Reading the Waveform
Nystagmus is a language. Once you can name what you see — the slow phase shape, the asymmetry of the beat, the response to fixation — half the diagnostic work is done. This chapter teaches the vocabulary, the grammar, and then the clinical patterns.
Six pages of nystagmus terminology can be condensed to a single principle: the diagnostic signal lives in the slow phase. The fast phase is just the brainstem resetting the eye in the orbit. Examine the slow phase's shape, response to gaze, and behaviour with fixation, and you will localise most lesions before any caloric is run.[Leigh & Zee 2006][McCaslin 2013]
Anatomy of a Waveform
Before identifying patterns, learn the names of the parts. Every nystagmus tracing is built from the same components: a slow phase, a fast phase, an amplitude, a frequency, and a direction defined by the fast phase.
The eye drifts, driven by the imbalanced vestibular tone. Its shape (linear, decelerating, accelerating) localises the lesion.
A resetting saccade generated by the brainstem to keep the eye in the orbit. Its direction names the nystagmus by convention — but it is not the diagnostic signal.
Amplitude: degrees of eye excursion (peak-to-peak). Frequency: beats per second. Useful descriptors but rarely diagnostic on their own.
Slow Phase Velocity (SPV) — the Diagnostic Number
SPV is the slope of the slow phase, in degrees per second. It is the single most important quantitative measure in VNG. The Jongkees formula uses peak SPV; the spontaneous nystagmus norm is < 6°/s; gaze-evoked nystagmus is graded by SPV at each gaze position.
The window above samples 0.4 s of the trace and fits a straight line; SPV is the absolute slope of that line. Note how SPV varies across the slow phase of a decreasing-velocity waveform — high at the start, low at the end. Convention is to report peak SPV, taken from the steepest part of the slow phase.
Alexander's Law
Spontaneous peripheral nystagmus does not appear with the same intensity in every gaze position. It intensifies on gaze toward the fast phase, attenuates on gaze toward the slow phase. This is the law that explains why peripheral nystagmus appears 'direction-fixed' yet variable in size.
All three traces show the same nystagmus — same direction, same character — but the intensity varies systematically. The fast phase here beats rightward; per Alexander's law, intensity is greatest on gaze toward the right (the direction of the fast phase) and reduced on gaze leftward.[Alexander 1912]
Fixation Suppression
The single most useful bedside discriminator between peripheral and central nystagmus. Peripheral nystagmus is suppressed by visual fixation; central nystagmus is not. VNG goggles eliminate fixation, allowing peripheral nystagmus to emerge.
With eyes open in a lit room, peripheral nystagmus may be subtle or absent. Place the patient in goggles (or in a dark room with Frenzel lenses), and the nystagmus reveals itself. Central nystagmus does not change — it remains visible in light and dark alike.
Direction-fixed vs Direction-changing Nystagmus
Peripheral nystagmus has a single direction of fast phase, regardless of gaze. Central gaze-evoked nystagmus changes direction with the direction of gaze — beating right on right gaze, left on left gaze. This contrast is one of the cleanest peripheral-vs-central discriminators.
The Waveform Library
Twelve clinically important waveforms, each with an animated tracing, an annotated static plate, identifying features, mimics, pitfalls, and a localising pearl. Read in sequence or jump to the patterns that matter most for your patient.
The patterns below are organized so peripheral patterns come first (most common), then central, then ocular-motor. Within each, look for the slow-phase shape (linear, decelerating, accelerating, enveloped) and the response to gaze and fixation.
Jerk nystagmus — linear slow phase
A constant-velocity slow phase followed by a fast resetting phase. The eye drifts toward the lesioned side at a steady rate, then the brainstem resets it with a saccade. Linear slow phase is characteristic of compensated peripheral lesions or acute central pathology.
- Slow phase has a constant slope (linear ramp on the trace).
- Asymmetric beat: clear fast-phase reset visible.
- Direction is fixed across gaze positions in primary lesion (Alexander's law applies).
