Nystagmus Nystagmus has traditionally been divided into twotypes on the basis of the clinical impression of thewaveform. Thus, if the eyes appeared to oscillatewith "equal speed" in either direction, it was called"pendular" nystagmus; if movement in one directionwas faster than in the other, it was called "jerk"nystagmus. True pendular nystagmus is sinusoidal, whereas jerknystagmus has a slow phase away from the object ofregard, followed by a fast (saccadic) phase towardthe target. The direction of the fast component, byconvention, defines the nystagmus direction.
Nystagmus Nystagmus should be described not only byits waveform and direction but also by itsamplitude (A) and frequency (F), the productof which is intensity (I). The examiner shouldalso note the positions of gaze in which thenystagmus occurs and whether the intensitychanges with gaze direction. Jerk nystagmusis usually accentuated in amplitude upongaze in the direction of the fast component, acharacteristic referred to as Alexanders law
"null zone" The field of gaze in which nystagmus intensityis minimal is termed the "null zone". The "neutral zone" is that eye position inwhich a reversal of direction of jerknystagmus occurs and in which nonystagmus, any of several bidirectionalwaveforms, or pendular nystagmus ispresent. The null and neutral zones usuallyoverlap; however, several cases have beenrecorded where they do not.
High gain instabilityIn some persons, because of abnormally high gain in the SEMsubsystem, a runaway (increasing velocity) movement or apendular oscillation is evoked. In this chapter, the term "highgain" can also imply excessive delay for the gain present (i.e.,the control loop may have a normal gain but an increaseddelay). Control theory suggests how particular changes in gaincan result in either a pendular or a jerk nystagmus. Pendular nystagmus can be congenital or acquired, whereashorizontal jerk nystagmus with slow phases of increasingvelocity usually is congenital; however, the latter may resultfrom an Arnold-Chiari malformation. Vertical nystagmus with an exponential slow phase ofincreasing velocity may be secondary to acquired cerebellardisease.
Vestibular tone imbalance. The nystagmus of vestibular tone imbalance results from theimposition of asymmetric vestibular input on an inherentlynormal horizontal gaze generator. This asymmetric input occursif one vestibular apparatus (labyrinths, nerve, or brain stemnuclei) functions abnormally or if both sides are asymmetricallydefective. The nystagmus recording always shows a linear(straight line) slow phase, reflecting a persistent tone to drivethe eyes toward the side of the relatively damaged vestibularapparatus. The slow-phase amplitude is reduced by fixation andenhanced by darkness, Frenzel (high-plus) lenses, or closingthe eyes. Fixation inhibition may be related to an opposingsmooth-pursuit force and requires the integrity of the cerebellarflocculus.
Integrator leak. Nystagmus caused by a "leaky integrator" occursonly in an eccentric gaze position; thus, it is gaze-evoked. The eyes are unable to maintain theeccentric position and drift back to the primaryposition with a decreasing velocity, reflecting apassive movement resisted by the viscous forces oforbital soft tissues. The defect may reside in the brainstem "neural integrator" or its connections (such as inthe cerebellum), which mediates eye deviation. Thisform of gaze-evoked nystagmus is called "gaze-paretic" nystagmus
The localizing significance ofnystagmus The localizing significance of nystagmus isoften a mere indication of dysfunctionsomewhere in the posterior fossa (i.e.,vestibular end-organ, brain stem, orcerebellum). However, certain nystagmuspatterns are quite specific and permitreasonably accurate neuroanatomicdiagnosis. When possible, the specific andnonspecific forms are separated on the basisof clinical appearance and associated signsand symptoms.
NYSTAGMUS IN INFANCY There are several types of benignnystagmus usually seen in infancy.Congenital nystagmus (CN) is the mostcommon infantile nystagmus. Othersare latent/manifest latent nystagmus(LMLN) and the pendular nystagmus ofspasmus nutans.
