Nystagmus patterns and investigation

Classification & Waveform Electrophysiology of Nystagmus

Nystagmus is defined as an involuntary, rhythmic, biphasic oscillatory movement of one or both eyes, initiated by a slow drift away from the target of fixation. It is classified broadly into physiological forms and pathological forms.

Physiological Nystagmus

1. Optokinetic Nystagmus (OKN): A physiological tracking response to a continuously moving visual scene. It consists of a smooth pursuit slow phase tracking the target, followed by a fast saccadic refixation phase resetting the eye in the opposite direction.
2. Vestibulo-Ocular Reflex (VOR) Nystagmus: Elicited by angular or linear head acceleration stimulating the semicircular canals, or via caloric thermal stimulation of the horizontal semicircular canals (summarized by the COWS mnemonic: Cold Opposite, Warm Same for the direction of the fast reset phase).
3. End-Point Nystagmus: A fine, low-amplitude horizontal jerk nystagmus occurring at extreme angles of eccentric horizontal gaze ($>35^\circ$). It is non-sustained, fatigue-prone, and absent in primary position.

Pathological Forms: Infantile & Childhood Nystagmus

1. Infantile Nystagmus Syndrome (INS)

Formerly termed congenital motor nystagmus, INS typically manifests between 6 weeks and 3 months of life:

• Waveform Characteristics: Electro-oculography reveals a characteristic accelerating slow-phase velocity (the eyes drift away from fixation with exponentially increasing speed until a corrective saccade intervenes), reflecting congenital instability of the neural integrator. Oscillation may be pendular or jerk.
• Plane-Locked Horizontal Motion: The oscillation remains strictly horizontal even during upgaze and downgaze (plane-locked).
• Convergence Dampening: Nystagmus amplitude and frequency decrease significantly during convergence, explaining why children with INS often hold reading material close to achieve optimal visual acuity.
• Fixation & Anxiety Accentuation: Nystagmus intensifies during active fixation effort, visual stress, or emotional anxiety.
• Inverted Optokinetic Response: Rotating an OKN drum elicits an anomalous fast phase beating in the same direction as the moving drum stripes (opposite to the normal physiological response).
• The Null Zone: An eccentric gaze angle where nystagmus intensity is minimized and visual foveation time is maximized. To utilize this null zone, the patient adopts a compensatory abnormal head posture (AHP) (for instance, a face turn to the left to place the visual axes in right gaze).

2. Latent Nystagmus (LN) & Manifest Latent Nystagmus (MLN)

• Intimately associated with infantile esotropia and dissociated vertical deviation (DVD).
• Electrophysiologically characterized by a decelerating slow-phase velocity.
• Evoked when one eye is covered (or in MLN, when retinal illumination is unequal). The fast phase always beats toward the uncovered, fixating eye.

3. Spasmus Nutans

• Presents between 6 and 18 months of age with the classic diagnostic triad:
  1. Asymmetric, disconjugate, high-frequency, low-amplitude "shimmering" horizontal-torsional nystagmus.
  2. Intermittent head nodding.
  3. Compensatory torticollis.
• Although idiopathic spasmus nutans is a benign condition that resolves spontaneously by age 3 to 5 years, its presentation is clinically indistinguishable from nystagmus caused by optic pathway gliomas (pilocytic astrocytomas of the chiasm or hypothalamus). Assess visual function, discs, pupils, refraction and neurological development. Consider MRI to exclude a visual-pathway lesion, urgently with optic nerve dysfunction or neurological red flags; ERG and genetic testing may be appropriate when a retinal disorder is suspected.

Acquired Pathological Nystagmus & Neuro-Anatomical Localization

Case: why a head turn changes the examination

A child consistently turns the face right while viewing a distance target. Measure acuity binocularly and monocularly, observe oscillation in different gaze positions, and document the position with least oscillation or best visual function. A null in left gaze can explain a rightward face turn, but habitual head posture also occurs with incomitant strabismus or poor vision. Assess convergence effects, refraction, ocular alignment and sensory function before attributing the posture to one mechanism. If a sustained posture limits function, optical and surgical options can shift the useful gaze position, but they do not necessarily eliminate the underlying oscillation. Record the preoperative null, target distance and binocular conditions so postoperative change can be judged fairly. Newly acquired nystagmus or neurological signs require a different investigation pathway from stable infantile nystagmus.