• Nystagmus

    Introduction

    Early nystagmus terminology was permeated with misleading, often-incorrect clinical descriptions and presumptions. Therefore, to avoid the misunderstandings and misrepresentations resulting from that older terminology, the newer terminology and descriptions established by a workshop held at the turn of this century are used.1 Thus, INS (infantile nystagmus syndrome) replaces “CN” (congenital nystagmus) and FMNS (fusion maldevelopment nystagmus syndrome) replaces “LMLN” (latent/manifest latent nystagmus).

    Ocular motor research in the past half-century has produced remarkable advances in our understanding of the underlying mechanisms of the several types of nystagmus seen in infancy and childhood. The confluence of accurate methods of recording eye movements, the application of the rules of physics, waveform analysis techniques, and control-systems analysis allowed replacement of inaccurate terminology, erroneous concepts, and the ensuing diagnostic and therapeutic quagmire. As a result, we now have a science-based approach to understanding the underlying cause and mechanisms of nystagmus and to the diagnosis and treatment of patients with nystagmus. For instance:

    1. the INS in all patients, regardless of any visual system comorbidity, is directly caused by instability in the damping portion of the smooth pursuit system
    2. that simple, pendular, velocity instability gives rise to all of the predominant INS waveforms (pendular and jerk) by control mechanisms inherent to the normal ocular motor system (OMS);
    3. INS can be definitively diagnosed by eye-movement recordings;
    4. combinations of INS and other types of nystagmus can also be definitively diagnosed by eye-movement recordings;
    5. the use of a mathematical function (the eXpanded Nystagmus Acuity Function, NAFX) allows a priori prediction of which patients can be provided with improved visual function by specific therapies;
    6. the NAFX and its derived function, the Longest Foveation Domain (LFD) can provide an estimate of the amount of improvement that will be produced by current accepted therapies; and
    7. INS therapies should be aimed at damping the primary velocity instability to allow the OMS to improve the quality of the foveation periods within INS waveforms but should neither diminish nor destroy any of the OMS functions necessary for appropriate real-world visual function.

    Specifically, the attempt to improve cosmesis by damping or eliminating the nystagmus itself should not override the patients’ ability to advantageously use their OMS for all normal functions. Overzealous parents of children especially should be educated to understand that the appearance of INS is not the major concern of either the patients (as they grow up) or of the child’s physician; the physican’s primary concern should be the visual well-being of the young patient and the physician should be the child’s adult advocate. Crippling the OMS to eradicate INS (even if that were possible) should never be an option imposed on a child who cannot give informed consent or even an adult until they are informed of the symptomatic deficits and potential dangers such a procedure would produce.

    This chapter will concentrate on translating the above research results to better enable the clinician to make accurate diagnoses and provide safe and effective treatment (ie, multiple improvements in visual function) of patients with nystagmus. It is both an introduction to and overview of nystagmus in infancy and childhood and provides a data-based foundation for clinical diagnosis and treatment. Hopefully, it will also pique the interest of future researchers. A more thorough and fully referenced presentation of ocular motor research foundations, diagnostic findings, clinical “pearls,” and therapeutic approaches and outcomes may be found in the book on the subject by Hertle and Dell’Osso2 and the many scientific citations contained therein.

    Prior to the application of ocular motor research to the nystagmus of infancy, only clinical observation (albeit astute observation by pioneers like Kestenbaum and Anderson)3-5 was available. Unfortunately, a convoluted and factually unsupported ophthalmological mythology evolved, was taught to pediatric ophthalmology residents, and subsequently used to guide their clinical practice. It and the contents of most early medical texts on infantile nystagmus were based solely on those clinical impressions, unwarranted presumptions, and contradictions to the laws of physics. Some of the putative “truths” they presented were:

    1. Congenital Nystagmus “CN” is an oscillation of the eyes across the line of regard (target);
    2. There are 2 types of “CN,” “sensory” and “motor;”
    3. “Sensory CN” is caused by a visual sensory deficit and has a pendular waveform;
    4. “Motor (aka “idiopathic”) CN” has no known cause and a jerk waveform;
    5. There are no treatments for “CN;” and
    6. Surgically shifting the “CN” null to correct a head turn will not improve either the “CN” or visual acuity.” That statement disregarded Anderson’s observations.5

    Ocular motor research demonstrated that all of the above ”truths” were false. That is:

    1. INS is an oscillation away from and back to the target, as simple physics dictates. Those with INS can have good and even normal visual acuity; therefore, their eyes must come to rest when the fovea is on target, not pass the target at maximum velocity, as the prior description necessitates;
    2. All patients with INS have the same motor oscillation whose characteristics fall within idiosyncratic ranges (ie, types of waveforms, frequencies, amplitudes, and gaze-angle variations). Thus, INS is the same condition in all of the clinical variations encountered in patients;
    3. INS in most patients has an underlying pendular oscillation that is transformed into the various pathognomonic waveforms by different types of normal ocular motor attempts to maintain target foveation;
    4. All INS has the same primary cause (an undamped smooth-pursuit oscillation) and is modified to an idiosyncratic extent (eg, the position and sharpness of the INS “null”) by visuo-vestibular, constant-velocity, slow-phase movement—thus, no patient’s INS can accurately be called “idiopathic,” a misnomer derived from the mistaken designations of “sensory” and “motor” CN. Most patients with solely jerk waveforms reflect the underlying pendular pursuit oscillation; only those rare cases with solely linear slow phases reflect the visuo-vestibular tonic imbalance;
    5. There are many treatments (optical, pharmacological, and surgical) that significantly improve visual function in those with INS, with or without associated sensory visual deficits; and
    6. Surgically shifting the point of maximum foveation quality will usually improve both the foveation quality of the INS waveform and visual acuity across a broader range of gaze angles.

    Chapters