NYSTAGMUS IN INFANCY CN is the direct result of an ocular motor controlinstability that may develop with or without anaccompanying sensory deficit. Thus, for those casesin which a sensory deficit exists, it can only be asubordinate factor in the development of CN, perhapsinterfering with the normal calibration of a key ocularmotor subsystem and thereby precipitating itsinstability. The common association of "pendular" CNwith a sensory defect and the "jerk" form with aprimary motor abnormality is both simplistic anderroneous.
Nystagmus BlockageSyndrome The nystagmus blockage syndrome(NBS) is both a poorly understood andan overdiagnosed phenomenon relatedto CN. As the name suggests, thenystagmus of these patients diminishesor disappears with the act of willedesotropia while fixating a distant target.
Latent/Manifest Latent Latent/manifest latent nystagmus (LMLN) is a jerk nystagmuswith either a linear or decreasingvelocity exponential slowphase identical to that of gaze-paretic nystagmus. Occasionally,when both eyes are closed, a jerk nystagmus with a linear slowphase is present. Classically, "pure" or "true" latent nystagmus(LN) occurs only with uniocular fixation. There is no nystagmus with both eyes viewing, but when oneeye is occluded, nystagmus develops in both eyes, with the fastphase toward the uncovered eye LN is always congenital. However, several cases of manifestlatent nystagmus (MLN) associated with retrolental fibroplasiahave been recorded
Acquired SECONDARY TO VISUAL LOSS. Nystagmusoccurring in early childhood consequent toprogressive bilateral visual loss should not beclassified as CN unless CN waveforms aredocumented. The conceptual problems in theclassification were discussed above. Usually,nystagmus secondary to visual loss cannotbe distinguished from CN in a patient withcoexisting primary visual abnormalities.
SPASMUS NUTANS Spasmus nutans is a rare constellation of ocularoscillation, head nodding, and torticollis that begins ininfancy (usually between 4 and 18 months of age)and disappears in childhood (usually before 3 yearsof age). The nystagmus is generally bilateral (but itcan differ in each eye and may even be strictlymonocular), and it oscillates in horizontal, torsional,or vertical directions. An instance of spasmus nutanspresenting with monocular nystagmus inmonozygous twins has been reported. Spasmus nutans may sometimes be mimicked bytumors of the optic nerve, chiasm, or third ventricle.
AQUIRED PENDULARNYSTAGMUS (ADULTS) Acquired pendular nystagmus may reflect brainstem or cerebellar dysfunction, or both. It occurs inpatients with vascular or demyelinating disease. Inthe latter, it has been regarded as a sign ofcerebellar nuclear lesions. The nystagmus ismultivectorial (i.e., horizontal, vertical, diagonal,elliptic, or circular) and usually is associated with ahead tremor. Marked dissociation between the twoeyes often exists and may not correlate withdifferences in visual acuity from coexisting opticneuropathy.Despite the dissociation, theoscillations of the two eyes are phase locked, evenwhen there is a difference in their frequencies.
ACQUIRED HORIZONTALJERK NYSTAGMUS Vestibular We generally delimit vestibular nystagmus as being consequentto dysfunction of the vestibular end-organ, nerve, or nuclearcomplex within the brain stem. It is a horizontal-torsional orpurely horizontal primary-position jerk nystagmus with a linearslow phase. The nystagmus intensity increases with gazetoward the fast phase (obeying Alexanders law); it decreasesand, with central lesions, may reverse directions upon gazetoward the direction of the slow phase. The symptom of vertigousually coexists. As might be expected, acute lesions of thecerebellar flocculus (the vestibulocerebellum) can produce asimilar nystagmus. Cases of discrete cerebellar infarction arequite rare. Nystagmus may accompany episodic attacks ofataxia.
Gaze-Evoked (Gaze-Paretic)Nystagmus Gaze-evoked nystagmus is elicited by the attempt to maintainan eccentric eye position, and it is the most common form ofnystagmus encountered in clinical practice. Patients recoveringfrom a central gaze palsy show a phase in which lateral gazemovement is possible but cannot be maintained in the deviatedposition; that is, the eyes drift back slowly toward primaryposition. A corrective saccade repositions the eyes eccentrically, andrepetition of this pattern produces nystagmus, aptly designated"gaze-paretic."
