# Nystagmus: Classification and Diagnostic Approach

## Definition and Classification

### Basic Terminology

Nystagmus: involuntary, rhythmic oscillation of the eyes. **Jerk nystagmus**: fast phase (saccade) and slow phase (drift); named by direction of fast phase. **Pendular nystagmus**: equal velocity in both directions; sinusoidal. Nystagmus can be physiological (optokinetic, vestibular) or pathological.

### Classification by Onset

**Infantile (congenital)**: onset within first 6 months of life. **Acquired**: develops after 6 months; more likely to indicate neurological disease.

| Feature | Infantile Nystagmus | Acquired Nystagmus |
|---|---|---|
| Onset | < 6 months of age | > 6 months of age |
| Direction | Horizontal in all gaze positions | Variable (vertical, torsional, direction-changing) |
| Oscillopsia | Absent (brain adapts) | Present (illusion of movement) |
| Slow phase | Accelerating | Constant velocity or decelerating |
| Null zone | Present (head turn) | Usually absent |
| Neurologic concern | Low (unless sensory cause) | High -- investigate urgently |
| Common causes | Idiopathic, albinism, sensory defect | Drugs, Chiari, MS, stroke, tumor |

## Infantile Nystagmus Syndrome (INS)

### Features

Onset within first 6 months of life. Usually horizontal, remains horizontal in all gaze positions (even vertical gaze) Conjugate (both eyes move together) Accelerating slow phase (distinguishes from vestibular nystagmus) Dampens with convergence. Null zone: gaze position where nystagmus is minimal, which leads to patient adopts head turn to use this position.

Does not have oscillopsia (brain adapts in infancy) Foveation periods: brief moments of foveal fixation during nystagmus cycle, which leads to usable vision.

### Etiology

**Idiopathic**: no underlying ocular or neurological abnormality. **Sensory defect nystagmus**: secondary to bilateral visual pathway disease. Bilateral congenital cataracts. Albinism (foveal hypoplasia, iris transillumination, chiasmal misrouting on VEP) Aniridia. Achromatopsia (rod monochromatism)

Optic nerve hypoplasia. Leber congenital amaurosis. Bilateral optic atrophy. Associated conditions: may coexist with strabismus (latent nystagmus)

### Evaluation

Complete ophthalmic exam: cycloplegic refraction, fundoscopy (foveal hypoplasia?), iris transillumination. OCT: foveal architecture (hypoplasia in albinism, achromatopsia) VEP: chiasmal misrouting pattern in albinism. ERG: if retinal dystrophy suspected (achromatopsia, LCA) MRI brain: if sensory cause not identified (rule out structural lesion)

### Management

Correct refractive error (often high) Treat amblyopia if asymmetric. Contact lenses: may dampen nystagmus (convergence and proprioceptive feedback) Prisms: base-out prisms to induce convergence, which leads to dampening. Surgery (Kestenbaum-Anderson procedure): moves null zone to primary position by bilateral recession-resection of horizontal recti; does not eliminate nystagmus but reduces head turn. Medical: no effective pharmacotherapy for INS.

<image>Eye movement recording showing the characteristic accelerating slow phase waveform of infantile nystagmus syndrome compared to the constant-velocity slow phase of vestibular nystagmus</image>

## Latent Nystagmus / Fusion Maldevelopment Nystagmus Syndrome (FMNS)

### Features

Appears or worsens when one eye is occluded. Conjugate jerk nystagmus with fast phase toward the viewing (uncovered) eye. Associated with infantile esotropia. Clinically significant: interferes with monocular VA testing (VA appears worse than binocular) Testing tip: use a +10 D lens (high plus) to fog one eye rather than occluding; reduces latent nystagmus effect.

## Acquired Nystagmus

### General Principles

Acquired nystagmus is MORE concerning than infantile nystagmus. Often causes oscillopsia (illusion of environmental movement) Must investigate for underlying neurological, vestibular, or toxic cause. Multiple types based on direction, waveform, and associated features.

### Gaze-Evoked Nystagmus

Jerk nystagmus appearing in eccentric gaze; fast phase toward direction of gaze. Caused by impaired neural integrator (brainstem/cerebellum) Most common type of acquired nystagmus. Causes: medications (sedatives, anticonvulsants, alcohol), cerebellar disease, brainstem lesion. Drug-induced gaze-evoked nystagmus: most benign cause; dose-related. Alexander law: intensity increases in direction of fast phase (eccentric gaze)

### Downbeat Nystagmus

Fast phase downward; worst in downgaze and lateral gaze. Most common cause: **Chiari I malformation** (cerebellar tonsils herniate through foramen magnum) Other causes: MS, spinocerebellar degeneration, lithium, magnesium deficiency, brainstem stroke. MRI of craniocervical junction mandatory. Treatment: 4-aminopyridine (potassium channel blocker) — first-line; also baclofen, clonazepam.

