNS)1 includes ocular oscillations, head nodding, and anomalous head positions that begin in infancy (usually between 4 and 18 months of age) and disappear clinically in childhood (usually before 3 years or age). The nystagmus is generally bilateral (but can differ in each eye and may even be strictly monocular), and it oscillates in horizontal, torsional, or vertical directions. The average duration of SNS is 12 to 24 months; rarely, it lasts a number of years. It was fi rst described by Raudnitz in 1897, 20 followed by others, including several longitudinal studies. 21–29 The pathogenesis of SNS remains obscure despite some ocular motor studies.16,30–33 SNS has been confused with other entities, 34–37 but eye-movement recordings allow accurate differentiation (e.g., FMN or uniocular pendular nystagmus). 38 Prior to subsequent ocular motility studies, diagnosis was delayed until the clinical symptoms of nystagmus and anomalous head posture resolved. 4.2.1 Characteristics of Spasmus Nutans Syndrome SNS appears as a high-frequency, asymmetric, dysconjugate ocular oscillation. It is usually horizontal in direction but may also be vertical or torsional. It is often described as an intermittent nystagmus that is asymmetrical in appearance and occasionally monocular. A key eye-movement recording observation is the variable phase difference between the two eyes, which is reflected clinically as an asymmetry in the oscillations between the two eyes. On lateral gaze, the dissociation may increase, with nystagmus of the abducting eye predominating. Nystagmus in Infancy and Childhood • 129 04_Hertle_Ch04.indd 129 9/6/2012 9:44:52 PM OUP UNCORRECTED PROOF – REVISES, 09/06/12, NEWGEN Some case series suggest an increased prevalence of esotropia in SNS. Gottlob et al. found a high incidence of esotropia, latent nystagmus, dissociated vertical divergence, and amblyopia in children with SNS.29 Conversely, rare patients with infantile esotropia display horizontal or vertical head oscillations that resolve following surgical realignment of the eyes. In contradistinction to INS, visual acuity is minimally affected in SNS. 4 . 2 .1.1 W AV E F O R M S A N D M EC H A N I S M S The nystagmus waveform in SNS is a dissociated pendular nystagmus, and this dissociation may be so great that the nystagmus is uniocular.39,40 Ages of onset of the seven patients studied ranged from birth to 14 months; five had head nodding. The dissociated nystagmus is usually of a higher frequency than INS nystagmus, and the result can be disjugate, conjugate, or uniocular. Early reports considered SNS to be pathogenetically related to diverse causes that included light deprivation, dietary factors, season, rickets, epilepsy, auto-arousal, and poor socioeconomic conditions.34 Hermann noted a strong predisposition for the onset of SNS to occur during the winter months, with 70% of cases having their onset during December, January, and February.41–44 In 1897, Raudnitz published the classical description of SNS in which he collated previously reported cases with 15 cases of his own. He emphasized the fact that virtually all of his patients belonged to a certain dark quarter of Prague. When this district was later sanitized, no further cases of SNS developed. Raudnitz viewed darkness as the primary etiologic factor, speculating that the eyes of affected children were somehow damaged by the “irritant effect” of insufficient light during a critical period of fi xation development. Raudnitz noted that pups that were reared in total darkness for several months developed eye nystagmus and head nystagmus. Lower socioeconomic status may represent a risk factor for the development of SNS. In a study comparing SNS with infantile nystagmus, Wizov found African American or Hispanic ethnicity to be significantly more common in SNS.45 Patients with SNS also had lower average gestational ages, lower home luminances at birth, fewer married parents living together, and more psychiatric disorders, including alcohol and drug abuse. For a century, numerous reports emphasized that SNS was a visually and systemically benign and self-limited clinical entity. 26,46–52 Since 1967, however, many infants with some of the features of spasmus nutans have been found to have congenital suprasellar tumors (most commonly chiasmal gliomas). Suprasellar tumors can produce a constellation of neuro-ophthalmologic signs that are clinically and electrophysiologically indistinguishable from SNS. Congenital head nodding and nystagmus has been reported with cerebrocerebellar degeneration. 53 The clinical fi ndings of hydrocephalus, café au lait spots, optic atrophy, or other clinical signs of neurofibromatosis make it more likely that a child with SNS will have a central nervous system glioma. A substantial proportion of patients presenting with SNS-like nystagmus have important underlying ocular, intracranial, or systemic abnormalities. Neurodegenerative disorders such as Pelizaeus–Merzbacher disease and Leigh disease may produce nystagmus and head nodding that are indistinguishable from SNS. 52,54–57 These disorders should be suspected in children with clinical signs of ataxia or developmental delay or with magnetic resonance evidence of white matter signal abnormalities. Achromatopsia, congenital stationary night blindness, and Bardet Biedl syndrome can also masquerade as SNS. Genetic factors were suggested by the descriptions of SNS in identical twins and the fi nding that it is more common in Black children.29,34,40,58,59 However, SNS occurs in neurologically normal children. Weissman et al. considered vergence, saccadic, and pursuit system abnormalities as possible causes of SNS but came to no defi nitive conclusion.40 SNS is a diagnosis that can only be made using a combination of clinical characteristics and eye-movement fi ndings, which exclude other visual or nervous system disease. The pathogenesis and neuroanatomical substrate of this developmentally acquired form of asymmetric, dysconjugate nystagmus are still unknown. We hypothesize that SNS reflects a yoking abnormality, perhaps due to delayed development. Recordings show that SNS nystagmus 130 • O T H E R T Y P E S O F N Y S T A G M U S O F I N F A N C Y 04_Hertle_Ch04.indd 130 9/6/2012 9:44:52 PM OUP UNCORRECTED PROOF – REVISES, 09/06/12, NEWGEN may not disappear completely but may recede to a subclinical level; neither INS nor FMNS disappears with age. 4 . 