A variety of diseases, drugs, or other factors may disrupt the systems that provide ocular stability. Abnormal eye movements may occur because of inability to maintain fixation, loss of the normal inhibitory influences on eye movement control, or loss of the normally symmetric input from one of the vestibular pathways to the ocular motor nuclei. Nystagmus refers to one form of excessive eye movements; this term should be reserved for rhythmic, to-and-fro (eg, horizontal, vertical, torsional, or mixed) eye movements that incorporate a slow phase. Jerk nystagmus has 2 phases: (1) a slow-phase drift from the visual target, followed by (2) a corrective saccade (fast phase) back to the target. Pendular nystagmus describes back-and-forth, slow-phase movements that occur without a fast phase (Fig 9-1). Abnormal saccadic movements may also drive the eyes off fixation. Collectively, these pathologic eye movements are termed saccadic intrusions or saccadic oscillations; because they have no slow phase, they do not conform to the definition of nystagmus.
Patients with acquired nystagmus often report oscillopsia, a sensation of environmental movement. Children with early-onset nystagmus typically do not have oscillopsia. Nystagmus in primary position may degrade visual acuity. Patients should be asked about any associated neurologic symptoms (eg, vertigo, ataxia, motor weakness, or sensory weakness) and any family history of abnormal eye movements or strabismus.
An examination of ocular motility begins with an assessment of ocular stability in primary gaze. Any abnormal eye movements in the cardinal positions should be examined to determine whether they are
monocular or binocular
conjugate (ie, the eyes behave similarly)
horizontal, vertical, torsional, or mixed in pattern
continuous or induced by a particular eye position
characterized by slow phases only (ie, pendular nystagmus), fast and slow phases (ie, jerk nystagmus), or fast phases only (ie, saccadic intrusion)
reduced at a null point (ie, the field of gaze at which the nystagmus is minimal)
Figure 9-1 Nystagmus waveforms are named for their slow-phase velocity profile. From top to bottom: Linear velocity is typical of vestibular nystagmus; increasing velocity exponential typifies congenital nystagmus; decreasing velocity exponential typifies gaze-evoked nystagmus; and pendular velocity typifies congenital or acquired nystagmus. The horizontal line indicates alignment with the object of fixation.
(Modified from Kline LB, Bajandas FJ. Neuro-Ophthalmology Review Manual. 6th ed. Thorofare, NJ: Slack; 2008.)
By convention, the direction of jerk nystagmus is reported as the direction of its fast-phase component; however, it is the slow-phase component that indicates the pathology. Dissociated or disconjugate nystagmus occurs when the amplitude of oscillations differs in each eye, whereas disjunctive nystagmus occurs when the direction of the oscillations differs between the 2 eyes.
The amplitude of nystagmus often changes with gaze position. A few beats of nystagmus are normally present at the extremes of horizontal gaze (beyond 45°), especially in older patients. This finding should not be considered pathologic unless the nystagmus is persistent, asymmetric (eg, present to the left but not the right), or accompanied by other features. Assessment for nystagmus can be complemented by strategies that search for subtler, smaller-amplitude eye movements. Illuminated Frenzel (high-magnification) goggles are extremely useful for detecting eye movements, but a 20.00 diopter (D) lens, slit lamp, or direct ophthalmoscope can also be used to block the patient’s fixation and magnify abnormal eye movements. Ocular motor recordings (eg, via electro- or video-oculography, infrared tracking, or electromagnetic scleral search-coil techniques) provide objective and highly sensitive measurements of eye movements but are often unavailable in standard clinical practice. The characteristics of eye movements can be recorded and communicated effectively via drawings and video recordings. Video recordings of many of the forms of nystagmus discussed in this chapter are available at http://novel.utah.edu.
In 2001, a National Eye Institute–sponsored committee published a document, “The classification of eye movement abnormalities and strabismus (CEMAS),” which provides a systematic classification of primary eye movement abnormalities and strabismus conditions. Formal definitions of the entities discussed in this chapter can be found in this document.
CEMAS Working Group. A National Eye Institute sponsored workshop and publication on the classification of eye movement abnormalities and strabismus (CEMAS). In: Hertle RW, ed. The National Eye Institute Publications. Bethesda, MD: National Eye Institute, National Institutes of Health; 2001.
Leigh RJ, Zee DS. The Neurology of Eye Movements. 5th ed. Contemporary Neurology Series. New York: Oxford University Press; 2015.
Serra A, Leigh RJ. Diagnostic value of nystagmus: spontaneous and induced ocular oscillations. J Neurol Neurosurg Psychiatry. 2002;73(6):615–618.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.