### Upbeat Nystagmus

Fast phase upward; usually in primary position. Lesions of the medulla or anterior vermis of cerebellum. Causes: brainstem stroke, MS, Wernicke encephalopathy, meningitis. Treatment: 4-aminopyridine, baclofen.

### Periodic Alternating Nystagmus (PAN)

Horizontal jerk nystagmus that reverses direction every 90–120 seconds. With intervening null period of ~10 seconds. Congenital or acquired (Chiari malformation, MS, drug toxicity, spinocerebellar degeneration) Treatment: **baclofen** (highly effective for acquired PAN); memantine.

### See-Saw Nystagmus

One eye elevates and intorts while the other depresses and extorts; then reverses. Pendular: associated with chiasmal/parachiasmal lesions (craniopharyngioma, pituitary adenoma) Jerk: brainstem lesion. MRI mandatory to evaluate sellar/parasellar region.

### Convergence-Retraction Nystagmus

Convergent jerking movements with globe retraction on attempted upgaze. Parinaud syndrome (dorsal midbrain syndrome) Due to co-contraction of horizontal recti. Causes: pinealoma (most common in young patients), MS, stroke, hydrocephalus. Associated features: light-near dissociation, eyelid retraction (Collier sign), upgaze palsy.

### Spasmus Nutans

Triad: nystagmus (asymmetric, small amplitude, high frequency), head nodding, torticollis. Onset: 4–12 months; resolves by age 3–5 years. Usually benign BUT must exclude chiasmal/hypothalamic glioma (can mimic spasmus nutans) MRI brain mandatory to rule out optic pathway glioma.

<image>Diagram illustrating different types of acquired nystagmus waveforms: gaze-evoked, downbeat, upbeat, periodic alternating, and see-saw nystagmus with their characteristic patterns and associated lesion locations</image>

## Vestibular Nystagmus

### Peripheral Vestibular Nystagmus

Unidirectional horizontal-torsional jerk nystagmus. Fast phase AWAY from the affected (lesioned) side. Associated with vertigo, nausea, hearing loss, tinnitus. Suppressed by visual fixation. Causes: BPPV, vestibular neuritis, Meniere disease, labyrinthitis.

### Central Vestibular Nystagmus

May be purely vertical, torsional, or direction-changing. NOT suppressed by visual fixation. Associated with brainstem/cerebellar signs. Causes: stroke, MS, tumor, Chiari malformation.

## Pharmacological Treatment of Nystagmus

### Available Agents

**4-Aminopyridine**: potassium channel blocker; best evidence for downbeat nystagmus and episodic ataxia type 2. **Baclofen**: GABA-B agonist; effective for PAN. **Gabapentin**: effective for acquired pendular nystagmus (especially in MS) **Memantine**: NMDA antagonist; effective for acquired pendular nystagmus. **Clonazepam**: benzodiazepine; broadly used but limited evidence; sedating.

## Clinical Pearls

Infantile nystagmus is horizontal in ALL gaze positions; vertical nystagmus in an infant is NOT INS — investigate urgently. Oscillopsia = acquired nystagmus; absence of oscillopsia = likely infantile onset. Spasmus nutans requires MRI to rule out optic pathway glioma — do not assume benignity. Downbeat nystagmus = MRI craniocervical junction for Chiari malformation. Convergence-retraction nystagmus = dorsal midbrain lesion (Parinaud syndrome); MRI brain. Drug-induced nystagmus (anticonvulsants, sedatives, lithium) is common — always review medication list.

Latent nystagmus: fog (do not occlude) the non-tested eye for accurate VA measurement. PAN responds dramatically to baclofen — one of the most gratifying treatments in neuro-ophthalmology. Head turn in INS: surgery (Kestenbaum) addresses the turn, not the nystagmus itself. Albinism workup: iris transillumination, foveal hypoplasia on OCT, crossed asymmetry on VEP.

## References

- Leigh RJ, Zee DS. The Neurology of Eye Movements. 5th ed. Oxford University Press; 2015.
- Hertle RW, Dell'Osso LF. Nystagmus in Infancy and Childhood. Oxford University Press; 2013.
- Strupp M, et al. Aminopyridines for the treatment of neurologic disorders. Neurol Clin Pract. 2017;7(1):65-76.
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 5: Neuro-Ophthalmology.