- Don't confuse linear slow phase with absence of a slow phase. Look at the asymmetry between the slow ramp and the fast reset.
- Linear slope can occur in central pathology — always check fixation suppression and the oculomotor battery.
Jerk nystagmus — decreasing-velocity slow phase
Slow phase begins fast and decelerates exponentially as the cupula returns toward neutral. This decreasing-velocity profile is the hallmark of an acute peripheral lesion and is the typical pattern seen in spontaneous nystagmus following vestibular neuritis.
- Slow phase begins with the steepest slope and decelerates exponentially.
- Often high-amplitude in the acute setting, decreases over days.
- Suppresses dramatically with fixation.
- Follows Alexander's law: more intense on gaze toward the fast phase.
- May fade as compensation occurs. A patient seen on day 2 may have dramatic nystagmus; on day 14, only on gaze testing.
- Examiners often miss the torsional component — peripheral nystagmus is typically mixed horizontal-torsional, not pure horizontal.
Pendular nystagmus
Sinusoidal back-and-forth — no slow/fast distinction. Both phases have similar velocity profiles. Most often congenital (idiopathic infantile nystagmus syndrome) but can be acquired with severe visual loss or central demyelination.
- Sinusoidal — slow phase and fast phase have similar velocity profiles.
- No clear "beat" direction; named by amplitude and frequency.
- Often present from infancy if congenital; may be asymmetric in MS.
- Symptoms of oscillopsia in adult-onset pendular suggest acquired pathology — work up MS, brainstem stroke.
- In children, pendular without nystagmus on lateral gaze should prompt ophthalmologic workup for sensory deprivation (cataracts, retinal disease).
Down-beat nystagmus
Fast phase beats DOWNWARD, with an upward slow drift, present in primary gaze. Classically associated with Arnold-Chiari malformation and inferior cerebellar/medullary lesions. May also be drug-induced (lithium, anticonvulsants).
- Fast phase beats DOWNWARD in primary gaze.
- Often increases on lateral and downward gaze.
- May be combined with cerebellar signs (dysmetria, ataxia).
- Always image — Arnold-Chiari I is a treatable cause and easily missed without dedicated cervicomedullary views.
- Vitamin deficiencies (B12, thiamine) and Wernicke encephalopathy can produce reversible downbeat.
Up-beat nystagmus
Fast phase beats UPWARD in primary gaze. Less common than down-beat. Localises to specific brainstem regions; often resolves with treatment of underlying cause.
- Fast phase beats UPWARD in primary gaze.
- Less common than downbeat; more localising.
- Often suppresses on convergence (a useful bedside test).
- Misinterpreted as drug effect — careful history of thiamine status, alcohol use, hyperemesis is mandatory.
- Convergence suppression is sometimes the only finding distinguishing upbeat from other vertical patterns.
Rebound nystagmus
Gaze-evoked nystagmus that reverses direction transiently when the eyes return to primary gaze. Pathognomonic for cerebellar dysfunction. Reflects failure of the gaze-holding neural integrator and Cogan's classic sign.
- Gaze-evoked nystagmus in eccentric gaze that REVERSES briefly when eyes return to primary.
- Cogan's classic sign of cerebellar dysfunction.
- Often accompanied by saccadic dysmetria and broken pursuit.
- Easy to miss — the reverse beat is brief (a few seconds) and requires the examiner to watch the eye carefully on return to primary.
- May appear in any cerebellar disease; combine with HINTS, head-impulse, and oculomotor battery for full picture.
Positional — canalithiasis
Triggered by provocative head positioning (Dix–Hallpike, supine roll). Brief latency (1–5 s), crescendo–decrescendo intensity, duration < 60 s, fatigues with repetition. The shape on the trace is a clear envelope: builds, peaks, declines.
- Latency of 1–5 seconds between assuming the position and onset.
- Crescendo–decrescendo intensity over 10–40 seconds.
- Fatigues with repeated positioning.
