Nystagmus

Clinic

• Nystagmus is a condition of involuntary eye movement

• In normal eyesight, while the head rotates about an axis, distant visual images are sustained by rotating eyes in the opposite direction of the respective axis.
• The semicircular canals in the vestibule of the ear sense angular acceleration, and send signals to the nuclei for eye movement in the brain.
• From here, a signal is relayed to the extraocular muscles to allow one's gaze to fix on an object as the head moves.
• Nystagmus occurs when the semicircular canals are stimulated (e.g., by means of the caloric test, or by disease) while the head is stationary.
• The direction of ocular movement is related to the semicircular canal that is being stimulated.

Pathological vs physiological

• Pathological nystagmus is characterized by "excessive drifts of stationary retinal images that degrades vision and may produce illusory motion of the seen world:
• Oscillopsia (an exception is congenital nystagmus)
• Pathological nystagmus is the result of damage to one or more components of the vestibular system, including
  • Semicircular canals
  • Otolith organs
  • Vestibulocerebellum

Variations

• Central nystagmus occurs as a result of either normal or abnormal processes not related to the vestibular organ. For example, lesions of the midbrain or cerebellum can result in up- and down-beat nystagmus.
  • Gaze induced nystagmus occurs or is exacerbated as a result of changing one's gaze toward or away from a particular side which has an affected central apparatus.
• Peripheral nystagmus occurs as a result of either normal or diseased functional states of the vestibular system and may combine a rotational component with vertical or horizontal eye movements and may be spontaneous, positional, or evoked.
  • Positional nystagmus occurs when a person's head is in a specific position. An example of disease state in which this occurs is Benign paroxysmal positional vertigo (BPPV).
  • Post rotational nystagmus occurs after an imbalance is created between a normal side and a diseased side by stimulation of the vestibular system by rapid shaking or rotation of the head.
  • Spontaneous nystagmus is nystagmus that occurs randomly, regardless of the position of the patient's head.

Physiological nystagmus[edit]

A diagram of the caloric reflex test, which tests the vestibulo-ocular reflex (VOR) by irrigating the ear canal with warm or cold water to induce physiological nystagmus. Physiological nystagmus is a form of involuntary eye movement that is part of the vestibulo-ocular reflex (VOR), characterized by alternating smooth pursuit in one direction and saccadic movement in the other direction.

Variations[edit]

The direction of nystagmus is defined by the direction of its quick phase (e.g. a right-beating nystagmus is characterized by a rightward-moving quick phase, and a left-beating nystagmus by a leftward-moving quick phase). The oscillations may occur in the vertical, horizontal or torsional planes, or in any combination. The resulting nystagmus is often named as a gross description of the movement, e.g. downbeat nystagmus, upbeat nystagmus, seesaw nystagmus, periodic alternating nystagmus.

These descriptive names can be misleading, however, as many were assigned historically, solely on the basis of subjective clinical examination, which is not sufficient to determine the eyes' true trajectory.

• Optokinetic (syn. opticokinetic) nystagmus: a nystagmus induced by looking at moving visual stimuli, such as moving horizontal or vertical lines, and/or stripes. For example, if one fixates on a stripe of a rotating drum with alternating black and white, the gaze retreats to fixate on a new stripe as the drum moves. This is first a rotation with the same angular velocity, then returns in a saccade in the opposite direction. The process proceeds indefinitely. This is optokinetic nystagmus, and is a source for understanding the fixation reflex.
• Postrotatory nystagmus: if one spins in a chair continuously and stops suddenly, the fast phase of nystagmus is in the opposite direction of rotation, known as the "post-rotatory nystagmus", while slow phase is in the direction of rotation.