SPECIAL NYSTAGMUSTYPES Physiologic (End-Point) There are three basic types ofnystagmus that are regarded as normal(physiologic) phenomena
Fatigue nystagmus begins during extended maintenance of anextreme gaze position and has been found inup to 60% of normals when horizontal gaze ismaximally deviated for a time exceeding 30seconds. It may become increasinglytorsional with prolonged deviation effort andmay be greater in the adducting eye. Fatiguenystagmus is not a clinically importantphenomenon, because routine examinationsdo not include the maintenance of fareccentric gaze.
Unsustained end-pointnystagmus is certainly the most frequentlyencountered physiologic nystagmus. Itsincidence and characteristics havenever been studied quantitatively. Allexperienced clinicians recognize that afew beats of nystagmus are withinperfectly normal limits at gazedeviations of 30° or more.
Sustained end-pointnystagmus Sustained end-point nystagmus beginsimmediately upon, or within severalseconds of, reaching an eccentriclateral-gaze position. It has been foundin more than 60% of normal subjectswith horizontal-gaze maintenancegreater than 40°.
Torsional Torsional nystagmus describes a torsional movementof the globe about its anteroposterior axis; the term"rotary nystagmus" is used interchangeably. Mostnystagmus consequent to vestibular end-organdysfunction has a torsional component admixed witha major horizontal or vertical nystagmus. A purelytorsional nystagmus never occurs with vestibularend-organ disease. When of small amplitude,torsional nystagmus may reflect a medullary lesion.Larger amplitude torsional nystagmus may becongenital, but when it is acquired it often indicatesdiencephalic (thalamic) involvement, in which case itis the underlying pattern in see-saw nystagmus.
See-saw nystagmusSee-saw nystagmus is characterized by a conjugate, pendular,torsional oscillation with a superimposed disjunctive verticalvector. The intorting eye rises and the opposite, extorting eyefalls. Repetition of this sequence in the alternate directionprovides the see-saw effect. The torsional movementspredominate in all fields of gaze, but the see-saw feature maybe restricted to the primary position or, more commonly, todownward or lateral gaze. See-saw nystagmus can be of thejerk type (with one phase being slow and the other fast) withunilateral, mesodiencephalic lesions.Most patients with acquired see-saw nystagmus havebitemporal hemianopias consequent to large parasellar tumorsexpanding within the third ventricle. It is occasionally evokedtransiently after blinks or saccades
Convergence-retraction"nystagmus" Convergence-retraction "nystagmus" as amanifestation of the dorsal midbrainsyndrome here, the initiating convergencemovements are saccadic and thus not a truenystagmus. Fast divergent movementsfollowed by a slow convergence, associatedwith epileptic electroencephalographicactivity, occurred in a neonate with anintraventricular hemorrhage.
Periodic alternatingnystagmus (PAN)Periodic alternating nystagmus (PAN) is an extraordinary ocularmotor phenomenon in which a persisting horizontal jerknystagmus periodically changes directions. PAN may becongenital or acquired. The congenital variety, which may beassociated with albinism,has the slow-phase waveform of anincreasing-velocity exponential and usually lacks the well-defined stereotyped periodicity seen in acquired PAN (i.e., it isaperiodic alternating nystagmus). The periodicity of thecongenital PAN is markedly influenced by changes in gazeposition, supporting the hypothesis that the PAN is a result of atemporal shift in the null zone
Downbeat Downbeat nystagmus is defined asnystagmus gaze position with the fast phasebeating in a downward direction. Patients withbrain stem disease or drug intoxicationsusually lack gaze-evoked downwardnystagmus despite nystagmus in all otherfields of gaze. Thus, nystagmus beatingdownward in the primary position is a strikingphenomenon and is highly suggestive of adisorder of the craniocervical junction, suchas Arnold-Chiari malformations.