- Reverses direction on returning to upright.
- Goggles or fixation-blocking glasses are essential — positional nystagmus suppresses dramatically with fixation.
- Always test the side that reproduces symptoms first (Dix–Hallpike or roll test). Test the opposite side after for completeness.
Positional — cupulolithiasis
Same provocative positions as canalithiasis but no latency, no crescendo, and no fatigue. Persists as long as the head is held in the provocative position. The trace shows immediate, sustained, slowly-fatiguing nystagmus.
- No latency — nystagmus begins immediately on assuming the position.
- Sustained as long as the position is held.
- Does not fatigue with repetition.
- Often more vertigo than canalithiasis at the moment of positioning.
- Standard Epley may be less effective; consider Semont liberatory manoeuvre for cupulolithiasis variants.
- May coexist with canalithiasis in different canals — always test all positions.
See-saw nystagmus
Pendular waveform in which one eye intorts and elevates while the other extorts and depresses, then alternates. On a single-channel trace, both eyes appear to move in a sinusoidal pattern. Classically associated with chiasmal lesions (pituitary tumours, craniopharyngioma) — bitemporal hemianopsia is the companion sign.
- Conjugate eyes move oppositely: one elevates and intorts, other depresses and extorts.
- Pendular waveform on each eye individually.
- Often associated with bitemporal hemianopsia (chiasmal lesions).
- Often missed without slow-motion video review of binocular tracings.
- Always pair with formal visual fields — bitemporal hemianopsia is the major diagnostic clue.
Periodic alternating nystagmus (PAN)
Horizontal jerk nystagmus that periodically reverses direction every 90–120 seconds, with a short null period between reversals. Reflects loss of cerebellar inhibition of velocity-storage. Classically improves with baclofen, which is itself a useful diagnostic clue.
- Horizontal jerk nystagmus that reverses direction every 90–120 seconds.
- Brief null period (5–10 s) between reversals.
- Persists in darkness; fixation has limited effect.
- Easy to miss in a short examination — observe for at least 2 minutes in primary gaze.
- Patients often have head turn behaviour to use the null period for visual tasks.
Congenital (infantile) nystagmus syndrome
Onset before 6 months. Increasing-velocity slow phase (the hallmark) and brief foveation periods of clear vision between beats. Direction may reverse with gaze (null point gives best vision). Patients often have a head turn to use the null position.
- Onset before 6 months of age.
- Increasing-velocity (accelerating) slow phase — the pathognomonic feature.
- Brief foveation periods between beats (visible as a flat segment on the trace).
- Often a null point with direction reversal on lateral gaze.
- Patients may adopt a head turn to use the null position.
- Patients often have NORMAL visual function during foveation periods — visual acuity may be much better than the nystagmus would suggest.
- Always assess for sensory deprivation: cataracts, retinal disease, optic nerve hypoplasia, albinism.
Internuclear ophthalmoplegia (INO) — dissociated
On attempted lateral gaze, the abducting eye shows dissociated jerk nystagmus while the adducting eye is slowed or fails to fully adduct. The trace is conventionally drawn for the abducting eye. In MS, INO is often bilateral; in older patients, unilateral INO suggests brainstem stroke.
- Dissociated jerk nystagmus in the abducting eye on attempted lateral gaze.
- Slowed or absent adduction in the contralateral eye.
- Convergence usually preserved (distinguishing from medial rectus weakness).
- Bilateral INO in a young adult = MS until proven otherwise — order brain and spinal cord MRI.
- Unilateral INO in older patients (>50) suggests brainstem stroke; image urgently.
Self-test — Identify the Trace
Six unlabelled tracings. Read the slow phase, choose the diagnosis, then check your reasoning. Patterns repeat across questions; the goal is to confirm you can distinguish similar-looking waveforms by their slow-phase profile and clinical context.
A 45-year-old presents on day 2 of severe vertigo. This is the spontaneous nystagmus recording (eyes closed, in goggles). Identify the pattern.