2.1. 2 VA R I A B L E I N T E R O CU L A R P H A S E The nystagmus of the SNS tends to be asymmetric in the two eyes, to vary in different directions of gaze, and to be rapid and of small amplitude. The pendular oscillation of SNS is characterized by a variable phase difference between the oscillations of each eye.40 These phase differences can appear from minute to minute and during the child’s development. As Figure 4.8 shows, the pendular oscillations of each eye may be phase locked, slightly out of phase, or even totally out of phase, and this can vary during the course of the recording anywhere from pure conjugacy to pure disconjugacy (0–180° phase shift).40 The phase variation usually varies from minute to minute. Distinguishing this variable phase relationship between the pendular oscillations of both eyes requires DC-coupled, high-bandwidth recordings of both eyes simultaneously. 4 . 2.1.3 H E A D N O D D I N G The head nodding in SNS is curious. It is inconstant and irregular and can be horizontal or vertical, or both. The head nodding associated with SNS is a combination of vertical head nodding together with a lateral shaking of the head in an unpredictable pattern. 57,60–62 The head nodding is of lower frequency than the nystagmus and becomes prominent when the child attempts to inspect something of interest. It disappears during sleep but may persist when the child is lying down. Since some children with INS also have head nodding, this fi nding alone cannot be used to confi rm the diagnosis of SNS in the child with nystagmus. Studies of quantitative head- and eye-movement recordings indicate that the head movement may, using the normal VOR, actually serve to abolish the eye movements (see Fig. 4.9). 30 In some patients, it may be only compensatory with suppression of the VOR. Compare this to INS, where the head oscillation is an extension of the nystagmus and the VOR is normal (see Chapter 2, Section 2.1.8). Gresty et al. examined patients with SNS in whom head nodding abolished the nystagmus, and a normal VOR stabilized the eyes during head movements. 32 They demonstrated with eye-movement recordings that the head nodding in SNS is an adaptive behavior that serves to improve visual acuity by suppressing the nystagmus, rather than a separate pathological phenomenon. Eye-movement recordings from these patients demonstrated that the head nodding in SNS functions to abolish the nystagmus through some mechanism independent of the vestibulo-ocular response. Gott lob et al. confi rmed and refi ned these conclusions in a large number of patients with SNS using eye-movement recordings.29,60 In their patients, the head nodding changed the SNS waveform from a fi ne, pendular, dissociated nystagmus of high frequency to a larger slower waveform that is symmetrical between the two eyes. There is now general agreement that head nodding in SNS is compensatory. Because the vestibulo-ocular reflex (VOR) of these patients is normal, by willfully shaking of the head, the nystagmus is switched off and the eyes become stable in space because of a good VOR. A patient may have convergence nystagmus, one eye going left, the other eye going right at the same time (180° out of phase), while the head is still. When the patient starts shaking his or her head, the nystagmus stops and, owing to the normal VOR, the eyes begin moving conjugately equally and oppositely to the head, so gaze remains constant. “Cultured” Clinical Pearl: Based on the observation that head nodding is compensatory in the SNS, if further research on the eye movements of the “SN-like” nystagmus associated with brainstem gliomas demonstrates that no head nodding is exhibited by these patients, the presence of deliberate, compensatory head nodding is an indication of SNS and is benign. The head tilt in SNS is a variable fi nding that is present in less than half of cases. Although the reason for the associated head tilt is unclear, Gott lob et al. suggested that it may serve to directionalize the head nodding to its optimal trajectory.60 Although early authors stated that Nystagmus in Infancy and Childhood • 131 04_Hertle_Ch04.indd 131 9/6/2012 9:44:52 PM OUP UNCORRECTED PROOF – REVISES, 09/06/12, NEWGEN FIGURE 4.8 First example of the phase variation between the two eyes in spasmus nutans syndrome. (Left) A uniocular, pendular nystagmus in the LE. (Middle) A binocular, pendular nystagmus with both eyes in phase. (Right) A binocular, pendular nystagmus with the eyes 180° out of phase. The three intervals were within seconds of each other. Time markers indicate 1-sec intervals. LE, left eye; POS, eye position; RE, right eye; VEL, eye velocity. the head nodding was the fi rst sign of SNS to appear and the last to resolve, it is now generally agreed that the nystagmus is the most constant feature of SNS and that it probably precedes the head nodding, although the head nodding may be the abnormality that fi rst att racts attention. Weissman found persistence of the nystagmus in some of their patients, and Gott lob et al. found persistence of nystagmus using eye-movement recordings in all patients who had clinical resolution of the condition, suggesting that the nystagmus diminishes to a subclinical level but does not entirely resolve.40 4.2.2 Treatment of Spasmus Nutans Syndrome In patients with “SN-like” nystagmus, accurate diagnosis is the most important factor. Therefore, until definitive eye-movement-based criteria are FIGURE 4.9 First demonstration that the head movements in spasmus nutans syndrome are both willful and compensatory. 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