downbeat nystagmus The other major cause of downbeat nystagmus isspinocerebellar degeneration. Indeed, it is difficult to ascertainfrom the literature whether an Arnold-Chiari malformation orspinocerebellar degeneration is the most common cause.However, because the latter is correctable, a defect of thecraniocervical junction must be carefully considered in allpatients with downbeat nystagmus. Downbeat nystagmus maycoexist with PAN, another type of nystagmus suggestive of anabnormality of the craniocervical junction. A variety ofmiscellaneous conditions have also been reported to producedownbeat nystagmus.92,112 These include anticonvulsant,alcohol, and lithium intoxication; magnesium deficiency; B12deficiency; brain stem encephalitis; alcoholic cerebellardegeneration; dolichoectasia of the ventral artery; and vertebralartery occlusion
Rebound Rebound nystagmus is either thediminution and direction change ofgaze-evoked horizontal nystagmusduring sustained ocular deviation or ahorizontal gaze-evoked nystagmus that,on refixation to primary position,transiently beats in the oppositedirection. The sign is often present inpatients with cerebellar disease.
Muscle-Paretic (Myasthenic) A paretic eye muscle, from whatever cause, can fatigue quicklyduring contraction, and muscle-paretic nystagmus can beobserved. This is often evident as gaze-evoked nystagmus inmyasthenia gravis, in which there is usually asymmetry betweenthe two eyes.Another form of oscillation in myasthenia is"nystagmus" of the abducting eye (may be saccadic pulsetrains, discussed below) coexisting with paresis of adduction;this mimics an internuclear ophthalmoplegia. Here theoscillation is not due to lateral rectus paresis but rather toexcessive innervation by increased central gain, the result ofparesis of the contralateral yoke medial rectus. Cessation ofboth muscle-paretic and contralateral yoke nystagmus inmyasthenia usually follows administration of anticholinesterasemedication.
Acoustic Neuroma Schwannomas of the eighth nerve grow so slowly that adaptivemechanisms often obscure clinical vestibular manifestations.Vestibular nystagmus beating contralateral to the lesion may bepresent, particularly if fixation is eliminated. As the tumorexpands to compress the brain stem, a slow, gaze-evokedipsilateral nystagmus is often added. The combination of asmall-amplitude, rapid primary-position jerk nystagmus beatingcontralateral to the lesion and a slower, larger-amplitude, gaze-evoked (Bruns) nystagmus ipsilateral to the lesion also occurswith other extra-axial masses, including cerebellar tumors,compressing the brain stem. Rarely, Bruns nystagmus isinverted
Albinism and Achiasma Ocular albinism is associated with anomalous visual projectionsthat result in a variety of eye movement disturbances, withconsiderable intersubject variability. These persons may havependular or jerk nystagmus, absent OKN, "inverted" pursuit, or"defective" pursuit (see section on CN) when targets areprojected onto the temporal half-retina. Periodic alternatingnystagmus may also occur.Albinism can also exist in theabsence of any nystagmus. Achiasma is a very rare condition, first documented in dogs,then in humans,that has associated with it the combination ofhorizontal CN and see-saw nystagmus. In this condition, allretinal fibers remain ipsilateral, passing to the ipsilateral lateralgeniculates and visual cortexes.
Lateral Medullary Syndrome The lateral medullary syndrome (Wallenberg) is adistinctive constellation of signs. The nystagmus inthis syndrome tends to be stereotyped. With the eyesopen there is horizontal-torsional jerk nystagmusbeating contralateral to the lesion; when recordedwith the eyes closed, the nystagmus beats ipsilateralto the lesion. Other rare manifestations, confined tosingle cases, are gaze-evoked eyelid and ocularnystagmus inhibited by the near reflex90 andhorizontal gaze-evoked monocular downbeatnystagmus.
Upbeat Primary-position nystagmus with the fastphase beating upward rarely reflects drugintoxication. Most often, the nystagmus isacquired and indicates structural disease,usually of the brain stem. The location of thelesions in patients with upbeat nystagmusafter meningitis, Wernickes encephalopathy,or organophosphate poisoning is uncertain.With convergence, upbeat may enhance orconvert to downbeat.The slow-phasewaveform is usually linear but may be anincreasing-velocity exponential.